U.S. patent application number 14/396798 was filed with the patent office on 2015-05-14 for method for producing molds and cores for metal casting and molds and cores produced according to this method.
The applicant listed for this patent is ASK Chemicals GmbH. Invention is credited to Diether Koch, Oliver Schmid.
Application Number | 20150129155 14/396798 |
Document ID | / |
Family ID | 48538933 |
Filed Date | 2015-05-14 |
United States Patent
Application |
20150129155 |
Kind Code |
A1 |
Koch; Diether ; et
al. |
May 14, 2015 |
Method for producing molds and cores for metal casting and molds
and cores produced according to this method
Abstract
The invention relates to a method for producing casting molds
and cores, in which a foundry base material comprising at least one
refractory material and a binder curable by CO.sub.2, preferably
based on water glass, is cured by gassing with CO.sub.2 and
flushing with a second gas. The invention further relates to molds
and cores produced according to this method.
Inventors: |
Koch; Diether; (Mettmann,
DE) ; Schmid; Oliver; (Langenfeld, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASK Chemicals GmbH |
Hilden |
|
DE |
|
|
Family ID: |
48538933 |
Appl. No.: |
14/396798 |
Filed: |
April 26, 2013 |
PCT Filed: |
April 26, 2013 |
PCT NO: |
PCT/DE2013/000223 |
371 Date: |
October 24, 2014 |
Current U.S.
Class: |
164/16 ; 164/349;
164/369 |
Current CPC
Class: |
B22C 1/162 20130101;
B22C 1/188 20130101; B22C 1/18 20130101; B22C 9/02 20130101; B22C
9/123 20130101 |
Class at
Publication: |
164/16 ; 164/349;
164/369 |
International
Class: |
B22C 1/16 20060101
B22C001/16; B22C 9/02 20060101 B22C009/02; B22C 1/18 20060101
B22C001/18 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2012 |
DE |
102012103705.1 |
Claims
1. A method for producing casting molds and cores, comprising the
steps of: preparing a molding mixture comprising at least one
refractory molding base material and an inorganic binder,
introducing the molding mixture into a mold, and flushing the
molding mixture while the molding mixture is curing in the mold,
through the steps of: flushing with a first gas, being gaseous
carbon dioxide or containing gaseous carbon dioxide; and flushing
with a second gas, the second gas having a lower percentage of
carbon dioxide, including no carbon dioxide, than the first
gas.
2. The method of claim 1, wherein: for introducing the molding
mixture into the mold, a core-shooting machine with compressed air
is used, where the mold is a molding tool that is flushed with the
first and second gas.
3. The method of claim 1, wherein the mold either is not heatable
or is heated to temperatures of less than 70.degree. C., preferably
less than 60.degree. C. and especially preferably less than
40.degree. C.
4. The method of claim 1, wherein the inorganic binder is water
glass, especially water glass with a SiO.sub.2/M.sub.2O molar ratio
of 1.6 to 4.0, preferably 2.0 to less than 3.5 with M is selected
from the group consisting of: lithium, sodium, potassium, and
mixtures thereof.
5. The method of claim 1, wherein the molding mixture contains a
maximum of 1 wt %, preferably a maximum of 0.5 wt % and
particularly preferably a maximum of 0.2 wt % organic
compounds.
6. The method of claim 1, wherein the second gas contains less than
10 vol % CO.sub.2, especially less than 2 vol % CO.sub.2, and
especially is air or nitrogen or a mixture thereof.
7. The method of claim 1, wherein the first gas contains at least
25 mol % CO.sub.2, especially at least 50 mol % CO.sub.2 and
preferably at least 80 mol % CO.sub.2.
8. The method of claim 1, wherein at least one of the first and the
second gas is applied at a gas flow that amounts to 0.5 to 600
L/min (standard liters), preferably to 0.5 to 300 L/min and
particularly preferably 0.5 L/min to 100 L/min.
9. The method of claim 1, wherein at least one of the first and the
second gas is used at a gas flow that amounts to 0.5 to 30 L/min
(standard liters), preferably to 0.5 to 25 L/min and particularly
preferably 0.5 L/min to 20 L/min, preferably at a gas temperature
of 15 to 40.degree. C.
10. The method of claim 1, wherein at least one of the first and
the second gas is at a charging pressure with reference to the mold
that is between 0.5 bar and 10 bar, preferably between 0.5 bar and
8 bar and particularly preferably between 0.5 and 6 bar.
11. The method of claim 1, wherein the first gas relative to the
second gas is applied at a ratio of gassing times of 2:98 to 90:10,
preferably 2:98 to 20:80 and particularly preferably 5:95 to 30:70
and especially with the first gas amounts maximally to 60% of the
sum of the gassing time with the first and second gas.
12. The method of claim 1, wherein the refractory molding base
material has a mean particle diameter in the range of from 100 to
600 .mu.m, preferably from 150 to 500 .mu.m, and is selected from
the group consisting of: quartz-, zirconium- or chrome ore sand,
olivine, vermiculite, bauxite, fireclay and combinations
thereof.
13. The method of claim 1, wherein the binder, especially water
glass, is contained in the refractory molding base material at up
to 0.5 to 5 wt %, preferably 1 to 3.5 wt %, based on the refractory
molding base material, in the case of water glass, based on a
solids fraction of 25 to 65 wt %, preferably 30 to 60 wt %.
14. The method of claim 1, wherein the molding mixture further
comprises amorphous SiO.sub.2, especially synthetic amorphous
SiO.sub.2 and preferably with a mean particle size of between 0.05
.mu.m and 10 .mu.m, especially between 0.1 .mu.m and 5 .mu.m,
particularly preferably between 0.1 .mu.m and 2 .mu.m, and
independently thereof, the amorphous SiO.sub.2 has a BET surface
area amounting to 1 to 200 m.sup.2/g, especially 1 to 50 m.sup.2/g
and particularly preferably 1 to 30 m.sup.2/g.
15. The method of claim 14, wherein the amorphous SiO.sub.2 is
present in the range of 0.1 to 2 wt %, preferably 0.1 to 1.5 wt %,
in each case based on the refractory molding base material and
independently thereof, based on the weight of the binder with 2 to
60 wt %, particularly preferably 4 to 50 wt %.
16. The method of claim 1, wherein in the flushing step, the first
gas is introduced into the mold at a temperature of 15 to
120.degree. C., preferably from 15 to 100.degree. C. and
particularly preferably from 25 to 80.degree. C., and independently
thereof, the second gas is introduced into the mold at a
temperature within the same temperature interval or a temperature
of 40 to 250.degree. C., and preferably the temperature of the
second gas upon introduction into the mold is greater than that of
the first gas.
17. The method of claim 14, wherein: wherein the amorphous
SiO.sub.2 has a water content of less than 15 wt %, especially less
than 5 wt % and particularly preferably less than 1 wt % and
independently of this is used especially as a powder.
18. The method of claim 1, wherein: the first gas and the second
gas are introduced into the mold for flushing in any arbitrary
order and number of introduction processes, but at least
temporarily separate from one another, wherein preferably the
second gas is conducted through the mold last, and especially first
and only once the first gas and subsequently the second gas is
conducted through the mold.
19. A mold or core produced according to claim 1.
Description
[0001] The invention relates to a method for producing casting
molds and cores, in which a molding mixture consisting of at least
one refractory material and a binder curable with CO.sub.2,
preferably containing or consisting of water glass, is cured by
gassing with CO.sub.2 or a CO.sub.2-containing gas and purging with
a second gas containing no CO.sub.2, or at least a minor fraction
of CO.sub.2. In addition the invention relates to the molds and
cores produced according to this method.
PRIOR ART
[0002] Molds and cores are generally produced using a refractory
molding base material, e.g., quartz sand, and a suitable binder.
The refractory molding base material in such processes is
preferably present in free-flowing form, so that the mixture of
molding base material and binder, the so-called molding mixture,
can be filled into a hollow mold, compressed there, and cured. The
binder produces solid adhesion between the particles of the molding
base material, so that the molds and cores attain the required
mechanical stability.
[0003] During casting, molds form the outer walls for the casting;
cores are used when cavities within the casting are necessary. It
is not absolutely necessary for molds and cores to consist of the
same material. For example, the external shape of the casting is
created using permanent metal molds. A combination of molds and
cores produced in various ways is also possible. As will be
explained in the following based on cores, similar statements are
also true for molds (casting molds) produced according to the same
method and vice-versa.
[0004] For producing cores, both organic and inorganic binders
curable by cold or hot methods may be used. Cold methods are
methods which are essentially performed at room temperature without
heating the molding tool used for core manufacture.
[0005] In this process, curing is usually performed by a chemical
reaction induced, for example, by the fact that a gas is passed
through the molding mixture to be cured. In hot methods, after
molding, the molding mixture is heated by the heated molding tool
to a sufficiently high temperature in order, for example, to expel
the solvent contained in the binder and/or to initiate a chemical
reaction by which the binder is cured, for example, by
cross-linking.
