U.S. patent application number 15/534551 was filed with the patent office on 2017-11-09 for method for constructing molds and cores layer by layer by means of a binder containing water glass, and a binder containing water glass.
The applicant listed for this patent is ASK Chemicals GmbH. Invention is credited to Heinz DETERS, Henning ZUPAN.
Application Number | 20170320128 15/534551 |
Document ID | / |
Family ID | 55272192 |
Filed Date | 2017-11-09 |
United States Patent
Application |
20170320128 |
Kind Code |
A1 |
DETERS; Heinz ; et
al. |
November 9, 2017 |
METHOD FOR CONSTRUCTING MOLDS AND CORES LAYER BY LAYER BY MEANS OF
A BINDER CONTAINING WATER GLASS, AND A BINDER CONTAINING WATER
GLASS
Abstract
The invention relates to a binder, which contains water glass
and further a phosphate or a borate or both. The invention further
relates to a method for constructing molds and cores layer by
layer, the molds and cores comprising a construction material
mixture, which at least comprises a refractory molding base
material, and the binder. In order to produce the molds and cores
layer by layer in 3-D printing, the refractory molding base
material is applied layer by layer and is selectively printed with
the binder layer by layer, and consequently a body corresponding to
the molds or cores is constructed and the molds or cores are
released after the unbonded construction material mixture has been
removed.
Inventors: |
DETERS; Heinz; (Dusseldorf,
DE) ; ZUPAN; Henning; (Wuppertal, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ASK Chemicals GmbH |
Hilden |
|
DE |
|
|
Family ID: |
55272192 |
Appl. No.: |
15/534551 |
Filed: |
December 11, 2015 |
PCT Filed: |
December 11, 2015 |
PCT NO: |
PCT/DE2015/000589 |
371 Date: |
June 9, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C04B 2235/5436 20130101;
B32B 18/00 20130101; C04B 2237/704 20130101; B33Y 50/02 20141201;
C04B 35/6316 20130101; C04B 2235/6026 20130101; C04B 2235/5427
20130101; B29C 64/165 20170801; B33Y 70/00 20141201; C04B 2235/3427
20130101; B28B 1/001 20130101; C04B 2235/667 20130101; C04B 35/14
20130101; C04B 35/6313 20130101; B22C 1/188 20130101; B22C 9/10
20130101; B33Y 10/00 20141201; B22C 9/02 20130101; B33Y 80/00
20141201; B28B 11/241 20130101; C04B 2235/3201 20130101; C04B
35/6303 20130101; C04B 2235/3409 20130101; Y02P 10/292 20151101;
Y02P 10/25 20151101 |
International
Class: |
B22C 1/18 20060101
B22C001/18; B28B 1/00 20060101 B28B001/00; B33Y 80/00 20060101
B33Y080/00; B33Y 10/00 20060101 B33Y010/00; B33Y 70/00 20060101
B33Y070/00; B33Y 50/02 20060101 B33Y050/02; B28B 11/24 20060101
B28B011/24; B22C 9/10 20060101 B22C009/10 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 12, 2014 |
DE |
10 2014 118 577.3 |
Claims
1. A method for constructing bodies layer by layer, comprising at
least the following steps: a) providing a refractory molding base
material as a component of a construction material mixture; b)
spreading a thin layer of the construction material mixture with a
layer thickness of 0.05 mm to 3 mm, preferably 0.1 mm to 2 mm and
particularly preferably 0.1 mm to 1 mm; c) printing selected areas
of the thin layer with a binder having a dynamic viscosity at
25.degree. C. of greater than 1 mPas and less than 25 mPas,
comprising: water glass in the form of an aqueous alkali silicate
solution, and at least one phosphate or at least one borate or
phosphate and borate; and d) repeatedly repeating steps b) and
c).
2. The method according to claim 1, further comprising the step of
curing the printed areas, in particular by temperature increase,
preferably effected by microwaves and/or infrared light.
3. The method according to claim 2, wherein the curing of the
printed areas takes place in each case after constructing 1 to 10
printed layers of the construction material mixture, preferably 3
to 8 printed layers.
4. The method according to claim 2, further comprising the
following steps: i) curing the body after completion of the layer
by layer construction in a furnace or by means of a microwave, and
subsequent ii) removing the unbonded construction material mixture
from the at least partially cured casting mold.
5. The method according to claim 1, wherein the refractory molding
base material comprises quartz, zirconium or chromium ore sand,
olivine, vermiculite, bauxite, fire clay, glass beads, glass
granules, hollow aluminum silicate microspheres and mixtures
thereof, and preferably consists of more than 50% by weight of
quartz sand, based on the refractory molding base material.
6. The method according to claim 1, wherein more than 80% by
weight, preferably greater than 90% by weight, and particularly
preferably greater than 95% by weight, of the construction material
mixture is refractory molding base material.
7. The method according to claim 1, wherein the refractory molding
base material has average grain diameters from 50 .mu.m to 600
.mu.m, preferably between 80 .mu.m and 300 .mu.m, determined by
sieving analysis.
8. The method according to claim 1, wherein the water glass,
including solvent/diluent, is used in an amount of between 0.5% by
weight and 7% by weight, preferably between 0.75% by weight and 6%
by weight, and particularly preferably between 1% by weight and
5.0% by weight, based on the molding base material.
9. The method according to claim 1, wherein an inorganic curing
agent for water glass-based binders is added to the construction
material mixture, and the inorganic curing agent is preferably a
phosphate.
