Title of Invention

A NOVEL BINDER COMPOSITION FOR METAL INJECTION MOULDING (MIM)

Abstract

This invention provides a novel binder consisting a synergistic mixture of PMMA (polymethyl methacrylate) and commercial wax and a process of preparation of binder from the said synergistic composition, in a controlled temperature range having a weight percentage range of ingredients to secure a consistency and appropriateness for metal injection moulding (MIM) process. This specification also discloses a MIM process to remove (debinding) this binder at an appropriate stage, when the final product out of the MIM process is free from binder material. The novel binder and the process of debinding have been developed through research to solve technical problems as obvious in the prior art. This invention describes a new binder composition, the method of preparation of the same and also describes the MIM process involving debinding the binder from the green part prior to conventional sintering.

Full Text

The present invention relates to a novel binder composition for metal injection moulding (MIM), a process for the preparation of novel binder from the composition and a MIM process using the said binder. Metal injection moulding (MIM) is an emerging technology using the advantages of intricate geometry production by injection moulding and is a new powder metallurgy (P/M) process for near net-shape metal parts. The main usage of the novel binder of the present invention is in the intermediate stage of metal injection moulding process. The low cost forming of plastic consumer products by injection moulding is a wellknown industrial production process. A flow chart of the process route for general metal injection moulding (MIM) has been shown in figure 1 of the drawings accompanying this specification. As referred in figure 1, the steps involved in forming a product[10] by MIM include the following: a) selecting and tailoring a metal powder[l] for the process; b) design of a suitable binder[2]; c) mixing[3] the powder[l] with melted binder[2] for feedstock preparation that is homogeneous mixture of the powder[l] and binder[2] which becomes hard at room temperature and has tooth paste like viscosity at moulding temperature; d) pelletizing[4] that is producing of granular pellets of feedstock for ease of feeding in moulding machine hopper ; e) forming the part by injection moulding[5] in a closed die; f) processing the formed green part[6] to remove the binder[2] that is debinding[7]; g) densifying the debound part[8] by high-temperature sintering[9] to achieve the product[10]. The MIM process involves a lot of different production steps. Some extremely important aspects of MIM process need to be improved before the technology is fully accepted as an industrial production route. One of these is binder technology. The binder is a major key for successful MIM production. The binder[2] is a temporary vehicle for homogeneously packing the powder[l] into the desired shape during injection moulding[5] by reducing viscosity and then holding the particles in that shape until the beginning of sintering[9]. Without binder[2], powder[l] alone cannot be injected into the die due to high viscosity and faulty binder[2] design will cause distortion, cracking and shape preservation difficult during debinding[7] in the green part[6] formed by injection moulding. Accordingly, though the binder[2] should not dictate the final composition of the MIM product [10], it has a major influence on the success of the processing and obtaining the desired end-product. Reference may be made to US patent no.4898902, titled "Binder composition for injection moulding", wherein a binder composition for injection moulding of at least one of metal powder and ceramic powder which comprises polyoxyethylene / polyoxypropelene condensation polyether as the main component together with from 10 to 100 parts by weight of at least one auxiliary component selected from the group consisting of natural waxes, synthetic waxes, fatty acids and esters per 100 parts of said polyoxyethylene / polyoxypropelene condensation polyether. The drawbacks are: i) The patent does not talk about mouldability of feedstock with the above binder and shape preservation during thermal debinding. ii) The binder system particularly contains water as auxiliary component and water gets vapourised during moulding, making recycling of the feedstock difficult. Reference may be made to another US patent no. 5421853, titled "High performance binder / molder compounds for making precision metal parts", wherein a binder composition for use in making metal parts via metal powder injection moulding process consists of the following components: (a) a first polymer with a relatively low solubility parameter such as polyethylene and polypropylene; (b) a second polymer with a relatively high solubility parameter such as polystyrene and poly( methyl methacrylate); and (c) and a block copolymer containing blocks of the first and second or other structurally similar, constituting monomeric units. Examples of the block polymers include ethylene / styrene copolymer, propylene / styrene copolymer and isoprene / styrene copolymer etc. The binder composition is dispersed in an appropriate dispersant, such as an oil or wax, then blended with a metal powder to form a metal powder injection composition. The metal powder injection forms a green compact with a predetermined shape and dimension using an injection moulding machine. Finally the green compact is sintered to form the final product. The drawbacks are: i) The above work uses four constituents in its binder system which adds more cost to the production of MIM parts. ii) The patent does not talk about the debinding of the binder system iii) The patent does not talk about shrinkage upon sintering of the green