Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B28—WORKING CEMENT, CLAY, OR STONE
  • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
  • B28B7/00—Moulds; Cores; Mandrels
  • B28B7/34—Moulds, cores, or mandrels of special material, e.g. destructible materials
  • B28B7/344—Moulds, cores, or mandrels of special material, e.g. destructible materials from absorbent or liquid- or gas-permeable materials, e.g. plaster moulds in general
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B28—WORKING CEMENT, CLAY, OR STONE
  • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
  • B28B1/00—Producing shaped prefabricated articles from the material
  • B28B1/26—Producing shaped prefabricated articles from the material by slip-casting, i.e. by casting a suspension or dispersion of the material in a liquid-absorbent or porous mould, the liquid being allowed to soak into or pass through the walls of the mould; Moulds therefor ; specially for manufacturing articles starting from a ceramic slip; Moulds therefor
  • B28B1/261—Moulds therefor
  • B28B1/262—Mould materials; Manufacture of moulds or parts thereof
  • B28B1/263—Plastics
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B28—WORKING CEMENT, CLAY, OR STONE
  • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
  • B28B7/00—Moulds; Cores; Mandrels
  • B28B7/34—Moulds, cores, or mandrels of special material, e.g. destructible materials
  • B28B7/346—Manufacture of moulds

Definitions

• the invention relates to porous mould materials and moulds, their production and their use for forming ceramic castings from ceramic materials ranging from liquid to plastic consistency by casting, pressure casting, rotation and pressing.
• moulds have up to the present time been advantageously produced from plaster of Paris, which allows simple mould production by casting and because of its absorption properties it presupposes that a sufficiently firm ceramic body will be formed.
• the water taken from the ceramic material by the mould material is removed by drying after one or more casting operations.
• plaster of Paris moulds are also used as pressure casting moulds and press moulds.
• the plaster of Paris moulds are provided with a channel system extending over the inner mould surface, and through which vacuum and compressed air can be fed into the mould. Compressed air releases the casting from the surface, by which water taken up from the mould is blown out. Vacuum serves to retain the casting in a part of the mould until it is desired to release it.
• the required moulds can be produced from open-pore hardened polyurethane foam by machining.
• the cost involved is however relatively high.
• the material tends to fracture under high pressure-stressing because of its brittleness. Moulds of hardened polyurethane foam are therefore not really suitable as mould material for producing ceramic castings.
• CH 490 960 A describes a process in which highly filled resins such as epoxy resins, phenolic resins and furan resins, are used as the binder in the production of mould materials. Glass micro-heads or powdered quartz are used as the filler. The quantity of resin is determined such that it only wets the filler particles, so that open pores result in the free intermediate spaces.
• highly filled resins such as epoxy resins, phenolic resins and furan resins
• EP 165 952 A describes the production of porous mould materials having the properties typical of plaster of Paris, by which ceramic bodies can be produced from ceramic materials in a short time without the use of pressure.
• the emulsion to be used for producing the mould materials is adjusted to a determined degree of dispersion, and in particular to a viscosity within the range of 1600-5000 cP, particularly by adjustinging the stirring or mixing conditions and the duration of mixing.
• the ceramic body formation rate is higher the higher the emulsion viscosity set during the production of the water-in-oil emulsion.
• the ceramic body formation rate can be increased by adding substances such as calcium sulphate dihydrate, sodium disilicate or disodium tetraborate as regulators for the resultant porosity.
• substances such as calcium sulphate dihydrate, sodium disilicate or disodium tetraborate as regulators for the resultant porosity.
• the ceramic body formation time can be additionally shortened using a pressure in the range of up to about 3 bar gauge.
• the added pore-width regulator substances have the drawback of leading to considerable shrinkage, which is extremely unfavourable in the production of castings from ceramic materials, in which particular dimensional accuracy is required throughout.
• porous plastics mould materials are produced from hardenable water-in-oil emulsions corresponding to DE 19 28 026 A, however no special polymer powder for making the emulsions unstable is used.
• the mould material of EP 165 952 A behaves in this respect as plaster of Paris, which likewise does not allow release of the casting with compressed air on account of its fine porosity. In this case this is aided by producing additional porosity in the plaster of Paris mould. Compressed air is blown in through a channel system provided in the mould and penetrates by pressure through the mould. This produces a coarse pore system through which the compressed air can escape. In this way a permeability is achieved which makes it possible to release the ceramic body with compressed air.
• the object of the invention is to provide plastics-based porous mould materials which can be produced with little or no shrinkage and which are suitable for both pressure-less and pressurized forming of ceramic castings.