10. The method according to claim 9, wherein the proportion of the
curing agent relative to the binder is greater than or equal to 5
to less than 25% by weight, preferably greater than or equal to 8
to less than 20% by weight, particularly preferably greater than or
equal to 10 to less than 15% by weight.
11. The method according to claim 9, wherein between 0.05% by
weight and 2% by weight of the inorganic curing agent is used,
preferably between 0.1% by weight and 1% by weight, particularly
preferably between 0.1% by weight and 0.6% by weight, in each case
based on the molding base material.
12. The method according to claim 1, wherein the printing is
carried out with a printing head having a plurality of nozzles, the
nozzles preferably being individually selectively controllable.
13. The method according to claim 12, wherein the print head is
movable in at least one plane controlled by a computer, and the
nozzles apply the liquid binder layer by layer.
14. The method according to claim 12, wherein the print head is a
drop-on-demand print head with bubble-jet or piezo technology.
15. The method according to claim 1, wherein the at least one
phosphate is at least water-soluble at 25.degree. C. and is
preferably an alkali phosphate, particularly preferably sodium
polyphosphate or sodium metaphosphate or both.
16. The method according to claim 1, wherein the binder contains
phosphates, preferably in a molar ratio of P.sub.2O.sub.5/SiO.sub.2
(calculated as P.sub.2O.sub.5 and relative to SiO.sub.2 in the
binder) of greater than 0.01 and less than 0.5.
17. The method according to claim 1, wherein the binder contains
borates and preferably the molar ratio of B.sub.2O.sub.3/SiO.sub.2
(calculated as B.sub.2O.sub.3 and SiO.sub.2 in the binder) is from
0.01 to 0.5.
18. The method according to claim 1, wherein the at least one
borate is at least water-soluble at 25.degree. C. and is preferably
an alkali borate, more preferably disodium tetraborate
decahydrate.
19. A mold or core, producible according to claim 1, for metal
casting, in particular iron, steel, copper or aluminum casting.
20. A binder, comprising water glass with a molar ratio of
Na.sub.2O/M.sub.2O of greater than 0.4, preferably greater than 0.5
and more preferably greater than 0.6 and particularly preferably
greater than 0.7, and a molar modulus SiO.sub.2/M.sub.2O of greater
1.4 to less than 2.8, preferably greater than 1.6 to less than 2.6,
preferably greater than 1.8 to less than 2.5 and more preferably
greater than 1.9 and less than 2.4, wherein in each case M.sub.2O
is the sum of the molar amounts of lithium, sodium and potassium
calculated as oxide, based on the amorphous alkali silicates,
alkali oxides, alkali hydroxides, alkali phosphates and/or alkali
borates contained in the binder; and wherein any particulate
constituents in the binder have a D90 value of less than 70 .mu.m;
the binder contains at least one phosphate or at least one borate
or phosphate and borate; and the binder has a dynamic viscosity at
25.degree. C. of greater than 1 mPas and less than 25 m Pas.
21. The binder according to claim 20, wherein the binder has a
solids content of greater than 22 to less than 40% by weight,
preferably greater than 24 to less than 38% by weight, preferably
greater than 26 to less than 36% by weight, particularly preferably
greater than 28 to less than 35% by weight.
22. The binder according to claim 20, wherein particles, if any, in
the binder have a D90 value of less than 40 .mu.m, preferably less
than 30 .mu.m, particularly preferably less than 25 .mu.m, and more
preferably less than 20 .mu.m, and/or have a D100 value of less
than 250 .mu.m, preferably less than 120 .mu.m, preferably less
than 50 .mu.m, more preferably less than 40 .mu.m, particularly
preferably less than 35 .mu.m, and more preferably less than 30
.mu.m.
23. The binder according to claim 20, wherein the at least one
phosphate is at least water-soluble at 25.degree. C., preferably is
an alkali phosphate, particularly preferably sodium polyphosphate
or sodium metaphosphate or both.
24. The binder according to claim 20, wherein the binder contains
phosphates, preferably in a molar ratio of P.sub.2O.sub.5/SiO.sub.2
(calculated as P.sub.2O.sub.5 and relative to the SiO.sub.2 in the
binder) of greater than 0.01 and less than 0.5.
25. The binder according to claim 20, wherein the binder contains
borates and preferably the molar ratio B.sub.2O.sub.3/SiO.sub.2
(calculated as B.sub.2O.sub.3 and SiO2 in the binder) is from 0.01
to 0.5.
26. The binder according to claim 20, wherein the at least one
borate is at least water-soluble at 25.degree. C. and is preferably
an alkali borate, particularly preferably disodium tetraborate
decahydrate.
27. The binder according to claim 20, wherein the binder further
contains surface-active substances, preferably surfactants, in
particular between 0.01 and 4.0% by weight, preferably between 0.1
and 3.0% by weight.
28. (canceled)
Description
[0001] The invention relates to a method for constructing molds and
cores layer by layer, the molds and cores comprising a refractory
molding base material and a binder containing least one aqueous
alkali silicate solution and further a phosphate or a borate or
both. In order to produce molds and cores layer by layer in 3-D
printing it is necessary to apply a refractory molding base
material layer by layer and to selectively print each with the
binder. Furthermore, the invention relates to molds or cores
produced in this way.
PRIOR ART
[0002] Casting molds essentially consist of cores and molds, which
represent the negative forms of the casting to be produced. These
cores and molds consist of a refractory material, for example
quartz sand, and a suitable binder which imparts sufficient
mechanical strength to the casting mold after removal from the
forming die. For the production of casting molds, therefore, a
refractory molding base material is used, which is coated with a
suitable binder. The refractory molding base material is preferably
in a free-flowing form so that it can be filled into a suitable
hollow mold. The binder creates a firm cohesion between the
particles/granules of the molding base material so that the casting
mold is given the required mechanical stability.