[0006] Curing water glass-binders with CO.sub.2 is known, for
example, from GB 654817. In the 1950s and 1960s, the water
glass-CO.sub.2 method was widely used. One of the weaknesses of
this method is that the cores produced from it have relatively low
strengths, especially immediately after manufacturing. Longer
gassing times with CO.sub.2, to be sure, produce higher initial
strengths, but at the same time the strengths after 1 or 2 days of
storage are diminished. In addition to relatively low initial and
final strengths, the water glass-CO.sub.2 method permits only low
to moderate manufacturing speeds.
[0007] Therefore it has been suggested that standard CO.sub.2
curing be combined with a subsequent so-called "hot air process,"
as described by Y. A. Owusu (Y. A. Owusu, Ph.D. Dissertation
"Sodium Silicate Bonding in Foundry Sands," Pennsylvania State
University, May 1, 1980, p. 88, 102-103 and AFS Transactions, vol.
89, 1981, pp. 47-54). The hot air process is defined as oven curing
following the CO.sub.2 gassing. This is also confirmed by the
dissertation of Y. A. Owusu. This method, at least in the case of
some water glass binders, gives better strengths than in the case
of pure CO.sub.2 curing, but has the drawback that the
manufacturing time for the cores is extended from a few seconds to
several minutes.
[0008] DE 102011010548-A1 describes a method for curing water
glass-bonded molding mixtures wherein a combination of air and
carbon dioxide flow is used. It was found in this instance that the
molding mixture must first be gassed with air and then with
CO.sub.2 or an air-carbon dioxide mixture. Furthermore it is of
great significance for this invention that the alkali silicate
solutions used have a weight ratio of SiO.sub.2 to metal oxide in
the range of 1.5:1 to 2.0:1.
[0009] An additional patent that describes a method of this type is
WO 80/01254 A1. This discloses a method for curing a water
glass-containing molding mixture which is heated at a temperature
of 110 to 180.degree. C. while simultaneously passing through a
CO.sub.2 or a CO.sub.2-air mixture.
[0010] PL 129359 B2 describes a binder for producing casting molds
for metal casting made of water glass and urea resin. In this case
the curing is performed by flushing the molding mixture with a
CO.sub.2-air mixture. In this process it is advantageous for the
gases to be heated to temperatures of 60-200.degree. C. An
additional publication describing the use of a CO.sub.2-air mixture
is CN 94111187 A.
[0011] From EP 2014392 B1 the use of amorphous spherical SiO.sub.2,
which is available in more than two particle size classes, is
known. EP 2014392 B1, however, does not disclose the use of a
second gas stream and introduces the SiO.sub.2 in aqueous
suspension, which as a result of the increased introduction of
water is disadvantageous for the (early) strengths of molds
produced according to the method of the invention provided
here.
[0012] Good strengths even after brief curing times are necessary
to reliably manage the increasingly complex, thin-walled casting
molds that are required more and more frequently today and at the
same time guarantee high productivity. Therefore it is not
surprising that the water glass-CO.sub.2 method rapidly lost its
significance with the arrival of processes based on organic
binders, especially the so-called Ashland Polyurethane ColdBox
method.
[0013] However, all organic binders have the drawback that they
undergo thermal decomposition during casting and can release
harmful materials such as benzene, toluene or xylenes. In addition
the organic binder systems already release solvents to the
environment during core manufacturing and storing, or
unpleasant-smelling gases are used as curing catalysts. To be sure
it has become possible as a result of various measures to avoid all
these emissions, but they cannot be avoided in the case of organic
binders.
[0014] For this reason, for years there have been increasing
attempts to develop binders based completely on inorganic materials
or containing at most a small fraction of organic compounds. To
achieve high strengths within short time periods, for example, the
pathway was followed of performing the curing in a hot tool and if
desired additionally passing hot air through the molding mixture to
remove the water present as solvent as completely as possible. One
such system is described, for example, in EP 1802409 B1 (U.S. Pat.
No. 7,770,629). However, these methods have the drawback that the
tools must be designed so that they can be heated and the heating
causes additional energy consumption, which represents a
considerable cost burden for the method.
Problem of the Invention
[0015] The inventors have therefore posed the problem of developing
a method that would make it possible to manufacture molds and cores
using an inorganic-based, CO.sub.2-curable binder even in unheated
tools, wherein the strengths should already be substantially higher
with the same binder and identical binder content and without
subsequent heat treatment than in the previously known curing with
CO.sub.2, especially immediately after removal from the tool.
SUMMARY OF THE INVENTION
[0016] This task will be accomplished using a method with the
features of claim 1. Advantageous further embodiments of the method
according to the invention are the subject of the independent
claims or will be described in the following.
[0017] Surprisingly it was found that by the combination of
CO.sub.2 curing with a first gas and flushing the molding mixture
with a second gas, in the following also called flushing gas, cores
with good strengths can be achieved. In this process the first gas
and the second gas are not to be set up so that the first gas is
applied before the second gas; on the contrary, the sequence may be
as desired and in addition the first and/or the second gas can also
be applied several times. However, it is preferred that the second
gas be introduced last, and independently of this that the first
gas be introduced into the mold first.
[0018] For the first gas, also called CO.sub.2 gas in the
following, gas temperatures during gassing of between 15.degree. C.
and 120.degree. C., preferably 15.degree. C. and 100.degree. C.,
and particularly preferably between 25.degree. C. and 80.degree. C.
are advantageous. The gas temperature, as used in the further
description of the method according to the invention as well, means
the temperature of the gas upon entry into the molding tool. The
second gas preferably also has gas temperatures of 15 to
120.degree. C., preferably between 15.degree. C. and 100.degree.
C., and particularly preferably between 25.degree. C. and
80.degree. C., but higher temperatures frequently make it possible
to shorten the flushing time. There is no upper limit to this based
on the curing mechanism. In practice, depending on the shape and
size of the core or the mold, the temperatures will be between
40.degree. C. and 250.degree. C., preferably between 50.degree. C.
and 200.degree. C. First and foremost, financial considerations
oppose the use of very high temperatures, since the costs of the
heaters required for this purpose increase greatly with increasing
power and the cost for effectively insulating the lines is very
high. For example, the temperature of the first gas is
approximately the same as the temperature of the second gas.
Preferably, however, the temperature of the second gas is higher
than the temperature of the first gas.
[0019] As a result of the above-named measures it is advantageous
to control the cooling of the molding mixtures seen during gassing
by controlling the temperatures of the two gas streams.
[0020] The use of available heatable tools is by no means excluded
in the methods described; through the method of the invention the
discloses possibility of operating the tools either cold, i.e., at
ambient temperature or room temperature of 15 to 30.degree. C. or
at lower than usual temperatures, i.e., less than 200.degree. C. or
less than 120.degree. C. or even less than 100.degree. C., to lower
the costs. In particular, non-heatable tools may advantageously be
used. Such tools are not heatable, i.e., they have no heating
devices of their own such as an electrical heater, but can be
heated by the gas introduced at controlled temperature. The method
according to the invention also does not rule out the possibility
of subsequently subjecting the cores or molds to additional heat
treatment.
[0021] The method according to the invention comprises the
following steps: [0022] producing a molding mixture from at least
one refractory molding base material and a CO.sub.2-curable binder,
preferably based on water glass [0023] molding the molding mixture
[0024] flushing the molding mixture with the CO.sub.2 gas [0025]
optionally flushing the molding mixture with CO.sub.2 gas and the
flushing gas alternately [0026] flushing the molding mixture with a
second gas (flushing gas).
DETAILED DESCRIPTION OF THE INVENTION
[0027] The procedure generally followed in producing the molding
mixture is that the refractory molding base material is taken
initially and the binder is added under agitation. The agitation is
continued until uniform distribution of the binder on the molding
base material is guaranteed.
[0028] Then the molding mixture is placed in the desired mold. In
this process, the usual methods for molding are used. For example,
the molding mixture can be shot into the core-molding machine using
compressed air. The curing is performed subsequently, in that
(especially) first the CO.sub.2 gas is passed through the mold
filled with the molding mixture, followed by flushing with the
second gas. The expert can immediately see that a variety of
designs are possible for this process.
[0029] For example, it is by no means necessary (but is frequently
advantageous) to use pure CO.sub.2 (for example, industrial-grade)
as the CO.sub.2 gas. To achieve the shortest possible curing times,
however, it is advantageous for the CO.sub.2 gas to contain at
least 50 vol % CO.sub.2, preferably at least 80 vol % CO.sub.2.
[0030] On the other hand it is not necessary for the second gas to
be completely CO.sub.2-free, for example, synthetic air or
nitrogen. Air is preferred for cost reasons. CO.sub.2 can even be
added to the flushing gas, but no more than 10 vol %, particularly
preferably no more than 5 vol %, especially no more than 2 vol % or
even no more than 1 vol %.
[0031] The transition from CO.sub.2 gas to the second gas need not
take place in a single step; stepwise or fluid transitions are
likewise possible. In addition, both gases or even one of the two
may be pulsed through the molding mixture.