part. Reference may be made to yet another US patent no. 5002988, titled "Polyamide base binder for use in metal powder injection moulding process", wherein a polyamide binder for use in injection moulding a metal powder comprising in combination: (a) 40 to 50 % of a polyamide resin component having an average molecular weight of not less than 20000 and prepared by co-polycondensation of a mixture of dimmer acid, azelaic acid, ethylenediamine and xylylenediamine each being mixed in a substantially equal molecular equivalent ratio; (b) 20 to 30 wt % of ethylene-bis-laurylamide; and (c) the balance of N,N-diacetylpiperazine. The drawbacks are: i) The feedstock made with this binder system requires very high moulding pressure(660 kgf/mm2) which can cause considerable damage to the moulding machine in the continious production of MIM parts and chances are there for powder binder separation due high moulding pressure during moulding. ii) The binder system uses three constituents which adds more cost to the production. Reference may be made to still another US patent no. 5380179, titled "Binder system for use in the injection moulding of sinterable powders and molding compound containing the binder system", wherein the improved binder system for use in the injection moulding of sinterable powder such as metal powders, ceramic powders and cermet powders comprises a (co)-polymer, a (co)- polymer other than said (co)-polymer and an inorganic compound having a molecular weight up to 2000. Using a binder system comprising (a) 3-8 wt% of a (co)polymer that has a molecular weight in excess of 2000 and which has at least one epoxy group in the molecule, (b) up to 70 wt% of a (co)polymer other than component (a), and (c) 20-80 wt% of an organic compound having a molecular weight of not more than 2000, there is provided a composition for injection moulding of sinterable powder. The drawbacks are: i) The binder system consists of three constituents which adds cost to the production of MIM parts, ii) The patent does not talk about mouldability of the feedstock made out of the binder system. Reference may be made to still yet another US patent no.5034451, titled "Water soluble binder and hot-melt binder for ceremics molding", wherein the invention provides a water soluble binder and a hotmelt binder for ceramics moulding, said being composed of a polyvinyl alcohol-based copolymer containing (metha)allyl group-containing nonionic mononer units or nitrogeneous group-containing nonionic monomer units. The drawbacks are: i) The binder system is designed for ceramic powder and whether this binder system can be used for metal powder or not is not stated in the patent. Reference may be made to one more US patent no.5332537, titled "Method and binder for use in powder moulding", wherein a binder is mixed with powder to form feedstock for moulding. The binder includes a first component which is soluble in water and a second component which is insoluble in water. The two binder components are miscible with each other in a liquid state. The two binder components form a uniform heterogeneous mixture when mixed in liquid state. The binder is mixed with suitable powder to form the feedstock. The feedstock is moulded to form a compact. The polymer units of the binder are cross linked after the compact has been formed. The cross linking of the polymer units of a component of the binder is accomplished by exposing the compact to ultraviolet radiation or by the use of a catalyst. The compact is partially debound by exposing the compact to water, that is, to a water based solvent. Thereafter, the compact is further debound to remove the component which is insoluble in water. The powder is then sintered to form an article. The drawbacks are: i) The patent does not talk about mouldability of feedstock with the above binder and also the feedstock requires higher temperature(200°C) for moulding, ii) For cross linking of the polymer unit of the binder system requires by exposing the compact to ultra violet radiation to eliminate deformation during debinding and it adds extra cost to production of MIM parts, iii) There is a minimal tendency to for the compact to deform during cross linking reaction of the polymer unit of the binder system by exposing to ultraviolet radiation minimal. The main object of the present invention is to provide a novel binder composition for metal injection moulding (MIM) process which obviates the drawbacks as detailed above. Another object of the present invention is to provide a process for the preparation of novel binder from the novel composition. Yet another object of the present invention is to provide a metal injection moulding(MIM) process, using the novel binder of the present invention. The present invention provides a novel binder consisting a synergistic mixture of PMMA (polymethyl methacrylate) and commercial wax and a process of preparation of binder from the said synergistic composition, in a controlled temperature range having a weight-percentage range of ingredients to secure a consistency and appropriateness for metal injection moulding(MIM) process. This specification also discloses a MIM process to remove (debinding) this binder at an appropriate stage, when the final product out of the MIM process is free from the binder material. The novel binder and the process of debinding have been developed through research to solve technical problems as obvious in the prior art. This invention describes a new binder composition, the new method of preparing the same and also describes the MIM process involving debinding the binder from the green part prior to conventional sintering. In figures 2 and 3 of the drawings accompanying this specification the flow chart of the present invention has been described. Figure 2 depicts the synergistic binder material composition. Figure 