U.S. patent application number 14/391094 was filed with the patent office on 2015-03-05 for salt-based cores, method for the production thereof and use thereof.
This patent application is currently assigned to Emil Muller GmbH. The applicant listed for this patent is Emil Muller GmbH. Invention is credited to Thorsten Hartig.
Application Number | 20150060005 14/391094 |
Document ID | / |
Family ID | 48607195 |
Filed Date | 2015-03-05 |
United States Patent
Application |
20150060005 |
Kind Code |
A1 |
Hartig; Thorsten |
March 5, 2015 |
SALT-BASED CORES, METHOD FOR THE PRODUCTION THEREOF AND USE
THEREOF
Abstract
Cores that are inserted into the mold during the die casting of
workpieces from metal in order to keep the cavities provided in the
workpieces free during the filling of the molds with the melt have
to meet demanding requirements with regard to the dimensional
stability and suitability thereof for removal from the cavities.
Therefore, salt-based cores which can be produced by molding and
compressing a core material mixture are provided according to the
invention, the core materials thereof being selected from at least
one salt, at least one binder system comprising a combination of
binder/binding agent and optionally auxiliary substances such as
additives, fillers, wetting agents and catalysts, wherein the salt,
the binder system and the optionally used auxiliary substances of
the core material mixture are inorganic, and these core materials
are soluble with water as the solvent.
Inventors: |
Hartig; Thorsten;
(Puschendorf, DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Emil Muller GmbH |
Wilhermsdorf |
|
DE |
|
|
Assignee: |
Emil Muller GmbH
Wilhermsdorf
DE
|
Family ID: |
48607195 |
Appl. No.: |
14/391094 |
Filed: |
April 10, 2013 |
PCT Filed: |
April 10, 2013 |
PCT NO: |
PCT/EP2013/001054 |
371 Date: |
October 8, 2014 |
Current U.S.
Class: |
164/528 ;
164/369 |
Current CPC
Class: |
B22C 1/167 20130101;
B22C 1/18 20130101; B22C 1/02 20130101; B22C 1/185 20130101; B22C
9/105 20130101 |
Class at
Publication: |
164/528 ;
164/369 |
International
Class: |
B22C 9/10 20060101
B22C009/10; B22C 1/16 20060101 B22C001/16; B22C 1/18 20060101
B22C001/18; B22C 1/02 20060101 B22C001/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 10, 2012 |
DE |
10 2012 205 767.6 |
Claims
1-47. (canceled)
48. Salt-based cores comprising a core material mixture; wherein
the core material mixture comprises a salt and a binder system;
said binder system comprising a binder and a drying agent; wherein
the salt and the binder are inorganic; and wherein the core
material mixture is shaped into the salt cores and compacted via a
dry pressing method.
49. The salt-based cores according to claim 48, wherein salts are
used which have a decomposition or melting point above the
temperature of the liquid metal that is poured around the
cores.
50. The salt-based cores according to claim 48, wherein the salt is
selected from the group consisting of an alkali chloride, an
alkaline earth chloride, a sulfate of an alkali element, a sulfate
of an alkaline earth element, a nitrate of an alkali element and a
nitrate of an alkaline earth element.
51. The salt-based cores according to claim 48, wherein the salt is
selected from the group consisting of sodium chloride, potassium
chloride, magnesium chloride sulfates and nitrates of the alkali
elements and alkaline earth elements, in particular potassium
sulfate and/or magnesium sulfate, ammonium salts, in particular
ammonium sulfate, or mixtures of these salts.
52. The salt-based cores according to claim 48, wherein the salt is
sodium chloride.
53. The salt-based cores according to claim 48, wherein the salt
has a grain size in the range from 0.01 mm to 2 mm.
54. The salt-based cores according to claim 48, wherein the salt is
present in a bimodal or trimodal grain size distribution.
55. The salt-based cores according to claim 48, wherein the salt
has a grain size distribution of from 0.01 to 0.29 mm.
56. The salt-based cores according to claim 48, wherein the salt
has a grain size distribution of from 0.3 to 1.3 mm.
57. The salt-based cores according to claim 48, wherein the salt
has a grain size distribution of from 1.31 to 2.0 mm.
58. The salt-based cores according to claim 48, wherein the binder
comprises at least one compound selected from the group consisting
of an inorganic phosphate, an inorganic borate and a silicate
compound which can be removed with water without leaving any
residue.
59. The salt-based cores according to claim 58, wherein the binder
is selected from the group consisting of an alkali phosphate,
ammonium phosphate, monoaluminum phosphate, boron phosphate,
trisodium phosphate, tetrapotassium pyrophosphate and sodium
polyphosphate.