[0032] An additional variant consists of the fact that, first, part
of the water present in the molding mixture is removed by brief
gassing with the gas of low CO.sub.2 content as a flushing gas,
then the binder is hardened with the CO.sub.2 gas and optionally,
for further drying, the core is treated again with the low CO.sub.2
gas.
[0033] The addition of CO.sub.2 can take place either by
establishing a certain CO.sub.2 flow for the CO.sub.2 gas or a
certain gassing pressure. Which of the two possibilities is
selected depends on many factors in practice, for example, the
geometry and size of the core, the tightness of the molding tool,
the ratio of gas inlet to gas outlet, the gas permeability of the
molding base material, the diameter of the gas line, the binder
content, the desired gassing time, etc. To optimize the properties,
the gassing parameters can be adjusted depending on the
requirements of the selected core or mold geometry within the
framework of the present disclosure and the knowledge that is
customarily possessed by a person skilled in the art. As a rule a
CO.sub.2 flow of between 0.5 L/min and 600 L/min will be selected,
preferably between 0.5 L/min and 300 L/min and particularly
preferably between 0.5 L/min and 100 L/min (in each case standard
liters). According to another embodiment the CO.sub.2 flow can be
selected between 0.5 L/min and 30 L/min, preferably between 0.5
L/min and 25
[0034] L/min and particularly preferably between 0.5 L/min and 20
L/min. This embodiment is particularly advantageous at low gas
temperatures for the CO.sub.2 or the CO.sub.2-containing gas of 15
to 40.degree. C.
[0035] In the case of pressure regulation the pressures of the
CO.sub.2 gas usually vary between 0.5 bar and 10 bar, preferably
between 0.5 bar and 8 bar and particularly preferably between 0.5
bar and 6 bar.
[0036] In the case of air as the flushing gas this can be taken
from the pressure line usually available in foundries, so that for
purely practical reasons the pressure prevailing therein represents
the upper limit for gassing. The lower limit for effective gassing
with air is about 0.5 bar. At a lower pressure the gassing time
would be greatly prolonged, which would be associated with a loss
of productivity.
[0037] All pressure statements, unless indicated otherwise, relate
to gauge pressure, i.e., a pressure above ambient pressure.
[0038] It is possible for the ratio of the gassing times of the
first gas (CO.sub.2 gas) and the second gas (flushing gas) to one
another to vary, for example, between 2:98 to 90:10, preferably
between 2:98 to 20:80 and particularly preferably between 5:95 to
30:70. However, since it is also a goal in this application to keep
the CO.sub.2 consumption as low as possible, the gassing time with
the CO.sub.2 gas preferably should amount to no more than 60% of
the total gassing time, particularly preferably no more than
50%.
[0039] By suitably selecting the gassing parameters and the layout
of the tool it is possible to ensure that even for larger cores,
manufacturing times that correspond to those with organic binders,
e.g., less than 3 minutes, preferably less than 2.5 minutes and
particularly preferably less than 2 minutes are possible.
Optionally such optimization can also be performed with the aid of
computer simulation.
[0040] It is easily possible to modify the curing of the molding
material by known methods, for example, by applying a vacuum. In
addition other known steps may follow the actual curing process,
for example, treatment with microwaves or heating in an oven.
[0041] The usual materials may be employed as molding base
materials for producing casting molds. Suitable materials are, for
example, quartz, zirconium or chromium ore sand, olivine,
vermiculite, bauxite and fireclay. It is not necessary to use
exclusively new sands in such cases. For the sake of resource
conservation and to avoid waste disposal costs it is even
advantageous to use the highest possible fraction of regenerated
old sand.
[0042] For example, a suitable sand is described in WO 2008/101668
(=US 2010/173767 A1). Also suitable are regenerates obtained by
washing and subsequent drying. Usable but less preferred are
regenerates obtained by purely mechanical treatment. As a rule the
regenerate can replace at least about 70 wt % of the new sand,
preferably at least about 80 wt % and particularly preferably at
least about 90 wt %.
[0043] Also usable as refractory molding base materials are
synthetic molding materials such as glass beads, glass granulate,
the spherical ceramic building base materials known under the name
of "Cerabeads" or "Carboaccucast" or aluminum silicate hollow
microspheres (so-called microspheres). Such aluminum silicate
hollow microspheres are marketed, for example, by Omega Minerals
Germany GmbH, Norderstedt, in various qualities with different
aluminum oxide contents under the name of "Omega-Spheres."
Corresponding products are available from PQ Corporation (USA)
under the name of "Extendospheres."
[0044] The mean diameter of the molding base material is generally
between 100 .mu.m and 600 .mu.m, preferably between 120 .mu.m and
550 .mu.m and particularly preferably between 150 .mu.m and 500
.mu.m. The particle size can be determined, for example, by
screening according to DIN ISO 3310.
[0045] In casting experiments with aluminum it was found that when
synthetic molding base materials are used, for example, in the case
of glass beads, glass granulates or microspheres, less molding sand
remains sticking to the metal surface than when pure quartz sand is
used. The use of artificial molding base materials therefore makes
it possible to achieve smoother casting surfaces, so that expensive
after-treatment by blasting is not necessary, or at least is
necessary to a considerably lesser extent.
[0046] It is not necessary in such instances for all of the molding
base material to consist of the synthetic molding base
materials.
[0047] The preferred fraction of the synthetic molding base
material is at least about 3 wt %, particularly preferably at least
5 wt %, especially preferably at least 10 wt %, advantageously at
least about 15 wt %, particularly advantageously at least about 20
wt %, based on the total quantity of the refractory molding base
material. The refractory molding base material preferably is in the
free-flowing state, so that the molding mixture according to the
invention can be processed in conventional core shooting
machines.
[0048] As an additional component the molding mixture according to
the invention contains a binder based on water glass. The water
glass that may be used includes the usual water glasses such as
those already in use as binders in molding mixtures.
[0049] Water glasses are aqueous solutions of alkali silicates,
especially lithium, sodium and potassium silicates, and are also
used as binders in other areas, for example, in construction. The
water glass is manufactured, for example, on a large industrial
scale by melting quartz sand and alkali carbonates at temperatures
of 1350.degree. C. to 1500.degree. C. In this process the water
glass is initially obtained in the form of a blocky, solid glass,
which is dissolved in water under the application of temperature
and pressure. An additional method for producing water glasses is
the direct dissolution of quartz sand with sodium hydroxide.
[0050] The alkali silicate solution obtained can then be adjusted
to the desired SiO.sub.2/M.sub.2O molar ratio by adding alkali
hydroxides and/or alkali oxides or the hydrates thereof. In
addition the composition of the alkali silicate solution can be
adjusted by dissolving alkali silicates with a different
composition. In addition to alkali silicate solutions,
water-containing alkali silicates in solid form, for example, the
product groups of Kasolv, Britesil or Pyramid from PQ Corporation
are available.
[0051] The binders can also be based on water glasses containing
more than one of the alkali ions mentioned, for example, the
lithium-modified water glasses known from DE 2652421 A1 (=GB
1532847). Furthermore the water glasses may also contain polyvalent
ions such as boron or aluminum (corresponding compounds are
described in EP 2305603 A1 (=WO2011/042132 A1).
[0052] The water glass preferably has an SiO.sub.2/M.sub.2O molar
ratio in the range of 1.6 to 4.0, especially 2.0 to less than 3.5,
wherein M represents lithium, sodium or potassium.
[0053] The water glasses preferably have a solids fraction of
greater than or equal to 30 wt %, particularly preferably greater
than or equal to 33 wt % and especially preferably greater than or
equal to 36 wt %. The upper limits for the solids content of the
preferred water glass are less than or equal to 65 wt %,
particularly preferably less than or equal to 60 wt % and
especially preferably less than 55 wt %. The solids fraction is
determined on a Sartorius MA30 Moisture Analyzer, wherein about 3-4
g of the binder is heated on an aluminum dish (diameter=10 cm,
height=0.7 cm) at a temperature of 140.degree. C. to constant
weight.
[0054] In addition the water glasses have a solids fraction,
calculated as M.sub.2O and SiO.sub.2, in the range of 25 to 65 wt
%, preferably 30 to 60 wt %. The solids fraction is based on the
quantity of alkali silicates, calculated as SiO.sub.2 and M.sub.2O,
contained in the water glass.
[0055] Depending on the application and desired liquid level,
between 0.5 wt % and 5 wt % of the binder based on water glass are
used, preferably between 0.75 wt % and 4 wt %, particularly
preferably between 1 wt % and 3.5 wt %, in each case based on the
molding base material. The information here is based on water
glasses with solids contents as given above and includes the
diluent, water.
[0056] Based on the amount of alkali silicates, calculated as
M.sub.2O and SiO.sub.2, which are added to the molding base with
the inorganic binder according to the invention, without
considering the diluent, the quantity of the binder used is 0.2 to
2.5 wt %, preferably 0.3 to 2 wt % relative to the molding base
material, wherein M.sub.2O has the meaning stated above.
