MOULDING FINELY DIVIDED SINTERABLE MATERIAL

Abstract: The invention relates to a method of hot injection-moulding a dispersion of sinterable powder (for example, of metal or high-melting-point non-metal such as silicon, or a ceramic substance such as silicon carbide, silicon nitride or alumina), using a dispersion of the sinterable material in a binder which, like the dispersion, is solid at room temperature and fluid at the temperature of moulding, with formation of a moulded product of uniform density. The novel binder employed comprises (i) polystyrene, which forms the predominant weight proportion of the binder, and (ii) in lowr weight proportion, an organic compound (e.g. diphenyl carbonate) which is solid at room temperature, is soluble in the molten polystyrene, and is more soluble than the polystyrene is in a lower aliphatic alcohol or keton. After formation of the moulded product, the binder is removed in non-solid form (e.g. as vapour, or by dissolution in a solvent, or by oxidation to volatile materials) to leave a porous structure of uniform density.

Description: Mouldinσ finely divided sinterable material This invention relates to the moulding of finely divided sinterable material, such as a powder of metal or high-melting- point non-metal (e.g. silicon) or ceramic substance such as silicon carbide, silicon nitride or alumina. The invention provides a method by which a dispersion of sinterable material can be injection-moulded under moderate pressure to a product whose shape is of considerable complexity, the product being not only of uniform density but also such that the binder employed can be readily removed from it, so as to obtain a porous structure with density still uniform (though, of course, lower) and with shape essentially unaffected. This in turn enables subsequent sintering to be carried out with no substantial change in shape from that of the direct product of injection moulding. The method of the invention depends on the use of a novel binder of our discovery, which has the necessary property of being solid at room temperature and fluid at the elevated temperature of injection moulding. The binder comprises (i) a solid polystyrene, which forms the predominant weight ingredient of the binder, and (ii) in lower weight proportion, an organic compound which is soluble in the molten polystyrene and is more soluble than polystyrene in a lower aliphatic alcohol or ketone, examples of which are methanol, ethanol, propanol, isopropanol, acetone and methyl ethyl ketone. Preferably the organic compound has a melting point of at least 50°C. Preferred organic compounds are those whose molecular structure includes an aromatic ring, for example diphenyl carbonate (melting point about 80°C) . According to the invention there is provided a method which comprises injection-moulding at elevated temperature a dispersion of finely divided sinterable material in a binder, to obtain a moulded product of uniform density, both the dispersion and the binder being solid at room temperature and fluid at the temperature of moulding, said binder comprising (i) polystyrene, which forms the predominant weight proportion of the binder, and (ii) in lower weight proportion, an organic compound which is solid at room temperature, is soluble in molten polystyrene and is preferentially soluble in a lower aliphatic alcohol or ketone; and removing said binder in non-solid form from the moulded product, to leave a porous sinterable structure of uniform density. The solid dispersion may additionally include a wax and/or a dispersant. It may also include an ethylene/vinyl acetate copolymer suitably in an amount 0.5-2.5 w /o of the dispersion to improve the rigidity of the moulded product while the binder is being removed from it in non-solid form. In the latter expression we include removal by sublimation, by dissolution in a liquid solvent such as one of those (methanol etc.) previously referred to, and by controlled oxidation to form volatile products, e.g. benzoic acid, carbon dioxide and water vapour. According to a preferred feature, the binder is removed from the moulded product in two stages, in the first of which the major part of the organic compound is removed as vapour at elevated temperature; and subsequently the polystyrene, with any residue of the organic compound, is removed at further-elevated temperature. According to an alternative preferred feature, the binder is removed from the moulded product in two stages, in the first of which the major part of the organic compound is removed by dissolution in a solvent liquid; and subsequently the polystyrene, with any residue of the organic compound, is removed at elevated temperature. The solid dispersion may conveniently by prepared by mixing the finely divided sinterable material with a solution of the polystyrene and the organic compound in a relatively volatile organic solvent (preferably of boiling point below 100°C) , and evaporating said solvent from the mixture thus formed. Preferred proportions of the various ingredients of the dispersion are as follows:- Ingredient Main Function % Bv Weicrht Sinterable material 70-95 2. Polystyrene Major Binder Component 2-10 3. Diphenyl Carbonate Minor Binder Component 2-10 Wax Minor Binder Component 0.5-5 5. Dispersant Dispersion of Powder 0.5-5 in Organic Medium 6. Plasticiser Viscosity Modifier 0.2-2 7. Ethylene Vinyl Improves Structural 0-2.5 Acetate Co-polymer Rigidity During Binder Removal Moulding is preferably carried out with a screw-type injection moulder. The invention is further illustrated by the following examples. EXAMPLE 1 This example illustrates the application of the invention to the preparation of a porous sinterable structure from silicon powder of particle size below 20 microns (mean particle size 5 microns) . A solid-form dispersion was prepared from the following ingredients: Ingredient % Bv Weight 1. Silicon Powder 81.0 2. Polystyrene 6.8 ol wt (M = 150,000- 300,000 T G = 98°C 3. Diphenyl Carbonate 4.7 m.pt = 80°C 4. Wax 2.6 Ethylene Bis Stearoyl Amide 5. Surfactant 3.0 Polymerised Fatty Acid Ingredient By Weight 6. Plasticiser 0.9 Di Butyl Phthalate Ethylene/Vinyl l.o Acetate Copolymer Melt Index : 60dg/min VA content = 0wt% 100.0 The ingredients 2, 3 and 5-7 were dissolved in methyl ethyl ketone (MEK) , and a fine powder of the wax (4) was added to the resulting MEK solution. The resulting low-viscosity liquid thus obtained was gently churned with the silicon powder in a sigma blade mixer at ambient temperature. A free-flowing homogenous crumble was obtained from the resulting mixture by distilling off the MEK under reduced pressure. The crumble was fusible to a pseudoplastic suspension of a viscosity 600Pa.S measured at a shear rate of 1000s "1 , at a temperature of 120°C. It was subsequently moulded at that temperature in a standard screw-type machine [Negri Bossi NB90 or Arburg 270M-350-90] . The resulting products, which were of uniform density and robust enough to be handled in the usual way, were then treated by one of two methods in order to remove the binder in non-solid form (i.e. as vapour, liquid or gas) and leave a porous sinterable structure of uniform density. a. The products were heated at 50-70°C under reduced pressure (lmm) to cause the diphenyl carbonate (mp 80°C) to sublime without causing the moulded product to distort. Once the bulk of the diphenyl carbonate had been removed, the moulded products were heated in a forced air circulation oven at 20°C/hr to 400-450°C to oxidise the polystyrene to volatile material including benzoic acid, carbon dioxide and water vapour. b. The products were immersed in methanol for 24 hours at ambient temperature, to dissolve at least 65% of the diphenyl carbonate content (i.e. to remove it in liquid form) , and were then heated to 420°C at 20°C/hour to oxidise the polystyrene and residual diphenyl carbonate as in a. If the procedure of the above example is carried out without the minor proportion of diphenyl carbonate in the binder, then a considerably higher pressure is required to injection mould and moulding has to be performed at a much higher temperature (190°C) . Additionally more than twice as long a time is needed for removal of the binder - a removal which has to be effected entirely by thermal degradation. The porous products resulting from the above procedures which had good strength and rigidity were heated to 1360°C in a nitrogen atmosphere over a period of 38 hours to yield reaction bonded silicon nitride with a final density of 2.40g/cc. EXAMPLE 2 Following generally the procedure of Example 1, a solid-form dispersion of the following composition was prepared from a stainless steel powder of particle size below 44 microns (mean particle size 11 microns) : Ingredient % By Weight 1. Stainless Steel Powder 93.0 (316L) 2. Polystyrene 2.5 3. Diphenyl Carbonate 2.2 4. Wax 1.0 5. Surfactant 1.1 6. Plasticiser 0.2 100.00 The crumble obtained generally as in Example 1 was injection moulded this time at 100°C in a standard screw-type machine as in Example 1. The moulded products obtained were immersed in a stirred solution of industrial methylated spirit for 120 hours at ambient temperature to dissolve at least 95% of the diphenyl carbonate content, and were then heated to 450°C at 15°C/hour in a stream of 75%H 2 /25%N 2 , held at this temperature for three hours and finally heated at 100°C/hour to 750°C in the same atmosphere. The porous, easily handleable structures thus obtained exhibited good shape retention, and were sintered in a conventional manner at a temperature of 1280°C to yield a product of 94% theoretical density.