60. The salt-based cores according to claim 58, wherein the binder
is a water-soluble silicate compounds.
61. The salt-based cores according to claim 48, wherein the binder
is a water glass having a water glass module of from 1 to 5.
62. The salt-based cores according to claim 48, wherein the content
of binder is between 0.5% by weight and 15% by weight, based on the
salt.
63. The salt-based cores according to claim 48, wherein the content
of the binder is between 0.5% by weight and 15% by weight, based on
the salt.
64. The salt-based cores according to claim 48, wherein water glass
is present as the binder in a content of 0.5% by weight to 15% by
weight, based on the salt that is used, as a function of the grain
size distribution and tailored to the water glass module.
65. The salt-based cores according to claim 48, wherein the binder
is tetrapotassium pyrophosphate.
66. The salt-based cores according to claim 48, wherein the binder
is tetrapotassium pyrophosphate in liquid form.
67. The salt-based cores according to claim 48, wherein the binder
is tetrapotassium pyrophosphate in an aqueous 60% solution.
68. The salt-based cores according to claim 48, wherein the binder
used in the binder system is tetrapotassium pyrophosphate in an
aqueous 60% solution and in an amount of from 1 to 5% by weight,
based on the amount of the salt.
69. The salt-based cores according to claim 48, wherein the binder
is tetrapotassium pyrophosphate in an aqueous 60% solution and in
amount of from 1 to 5% by weight, and tetrapotassium pyrophosphate
in solid form is present in the same amount or in a larger
amount.
70. The salt-based cores according to claim 48, wherein the binder
is present in the binder system in a content of 1 to 15% by weight,
based on the amount of the salt, and the drying agent is present in
a content of 0.3 to 4.5% by weight, based on the amount of the
salt.
71. The salt-based cores according to claim 48, wherein hydrophilic
substances which are able to reversibly bind water are used in the
binder system as drying agents.
72. The salt-based cores according to claim 48, wherein the drying
agent is selected from the group consisting of silicic acid, silica
gel, a zeolite, an anhydrous sodium sulfate and magnesium
sulfate.
73. The salt-based cores according to claim 48, further comprising
a catalyst.
74. The salt-based cores according to claim 48, wherein the
catalyst is particularly fine-grained salt, and preferably powdered
salt having a particle size of less than 100 nm.
75. The salt-based cores according to claim 48, wherein the salt is
sodium chloride, which is preferably present in a bimodal or
trimodal grain size distribution, particularly preferably in a
grain size distribution of 0.01 to 0.29 mm, 0.3 to 1.3 mm and/or
1.31 to 2.0 mm, the binder system is composed of the combination of
water glass as the binder and Aerosil as the drying agent, the
catalyst is particularly fine-grained salt, and preferably powdered
salt having a particle size of less than 100 nm, optionally further
auxiliary substances such as additives, fillers, wetting agents
and/or further catalysts are present, and the mixture of the core
materials is free-flowing.
76. The salt-based cores according to claim 48, wherein the cores
are heat-treated after shaping.
77. The salt-based cores according to claim 48, wherein, after
shaping, the cores are heat-treated at a temperature up to
600.degree. C., preferably at temperatures of 500 to 600.degree.
C., and preferably at a temperature of 580.degree. C.
78. The salt-based cores according to claim 48, wherein the shaped
cores have a density of 1.5 g/cm.sup.3 to 2.1 g/cm.sup.3.
79. The salt-based cores according to claim 48, wherein the shaped
cores have a porosity of 10% to 40%.
80. The salt-based cores according to claim 48, wherein the shaped
cores have a flexural strength between 400 N/cm.sup.2 and 1500
N/cm.sup.2.
81. A method for producing salt-based cores comprising the steps
of: homogenously mixing the core material mixture; shaping the core
material mixture into shaped cores; and compacting the shaped cores
view dry pressing to form the salt-based cores; wherein the core
material mixture comprises a salt and a binder system; said binder
system comprising a binder and a drying agent; wherein the salt and
the binder are inorganic.
82. A method according to claim 81, wherein salt having grain sizes
with differing distribution curves, preferably in a bimodal or
trimodal grain size distribution, is used and mixed.
83. A method according to claim 81, wherein the salt is selected
from the group consisting of an alkali chloride, an alkaline earth
chloride, a sulfate of an alkali element, a sulfate of an alkaline
earth element, a nitrate of an alkali element and a nitrate of an
alkaline earth element.
84. A method according to claim 81, wherein the binders/binders
used in the binder system are inorganic phosphates, inorganic
borates or silicate compounds which can be removed without leaving
any residue, using water, or mixtures of these binders/binders.