• the corresponding process for producing ceramic castings using these mould materials and suitable processable compositions will also be defined.
• the porous, open-pore water absorbent mould materials of the invention consist of a hardened, filler-containing plastics material and have a water-fillable pore volume of at least 10% of the total volume. They are characterised by containing, as filler, short fibres having a length of 1-6 mm and in a quantity of up to 4 weight% with respect to the weight of the plastics content excluding additives.
• the basic concept of the present invention is that by adding even very small quantities of short fibres a considerable reduction in shrinkage can be achieved, which in view of the current state of the art is surprising, and in particular the effectiveness of even very small quantities of added short fibres.
• the mould materials contain between 0.5 and 10 weight% of short fibres with respect to the weight of the plastics content excluding additives. If plastics filler particles are added to the polymerization system for producing the plastics matrix, these are to be considered as additives and not to be taken as part of the weight of the actual plastics content.
• Particularly advantageous is a short-fibre content in the range of 1-1.5 weight% with respect to the emulsion.
• short fibres of textile materials, carbon and/or glass are particularly preferred as fillers, and especially glass staple fibres.
• the mould materials according to the invention can also contain further fillers, which can be of granular, spherical or spheroidal shape. Suitable such additional fillers are for example glass micro-beads, ceramic micro-beads, hollow glass micro-beads, hollow ceramic micro-beads and/or granulated polymers from powder form to fine granular form, in particular those which are only difficultly soluble or only swellable in the polymerization system used for producing the mould materials.
• These further fillers can be contained in a quantity of up to 15 weight% with respect to the total dry weight.
• the plastics material consists advantageously of a homopolymer or a copolymer of monomer units deriving from styrene, ⁇ -methylstyrene, allyl phthalate, acrylic esters, in particular methylacrylate or ethylacrylate, and/or methacrylic esters, in particular methylmethacrylate or ethylmethacrylate, or contains such monomer units.
• the plastics material can be advantageously cross-linked with a multi-functional crosslinking agent such as divinylbenzene or a diacrylate or dimethylacrylate. It can also contain in-polymerized or grafted homopolymer blocks.
• a multi-functional crosslinking agent such as divinylbenzene or a diacrylate or dimethylacrylate. It can also contain in-polymerized or grafted homopolymer blocks.
• plastics material contains polymer blocks deriving from an unsaturated polyester.
• the pore size of the mould materials is advantageously in the range from 0.1 to 0.5 ⁇ and is adjustable.
• the mould materials of the invention can be produced by any desired emulsion polymerization process, it being particularly advantageous to carry out the following process steps:
• the further fillers are added to the polymerization system, in particular to the W/O emulsion, preferably in a quantity of up to 35 vol% with respect to the polymerization system including the water. (Vers. 7).
• W/O emulsions with an oil phase in which a polyester resin, in particular an unsaturated polyester resin, and/or a liquid prepolymer predominantly of methylmethacrylate are dissolved in a quantity of 40-70 weight%.
• calcium sulphate dihydrate, sodium disilicate and/or disodium tetraborate can be added to the W/O emulsion as permeability regulator.
• this regulator further unfavourably increases the shrinkage of the mould materials, this shrinkage is restricted by the addition of short fibres according to the invention, so that the advantageous effects of the permeablity regulator can be fully utilized.
• These regulators are advantageously added in a quantity of between about 2 and 12 weight% with respect to the weight of the polymerizable part of the emulsion.
• the polymerizable liquid monomer or monomers are advantageously used according to the invention in a quantity of between 30 and 78 weight% with respect to the weight of the emulsion.
• Powdered polymers swellable in the oil phase of the emulsion, polymethylmethacrylate-polymer beads, barite and/or ground quartz can be used as further fillers.
• the surface of the mould materials or the outer surfaces of the corresponding moulds can be sealed by applying aqueous synthetic resin solutions, in order to direct the pressure in a determined direction (onto the inner surface of the mould) and to prevent the pressure medium (compressed air, water) from leaking out.
• aqueous synthetic resin solutions for example melamine-urea-formaldehyde resins, epoxy resins or film-forming synthetic resin dispersions are suitable for this purpose.
• glass fibres or glass beads treated with an adhesive is to be preferred because of their better adhesion to the matrix which surrounds them.