[0003] Casting molds have to meet various requirements. In the
casting process itself, they firstly have to have sufficient
stability and heat resistance to accommodate the liquid metal in
the hollow space formed by one or more (parts of) casting molds.
After commencement of solidification, the mechanical stability of
the casting mold is ensured by a solidified metal layer which forms
along the walls of the casting mold. The material of the casting
mold then has to decompose under the action of the heat given off
by the metal so that it loses its mechanical strength, i. e.
cohesion between individual particles/granules of the refractory
material is lost. In the ideal case the casting mold disintegrates
again to leave a fine sand which can be poured from the casting
mold of the casting.
[0004] The designation "rapid prototyping" includes various known
methods for the production of three-dimensional bodies by
constructing them layer by layer. An advantage of these methods is
the possibility of producing even complex single-piece bodies with
undercuts and cavities. By conventional methods these bodies would
have to be assembled from several, individually produced parts. A
further advantage is that the methods are capable of producing the
bodies without molding dies directly from the CAD data.
[0005] As a result of the 3-dimensional printing methods, novel
requirements are imposed on binders which hold the casting mold
together when the binder or a binder component is to be applied
through the nozzles of a print head. The binders must then not only
lead to a sufficient strength level and to good disintegration
properties after the metal casting, and also have sufficient
thermal and storage stability, but also now must be "printable",
that is, the nozzles of the print head can not be blocked by the
binder, on the other hand, the binder should also not be able to
flow directly out of the print head but instead form single
droplets.
[0006] Furthermore, more and more frequently, it is required that
no emissions in the form of CO.sub.2 or hydrocarbons occur during
the production of the casting molds as well as during casting and
cooling in order to preserve the environment and to limit the odor
pollution of the environment by hydrocarbons, mainly by aromatic
hydrocarbons. In order to meet these requirements, inorganic
binding systems have been developed or further developed in recent
years. Their use leads to the avoidance or at least minimization of
emissions of CO.sub.2 and hydrocarbons during the production of
metal molds.
[0007] EP 1802409 B1 discloses an inorganic binder system with
which it is possible to produce molds with sufficient stability.
However, the binder system is particularly suitable for thermal
curing in a core firing machine in which a previously mixed molding
material mixture (mixture of refractory material and binder) is
conveyed into the heated molding die by means of pressure.
[0008] WO 2012/175072 A1 discloses a method for constructing models
layer by layer, wherein an inorganic binder system is used. The
particulate material applied layer by layer comprises a particulate
construction material and a spray-dried alkali silicate solution.
The selective activation of the curing is carried out by means of a
solution comprising water, which is added by means of the print
head. Both pure water and modified water containing rheological
additives are disclosed. Rheological additives mentioned are
exemplified by thickening agents, such as glycerol, glycol or
layered silicates, the layered silicates being particularly
emphasized. WO 2012/175072 A1 does not disclose the use of aqueous
alkali silicate solutions. The binder or the water-glass solution
is not dosed via the print head, but is already contained in the
particulate material applied layer by layer. The selective wetting
or setting of a material applied layer by layer by means of a
binder is achieved according to WO 2012/175072 A1 only by a detour
and not directly by means of an aqueous alkali silicate solution.
The process described in WO 2012/175072 A1 provides the binder, the
spray-dried alkali silicate solution, not only at the intended
destination, but also in areas in which it is not required. Thus,
the binder is consumed unnecessarily.
[0009] DE 102011053205 A1 discloses a method for producing a
component in deposition technology in which, inter alia, water
glass is used as a printing liquid in addition to many other
options. Accordingly, the water glass can be dosed by means of a
print head and applied to a predetermined section of the
respectively uppermost layer. DE 102011053205 A1, however, does not
provide information as to which water glass compositions can be
used. The person skilled in the art is also not given any
information about the physical properties of the water glasses
used, which could have indicated a chemical composition. Only in
the described prior art inorganic binders (such as, e. g.,
free-flowing water glass) are mentioned quite generally, which
generally contain large amounts of moisture--as an example, only up
to 60% by weight water is mentioned. The large amounts of water (e.
g. up to 60% by weight water) are thought to be disadvantageous
since they are difficult to handle.
[0010] The person skilled in the art is not given any information
as to which water glass compositions are suitable for 3-D printing
by DE 102011053205 A1.
[0011] WO 2013/017134 A1 discloses an aqueous alkali silicate
solution having a viscosity of 45 mPas or less at 20.degree. C.,
which has a solids content with respect to the alkali silicate of
39% by weight. The ratio between SiO.sub.2 and M.sub.2O (M.sub.2O
is Na.sub.2O and/or K.sub.2O) is stated as the weight ratio. The
narrowest limits of this weight ratio are between 1.58 and 3.30. In
the examples of W02013/017134 A1, a method is disclosed with which
it appears possible to lower the viscosity of water glass binders
by means of a ball mill. However, such a method is very complex and
cost-intensive.
THE OBJECT OF THE INVENTION
[0012] The inventors are therefore faced with the object of
developing a water glass binder or an aqueous alkali silicate
solution which is suitable for the 3-dimensional printing of
casting molds, i. e., the water glass binder can be selectively
dosed directly via a print head without blocking or clogging the
nozzles or the module of the print head. Furthermore, the binder
should be applied as finely, punctiformly and in a precisely
defined dose. Moreover, the use of the water glass according to the
invention leads to positive properties of the molds produced
therewith.