85. A method according to claim 81, wherein the binder comprises at
least one member selected from the group consisting of an alkali
phosphate, ammonium phosphate, monoaluminum phosphate, boron
phosphate, trisodium phosphate, tetrapotassium pyrophosphate and
sodium polyphosphate which can be removed with water without
leaving any residue.
86. A method according to claim 81, wherein the binder is a
water-soluble silicate compound.
87. A method according to claim 81, wherein the binder in the
binder system is water glass having a water glass module of 1 to 5,
and/or a mixture of water glasses having differing water glass
modules.
88. A method according to claim 81, wherein hydrophilic substances
which are able to reversibly bind water are used in the binder
system as drying agents.
89. A method according to claim 81, wherein the drying agent is
selected form the group consisting of a silicic acid, silica gel, a
zeolite, an anhydrous sodium sulfate and magnesium sulfate.
90. A method according to claim 81, wherein a catalyst is added as
an auxiliary substance.
91. A method according to claim 81, wherein the catalyst is
particularly fine-grained salt, and preferably powdered salt having
a particle size of less than 100 nm.
92. A method according to claim 81, wherein the salt is sodium
chloride, which is present in a bimodal or trimodal grain size
distribution, the binder system is composed of the combination of
water glass as the binder and Aerosil as the drying agent, the
catalyst is particularly fine-grained salt, and preferably powdered
salt having a particle size of less than 100 nm.
93. A method according to claim 81, wherein the core materials are
homogeneously mixed, shaped into a core, and compacted in a dry
pressing method.
94. A method according to claim 81, wherein the core materials have
a grain size ranging from 0.01 mm to 2 mm.
95. A method according to claim 81, wherein the cores are
heat-treated after shaping.
96. A method according to claim 81, wherein, after shaping, the
cores are heat-treated at a temperature up to 600.degree. C.
97. A method comprising providing the salt-based cores according to
claim 48 as cavity placeholder in a mold during production of metal
cast part.
98. A method according to claim 81, wherein the salt-based cores
are heat treated.
99. The salt-based cores according to claim 48, wherein the binder
is a mixture of water glasses having differing water glass
modules.
100. The salt based cores adoring to claim 48, wherein the binder
is a water glass.
101. The salt-based cores according to claim 48, further comprising
an auxiliary substance.
102. A method according to claim 81, wherein the binder is a water
glass.
103. A method according to claim 81, wherein the salt is sodium
chloride, which is present in a bimodal or trimodal grain size
distribution, particularly preferably in a grain size distribution
of 0.01 to 0.29 mm, 0.3 to 1.3 mm and/or 1.31 to 2.0 mm, the binder
system is composed of the combination of water glass as the binder
and Aerosil as the drying agent, the catalyst is particularly
fine-grained salt, and preferably powdered salt having a particle
size of less than 100 nm, optionally further auxiliary substances
such as additives, fillers, wetting agents and/or further catalysts
are present, and the mixture of the core materials is free-flowing.
Description
[0001] The invention relates to salt-based cores, to methods for
producing salt-based cores, and to the use of such cores as cavity
placeholders in the production of metal cast parts, preferably in
permanent mold casting technology, which can be completely and
easily removed from the workpieces using solvents, without any
solid residue remaining.
[0002] Cores that are inserted into the molds when casting
workpieces made of metal so as to keep the cavities provided in the
workpieces free when filling the molds with the melt are subject to
demanding requirements. The cores must be easy to produce, be
dimensionally stable and have precise contours, and the materials
used for the production of the same and the solvents dissolving
them should not adversely affect the casting quality or the
environment, and should not cause any health hazards.
[0003] If special demands are imposed on the surface and the
contour accuracy of the cavities of the workpieces, the surface of
the cores must be particularly smooth and have precise contours,
and the cores must be dissolved completely in a suitable solvent
and be easy to remove from the cavities of the workpieces, without
any solid residue remaining. Residues of cores containing insoluble
components, such as silica sand, can result in damage to the
surfaces to be finished or cause units to fail, for example when
core residue results in the clogging of an injection nozzle in the
common rail system of a diesel engine.
[0004] Salt-based cores that withstand both the extreme thermal
stresses and the mechanical stresses that occur during overcasting
with fusible metals, and cores that not only have high strength,
but can also be easily removed from the cast part after the casting
process and leave the best possible smooth surface finish on the
cast part, can now be produced using the so-called dry pressing
process, without a subsequent sintering or recrystallization
process.
[0005] It is the object of the present invention to produce
salt-based cores that have low porosity, a good surface quality and
the highest possible strength, and can be easily and completely
removed from the workpieces after the workpieces have been
cast.
[0006] It is a further object of the present invention to produce
such cores in a shaping method that is as simple and cost-effective
as possible, preferably using the so-called dry pressing
method.