[0057] The molding mixture also preferably contains a quantity of a
particulate metal oxide selected from the group of silicone
dioxide, aluminum dioxide, titanium dioxide and zinc oxide and
mixtures thereof or mixed oxides, especially silicone dioxide,
aluminum dioxide and/or alumosilicate. The particle size of these
metal oxides preferably amounts to less than 300 .mu.m, preferably
less than 200 .mu.m, particularly preferably less than 100 .mu.m
and has, for example, a mean primary particle size between 0.05
.mu.m and 10 .mu.m.
[0058] The particle size can be determined by screen analysis.
Particularly preferably the screen residue on a screen with a mesh
size of 63 .mu.m amounts to less than 10 wt %, preferably less than
8 wt %. Particularly preferably, silicone dioxide is used as the
particulate metal oxide, wherein synthetically produced amorphous
silicone dioxide is particularly preferred.
[0059] Precipitated silica and/or pyrogenic silica is preferably
used as the particulate silicone dioxide.
[0060] Optionally the molding mixture can contain amorphous
SiO.sub.2. Surprisingly it was found that the addition of amorphous
SiO.sub.2 to the molding mixture not has only positive effects in
the hot curing described in EP 1802409 B1 (=U.S. Pat. No.
7,770,629), but also in the curing with CO.sub.2 gas and flushing
gas. In terms of the initial strengths, the strength increase comes
out to be much greater than that of raising the binder content by
the same amount, while in terms of the final strength the higher
binder content turns out more advantageously, so that alternatives
are available according to the desired effect.
[0061] The amorphous SiO.sub.2 preferably used according to the
present invention has a water content of less than 15 wt %,
especially less than 5 wt % and particularly preferably less than 1
wt %. In particular, the amorphous SiO.sub.2 is used as a
powder.
[0062] Both synthetically manufactured and naturally occurring
silicas can be used as the amorphous SiO.sub.2. However, the
latter, known, for example, from DE 102007045649 are not preferred,
since they generally contain appreciable crystalline fractions and
therefore are classified as carcinogenic.
[0063] Synthetic means non-naturally occurring amorphous SiO.sub.2,
i.e., the manufacturing thereof comprises a chemical reaction,
e.g., the manufacturing of silica sols by ion exchange processes
from alkali silicate solutions, precipitation from alkali silicate
solutions, flame hydrolysis of silicone tetrachloride or the
reduction of quartz sand with coke in an electric furnace during
the manufacturing of ferrosilicon and silicon. The amorphous
SiO.sub.2 produced according to the two last-named methods is also
called pyrogenic SiO.sub.2.
[0064] Sometimes synthetic amorphous SiO.sub.2 is defined
exclusively as precipitated silica (CAS No. 112926-00-8) and
SiO.sub.2 produced by flame hydrolysis (pyrogenic silica, fumed
silica, CAS No. 112945-52-5), whereas the product produced in
ferrosilicon or silicon manufacturing is known merely as amorphous
SiO.sub.2 (Silica Fume, Microsilica, CAS No. 69012-64-12). For the
purposes of the present invention the product produced in
ferrosilicon or silicon manufacturing will also be defined as
synthetic amorphous SiO.sub.2.
[0065] Precipitated silicas and pyrogenic, i.e., flame-hydrolytic
or electric arc-manufactured SiO.sub.2 are preferably used.
Particularly preferably used is amorphous SiO.sub.2 produced by
thermal decomposition of ZrSiO.sub.4 (see DE 102012020509) as well
as SiO.sub.2 produced by oxidation of metallic Si with an
oxygen-containing gas (see DE 102012020510). Also preferred is
quartz glass powder (principally amorphous SiO.sub.2) that was
produced by melting and rapid cooling from crystalline quartz, so
that the particles are spherical and not present as splinters (see
DE 102012020511). The mean primary particle size of the synthetic
amorphous silicon dioxide can be between 0.05 .mu.m and 10 .mu.m,
especially between 0.1 .mu.m and 5 .mu.m, particularly preferably
between 0.1 .mu.m and 2 .mu.m. The primary particle size was
determined by dynamic light scattering on a Horiba LA 950 and
checked by scanning electron microscopy (SEM microscopy) on a Nova
NanoSEM 230 from the firm of FEI. Furthermore, with the aid of the
SEM images, details of the primary particle size could be made
visible down to the order of magnitude of 0.01 .mu.m. The SiO.sub.2
samples were dispersed in distilled water for the SEM measurements
and then applied to an aluminum holder bonded to a copper strip
before the water was evaporated.
[0066] Furthermore the specific surface area of the synthetic
amorphous silicone dioxide was determined with the aid of gas
adsorption measurements (BET method) according to DIN 66131. The
specific surface of the synthetic amorphous SiO.sub.2 is between 1
and 200 m.sup.2/g, especially between 1 and 50 m.sup.2/g,
particularly preferably between 1 and 30 m.sup.2/g. Optionally the
products can also be mixed, for example, to systematically obtain
mixtures with certain particle size distributions.
[0067] The amorphous SiO.sub.2 types mentioned easily form larger
aggregates by agglomeration. For a uniform distribution of the
amorphous SiO.sub.2 in the molding material mixture it is important
that the aggregates can break up again into smaller units upon
mixing or do not exceed a certain size from the beginning.
Preferably the screen residue--used to describe the extent of
aggregation--amounts to no more than about 10 wt %, particularly no
more than about 5 wt % and most particularly preferably no more
than about 2 wt % upon passage through a screen with a 45 .mu.m
mesh size (325 mesh).
[0068] Depending on the manufacturing method and the manufacturer,
the purity of the amorphous SiO.sub.2 can vary greatly. Types
containing at least 85 wt % SiO.sub.2 proved suitable, preferably
at least 90 wt % and particularly preferably at least 95 wt %.
[0069] Depending on the application and the desired level of
strength, between 0.1 wt % and 2 wt % of the particulate amorphous
SiO.sub.2 are used, preferably between 0.1 wt % and 1.8 wt %,
particularly preferably between 0.1 wt % and 1.5 wt %, in each case
based on the molding base mixture.
[0070] The ratio of water glass binder to amorphous SiO.sub.2 can
be varied within broad limits. This offers the advantage of greatly
improving the initial strengths of the cores, i.e., the strength
immediately after removal from the tool, without substantially
affecting the final strengths. This is of particularly great
interest in light metal casting. On one hand high initial strengths
are desirable to transport the cores without problems after they
are manufactured or to make up entire core packets, but on the
other hand the final strengths should not be too high in order to
prevent problems with core disintegration after casting.
[0071] Based on the weight of the binder, the amorphous SiO.sub.2
is preferably present in a fraction of 2 to 60 wt %, preferably of
3 to 55 wt % and particularly preferably between 4 and 50 wt %, or
particularly preferably based on the ratio of the solid fraction of
water glass to amorphous SiO.sub.2 of 10:1 to 1:1.2 (parts by
weight).
[0072] According to EP 1802409 B1, the addition of the amorphous
SiO.sub.2 can take place both before and after the binder addition,
directly to the refractory material, but it is also possible, as
described in EP 1884300 A1 (=US 2008/029240 A1), to first produce a
premix of the SiO.sub.2 with at least part of the binder and then
mix this in with the refractory material. The binder or binder
fraction that may still be present and is not used for the premix
can be added to the refractory material before or after the
addition of the premix or together with it. Preferably the
amorphous SiO.sub.2 is added directly to the refractory material
before the binder addition.
[0073] Without wishing to be bound to this, the inventors assume
that the highly alkaline water glass can react with the silanol
groups arranged on the surface of the amorphous silicone dioxide,
and that when the water is evaporated an intensive bond is produced
between the silicone dioxide and the water glass, which is solid at
that point.
[0074] In an additional embodiment, barium sulfate may be added to
the molding mixture to further improve the surface of the casting,
especially in light metal casting such as aluminum casting. The
barium sulfate may be synthetically produced or added as natural
barium sulfate, i.e., in the form of minerals containing barium
sulfate, such as heavy spar or barite. This and other features of
the suitable barium sulfate as well as the solid mixture produced
with it will be described in greater detail in DE 102012104934, and
the disclosure content thereof is thus incorporated by reference in
the disclosure of the present patent.
[0075] The barium sulfate is preferably added in a quantity of 0.02
to 5.0 wt %, particularly preferably 0.05 to 3.0 wt %, especially
preferably 0.1 to 2.0 wt % or 0.3 to 0.99 wt %, in each case based
on the total molding mixture.
[0076] According to an additional embodiment other substances may
also be added to the molding mixture, which are characterized by a
low degree of wetting by molten aluminum, for example, boron
nitrite.
[0077] Such a mixture of poorly wettable substances, including
among other things barium sulfate as a poorly wettable agent, can
likewise lead to a smooth, casting surface free from sand adhesion.
Based on the total quantity of non-wettable/poorly wettable
substances the fraction of the barium sulfate should be larger than
5 wt %, preferably larger than 10 wt %, particularly preferably
larger than 20 wt % or greater than 60 wt %.