SUMMARY OF THE INVENTION
[0013] This object is achieved with a binder having the features of
the independent claims. Advantageous further developments of the
method according to the invention are the subject matter of the
dependent claims or described below. The method for constructing
bodies layer by layer comprises at least the following steps:
[0014] a) providing a refractory molding base material as a
component of a construction material mixture;
[0015] b) spreading a thin layer of the construction material
mixture with a layer thickness of 0.05 mm to 3 mm, preferably 0.1
mm to 2 mm and particularly preferably 0.1 mm to 1 mm of the
construction material mixture;
[0016] c) printing selected areas of the thin layer with a binder,
comprising [0017] water glass in the form of an aqueous alkali
silicate solution, and [0018] at least one phosphate or at least
one borate or phosphate and borate, preferably in a dissolved form
in a at least partial, in particular completely aqueous solution;
and
[0019] d) repeatedly repeating steps b) and c).
[0020] When the water-glass binder according to the invention is
used, the casting molds produced therewith have the following
properties: [0021] 1. good strength, especially after thermal
curing [0022] 2. sufficient thermostability, particularly suitable
for metal casting, to prevent deformation of the casting mold
during the casting process. [0023] 3. good storage stability [0024]
4. good disintegration properties after metal casting [0025] 5. no
emissions of CO.sub.2 or other organic pyrolysis products during
the casting operation and cooling, unless organic additives are
present in the particulate material applied layer by layer, or in
the water glass binder.
[0026] Surprisingly, it has been found that the binder according to
the invention is very readily "printable", that is to say the
nozzles of the printing head are not rapidly blocked by the binder.
At the same time, the binder can be applied very finely. A clogging
of the nozzles of the print head would lead to a poor print result.
This is avoided by the binder according to the invention.
[0027] A rapid clogging of the nozzles or a rapid film formation at
the module of the print head is due to an increased reactivity of
the binder. The reactivity of the binder can be controlled by its
chemical composition as well as the thermal stability of the
binder. In view of the application in the area of the 3-D printing
method, on the one hand, a low reactivity of the binder is required
in order to avoid impairment of the print head and to control the
curing of the binder, but on the other hand, high thermal stability
is desired also in order to prevent the produced casting molds from
deforming during the casting operation and thus resulting in poor
dimensional stability of the casting. The thermostability and
reactivity of the binder depend in the same manner on the chemical
composition of the binder, i. e., the higher the reactivity, the
higher the thermal stability. It is the subject matter of this
invention to disclose a suitable binder composition which ensures a
sufficiently high thermostability of the casting molds at
sufficiently low reactivity of the binder.
DETAILED DESCRIPTION OF THE INVENTION
[0028] The binder according to the invention is provided for the
3-dimensional printing of casting molds. The binder serves as a
printing liquid, by means of which a material applied layer by
layer, such as, for example, a refractory molding base material (e.
g. quartz sand) and optionally one or more additives, collectively
referred to as a construction material mixture, is selectively
printed. The construction material mixture does not yet contain the
binder. Usually a selective printing operation follows the layer by
layer application of the construction material mixture--this
operation is repeated until the entire printing operation is
completed and the casting mold can be obtained.
[0029] The curing of the binder can be carried out in conventional
ways. Thus, on the one hand, it is possible to add one or more
water glass curing agents to the construction material mixture
applied layer by layer, which produce the immediate curing of the
printed water glass-containing binder by chemical means.
[0030] It is also possible to cure the applied water glass by means
of acidic gases, such as CO.sub.2--but this variant is less
preferred.
[0031] On the other hand, thermal curing can also take place. It
is, for example, possible for thermal curing to be carried out
after completion of a printing operation (immediately before,
during or after the next layer of the construction material mixture
is applied), by irradiating the mixture of construction material
mixture and binding agent, for example, by means of an infrared
light. In this layer by layer curing, the infrared light, for
example in the form of a spot, can follow the print head.
[0032] It is, of course, also possible to carry out this type of
thermal curing step by step after several layers have been applied.
It is also possible to carry out the thermal curing only after
completion of the last printing operation. The steps "applying a
layer of the construction material mixture" and subsequent
"printing operation" alternate until the last layer has been
printed which is necessary to produce the casting mold completely.
For this purpose, the applied and partially printed layers remain,
for example in a so-called "job box", which subsequently can be
transferred to a microwave oven to perform thermal curing.
[0033] Thermal curing is preferred, in particular drying by means
of microwaves and preferably after completion of the entire
printing operation in the microwave oven.
[0034] Customary and known materials can be used as the refractory
molding base materials for the production of casting molds.
Suitable materials are, for example, quartz, zirconium or chromium
ore sand, olivine, vermiculite, bauxite, fire clay as well as
artificial molding base materials, in particular more than 50% by
weight quartz sand based on the refractory molding base material.
In order to keep costs low, the proportion of quartz sand of the
refractory molding base material is greater than 70% by weight,
preferably greater than 80% by weight and particularly preferably
greater than 90% by weight. It is not necessary, however, to use
exclusively new sands. In the sense of saving resources and
avoiding landfill costs, it is even advantageous to use as high a
proportion of regenerated old sand as is obtainable from used forms
by recycling.
[0035] A refractory molding base material is understood to mean
substances which have a high melting point (melting temperature).
The melting point of the refractory molding base material is
preferably greater than 600.degree. C., preferably greater than
900.degree. C., particularly preferably greater than 1200.degree.