[0007] It is a further object of the present invention to improve
this dry pressing method and to provide cores that have
considerably improved strength and are nonetheless easily removed
from the cast part after casting, and that leave a good, smooth
surface finish on the cast part.
[0008] According to the invention, these objects are achieved by
the main claim and by the method according to claim 31.
Advantageous embodiments of the invention are characterized in the
dependent claims.
[0009] The cores according to the invention are composed of a salt,
with which a special binder system, and optionally auxiliary
substances such as fillers, additives, wetting agents and
catalysts, can be admixed. These cores are preferably intended for
workpieces that are cast from nonferrous metals, such as aluminum,
brass or copper, using the permanent mold casting method. The cores
according to the invention are composed of substances that can be
removed from the cavities of the workpieces without leaving any
residue, using water, which is the preferred solvent for
environmental protection reasons.
[0010] The cores according to the invention have the advantage that
they are composed of substances (core materials) that, when handled
properly, have no gas-liberating reactions which would harm the
environment, neither during the production thereof nor during the
casting process. In addition, since no cracking products of an
organic binder develop during casting, the quality of the cast
parts is improved due to the fact that casting defects such as
blowholes, gas pores or the like resulting from developing core
gases can be prevented. No residue that would require special
disposal is created during removal of the cores from the
workpieces. Depending on the composition, the substances can be
recovered from the liquid phase by using suitable methods; for
example, the salt can be recovered by spray drying or
concentration.
[0011] All compositions of the core materials can be processed in
conventional mechanical or hydraulic presses by compaction. The
complexity of the geometry of the cores determines the
manufacturing parameters as well as the configuration and design of
the tool used to produce the cores, and of the press.
[0012] Suitable materials for the cores according to the invention
are the salts of alkali elements and alkaline earth elements such
as in particular sodium chloride, potassium chloride and magnesium
chloride, the sulfates and nitrates of the alkali elements and
alkaline earth elements such as in particular potassium sulfate,
magnesium sulfate, as well as ammonium salts such as in particular
ammonium sulfate. The water-soluble compounds of these core
materials are preferred. These substances can be used individually
or as mixtures, provided that they do not react with each other and
thereby adversely affect the desired properties, since the core
material should not undergo any substance transformation during
production of the core which would adversely affect the
residue-free removal of the core material. In general, all readily
soluble salts having a decomposition or melting point above the
temperature of the liquid molten metal are suitable. Similarly to
sand, the core materials can be easily and simply divided into the
desired particle sizes or grain size classes. The selected grain
size distribution and the selected degree of compaction influence
in particular the surface finish of the cores. The smaller the
grain size, the smoother the surface will be. In general, as high a
degree of compaction as possible is desirable, which can be
achieved by mixing different salts, and optionally the additional
substances, having different distribution curves, for example by a
bimodal or trimodal grain size distribution in the mixture.
[0013] According to the invention, grain sizes in the range from
0.01 mm to 2 mm are preferred, depending on the core material, the
desired surface quality, and the contour accuracy of the workpiece
to be cast. Depending on the desired degree of compaction, grain
size fractions of 0.01 mm to 0.29 mm, 0.3 mm to 1.3 mm and/or 1.31
mm to 2.0 mm are mixed in different proportions.
[0014] Fillers, which can likewise be completely removed without
leaving any residue when using water as the solvent, can optionally
replace a portion of the salt to the extent that the density and
strength are not adversely affected. According to the invention, it
has been shown that as much as 30% by weight of the salt can be
replaced with appropriate fillers. The grain size of the filler is
advantageously matched to the grain size or the grain size
distribution of the salt.
[0015] So as to ensure the necessary stability of the cores after
the shaping process, at least one suitable binder system is added
to the salt prior to compaction.
[0016] The approach according to the invention for achieving the
object underlying the invention provides for the use of a special
binder system which comprises a binder/binding agent and a drying
agent tailored thereto.
[0017] Essentially all binders/binding agents which after the
curing process can be removed without leaving any residue, using
water as the solvent, and which wet the salt and optionally the
additional substances, can be used in this binder system according
to the invention, wherein the mixture of these substances must be
shapeable into lost cores by compaction. In general, inorganic
phosphates, inorganic borates, silicate compounds or mixtures of
these binding agents are suitable as binders/binding agents, if
they can be removed without leaving any residue, using water as the
solvent. For example, alkali phosphate or ammonium phosphate,
monoaluminum phosphate, boron phosphate, trisodium phosphate,
tetrapotassium pyrophosphate or sodium polyphosphate can be used as
the inorganic phosphate.