• the moulds used for producing ceramic castings can be formed from the mould materials according to the invention either by machining, in particular by turning, milling and/or drilling, or by direct casting in a mould. In this latter case the procedure is as follows:
• the method of the invention for producing ceramic castings from water-containing ceramic moulding materials from liquid to plastic consistency using moulds formed from a mould material produced by polymerization of a water-in-oil emulsion of a porous, open-pore, water-absorbent and hardened plastics with a pore volume of at least 10% of the total volume can be carried out by pouring, loading or forcing the ceramic moulding material into the mould, possibly applying pressure in excess of atmospheric to the mould, and particularly a pressure of up to about 5 bar gauge, to if necessary empty the residual liquid slip from the mould and to release the formed ceramic body from the mould after sufficient dewatering.
• a mould formed from a mould material as heretofore defined is used.
• the release from the mould is simplified if, before pouring the moulding material into a mould formed from the mould material according to the invention, powdered plaster of Paris or an aqueous plaster of Paris suspension with a plaster of Paris content of at least 1 weight% is applied to the mould, for example by spraying.
• the use of short fibres in accordance with the invention considerably reduces shrinkage, as can be seen from the ensuing experimental part.
• the permeability of the mould materials towards gases and liquids can be controlled such that under pressurization by compressed air or by suction, the liquid taken up from the mould can be squeezed out or forced from the mould surface towards the rear, to be sucked off, the ceramic body being easy to loosen with compressed air, which in known manner is fed via a channel system located in the vicinity of the inner mould surface.
• the mould materials according to the invention are also suitable for pressure-less ceramic body formation. The ceramic body formation can be further accelerated by pressurization with a low pressure of up to about 5 bar gauge.
• the reduction in shrinkage achieved in accordance with the invention presupposes that the predominantly open pore volume obtained with the W/O emulsion is not altered when the product is hardened. In addition the corresponding emulsions must be easily pourable.
• the short fibres are mixed into the emulsion during its preparation as a casting material. It has been further shown that the mixing of fibres with the emulsion achieves the best effect if the fibres remain uniformly distributed within the emulsion until a sufficiently high viscosity is reached.
• This uniform working-in of the fibres is simplified by adding spherical fillers such as glass micro-beads or ceramic micro-beads. Moreover, by this means the shrinkage can be additionally reduced by some ten percent.
• Sedimentation of the fibres in the emulsion can be opposed by using light-weight fillers such as hollow glass micro-beads or hollow ceramic micro-beads, and by employing a higher emulsion viscosity.
• polymers from fine-grain to powder consistency can be used. These should only be surface-soluble and must not destabilize the emulsion. As the particle size and the solubility of the polymers in the monomer part of the emulsion influence the viscosity, the viscosity variation of the hardenable part must be measured. As a rule of thumb it can be stated that a mixture of polymers and the hardenable part of the emulsion in the ratio of 1:1 should show a clear viscosity increase of about 20% only after about 10-15 minutes.
• the reduction in shrinkability achieved by the invention occurs both in porous mould materials produced in accordance with EP 165 952 A and in mould materials produced using the polymer-monomer system, for example in accordance with DE 19 28 026. With this latter process a shrinkage of about 1% can be reckoned for a 1:1 ratio of polymerizable matter to polymer. If only 1-1.5 weight% of glass fibres (length 3 mm) with respect to the weight of the plastics content is added, this shrinkage is reduced to about 0.1%. In this respect, the polymer quantity can be reduced without influencing shrinkage.
• moulds of porous plastics can be produced by which considerably quicker ceramic body formation without pressure or under a low pressure of about 0.5-5 bar gauge can be achieved. This effect is particularly due to the small pore width of about 0.1 to 0.2 ⁇ m achievable by this process.
• the permeability of the mould to gases and liquids is considerably reduced. It is therefore not possible to release the casting from the mould by the use of compressed air; the ceramic body loosens by shrinkage.
• the release is expedited by the aforesaid spraying of a plaster of Paris suspension before filling the mould with slip, so that the ceramic body can be removed as in the case of a plaster of Paris mould, but after a shorter drying time.
• mould materials usable, either without pressure or with a pressure of up to 6 bar, to form a ceramic body which can be released from the mould by compressed air are produced in which the water-in-oil emulsion either with or without the addition of a dispersing agent is adjusted to a degree of dispersion or a corresponding viscosity such that after moulding and hardening, pore widths of between 1 and 5 ⁇ m are achieved. Depending on the application, pore widths of about 0.1-0.2 ⁇ m can also be achieved.
• the aforesaid compounds for accelerating the ceramic body formation with which the pore diameter can be regulated, and in particular disodium tetraborate decahydrate, are used in a quantity of up to 4.5 weight% with respect to the hardenable content of the W/O emulsion, for which equally advantageously an unsaturated ester dissolved in styrene and methylacrylate as monomers is used.