C. and very particularly preferably greater than 1500.degree.
C.
[0036] The refractory molding base material preferably makes up
greater than 80% by weight, in particular greater than 90% by
weight, particularly preferably greater than 95% by weight, of the
construction material mixture.
[0037] A suitable refractory molding base material is described,
for example, in WO 2008/101668 A1 (=U.S. 2010/173767 A1). Likewise
suitable are regenerates which are obtainable by washing and
subsequent drying of comminuted used molds. Generally, the
regenerates can make up 70% by weight of the refractory molding
base material, preferably at least about 80% by weight and
particularly preferably greater than 90% by weight.
[0038] According to one embodiment of the invention, it is
advantageous to use regenerates which have been obtained by purely
mechanical treatment. A mechanical treatment is understood to mean
that at least a portion of the binder remaining in the old sand is
removed from the sand grain by a grinding or impacting principle.
These regenerates can be used as needed. The proportion of these
regenerates can, for example, be greater than 5% by weight,
preferably greater than 20% by weight, more preferably greater than
50% by weight, particularly preferably greater than 70% by weight
and very particularly preferably greater than 80% by weight, of the
refractory molding base material. Such regenerates are used, for
example, to effect a (pre- or partial) curing of the applied
binder.
[0039] According to one embodiment of the invention, it is
advantageous to use salts as a molding base material. Salts are
understood to mean alkali halides and alkaline earth halides.
Alkali halides are preferred, of which the alkali metal chlorides
are more preferred. Particularly preferably, sodium chloride is
used. The proportion of the salts or the salt can make up, for
example, greater than 5% by weight, preferably greater than 20% by
weight, more preferably greater than 50% by weight, more preferably
greater than 80% by weight, of the refractory molding base
material. It is particularly preferred in this embodiment to use
only the salt as a refractory molding base material. Salts are
used, for example, when the casting mold is to be removed after the
metal casting by means of water.
[0040] The mean grain diameter of the refractory molding base
material is generally between 50 .mu.m and 600 .mu.m, preferably
between 70 .mu.m and 400 .mu.m, preferably between 80 .mu.m and 300
.mu.m, and particularly preferably between 100 .mu.m and 200 .mu.m.
The grain diameter can be determined, for example, by sieving
according to DIN ISO 3310. Particularly preferred are particle
shapes/grains having the greatest longitudinal extent to the
smallest longitudinal extent (at right angles to one another and in
each case for all spatial directions) of 1:1 to 1:5 or 1:1 to 1:3,
i. e., those which are non-fibrous, for example.
[0041] The refractory molding base material is in a free-flowing
state.
[0042] The binder according to the invention contains water glasses
which are prepared, for example, by dissolving glassy lithium,
sodium and/or potassium silicates in water. Preferred water glasses
are those which contain at least sodium silicates.
[0043] The ratio Na.sub.2O/M.sub.2O in the binder is preferably
greater than 0.4, preferably greater than 0.5 and more preferably
greater than 0.6 and particularly preferably greater than 0.7,
wherein M.sub.2O is the sum of the mass quantities of lithium,
sodium and potassium calculated as oxide in the binder.
[0044] According to the present invention, the amounts of the
alkali metals, calculated as oxides M.sub.2O, are calculated
exclusively from the molar amounts of amorphous alkali silicates,
alkali oxides, alkali hydroxides, alkali phosphates and alkali
borates present in the binder. This means that any additions such
as alkali chloride or alkali carbonates to the water glass solution
are not included in the calculation of M.sub.2O (M.sub.2O in each
case as defined in the previous paragraph).
[0045] The water glass has a molar modulus SiO.sub.2/M.sub.2O of
greater than 1.4, preferably greater than 1.6, preferably greater
than 1.8, more preferably greater than 1.9, and particularly
preferably greater than 2.0. The water glass preferably has a molar
modulus SiO.sub.2/M.sub.2O of less than 2.8, preferably less than
2.6, preferably less than 2.5, particularly preferably less than
2.4. In this context, it is surprising for the person skilled in
the art that such low molar ratios SiO.sub.2/M.sub.2O of the water
glass solutions according to the invention lead to a sufficiently
high thermostability of the casting mold, in particular in metal
casting.
[0046] The binder has a solids content of less than 40% by weight,
preferably less than 38% by weight, preferably less than 36% by
weight, particularly preferably less than 35% by weight. The
remainder of the binder preferably consists of water.
[0047] The binder has a solids content of greater than 22% by
weight, preferably greater than 24% by weight, preferably greater
than 26% by weight, particularly preferably greater than 28% by
weight, very particularly preferably greater than 29% by weight and
particularly preferably greater than 29.5%.
[0048] The solids content is determined by gently evaporating the
liquid, drying the binder and then heating it at 600.degree. C. for
1 h in an air atmosphere. The remaining oxidic material is weighed
to determine the solids content.
[0049] Irrespective of this, the amount of material of SiO.sub.2
and M.sub.2O (calculated as mol %) in the binder is less than 16
mol %, preferably less than 15 mol %, preferably less than 14 mol
%, particularly preferably less than 13.5 mol %. Furthermore, this
amount of material is greater than 7 mol %, preferably greater than
8 mol %, preferably greater than 9 mol %, particularly preferably
greater than 10 mol % and very particularly preferably greater than
10.5 mol %.