[0018] Binders/binding agents made of water-soluble silicates, such
as water-soluble water glass having a water glass module of 1 to 5,
are preferably used, wherein water glasses having differing water
glass modules can also be present as a mixture. The amount added is
dependent on the water glass module that is used and, depending on
the wetting behavior, is between 0.5% by weight and 15% by weight,
preferably between 5% by weight and 8% by weight. So as to achieve
the properties necessary for the subsequent casting process, such
as strength and dimensional stability, it is also possible to use
special mixtures of binders.
[0019] For the further processing of the core material to form the
usable core, the form in which the core material is present is of
essential importance. If solid core materials are required, as is
the case with the present invention, it is crucially important
whether the core materials are present in agglomerated or
deagglomerated form, and whether they are present in free-flowing
form. Only free-flowing core material mixtures are able to
independently and fully fill so-called filling shoes used in the
dry pressing method, the preferred shaping method according to the
invention. Only free-flowing mixtures comprising the salt, the
binder system that is used, and the other admixed substances are
therefore usable as the core material for use in the dry pressing
method.
[0020] However, so as to substantially improve the usable core
materials for the dry pressing method in the manner according to
the invention, it is important to further improve the free
flowability of the core materials, in particular when using the
above-mentioned binders/binding agents.
[0021] This is surprisingly achieved according to the invention by
adding suitable drying agents in appropriate amounts as a function
of the selected binders/binding agents. The combination of
binders/binding agents and the drying agent forms the special
binder system provided according to the invention. This binder
system surprisingly allows the object underlying the invention to
be achieved.
[0022] All hydrophilic substances that are able to reversibly bind
water, which is to say that are able to release the absorbed water
through suitable treatment, are suitable as drying agents that can
be used according to the invention. According to the invention, for
example, finely dispersed silicic acids, such as Aerosil, silica
gel, zeolites, anhydrous sodium sulfate and/or magnesium sulfate
can be used. Due to the chemical and structural properties of these
drying agents, they trap water molecules and subsequently change
the physical molecular structure thereof due to intermolecular
forces. Water molecules can thus no longer escape from the
structure and remain bound during the preparation of the core
materials. The bound water can be released again by the application
of heat.
[0023] According to the invention, the amount of the drying agents
added is always dependent on the type and amount of binders/binding
agents used, and can be easily determined by simple
experimentation. Slight overdosing of the drying agent can be
tolerated.
[0024] For example, when using 1 to 5% by weight water glass as the
binder/binding agent, based on the amount of salt that is used, 0.3
to 1.5% by weight Aerosil, based on the amount of salt that is
used, is sufficient as the drying agent that can be used according
to the invention to not only ensure the free flowability of the
core material, but also to enable an improvement in the free
flowability of the core material, compared to core materials that
comprise 1 to 5% by weight water glass as the binder/binding agent,
based on the amount of salt that is used, but that comprise no
drying agent.
[0025] By using the special binder system according to the
invention, comprising the combination of binder/binding agent and
drying agent, it is also possible for the first time to use
binders/binding agents in liquid form in the preparation of the
core materials. Wetting of the core material constituents with the
binder/binding agent is considerably improved due to the use of
liquid binders/binding agents. The core material constituents, in
particular the salt grains which are enveloped by the
binder/binding agent, are thus coated. The result is a finished
salt core having considerably improved strength. According to the
invention, for example, an aqueous 60% tetrapotassium pyrophosphate
solution can be used as the liquid binding agent. Based on the
amount of salt that is used, 1 to 5% by weight, preferably 2 to 4%
by weight, and particularly preferably 2.5% by weight, of aqueous
60% tetrapotassium pyrophosphate solution is added in this variant.
So as to still ensure the desired free flowability of the core
material, the drying agent that is provided according to the
invention is added in a sufficient amount to the core material
coated in this way with the binding agent. Slight overdosing of the
drying agent can be tolerated in this case as well.
[0026] It is particularly advantageous when tetrapotassium
pyrophosphate in solid form is additionally added to the aqueous
60% tetrapotassium pyrophosphate solution in the same amount, or
also in a larger amount.
[0027] The properties of the mixture according to the invention
comprising salt, optionally auxiliary substances such as additives,
fillers, wetting agents and/or catalysts, and the special binder
system according to the invention can be influenced by the targeted
addition of additives. Here as well, the prerequisite is that these
additives, or the reaction products of these additives, can be
completely removed from the cavity of a workpiece without
difficulty and without leaving any residue, using water as the
solvent, and that during casting, no gases that impair the casting
process are released which can result in casting defects. Depending
on the composition of the core materials, these additives can be
selected from: wetting agents, for example surfactants, additives
that influence the consistency of the mixture, lubricants,
deagglomeration additives, gelling agents, additives that modify
the thermophysical properties of the core, for example the thermal
conductivity, additives that prevent the metal from adhering to the
cores, additives that result in improved homogenization and
miscibility, additives that increase the shelf life, additives that
prevent premature curing, additives that prevent the formation of
smoke and condensate during casting, and additives that result in
accelerated curing. These additives are known to a person skilled
in the art from the production of conventional cores. The amount
added depends on the type and composition of the core material.