[0078] The upper limit represents pure barium sulfate--the fraction
of non-wettable substances in barium sulfate in this case is 100 wt
%. The mixture of non-wettable/poorly wettable substances,
especially barium sulfate, is preferably added in a quantity of
0.02 to 5.0 wt %, particularly preferably 0.05 to 3.0 wt %,
especially preferably 0.1 to 2.0 wt % or 0.3 to 0.99 wt %, in each
case based on the molding mixture.
[0079] In an additional embodiment the additive component of the
molding mixture according to the present invention can also contain
at least 1 particulate or 1 particulate mixed metal oxide of
aluminum or aluminum and zirconium, as described in DE 102012113073
or DE 102012113074. By means of such additives, castings,
especially made of iron or steel, with very high surface quality
can be obtained after metal casting, so that removal of the casting
mold requires little or no surface processing of the casting
afterwards.
[0080] The particulate metal oxide or particulate mixed metal oxide
at room temperature exhibits little or no tendency to react with
the inorganic binder, especially the alkaline water glass.
[0081] The particulate metal oxide in this can includes or consists
of in particular at least 1 aluminum oxide in the alpha phase
and/or at least 1 aluminum/silicone mixed oxide, with the exception
of aluminum-silicone mixed oxide with phyllosilicate structure.
Particulate metal oxides containing at least 1 aluminum oxide in
the alpha phase and/or at least 1 aluminum/silicone mixed oxide,
with the exception of aluminum/silicone mixed oxides with
phyllosilicate structure, are defined as not only particulate metal
oxides consisting of pure aluminum oxide or pure alumosilicates or
aluminosilicates, but also mixtures of the above metal oxides with
other oxides, for example, zirconium, zirconium incorporated into
the aluminum/silicone mixed oxides or heterogeneous substance
mixtures, i.e., those consisting of several thicknesses, which
among other things consists of at least 2 of the afore-mentioned
solids or bases: alumina oxide-containing and/or aluminum/silicone
oxide containing solids or phases.
[0082] Preferred is particulate metal oxide selected from the group
of corundum plus zirconium dioxide, zirconium mullite, zirconium
corundum and aluminum silicate (except for those with
phyllosilicate structure) plus zirconium dioxide and also
optionally contains other metal oxides.
[0083] Unsuitable as additives for the binders are
aluminum/silicone mixed oxides with layered structure, for example,
metakaolins, kaolins, and kaolinite. Also unsuitable is pyrogenic,
amorphous aluminum oxide.
[0084] The particulate mixed metal oxide is at least one
particulate mixed oxide or a particulate mixture of at least two
oxides or is present at least as a particulate mixed oxide along
with at least one additional particulate oxide, wherein the
particulate mixed oxide comprises at least one oxide of aluminum
and at least one oxide of zirconium.
[0085] Particulate mixed metal oxides containing in each case in
addition to an oxide of aluminum, also an oxide of zirconium are
defined not only to include pure zirconium oxides and zirconium
oxides, but also mixed oxides, for example, aluminum silicate and
zirconium oxide or heterogeneous substance mixtures, i.e.,
consisting of several phases, which among other things consist of
one or more aluminum oxide-containing and zirconium-containing
solids or phases.
[0086] Preferred is the particulate mixed metal oxide according to
the invention selected from one or more members of the group of a)
corundum plus zirconium dioxide, b) zirconium mullite, c) zirconium
corundum and d) aluminum silicate plus zirconium dioxide and may
also contain additional metal oxides.
[0087] Aluminum silicates are defined here as both alumosilicates
and aluminosilicates.
[0088] Both the aluminum/silicone mixed oxides and the aluminum
silicates, insofar as they are not amorphous (i.e., crystallinity
or partial crystallinity is present) are preferably island
silicates. In island silicates, the SiO.sub.4 units (tetrahedral)
present in the structure are not directly linked together (no
Si--O--Si bonds) but instead bonds of the tetrahedral SiO.sub.4
units to one or more Al atoms (Si--O--Al) are present. The Al atoms
are coordinated in this process with 4, 5 and/or 6 oxygen
atoms.
[0089] Typical representatives of these island silicates (according
to Systematik der Minerale after Strunz [Strunz' Classification of
Minerals], 9.sup.th edition), for example, mullite (fused and
sintered mullite are meant here as well as ZrO.sub.2-containing
mullite) and sillimanite as well as other members of the
sillimanite group (for example, kyanite or andalusite), wherein
kyanite is particularly preferably used from the sillimanite group.
Particularly preferred is an amorphous aluminum silicate (except
for those with silicate structure) with greater than 50 atom %
aluminum atoms based on the total of all silicone and aluminum
atoms, optionally also containing zirconium/zirconium oxide, or an
aluminum oxide-containing dust which is formed as a byproduct in
zirconium corundum manufacture and therefore can also contain
zirconium oxide in finely divided form. This and other features of
the suitable particulates metal oxides or particulate mixed metal
oxides of aluminum or aluminum and zirconium are described in
greater detail in DE 102012113073 or DE 102012113074) and these are
thus also incorporated by reference in the disclosure of the
present patent.
[0090] The particulate amorphous silicone dioxide that is added to
the molding mixture to increase the strength can be added as part
of the particulate mixed metal oxide or separately.
[0091] In any case the statements made here on the concentration of
the particulate mixed metal oxide and the particulate amorphous
silicon dioxide are in each case to be understood without the other
component(s). In case of doubt the components must be calculated on
this basis.
[0092] In a further embodiment the additive components of the
molding mixture in accordance with the invention may comprise a
phosphorus-containing compound.
[0093] Such an additive is preferred in the case of very
thin-walled sections of a casting mold and especially in the case
of cores, since in this way the thermal stability of the cores or
the thin-walled section of the casting mold can be increased. This
is especially significant if the liquid metal in counters and
inclined surface during casting and exerts a strong erosion effect
there because of the high metallostatic pressure or can lead to
deformation especially of thin-walled sections of the mold.
Suitable phosphorus compounds have little or no significant effect
on the processing time of the molding mixtures according to the
invention. Suitable representatives of this group as well as the
quantities in which they are to be added are described in detail in
WO 2008/046653 A1 and this is thus also incorporated by reference
in the disclosure of the present patent.
[0094] Binders, water-based binders generally have poor flow
capability compared with binders based on organic solvents. This
means that molding tools with narrow passages and several direction
changes cannot be filled as well. As a result the cores can have
sections with inadequate consolidation, which in turn can lead to
casting errors in the finished castings. According to an
advantageous embodiment the molding material according to the
invention contains a certain fraction of a flaky lubricant,
especially graphite or MoS.sub.2. Surprisingly it was found that
upon addition of such a lubricant, especially graphite, complex
shapes with thin-walled sections can also be produced, wherein the
casting molds always have a uniformly high density and strength, so
that essentially no casting defects were observed during casting.
The quantity of the flaky lubricant used, especially graphite, is
preferably 0.05 wt % to 1 wt %, based on the molding base
material.
[0095] Instead of the flaky lubricant, surface-active substances,
especially detergents, can be used to improve the flowability of
the molding mixture. Suitable representatives of these compounds
are described, for example, in WO 2009/056320 (=US 2010/0326620
A1). Surfactants with sulfuric acid or sulfonic acid groups in
particular may be mentioned here.
[0096] In addition to the components mentioned, the molding mixture
according to the invention may also contain other additives. For
example, internal release agents may be added, which facilitate
removal of the casting molds from the mold tool. Suitable internal
lubricants are, for example, calcium stearate, fatty acid esters,
waxes, natural resins or special alkyd resins.
[0097] Surprisingly it was found that the addition of an organic
additive leads to improvement of the surface quality of a casting,
especially in aluminum casting. The mechanism of action of the
organic additives is not clear. Without being tied to this theory,
the inventors assume that at least part of the organic additive
burns during the casting process and thus a thin gas cushion forms
between the liquid metal and the molding material forming the wall
of the casting mold, thus preventing reaction between the liquid
metal and the molding material. Furthermore the inventors assume
that part of the organic additive forms a thin layer of so-called
glossy carbon in the reducing atmosphere that prevails during
casting, and this likewise prevents reaction between the metal and
the molding material. As a further advantageous effect, addition of
the organic additive can cause an increase in the strengths of the
casting mold after curing.
[0098] It was surprising that the improvement in the surface of the
casting can be accomplished with a great variety of organic
additives. Suitable organic additives are, for example,
phenol-formaldehyde resins, for example, novolaks; epoxy resin, for
example, bisphenol-A-epoxy resins, bisphenol-F-epoxy resins or
epoxidized novolaks; polyols, for example, polyethylene glycols,
polypropylene glycols; glycerol or polyglycerols; polyolefins, for
example, polyethylene or polypropylene, co-polymers of olefins,
such as ethylene or propylene and additional co-monomers, such as
vinyl acetate or styrene and/or diene monomers;
[0099] Polyamides, for example, polyamide-6, polyamide-12 or
polyamide-6,6; natural resins, for example, balsam resin; fatty
acid esters, for example, cetyl palmitate; fatty acid amides, for
example, ethylene-diamine-bis-stearamide; carbohydrates, for
example, dextrins; and metal soaps, for example, oleates or oleates
of divalent or trivalent metals. The organic additives may be
present either as pure materials or as a mixture of various organic
compounds.