• the mould materials produced in this manner form within a few minutes a ceramic body which can be loosened with compressed air and removed from the mould,
• a pressure from about 3 to 5 bar is sufficient for forming the ceramic body.
• additional coarse pore formation is not required, and compared with conventional pressure-casting moulds formed with the polymer-monomer system, the pores according to the procedure of the invention are up to about ten percent smaller.
• the release procedure for the ceramic body is hence easier to control.
• the considerable water discharge from the mould connected with the release of the ceramic body is minimal according to the invention.
• the danger of clogging due to penetration of particles from the slip is reduced because of the finer porosity.
• the low pressure required for ceramic body formation means that expensive devices such as pressure-casting presses or strengthening of the moulds can be dispensed with. Hence moulds can be produced at considerably less cost and inserted for example into the moulding bank.
• Examples 2 and 3 show the possible reduction in shrinkage attained according to the invention by adding 1.3 weight% of glass fibres, with respect to the W/O emulsion, when a polymer-monomer system is used.
• Example 1 is a comparison example in which the mould material is produced without the addition of glass fibres.
• Examples 4-7 demonstrate the reduction in shrinkage achieved by adding glass fibres but without adding the regulator.
• Examples 8-12 relate to shrinkage with the addition of a regulator in a quantity of 2.6 weight%, with respect to the weight of the emulsion.
• Examples 13 and 14 show the low shrinkage achieved by the addition of the regulator in a quantity of 6.5 weight%, with respect to the resin, both without and with the addition of the polymer II.
• the water content of the emulsion was 47%.
• Examples 15 and 16 show for an emulsion with a water content of 35% the reduction in shrinkage and the differing permeability for regulator additions of 6.5 weight% and 2.6 weight%.
• the 16 examples of the following table are classified in 5 groups according to the water content of the emulsion, the permeability and the pore radius.
• the mould material does not enable a ceramic body to be formed without pressure.
• the corresponding mould is however well suitable for use in forming a ceramic body at a very high pressure of 15-50 bar.
• the ceramic body formation is slower, however the water can be forced out through the mould and the ceramic body can be released with compressed air after its formation.
• release of the ceramic body with pressure is hardly or no longer possible.
• the rate of ceramic body formation is in this case considerably higher. It cannot be additionally accelerated using pressure. In such cases the ceramic body can only be released from the mould by shrinkage.
• the shrinkage at the upper diameter of the crucible mould was measured and the percentage shrinkage on the inner diameter of the casting mould used was calculated.
• the pore radii were determined with a Hg porosimeter.
• Example 4-16 the sequence of mixing steps for the individual components was the same as in Examples 1-3. However it can if necessary be altered. For example the hardener and accelerator can be interchanged.
• Components 1, 2, 4 and 7 were mixed with the resin.
• Components 3a and 3b were mixed with component 5 (water) and them mixed with the premixed components 1, 2, 4 and 7 under stirring, by which a water-in-oil emulsion was obtained.
• the glass fibres were sprinkled in and the peroxide mixed in.
• the emulsion was cast into a mould and left therein to harden under cold conditions. After 40-60 minutes the mould could be opened. Porous moulded articles were obtained containing the water of the emulsion.
• mould materials produced in accordance with the basic process of EP 165 952 A can also be used for pressure-casting in the the low pressure range of up to about 6 bar, and for pressing.

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• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Ceramic Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Mechanical Engineering (AREA)
• Dispersion Chemistry (AREA)
• Compositions Of Macromolecular Compounds (AREA)
• Porous Artificial Stone Or Porous Ceramic Products (AREA)
• Producing Shaped Articles From Materials (AREA)
• Filtering Materials (AREA)
• Manufacture Of Porous Articles, And Recovery And Treatment Of Waste Products (AREA)
• Curing Cements, Concrete, And Artificial Stone (AREA)

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• 1991
  • 1991-05-30 DE DE4117745A patent/DE4117745C2/de not_active Expired - Fee Related
• 1992
  • 1992-05-21 EP EP92201457A patent/EP0516224B1/fr not_active Expired - Lifetime
  • 1992-05-21 ES ES92201457T patent/ES2087431T3/es not_active Expired - Lifetime
  • 1992-05-21 DE DE69209719T patent/DE69209719T2/de not_active Expired - Lifetime
  • 1992-05-21 AT AT92201457T patent/ATE136489T1/de active

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