[0028] So as to further improve the strength that is required after
dry pressing, it may be necessary, depending on the composition of
the core material, to employ catalysts that are tailored to the
core material and that initiate and accelerate curing of the core
material.
[0029] It has surprisingly been shown that the addition of in
particular fine-grained salt, and preferably the addition of
powdered salt having a particle size of less than 100 nm, acts as a
catalyst for curing.
[0030] If gaseous catalysts are employed according to the
invention, the gas influencing the core material, preferably
CO.sub.2 or air, can be blown into the mold while the same is still
closed after dry pressing, in particular for curing and drying the
cores. The pressure can be as high as 5 bar.
[0031] It is also possible to carry out thermal post-treatment of
the cores at temperatures up to 600.degree. C., preferably at
temperatures between 500.degree. C. and 600.degree. C., and
particularly preferably at temperatures of 580.degree. C.
[0032] The core material is composed of the salt and the binder
system and, if necessary, the additional substances such as
fillers, additives and catalysts, wherein the fillers and the
binder system are inorganic. All substances can be homogeneously
mixed using known mixing units. The added amounts of the binder
system and of the additional substances are to be selected as a
function of the intended purpose of the cores, and determine the
surface quality as well as the density and strength of the
cores.
[0033] Preparation of the core materials takes place separately
from the manufacturing process, wherein optionally suitable
protective measures may have to be provided to prevent
agglomeration and premature curing. For example, depending on the
composition of the core material, the preparation, transport and
storage can also take place under a protective gas or a vacuum.
[0034] The composition and the properties of a core decisively
influence the quality of the cast part.
[0035] The sodium chloride-based salt cores produced according to
the invention typically have a density of 1.5 g/cm.sup.3 to 1.9
g/cm.sup.3, and preferably of 1.2 g/cm.sup.3 to 1.8 g/cm.sup.3,
determined according to the buoyancy method. This corresponds to a
porosity of 10% to 35%, and preferably of 5% to 25%. The flexural
strength, measured according to VDG (Association of German Foundry
Experts) Technical Bulletin P73, is between 400 N/cm.sup.2 and 1500
N/cm.sup.2.
[0036] The most important properties are therefore discussed below,
based on one exemplary embodiment. The described properties refer
to the cores that are not coated with a black wash.
[0037] A core made of NaCl is used, comprising additional
substances such as water glass binder, Aerosil as the drying agent,
and further added substances such as release agents, retarding
agents, wetting agents and the like. The core was shaped at a
pressure of 50 to 120 bar on a hydraulic press. The core was
subjected to thermal post-treatment for 60 minutes at 580.degree.
C. for curing. The present core is particularly suitable for use in
aluminum permanent mold casting. The core must be dimensionally
stable in order to be able to withstand the temperatures and forces
that occur during casting. The mechanical properties of the core
were determined using a sample 180 mm long, 22 mm wide and 22 mm
high. Flexural strength, measured according to VDG Technical
Bulletin P73 (February 1996), was between 400 N/cm.sup.2 and 1500
N/cm.sup.2.
[0038] The surface of the core must not be flushed out or become
damaged when the metal flows in. For this reason, the core must
have appropriate surface strength. Porosity also plays a vital
role. The porosity in the present exemplary embodiment is 30%.
[0039] After the cast part has completely solidified, the core must
be removed. It is important that the core immediately dissolves
completely and easily without leaving any solid residue. (Note: If,
within the scope of the present invention, the terms
"water-soluble," "dissolve" or "completely dissolve" are mentioned,
this does not necessarily refer to the chemical concept of
dissolving. The decisive factor is that the constituents of the
cores according to the invention can be removed from the cavity of
a workpiece easily, completely and without leaving any residue,
using water as the solvent.) By nature, the dissolution rate of the
core is dependent on the core materials and the pretreatment of the
core, as well as the core size. For pure salt, the dissolution rate
can deviate from that of a composition comprising binders and
fillers. Experiments conducted with a test part have shown that a
core having the dimensions 22 mm.times.22 mm.times.180 mm can be
completely flushed out of the cast part with hot water within 1
minute to 2 minutes.