[0100] The organic additives are preferably added in a quantity of
0.01 to 1 wt %, or 0.1-1.0 wt %, especially preferably 0.05 to 0.5
wt %, particularly preferably 0.1-0.2 wt %, in each case based on
the molding material.
[0101] Furthermore, silanes may also be added to the molding
mixture according to the invention to increase the resistance of
the molds and cores to high atmospheric humidity and/or water-based
molding material coatings. According to an additional preferred
embodiment the molding mixture contains a fraction of at least one
silane. Suitable silanes are for example, aminosilanes,
epoxysilanes, mercaptosilanes, hydroxysilanes and ureidosilanes.
Examples of suitable silanes are
.gamma.-aminopropyl-trimethoxysilane,
.gamma.-hydroxypropyltrimethoxysilane,
3-ureidopropyltriethoxysilane,
.gamma.-mercaptopropyltrimethoxysilane,
.gamma.-glycidoxypropyltrimethoxysilane,
.beta.-(3,4-epoxycyclohexyl)trimethoxysilane and
N-.beta.-(aminoethyl)-.gamma.-aminopropyl-trimethoxysilane.
Typically 0.1 to 2 wt % silane based on the binder are used,
preferably 0.1 to 1 wt %.
[0102] Additional suitable additives are alkali metal siliconates,
e.g., potassium methylsiliconate, of which 0.5 to 15 wt %,
preferably 1 to 10 wt % and particularly preferably 1 to 5 wt %
based on the binder can be used. If the molding mixture contains an
organic additive, the addition of this can be done at any time in
the production of the molding mixture. The addition of the organic
solvent may be made in bulk or in the form of a solution.
Water-soluble organic additives can be used in the form of an
aqueous solution. As long as the additives are soluble in the
binder and are stable therein for several months without
decomposition, they can also be dissolved in the binder and thus
added together with this to the molding material. Water-insoluble
additives can be used in the form of a dispersion or a paste. This
dispersions or pastes preferably contain water as the liquid
medium.
[0103] If the molding mixture contains silanes and/or alkali
methylsiliconates, they are usually added in the form in which they
are incorporated in the binder in advance. However, they can also
be added to the molding material as separate components.
[0104] Organic additives can also have a positive effect on the
characteristics of the cores produced according to the method of
the invention, for example, the carbonates mentioned in AFS
Transactions, vol. 88, pp. 601-608 (1980), and vol. 89, pp. 47-54
(1981) increase the moisture resistance of the cores during
storage, while the phosphorus compounds known from WO 2008/046653
(=CA 2666760 A1) increase the thermal stability of the cores.
[0105] In manufacturing the molding mixture, the refractory molding
base material is placed in a mixture, and then preferably first the
liquid component is added and mixed with the refractory molding
base material until a uniform layer of the binder has formed on the
granules of the refractory molding base material. The mixing time
is selected so that intimate mixing of refractory molding base
material and liquid components take place. The mixing duration
depends on the quantity of the molding mixture to be produced and
the mixing device used. Preferably the mixing duration is selected
between 1 and 5 minutes. Then, preferably with further agitation of
the mixture, the solid component(s) is added in the form of the
particulate mixed metal oxides, and optionally amorphous silicone
dioxide, barium sulfate or additional powdered solids are added and
then the mixture is mixed further. Here also the mixing duration
depends on the quantity of the molding mixture to be produced and
the mixing unit used. Preferably the mixing time is selected
between 1 and 5 minutes. A liquid component may be either a mixture
of various liquid components or the total of all liquid individual
components, wherein the latter can be added together or
successively to the molding mixture. According to another
embodiment first the solid components can be added to the
refractory molding material and only then the liquid components be
added to the mixture.
[0106] The molding material mixture is then brought into the
desired form. In this process, the usual molding methods are used,
for example, the molding mixture can be shot into the molding tool
using a core-shooting machine with compressed air. A further
possibility consists of allowing the molding mixture to flow freely
from the mixture into the molding tool and compacting there by
shaking, stamping or pressing.
[0107] The methods according to the invention in and of themselves
are suitable for manufacturing all casting molds suitable for metal
casting, for example, cores and molds.
[0108] Despite the high strengths achievable with the method
according to the invention, the cores produced with the molding
mixture according to the invention exhibit good disintegration
after casting, especially in the case of aluminum casting.
[0109] However, the use of the molded articles produced from the
molding mixture according to the invention is not limited to light
metal casting. The casting molds are generally suitable for casting
metals. Such metals include, for example, nonferrous metals such as
brass or bronzes as well as ferrous metals.
[0110] The invention will be explained in greater detail based on
the following, non-limiting examples.
EXAMPLES
1. Production of the Molding Mixtures
[0111] 1.1 without Addition of Amorphous SiO.sub.2
[0112] In each case 5 kg of quartz sand H 32 from Quarzwerke
Frechen GmbH were placed in the bowl of a Hobart mixer (model HSM
10). Then the binder was added under agitation and mixed
intensively with the sand. The respective quantities added are
shown in the individual experiments.
[0113] 1.2 with Addition of Amorphous SiO.sub.2
[0114] The procedure described under 1.1 was followed with the
difference that before the binder addition, 0.5 PW amorphous
SiO.sub.2 calculated on the basis of sand was added and mixed with
this for 1 minute. The addition form is listed in the individual
experiments.
2. Production and Testing of the Test Pieces
[0115] Part of the molding mixtures produced according to 1.1 and
1.2 were placed in the reservoir of an H 1 core shooting machine
Roperwerke AG. The remainders of each of the molding mixtures were
stored in a carefully closed vessel before refilling the core
shooting machine to protect them from drying out and from premature
reaction with CO.sub.2 present in air. The molding mixtures were
shot from the reservoir by compressed air (4 bar) into a
non-temperature-controlled molding tool provided with 2 engraved
grooves for round cores of 50 mm diameter and 50 mm height. Then
the test cores were cured. Details of this are presented with the
individual experiments. After curing the test pieces were removed
from the molding tool and their compression strengths were
determined immediately with a Zwick Universal Testing Machine
(model Z 010) immediately, i.e., a maximum of 15 seconds after
removal, and after 24 hours of storage. The values listed in the
tables represent means from 8 cores in each cases. To largely
exclude the influence of climatic changes, all test pieces used for
determining the 24-hour strengths were stored in a
climate-controlled chamber at 23.degree. C. and 50% relative
humidity.
[0116] In the case of examples 4.01 to 4.07 the experiments were
performed on an L 1 core-shooting machine from the firm of Laempe
& Mossner GmbH, which was equipped with a heating tube (model
HT42-13) from Hillesheim GmbH. For testing the molding mixtures,
rectangular test bars with dimensions of 150 mm.times.22.36
mm.times.22.36 mm were produced (so-called Georg-Fischer bars). The
three-part molding tool used made it possible to produce four
rectangular test bars simultaneously. Part of the molding mixture
produced according to 1.2 was transferred into the reservoir of the
core-shooting machine, the molding tool of which was not
electrically heated. The remainder of the respective molding
material mixture was stored in a carefully closed container until
it was used for refilling the core-shooting machine to protect it
from drying and to avoid premature reaction with the CO.sub.2
present in the air. The molding mixtures were introduced by
compressed air (4 bar) from the storage container into the molding
tool and flushed with hot CO.sub.2 or hot air. Additional
information on the flushing time with compressed air or CO.sub.2
and the gas temperature are given in [section] 6. After curing the
molding tool was opened and the test bars removed.
[0117] For determining the bending strengths, the test bars were
placed in a Georg-Fischer strength testing apparatus equipped with
a three-point bending device and the force leading to breakage of
the test bars was measured. The bending strengths were determined
both immediately, i.e., a maximum of 15 seconds after removal
(initial strengths) and about 24 hours after manufacturing (final
strengths).
[0118] The results of the strength tests are shown in table 4. The
values presented here are means from multiple determinations on at
least 4 cores.
3. Curing with CO.sub.2
[0119] 3.1 To manufacture the test pieces, molding mixtures
consisting of quartz sand H 32 and 2.0 PW (PW=parts by weight), 2.5
PW and 3.25 PW, respectively of a sodium water glass with a molar
ratio of about 2.33 and a solids content of about 40 wt % were
used. For curing, CO.sub.2 (supplier and purity in each case: Linde
AG, at least 99.5 vol % CO.sub.2) passed through the molding
mixture. The temperature of the gas at the inlet into the molding
tool was between 22 and 25.degree. C. Table 1 presents the gassing
times, the CO.sub.2 flow rate and the compressive strengths
determined under these conditions (see Examples 1.01 to 1.21 and
1.29 to 1.42).
[0120] 3.2 Some of the experiments according to 3.1. were repeated
with the difference that 0.5 PW amorphous silicone dioxide in
powder form was mixed into the molding mixtures before addition of
binder. The results are also presented in Table 1 (Examples 1.22 to
1.28).