[0050] The viscosity of the binder must not be too low and not too
high. The dynamic viscosity is measured using a Brookfield rotation
viscometer. At a temperature of 25.degree. C., the binder according
to the invention has a viscosity of less than 25 mPas, preferably
less than 20 mPas, preferably less than 18 mPas, and particularly
preferably less than 16 mPas. At a temperature of 25.degree. C.,
the binder has a viscosity of greater than 1 mPas, preferably
greater than 2 mPas, preferably greater than 3 mPas, and
particularly preferably greater than 4 mPas.
[0051] The binder according to the invention should be a clear
solution and, if possible, free of coarser particles, which in
their greatest extent have a size between several micrometers to
several millimeters and can originate, for example, from
impurities. Commercially available water glass solutions generally
have these coarser particles.
[0052] Particle or grain sizes are determined by means of dynamic
light scattering in accordance with DIN/ISO 13320 (e. g. Horiba LA
950).
[0053] The determined D90 value (in each case based on volume) is
the measure for the larger particles--it means that 90% of the
particles are smaller than the specified value. The water glass
according to the invention has a D90 value (determined by dynamic
light scattering or laser diffractometry) of less than 70 .mu.m,
preferably less than 40 .mu.m, preferably less than 30 .mu.m,
particularly preferably less than 25 .mu.m, and very particularly
preferably less than 20 .mu.m.
[0054] Irrespective of this, the water glass according to the
invention has a D100 value (in each case based on volume) of less
than 250 .mu.m, preferably less than 120 .mu.m, preferably less
than 50 .mu.m, more preferably less than 40 .mu.m, particularly
preferably less than 35 .mu.m, and very particularly preferably
less than 30 .mu.m.
[0055] The binders described above containing water glasses can be
obtained, for example, by suitable filtration. For example, filters
with a sieve diameter of 50 .mu.m, preferably of 25 .mu.m,
preferably 10 .mu.m, and particularly preferably 5 .mu.m, are
suitable. Preferred binders are those which contain no particles
with a size of at least 1 .mu.m.
[0056] In one embodiment, the binder according to the invention can
have proportions of lithium ions. The molar ratio of
Li.sub.2O/M.sub.2O can vary over wide ranges, for example between
0.01 and 0.3. Preferably, the ratio is in the range between 0.03
and 0.17, preferably between 0.035 and 0.16, and particularly
preferably between 0.04 and 0.14.
[0057] In one embodiment, the binder according to the invention can
have proportions of potassium ions. The molar ratio of
K.sub.2O/M.sub.2O can vary over wide ranges, for example between
0.01 and 0.3. Preferably, the ratio is in the range between 0.01
and 0.17, preferably between 0.02 and 0.16 and particularly
preferably between 0.03 and 0.14.
[0058] Surprisingly, it has been shown that an addition of
network-former agents other than silicate can both increase the
thermostability and reduce the reactivity. A network-forming agent
from the group of phosphates is therefore added to the binder and
dissolved in the binder, in particular alkali phosphates (e. g.
sodium hexametaphosphate or sodium polyphosphates) have been found
to be beneficial. Among the alkali phosphates, alkali
orthophosphates such as trisodium phosphate (Na.sub.3PO.sub.4) are
not preferred. Sodium polyphosphates and/or sodium metaphosphates
are particularly preferred.
[0059] Other network-forming agents which can be added to the
binder alternatively or additionally are borates, in particular
alkali borates, e. g., disodium tetraborate decahydrate. These,
too, are dissolved in the binder.
[0060] The amounts of the alkali metals which result from the
proportions of the alkali borates and/or alkali phosphates in the
total amount of the binder (including diluent) are calculated as
oxides and contribute to the total amount of material (i. e. the
sum of the individual amounts of material) of lithium, sodium and
potassium oxide in the total aqueous solution. Consequently, the
molar modulus SiO.sub.2/M.sub.2O is also influenced by the addition
of alkali borates and/or alkali phosphates.
[0061] The content of borates in the binder, in particular the
content of alkali borates, is calculated as B.sub.2O.sub.3. The
molar ratio of B.sub.2O.sub.3/SiO.sub.2 can vary over wide ranges,
for example from 0 to 0.5. This ratio is preferably less than 0.3,
preferably less than 0.2, particularly preferably less than 0.1,
very particularly preferably less than 0.08 and most particularly
preferably less than 0.06. Preferably, this ratio is greater than
or equal to zero. In a further embodiment, this ratio is preferably
greater than 0.01, particularly preferably greater than 0.02.
Borates in the context of the invention are boron compounds in the
oxidation state III, which are only directly bonded to oxygen, i.
e., oxygen atoms are the direct bonding partners of the boron in
the compound.
[0062] The content of phosphates in the binder, in particular the
content of alkali phosphates, is calculated as P.sub.2O.sub.5. The
molar ratio of P.sub.2O.sub.5/SiO.sub.2 can vary over wide ranges,
for example from 0 to 0.5. This ratio is preferably less than 0.4,
preferably less than 0.3, more preferably less than 0.25,
particularly preferably less than 0.2 and very particularly
preferably less than 0.15. This ratio is preferably greater than 0,
preferably greater than 0.01, particularly preferably greater than
0.02.
[0063] Phosphates in the context of the invention are phosphorus
compounds in the oxidation state V, which are only directly bonded
to oxygen, i. e. oxygen atoms are the direct bonding partners of
the phosphorus in the compound.
[0064] In a further embodiment, the binder can also contain
aluminum, wherein the proportion of the aluminum being then
calculated as Al.sub.2O.sub.3. The proportion of Al.sub.2O.sub.3 is
then usually less than 2% by weight, based on the total weight of
the binder.