[0040] Based on the above discussion, the teaching according to the
invention relates to salt-based cores [0041] which can be produced
by shaping and compacting a core material mixture, the core
materials of which are selected from at least one salt, at least
one binder system comprising a binder/binding agent and a drying
agent, and optionally auxiliary substances such as additives,
fillers, wetting agents and catalysts, wherein the salt, the binder
system and the optionally used auxiliary substances are inorganic,
these core materials can be dissolved using water as the solvent,
and the core material mixture is shaped into cores and compacted in
a dry pressing method.
[0042] Preferred cores are those made of core material mixtures
[0043] in which salts are used which have a decomposition or
melting point above the temperature of the liquid metal that is
poured around the cores; [0044] in which the salts used are
chlorides of the alkali elements and alkaline earth elements, in
particular sodium chloride, potassium chloride and/or magnesium
chloride, sulfates and nitrates of the alkali elements and alkaline
earth elements, in particular potassium sulfate and/or magnesium
sulfate, ammonium salts, in particular ammonium sulfate, or
mixtures of these salts; [0045] in which the salt is sodium
chloride; [0046] in which the grain sizes of the salt that is used
range from 0.01 mm to 2 mm; [0047] in which the salt that is used
is present in a bimodal or trimodal grain size distribution; [0048]
in which the salt that is used is present in a grain size
distribution ranging from 0.01 to 0.29 mm, 0.3 to 1.3 mm and/or
1.31 to 2.0 mm; [0049] in which the binders/binding agents used in
the binder system are inorganic phosphates, inorganic borates or
silicate compounds which can be removed without leaving any
residue, using water, or mixtures of these binders/binding agents;
[0050] in which the binders/binding agents used in the binder
system are alkali phosphate or ammonium phosphate, monoaluminum
phosphate, boron phosphate, trisodium phosphate, tetrapotassium
pyrophosphate or sodium polyphosphate, which can be removed without
leaving any residue, using water, or mixtures of these
binders/binding agents; [0051] in which the binders/binding agents
used in the binder system are water-soluble silicate compounds, and
preferably water glasses; [0052] in which the binding agent in the
binder system is a water glass having a water glass module of 1 to
5, and/or a mixture of water glasses having differing water glass
modules; [0053] in which the content of binders/binding agents is
between 0.5% by weight and 15% by weight, based on the salt that is
used; [0054] in which the content of the binder/binding agent is
between 0.5% by weight and 15% by weight, based on the salt that is
used, as a function of the wetting behavior and the water glass
module; [0055] in which water glass is present as the
binder/binding agent in a content of 0.5% by weight to 15% by
weight, based on the salt that is used, as a function of the grain
size distribution and tailored to the water glass module; [0056] in
which the binding agent in the binder system is tetrapotassium
pyrophosphate; [0057] in which the binding agent used in the binder
system is tetrapotassium pyrophosphate in liquid form; [0058] in
which the binding agent used in the binder system is tetrapotassium
pyrophosphate in an aqueous 60% solution; [0059] in which the
binding agent used in the binder system is tetrapotassium
pyrophosphate in an aqueous 60% solution and in amounts of 1 to 5%
by weight, preferably in amounts of 2 to 4% by weight, and
particularly preferably in amounts of 2.5% by weight, based on the
amount of salt that is used; [0060] in which the binding agent used
in the binder system is tetrapotassium pyrophosphate in an aqueous
60% solution and in amounts of 1 to 5% by weight, preferably in
amounts of 2 to 4% by weight, and particularly preferably in
amounts of 2.5% by weight, based on the amount of salt that is
used, and additionally tetrapotassium pyrophosphate in solid form
is used in the same amount or also in a larger amount; [0061] in
which the binding agent is present in the binder system in a
content of 1 to 15% by weight, based on the amount of salt that is
used, and the drying agent is present in a content of 0.3 to 4.5%
by weight, based on the amount of salt that is used; [0062] in
which hydrophilic substances which are able to reversibly bind
water are used in the binder system as drying agents; [0063] in
which finely dispersed silicic acids, such as Aerosil, silica gel,
zeolites, anhydrous sodium sulfate and/or magnesium sulfate are
used in the binder system as drying agents; [0064] in which a
catalyst is added as an auxiliary substance; [0065] in which the
catalyst is particularly fine-grained salt, and preferably powdered
salt having a particle size of less than 100 nm; [0066] in which
the salt is sodium chloride, which is preferably present in a
bimodal or trimodal grain size distribution, particularly
preferably in a grain size distribution of 0.01 to 0.29 mm, 0.3 to
1.3 mm and/or 1.31 to 2.0 mm, the binder system is composed of the
combination of water glass as the binding agent and Aerosil as the
drying agent, the catalyst is particularly fine-grained salt, and
preferably powdered salt having a particle size of less than 100
nm, optionally further auxiliary substances such as additives,
fillers, wetting agents and/or further catalysts are present, and
the mixture of the core materials is free-flowing; [0067] in which
the cores are heat-treated after shaping; [0068] in which, after
shaping, the cores are heat-treated at a temperature up to
600.degree. C., preferably at temperatures of 500 to 600.degree.