[0121] The following are apparent from Table 1 (see Appendix):
[0122] Without addition of amorphous SiO.sub.2:
[0123] The strengths depend on the quantity of CO.sub.2 used for
curing, wherein the initial strengths increase with increasing
CO.sub.2 quantity, whereas on the other hand strengths after 24 h
of storage time decrease because of the known over-gassing effect
(see Examples 1.01 to 1.21), wherein the over-gassing effect as
expected occurs later at higher binder fractions.
[0124] At the same absolute CO.sub.2 volume, a low CO.sub.2 flow
has a predominantly positive effect on the initial strengths, while
a high CO.sub.2 flow on the other hand has a positive effect on the
final strengths (see Example 1.03/1.08, 1.04/1.09/1.15.
1.05/1.16/1.06/1.11/1.17, 1.07/1.12/1.18, 1.14/1.20).
[0125] With addition of amorphous SiO.sub.2:
[0126] The addition of amorphous SiO.sub.2 causes an increase in
strength compared with cores cured with the same gassing parameters
but containing amorphous SiO.sub.2 (see Example 1.22-1.28 compared
with 1.08-1.14).
[0127] The loss of strength after 24 hours of storage due to
over-gassing is reduced by the amorphous SiO.sub.2 (see Examples
1.22-1.28 compared with 1.08-1.14).
[0128] Other than at long gassing times the addition of amorphous
SiO.sub.2 produces a greater increase in the initial strengths than
increasing the binder content by the same amount. On the other
hand, the final strengths increase substantially more greatly at
elevated binder contents, but also decrease again more greatly at
long gassing times because of the over-gassing effect (see Examples
1.22-1.28 compared with 1.29-1.35).
[0129] Even at the same solids content the mixture of water glass
binders and amorphous SiO.sub.2, other than at long gassing times,
offers advantages in terms of the initial strengths relative to the
increased binder quantity without amorphous SiO.sub.2. In the case
of the final strengths, the increased binder content on the other
hand has a greater effect, wherein the decrease in strengths at
long gassing times is once again pronounced because of over-gassing
(see Examples 1.22-1.28 compared with 1.36-1.42).
4. Curing with air
[0130] 4.1 For producing the test pieces, molding mixtures
consisting of quartz sand H 32 and 2.0 PW, 2.5 PW or 3.25 PW of a
sodium water glass with a molar ratio of about 2.33 and a solids
content of about 40 wt % were used. For curing, compressed air was
conducted through the molding mixture.
[0131] The temperature of compressed air at the inlet to the
molding tool was between 22 and 25.degree. C. In Table 2 the
gassing times, the gassing pressure and the compressive strengths
found under these conditions are shown (see Examples 2.01-2.03 and
2.07-2.12).
[0132] 4.2 Some of the experiments corresponding to 4.1 were
repeated with the difference that 0.5 PW of powdered amorphous
silicone dioxide was mixed into the molding mixtures before
addition of binder. These results are likewise presented in Table 2
(see Examples 2.04-2.06).
[0133] From Table 2 it can be seen that:
[0134] Without addition of amorphous SiO.sub.2:
[0135] The strengths depend on the volume of air passed through,
wherein the initial strengths increase more greatly with increasing
air volume than the final strengths (see Examples 2.01-2.03 and
2.07-2.12).
[0136] A higher binder fraction does not necessarily result in
better strengths. This can presumably be explained by the poorer
compaction ability and the higher water fraction in the molding
mixture (see Examples 2.01-2.03 compared with 2.07-2.12).
[0137] With addition of amorphous SiO.sub.2:
[0138] The addition of amorphous SiO.sub.2 causes an increase in
strengths relative to cores cured with the same gassing parameters
but containing no amorphous SiO.sub.2, wherein a greater effect is
seen on the initial strengths than on the final strengths (see
Examples 2.04-2.06 compared with 2.01-2.03).
[0139] The increase in strengths caused by the amorphous SiO.sub.2
is greater than that of increasing the binder content by the same
amount (see Examples 2.07-2.09 compared with 2.04-2.06).
[0140] The increase in strengths produced by the amorphous
SiO.sub.2 is greater than that of increasing the binder content to
the same solids content (see Examples 2.10-2.12 compared with
2.04-2.06).
5. Curing with a Combination of CO.sub.2 and Air
[0141] 5.1 To produce the test pieces, mixtures of solids
consisting of quartz sand H 32 and 2.0 PW, 2.5 PW and 3.25 PW of a
water glass with a molar ratio of about 2.33 and a solids content
of about 40 wt % were used. For curing, first CO.sub.2 and then
compressed air were passed through the molding mixture. The
temperatures of both gasses upon entering the molding tool were
between 22 and 25.degree. C.
[0142] In Table 3 the gassing times of CO.sub.2 and air, the
CO.sub.2 flow, the gassing pressure (air) and the compressive
strengths found under these conditions are shown (see Examples
3.01-3.09, 3.19-3.27, 3.37-3.45).
[0143] 5.2 Some of the experiments corresponding to 5.1 were
repeated with the difference that 0.5 PW powdered amorphous
silicone dioxide was mixed with the molding mixtures before
addition of the binder. These results are also shown in Table 3
(see Examples 3.10-3.18, 3.28-3.36 and 3.46-3.48).
[0144] 5.3 Some of the experiments according to 5.1 were repeated
with the difference that the compressed air passed through the
molding mixtures was heated to about 100.degree. C., measured at
the entrance to the molding tool. The results are also presented in
Table 3 (see Examples 3.49-3.51).
[0145] 5.4 Some of the experiments according to 5.2 were repeated
with the difference that the compressed air passed through the
molding mixtures was heated to about 100.degree. C., measured at
the entry into the molding tool. The results are also shown in
Table 3 (see Examples 3.52-3.54).
[0146] The following are apparent from Table 3:
[0147] Without addition of amorphous SiO.sub.2:
[0148] As a result of the combined CO.sub.2/air gassing,
substantially better strengths are achieved than by gassing with
CO.sub.2 or air alone (see Examples 3.01-3.09 and 3.19-3.27
compared with 1.01-1.21 and 2.01-2.03, respectively).
[0149] Increasing the pressure during gassing with air results in a
further increase in strengths (see Examples 3.43-3.45 compared with
3.04-3.06).
[0150] Heating the air used for gassing increases the final
strengths (see Examples 3.49-3.51 compared with 3.04-3.06). The
fact that the initial strengths do not show the same effect can
probably be explained by the fact that they were still hot at the
time of strength testing.
[0151] Extending the time of gassing with CO.sub.2 does not always
have positive effects on the strengths because of the over-gassing
effect (see Examples 3.01-3.09 and 3.19-3.27).
[0152] Increasing the CO.sub.2 flow causes an increase in the
initial strengths, although this is disadvantageous for the final
strengths (see Examples 3.01-3.09 compared with 3.19-3.27).
[0153] A higher binder fraction results in higher final strengths
but not necessarily higher initial strengths. The latter can
presumably be explained by the increased water fraction in the
molding mixture (see Examples 3.37-3.42 compared with
3.04-3.06).
[0154] With addition of amorphous SiO.sub.2:
[0155] The addition of amorphous SiO.sub.2 causes an increase in
strengths compared with cores cured with the same parameters but
containing no amorphous SiO.sub.2, wherein the effect on the
initial strengths is greater than that on the final strengths. At
higher CO.sub.2 flow rates and/or longer CO.sub.2 gassing times,
the final strengths decrease to some extent because of the
over-gassing effect (see Examples 3.10-3.18 compared with 3.01-3.09
and Examples 3.28-3.36 compared with 3.19-3.27).
[0156] The addition of amorphous SiO.sub.2 causes a greater
increase in the initial strengths than does increasing the binder
content by the same amount. In the case of the final strengths,
however, the effect of the increased binder strength is greater
(see Examples 3.13-3.15 compared with 3.37-3.39).
[0157] Even at the same solids content the mixture of water glass
binder and amorphous SiO.sub.2 results in advantages in initial
strengths versus the correspondingly increased binder quantity
without amorphous SiO.sub.2. In the case of the final strengths, on
the other hand, the effect of the higher binder content is stronger
(see Examples 3.13-3.15 compared with 3.40-3.42).
[0158] An increase in the pressure in the case of gassing with air
brings about a further increase in strength (see Examples 3.46-3.48
compared with 3.13-3.15).
6 Curing with CO.sub.2, Air or a Combination of CO.sub.2 and Air at
a Gassing Temperature of 115 to 90.degree. C.
[0159] 6.1 For producing the test pieces, molding material mixtures
consisting of quartz sand H 32 and 2.0 PW of a water glass with a
molar ratio of about 2.33 and a solids content of about 40 wt %
were used. Furthermore 0.5 PW powdered amorphous silicone dioxide
was added to the molding mixtures before binder addition. For
curing, first CO.sub.2 and then compressed air was passed through
the molding mixture. Both gases were heated to temperatures of up
to 120.degree. C. using a heating tube. The temperatures of both
gases on entry into the molding tool initially amounted to
115.degree. C. and declined to 90.degree. C. during the 35-second
gassing. This temperature drop is attributable to the fact that the
heating tube is unable to keep the gas temperature constant during
gassing.