[0065] In a preferred embodiment, surface-active substances may be
added to the binder according to the invention in order to
influence the surface tension of the binder. The proportion of
these surface-active substances is generally between 0.01 and 4.0%
by weight, preferably between 0.1 and 3.0% by weight.
[0066] Suitable surface-active substances in the binder are, for
example, described in DE 102007051850 A1, including preferably
anionic surfactants which carry a sulfate and/or sulfonate group.
Further suitable surface-active substances are, for example,
polyacrylate salts (e. g. of sodium--for example Dispex N40-Ciba)
or silicone surfactants for aqueous systems (e. g. Byk 348,
Altana). Surface-active substances based on trisiloxane or glycol
(e. g. polyethylene glycol) can be used.
[0067] Depending on the application and the desired strength level,
preferably between 0.5% by weight and 7% by weight of binder, based
on the water glass is used, preferably between 0.75% by weight and
6% by weight, particularly preferably between 1% by weight and 5.0%
by weight, and particularly preferably between 1% by weight and
4.0% by weight, based in each case on the molding base material.
The data relate to the total amount of the water-glass binder,
including the (in particular aqueous) solvent or diluent and the
solids content (if any) (together=100% by weight).
[0068] In a preferred embodiment, the construction material mixture
may contain a proportion of a particulate amorphous silica to
enhance the strength level of the casting molds. An increase in the
strengths of the casting molds, in particular the increase in the
hot strengths, can be advantageous in the automated production
process. Synthetically produced amorphous silica is particularly
preferred.
[0069] The mean particle size (including any agglomerates) of the
amorphous silica is preferably less than 300 .mu.m, preferably less
than 200 .mu.m, particularly preferably less than 100 .mu.m. The
sieve residue of the particulate amorphous SiO.sub.2 is preferably
not more than 10% by weight, particularly preferably not more than
5% by weight, and very particularly preferably not more than 2% by
weight, when passing through a sieve of 125 .mu.m mesh (120
mesh).
[0070] Irrespective of this, the sieve residue on a sieve with 63
.mu.m mesh is less than 10% by weight, preferably less than 8% by
weight. The sieve residue is determined according to the machine
sieving method described in DIN 66165 (part 2), in which case a
chain ring is additionally used as sieving aid.
[0071] The particulate amorphous silicon dioxide preferably used
according to the present invention has a water content of less than
15% by weight, in particular less than 5% by weight and
particularly preferably less than 1% by weight.
[0072] The particulate amorphous SiO.sub.2 is used as a powder
(including dusts).
[0073] Both synthetically produced and naturally occurring silicic
acids can be used as amorphous SiO.sub.2. The latter are known, for
example, from DE 102007045649, but are not preferred, since they
usually contain not insignificant crystalline proportions and are
therefore classified as carcinogens. Synthetic is understood to
mean non-naturally occurring amorphous SiO.sub.2, i. e., its
preparation comprises a deliberately performed chemical reaction,
as is caused by a human, e. g. the production of silica sols by ion
exchange processes from alkali silicate solutions, the
precipitation from alkali silicate solutions, the flame hydrolysis
of silicon tetrachloride, the reduction of quartz sand with coke in
the arc furnace in the production of ferrosilicon and silicon. The
amorphous SiO.sub.2 produced by the two last-mentioned methods is
also referred to as pyrogenic SiO.sub.2.
[0074] On occasion, only precipitated silicic acid (CAS No.
112926-00-8) and SiO.sub.2 produced by flame hydrolysis (pyrogenic
silica, fumed silica, CAS No. 112945-52-5) are understood to mean
synthetic amorphous silicon dioxide, whereas the product formed in
ferrosilicon or silicon production is merely referred to as
amorphous silicon dioxide (silica fume, microsilica, CAS No.
69012-64-12). For the purposes of the present invention, the
product formed in ferrosilicon or silicon production is also
considered an amorphous SiO.sub.2.
[0075] Precipitated silicic acids and pyrogenic, i. e., silicon
dioxide produced by flame hydrolysis or in the electric arc are
used preferably. Amorphous silicon dioxide (described in DE
102012020509) produced by thermal decomposition of ZrSiO.sub.4 and
SiO.sub.2 (described in DE 102012020510) produced by oxidation of
metallic Si by means of an oxygen-containing gas are used
particularly preferably. Quartz glass powder (mainly amorphous
silicon dioxide), which has been produced by melting and rapid
re-cooling from crystalline quartz, so that the particles are
spherical and not splintered (described in DE 102012020511) are
also preferred.
[0076] The mean primary particle size of the particulate amorphous
silicon dioxide can be between 0.05 .mu.m and 10 .mu.m, in
particular between 0.1 .mu.m and 5 .mu.m, particularly preferably
between 0.1 .mu.m and 2 .mu.m. The primary particle size can be
determined, e. g., by means of dynamic light scattering (e. g.
Horiba LA 950) as well as by scanning electron micrographs (SEM
images with, e. g., Nova NanoSEM 230 from FEI). Furthermore,
details of the primary particle shape could be visualized up to the
order of 0.01 .mu.m using the SEM images. The silicon dioxide
samples were dispersed in distilled water for the SEM measurements
and then applied to an aluminum holder bonded with copper tape
before the water was evaporated.
[0077] Furthermore, the specific surface area of the particulate
amorphous silicon dioxide was determined by means of gas adsorption
measurements (BET method) according to DIN 66131. The specific
surface area of the particulate amorphous SiO.sub.2 is between 1
and 200 m.sup.2/g, in particular between 1 and 50 m.sup.2/g,
particularly preferably between 1 and 30 m.sup.2/g. Optionally, the
products can also be blended, e. g., in order to obtain mixtures
with specific particle size distributions.