C., and preferably at a temperature of 580.degree. C.; [0069] in
which the shaped cores have a density of 1.5 g/cm.sup.3 to 2.1
g/cm.sup.3, and preferably of 1.2 g/cm.sup.3 to 1.8 g/cm.sup.3;
[0070] in which the shaped cores have a porosity of 10% to 40%, and
preferably of 5% to 25%; [0071] in which the shaped cores have a
flexural strength between 400 N/cm.sup.2 and 1500 N/cm.sup.2.
[0072] The teaching according to the invention further relates to:
[0073] methods for producing salt-based cores, wherein a core
material mixture, the core materials of which are selected from at
least one salt, at least one binder system comprising a combination
of binder/binding agent and drying agent, and optionally auxiliary
substances such as additives, fillers, wetting agents and/or
catalysts, is homogeneously mixed, shaped into a core, compacted in
a dry pressing method, and optionally heat-treated.
[0074] Preferred methods are those in which [0075] salt having
grain sizes with differing distribution curves, preferably in a
bimodal or trimodal grain size distribution, is used and mixed;
[0076] the salts selected are chlorides of the alkali elements and
alkaline earth elements, in particular sodium chloride, potassium
chloride and/or magnesium chloride, sulfates and nitrates of the
alkali elements and alkaline earth elements, in particular
potassium sulfate and/or magnesium sulfate, and ammonium salts, in
particular ammonium sulfate, or mixtures of these salts; [0077] the
binders/binding agents used in the binder system are inorganic
phosphates, inorganic borates or silicate compounds which can be
removed without leaving any residue, using water, or mixtures of
these binders/binding agents; [0078] the binders/binding agents
used in the binder system are alkali phosphate or ammonium
phosphate, monoaluminum phosphate, boron phosphate, trisodium
phosphate, tetrapotassium pyrophosphate or sodium polyphosphate
which can be removed without leaving any residue, using water, or
mixtures of these binders/binding agents; [0079] the
binders/binding agents used in the binder system are water-soluble
silicate compounds, and preferably water glasses; [0080] the
binding agent in the binder system is a water glass having a water
glass module of 1 to 5, and/or a mixture of water glasses having
differing water glass modules; [0081] the content of
binders/binding agents is between 0.5% by weight and 15% by weight,
based on the salt that is used; [0082] the content of
binder/binding agent is between 0.5% by weight and 15% by weight,
based on the salt that is used, as a function of the wetting
behavior and the water glass module; [0083] water glass is present
as the binder/binding agent in a content of 0.5% by weight to 15%
by weight, based on the salt that is used, as a function of the
grain size distribution and tailored to the water glass module;
[0084] hydrophilic substances which are able to reversibly bind
water are used in the binder system as drying agents; [0085] the
drying agents used in the binder system are finely dispersed
silicic acids, such as Aerosil, silica gel, zeolites, anhydrous
sodium sulfate and/or magnesium sulfate; [0086] a catalyst is added
as an auxiliary substance; [0087] the catalyst is particularly
fine-grained salt, and preferably powdered salt having a particle
size of less than 100 nm; [0088] the salt is sodium chloride, which
is preferably present in a bimodal or trimodal grain size
distribution, particularly preferably in a grain size distribution
of 0.01 to 0.29 mm, 0.3 to 1.3 mm and/or 1.31 to 2.0 mm, the binder
system is composed of the combination of water glass as the binding
agent and Aerosil as the drying agent, the catalyst is particularly
fine-grained salt, and preferably powdered salt having a particle
size of less than 100 nm, optionally further auxiliary substances
such as additives, fillers, wetting agents and/or further catalysts
are present, and the mixture of the core materials is free-flowing;
[0089] the core materials are homogeneously mixed, shaped into the
core, and compacted in a dry pressing method; [0090] the core
materials, depending on the material, desired surface quality and
contour accuracy of the workpiece to be cast from metal, are used
in grain sizes ranging from 0.01 mm to 2 mm, shaped into the core,
and compacted in the dry pressing method; [0091] the cores are
heat-treated after shaping; [0092] after shaping, the cores are
heat-treated at a temperature up to 600.degree. C., preferably at
temperatures of 500 to 600.degree. C., and preferably at a
temperature of 580.degree. C. The cores according to the invention
can be used, for example, as cavity placeholders in the production
of metal cast parts, preferably in permanent mold casting
technology.
* * * * *