[0160] Immediately before the beginning of the experiments,
experiment 4.04 was first repeated about 80 times over a period of
50 minutes, so that the molding tool had reached the necessary
operating temperature of about 60.degree. C.
[0161] In Table 4 the gassing times of CO.sub.2 and air, the
CO.sub.2 flow, the gassing pressure (air) and the bending strengths
found under these conditions are presented (see Examples
4.01-3.07).
[0162] Table 4 shows:
[0163] The values for the strengths clearly confirm that a
combination of CO.sub.2 and air gassing is distinctly superior to
gassing with either air or CO.sub.2 alone. Especially Examples
4.01-4.03, in which the curing was performed exclusively with
CO.sub.2, show distinctly lower initial values and, except for
example, 4.01, also final strengths compared with Examples
4.05-4.07 of the method according to the invention.
[0164] The strengths of Example 4.04, which shows the values for
gassing with air alone, are likewise significantly lower than the
strengths for the combined CO.sub.2-air gassing according to the
invention. Whereas the final strengths for examples 4.05-4.07 are
10-60 N/cm.sup.2 above the values for example 4.04, their initial
strengths are 50-60
[0165] N/cm.sup.2 higher. Thus the distinctly higher initial
strengths of Examples 4.05 to 4.07 positively demonstrate the
effect according to the invention even at the elevated gas
temperature of 115 to 90.degree. C. for CO.sub.2 and air. Examples
4.05 to 4.07 show only slight differences from one another, but
these are not significant.
TABLE-US-00001 TABLE 1 (Not according to the invention) Storage
Example/Compression strengths [N/cm.sup.2] Binder Amorph. CO.sub.2
Time of after Exper. content SiO2 Flow Test 10 s 15 s 20 s 30 s 45
s 60 s 90 s. No. [PW] [PW] [L/Min.] Pieces CO.sub.2 CO.sub.2
CO.sub.2 CO.sub.2 CO.sub.2 CO.sub.2 CO.sub.2 3.1 1.01 1.02 1.03
1.04 1.05 1.06 1.07 2 2 15 s 7 15 19 26 31 38 40 (a) 24 h 76 53 50
31 29 31 25 3.1 1.08 1.09 1.10 1.11 1.12 1.13 1.14 2 4 15 s 12 20
24 35 40 42 43 (a) 24 h 64 57 51 44 46 45 38 3.1 1.15 1.16 1.17
1.18 1.19 1.20 1.21 2 6 15 s 15 24 23 35 40 42 45 (a) 24 h 60 62 49
46 39 35 36 3.2 1.22 1.23 1.24 1.25 1.26 1.27 1.28 2 0.5 4 15 s 31
36 46 49 57 55 52 (a) (b) 24 h 74 71 68 64 60 59 50 3.1 1.29 1.30
1.31 1.32 1.33 1.34 1.35 2.5 4 15 s 9 12 19 31 45 52 59 (a), (c) 24
h 153 144 126 98 74 60 57 3.1 1.36 1.37 1.38 1.39 1.40 1.41 1.42
3.25 4 15 s 10 17 29 42 59 67 77 (a), (d) 24 h 140 173 165 86 88 67
68 (a) Sodium water glass, molar ratio approx. 2.33 (molar), Solids
approx. 40% (b) Elkem 971 U, added as dry powder (c) Added
according to the amount of binder + amorph. SiO2 in experiments
1.22-1.28 (d) Solids (1.3 PW = 2 PW .times. 40% + 0.5)
corresponding to experiments 1.22-1.28 calculated from binder +
amorph. SiO2 h = hours s = seconds
TABLE-US-00002 TABLE 2 (not according to invention) Test
Example/Compression piece strengths in [N/cm.sup.2] Binder amorph.
Air stor- after Exper. content SiO.sub.2 pressure age 30 s 45 s No.
[PW] [PW] [bar] time Air Air 60 s Air 4.1 2.01 2.02 2.03 2 2 15 s
27 71 101 (a) 24 h 75 93 104 4.2 2.04 2.05 2.06 2 0.5 2 15 s 60 126
161 (a) (b) 24 h 175 200 214 4.1 2.07 2.08 2.09 2.5 2 15 s 16 35 76
(a), (c) 24 h 86 102 134 4.1 2.10 2.11 2.12 3.25 2 15 s 8 31 66 (a)
(d) 24 h 96 115 126 (a) Sodium water glass, molar ratio approx.
2.33 (molar), solids approx. 40% (b) Elkem 971 U, added as dry
powder (c) Addition corresponds to amount of binder + amorph. SiO2
of experiments 2.04-2.06 (d) Solids (1.3 PW) corresponds to that of
experiments 2.4-2.6 calculated from binder + amorph. SiO.sub.2 h =
hours s = seconds
TABLE-US-00003 TABLE 3 Example/Compression Storage strengths in
Binder- Amorph. CO.sub.2- CO.sub.2 Air- time of [N/cm.sup.2] after
content SiO.sub.2 flow time pressure test 60 s Exper. No. [PW] [PW]
[L/Min.] [s] [bar] pieces 30 s Air 45 s Air Air 5.1 3.01 3.02 3.03
2 2 2 2 15 s 20 52 90 (a) 24 h 212 211 230 5.1 3.04 3.05 3.06 2 2 6
2 15 s 28 67 117 (a) 24 h 236 233 238 5.1 3.07 3.08 3.09 2 2 10 2
15 s 32 70 134 (a) 24 h 211 194 183 5.2 3.10 3.11 3.12 2 0.5 2 2 2
15 s 99 192 233 (a) 24 h 296 293 295 5.2 (b) 3.13 3.14 3.15 2 0.5 2
6 2 15 s 92 200 229 (a) (b) 24 b 277 238 233 5.2 3.16 3.17 3.18 2
0.5 2 10 2 15 s 80 154 182 (a) (b) 24 h 211 165 139 5.1 3.19 3.20
3.21 2 6 2 2 15 s 44 95 133 (a) 24 h 181 181 205 5.1 3.22 3.23 3.24
2 6 6 2 15 s 46 117 152 (a) 24 h 216 225 220 5.1 3.25 3.26 3.27 2
0.5 6 10 2 15 s 41 114 165 (a) (b) 24 h 187 189 182 5.2 3.28 3.29
3.30 2 0.5 6 2 2 15 s 105 190 234 (a) (b) 24 h 311 317 322 5.2 3.31
3.32 3.33 2 0.5 6 6 2 15 s 91 192 229 (a) (b) 24 h 306 270 262 5.2
3.34 3.35 3.36 2 0.5 6 10 2 15 s 76 145 168 (a) (b) 24 h 205 178
160 5.1 3.37 3.38 3.39 2.5 2 6 2 15 s 28 79 140 (a) (c) 24 h 364
383 394 5.1 3.40 3.41 3.42 3.25 2 6 2 15 s 9 32 83 (a) (d) 24 h 424
414 414 5.1 3.43 3.44 3.45 2 2 6 4 15 s 101 158 196 (a) 24 h 255
280 271 5.2 3.46 3.47 3.48 2 0.5 2 6 4 15 s 194 245 290 (a) (b) 24
h 334 309 304 5.3 3.49 3.50 3.51 2 2 6 2 15 s 18 53 112 (a) (e) 24
h 258 289 301 5.4 3.52 3.53 3.54 2 0.5 2 6 2 15 s 31 78 151 (a) (b)
(e) 24 h 294 276 263 (a) Sodium water glass, molar ratio approx.
2.33 (molar), solids approx. 40 % (b) Elkem 971 U, added as dry
powder (c) Addition corresponds to amount of binder + amorph.
SiO.sub.2 of experiments 1.22-1.28 (d) Solids (1.3 PW = 2 PW * 40%
+ 0.5) corresponds to that of experiments 1.22-1.28 calculated from
binder + amorph. SiO.sub.2 (e) Air prejeated to 100.degree. C.
(measured at inlet to molding tool) (i) no complete curing h =
hours s = seconds
TABLE-US-00004 TABLE 4 Storage Bending strengtys in Binder Amorph.
CO.sub.2 Air Air time of [N/cm.sup.2] for a CO.sub.2 content
SiO.sub.2 time pressure time test flow of Exper. No. [PW] [PW] [s]
[bar] [s] pieces 10 L/min 200 L/min 450 L/min 6.1 4.01 4.02 4.03 2
0.5 35 4 0 15 s 10 15 28 (a) (b) 24 h 360 44 23 6.2 4.04 15 s 83 2
0.5 0 4 35 24 h 338 (a) (b) 6.3 4.05 4.06 4.07 2 0.5 1 4 35 15 s
145 138 135 (a) (b) 24 h 375 350 400 (a) Sodium water glass, molar
ratio approx. 2.33 (molar), solids approx. 40% (b) Microsilica POS
B-W 90 LD from the firm of Possehl Erzkontor GmbH, added as dry
powder
* * * * *