Claims: CLAIMS 1. A method which comprises injection-moulding at elevated temperature a dispersion of finely divided sinterable material in a binder, to obtain a moulded product of uniform density, both the dispersion and the binder being solid at room temperature and fluid at the temperature of moulding, said binder comprising (i) polystyrene, which forms the predominant weight proportion of the binder, and (ii) in lower weight proportion, an organic compound which is solid at room temperature, is soluble in molten polystyrene and is more soluble than polystyrene in a lower aliphatic alcohol or ketone; and removing said binder in non-solid form from the moulded product, to leave a porous sinterable structure of uniform density. 2. A method according to claim 1, in which said organic compound solid at room temperature is diphenyl carbonate. 3. A method according to claim 1 or 2, in which the solid dispersion additionally contains a wax. 4. A method according to claim 3, in which the wax is ethylene bis stearoyl amide. 5. A method according to any of claims 1 to 4, in which the solid dispersion additionally contains an ethylene/vinyl acetate copolymer. 6. A method according to any of claims 1 to 5, in which the binder is removed from the moulded product in two stages, in the first of which the major part of the organic compound is removed as vapour at elevated temperature; and subsequently the polystyrene, with any residue of the organic compound, is removed at further elevated temperature. 7. A method according to any of claims 1 to 5, in which the binder is removed from the moulded product in two stages, in the first of which the major part of the organic compound is removed by dissolution in a solvent liquid or vapour; and subsequently the polystyrene, with any residue of the organic compound, is removed at elevated temperature. 8. A method according to any of claims 1 to 7, in which the solid dispersion is prepared by mixing the finely divided sinterable material with a solution of the polystyrene and the organic compound in a relatively volatile organic solvent, and evaporating said solvent from the mixture thus formed. 9. A method according to any of claims 1 to 8, in which moulding is carried out with a screw-type injection moulder. 10. A method according to claim 1, substantially as described with reference to the Examples herein. 11. A method of making a shaped article, which comprises sintering a porous sinterable structure of uniform density obtained by the method of any of claims 1 to 10. 12. A solid binder suitable for use in the method of any preceding claim, which comprises: (i) a solid polystyrene, which forms the predominant weight ingredient of the binder, and (ii) in lower weight proportion, an organic compound which is solid at room temperature, is soluble in the molten polystyrene and is more soluble than polystyrene in a lower aliphatic alcohol or ketone.

Inventors: HOLME JOHN DAVID, HOLME, JOHN, DAVID
Applicants: T & N TECHNOLOGY LTD, T&N TECHNOLOGY LIMITED
Application year: 1992
Main classification: C04B38
IPC codes: B22F1, B22F3, B28B1, B29C45, C04B35, C04B38