[0078] Depending on the type of production and the producer, the
purity of the amorphous SiO.sub.2 can vary widely. Suitable types
have a silicon dioxide content of at least 85% by weight,
preferably at least 90% by weight and particularly preferably at
least 95% by weight.
[0079] Depending on the application and the desired strength level,
between 0.1% by weight and 2% by weight of the particulate
amorphous SiO.sub.2 are used, preferably between 0.1% by weight and
1.8% by weight, particularly preferably between 0.1% by weight and
1.5% by weight, based in each case on the molding base
material.
[0080] The ratio of water glass-based binder to particulate
amorphous silicon dioxide can be varied within wide limits.
[0081] Based on the total weight of the binder (including diluent
or solvent), the amorphous SiO.sub.2 is preferably present in a
proportion from 1 to 80% by weight, preferably from 2 to 60% by
weight, particularly preferably from 3 to 55% by weight and very
particularly preferably from 4 to 50% by weight. Or Irrespective of
this, based on the ratio of the proportion of solids of the water
glass-based binder (based on the oxides, i. e., the total mass of
alkali metal oxides M.sub.2O and silicon dioxide) to amorphous
SiO.sub.2 of 10:1 to 1:1.2 (weight parts) preferred.
[0082] The amorphous SiO.sub.2 is added to the refractory solid or
to the construction material mixture prior to the addition of the
binder.
[0083] Thus, the method according to the invention is furthermore
characterized by one or more of the following features when using
amorphous SiO.sub.2:
[0084] (a) the amorphous silicon dioxide is only added to the
construction material mixture.
[0085] (b) The amorphous silicon dioxide has a BET surface area
between 1 and 200 m.sup.2/g, preferably greater than or equal to 1
m.sup.2/g and less than or equal to 30 m.sup.2/g, particularly
preferably less than or equal to 15 m.sup.2/g.
[0086] (c) The amorphous silica is selected from the group
consisting of precipitated silicic acid, pyrogenic silicon dioxide
produced by flame hydrolysis or in the electric arc, amorphous
silicon dioxide produced by flame decomposition of ZrSiO.sub.4,
silicon dioxide produced by oxidation of metallic silicon by means
of an oxygen-containing gas, quartz glass powder having spherical
particles produced by melting and rapid re-cooling of crystalline
quartz, and mixtures thereof, and is preferably amorphous silicon
dioxide produced by thermal decomposition of ZrSiO.sub.4.
[0087] (d) The amorphous silicon dioxide is preferably used in
amounts of 0.1 to 2% by weight, particularly preferably 0.1 to 1.5%
by weight, based in each case on the refractory molding base
material.
[0088] (e) The amorphous silicon dioxide has a water content of
less than 5% by weight and particularly preferably less than 1% by
weight.
[0089] (f) The amorphous silicon dioxide is particulate amorphous
silicon dioxide, preferably with a mean primary particle diameter
determined by dynamic light scattering between 0.05 .mu.m and 10
.mu.m, in particular between 0.1 .mu.m and 5 .mu.m, and
particularly preferably between 0.1 .mu.m and 2 .mu.m.
[0090] In a further embodiment, an inorganic curing agent for water
glass-based binders is added to the construction material mixture
prior to the addition of the binder. Such inorganic curing agents
are, e. g., phosphates such as, for example, Lithopix P26 (an
aluminum phosphate from Zschimmer and Schwarz GmbH & Co KG
Chemsche Fabriken) or Fabutit 748 (an aluminum phosphate from
Chemische Fabrik Budenheim KG). Other inorganic curing agents for
water glass-based binders are, for example, calcium silicates and
their hydrates, calcium aluminates and their hydrates, aluminum
sulfate, magnesium and calcium carbonate.
[0091] The ratio of curing agent to binder may vary depending on
the desired property, e.g. processing time and/or dismantling time
of the construction material mixtures. Advantageously, the curing
agent proportion (weight ratio of curing agent to binder and in the
case of water glass the total weight of the silicate solution or
other binders incorporated in solvents) is greater than or equal to
5% by weight, preferably greater than or equal to 8% by weight,
particularly preferably greater than or equal to 10% by weight,
based in each case on the binder. The upper limits are less than or
equal to 25% by weight, based on the binder, preferably less than
or equal to 20% by weight, particularly preferably less than or
equal to 15% by weight.
[0092] Irrespective of this, between 0.05% by weight and 2% by
weight of the inorganic curing agent are used, preferably between
0.1% by weight and 1% by weight, particularly preferably between
0.1% by weight and 0.6% by weight, based in each case on the
molding base material.
[0093] As soon as the strengths allow, the unbonded construction
material mixture can then be removed from the casting mold and the
casting mold fed to the further treatment, for example, the
preparation for metal casting. The removal of the unbonded from the
bonded construction material mixture is achieved, for example, by
means of an outlet so that the unbonded construction material
mixture can trickle out. The bonded construction material mixture
(casting mold) can, for example, be freed from residues of the
unbonded construction material mixture by means of compressed air
or by brushing.
[0094] The unbonded construction material mixture can be re-used
for a new printing operation.
[0095] Printing is carried out, e. g., with a print head having a
plurality of nozzles, the nozzles preferably being individually
selectively controllable. According to a further embodiment, the
print head is moved in at least one plane controlled by a computer,
and the nozzles apply the liquid binder layer by layer. The print
head can be, e. g., a drop-on-demand print head with bubble-jet or
preferably piezo technology.
* * * * *