3 shows the debinding of moulded part in MIM process. The meaning of the different blocks in the figures 2 and3 are as follows: 2 Binder 6 Green part i.e. the moulded part made out of feedstock 8 Debound part i.e. the green part after removing the binder 11 PMMA 12 Commercial wax 13 Blending 14 Solvent debinding in acetone bath 15 Drying 16 Thermal debinding Accordingly the present invention provides a novel binder composition for metal injection moulding (MIM), which comprises polymethyl methacrylate(PMMA) in the weight percentage of 30 to 40 and commercial wax in the weight percentage of 60 to 70. In an embodiment of the present invention the polymethyl methacrylate(PMMA) is polymer made by free vinyl polymerisation from monomer methyl methacrylate. The novel binder composition of the present invention is not a mere admixture but a synergistic mixture having properties which are distinct and different from the mere aggregated properties of the individual ingredients of the said composition, further there is no chemical reaction involved in the composition. Accordingly the present invention provides a process for the preparation of novel binder for metal injection moulding(MIM) from the novel binder composition, which comprises heating the synergistic mixture, with the components polymethyi methacrylate (PMMA) in the weight percentage of 30 to 40 and commercial wax in the weight percentage of 60 to 70, at a temperature range of 90° to 130° C to obtain a homogeneous melt, allowing the melt to cool to room temperature to solidify to obtain binder. In an embodiment of the present invention the binder is solid at room temperature and the pour point is in the range of 90 to 130 degree centigrade. Accordingly the present invention provides a metal injection moulding (MIM) process using the novel binder as herein described, which comprises adding the binder to metal powder and subjecting to kneading at a temperature in the range of 90 to 130°C to obtain injection moulding feedstock, effecting injection moulding by conventional methods to obtain green moulded part with binder, subjecting the said part with binder to two stage debinding consisting of solvent debinding followed by thermal debinding, sintering the debound green part by conventional methods. In an embodiment of the present invention the ratio of binder to metal powder is dependent on particle size distribution of the metal powder employed and is controlled in conventional manner. In another embodiment of the present invention the metal powder is selected from iron or iron alloy powder, such as stainless steel powder, hard metal powder such as titanium, tungsten, boron, magnetic material powder. In yet another embodiment of the present invention, the two stage debinding process comprises immersing the green part in to a solvent bath such as acetone and allowing to stand for a period of 23 to 26 hours, taking out the part from the solvent and drying by conventional methods at a temperature in the range of 40 to 55 °C for a time period of 2 to 5 hours, subjecting the dried part to nitrogen atmosphere inside a conventional furnace, raising the furnace temperature to 75 to 90 °C at a step of 3 to 5 °C per minute, maintaining this temperature for a period of 50 to 120 minutes, raising the furnace temperature to 390 to 450 °C in steps of 1 to 5 °C per minute, maintaining this temperature for a period of 45 to 60 hours, allowing the furnace to cool to room temperature at a normal rate to obtain debound green part. Polymer made by free vinyl polymerization from monomer methyl methacrylate is the main component of the binder for MIM of the present invention. It is essential that the binder composition of the present invention for MIM comprises above mentioned polymer and commercial wax. It is preferable that the commercial wax is blended in an amount of 60 to 70 parts by weight with 40 to 30 parts by weight of PMMA for 100 parts of the binder. It is particularly preferable to blend these components in such a manner that the resulting binder of the present invention is solid at room temperature and has a pour point of 90° to 130 °C. This is because when the binder for MIM is solid at room temperature, a mixture obtained by kneading the same with a metal powder may be readily pelletized. The injection moulded- product thus obtained has excellent mouldability.Further more, when the pour point of the binder for injection moulding is 90° to 130 °C, the moulding can be effected at a temperature below 130 °C. When an injection moulding feedstock is to be prepared by using the binder of the present invention for injection moulding, it is preferable that said binder for injection moulding of the present invention is added to metal powder and kneaded with the same at a temperature of 90 to 130 °C. The amount of binder composition for injection moulding to be added may be controlled in a conventional manner depending on particle size distribution of the employed metal powder. Examples of said metal powder to which the binder of present invention for MIM is to be added include iron or iron alloy powder such as stainless steel powder, hard metal powder obtained from, for example, titanium, tungsten or boron or magnetic material powder. The feedstock containing the binder for MIM of the present invention may be injection moulded and sintered in conventional manner. By using the binder for MIM as described above debinding may be carried out as per flow chart as shown in figure 3 of the drawings: solvent debinding[14] followed by thermal debinding[16], in which PMMA[ll] is removed by solvent debinding[14] and commercial wax[12] is removed by thermal debinding[16]. In solvent debinding[14], the injection moulded green part[6] made out of the feedstock is immersed in a acetone(solvent) bath where one component of the binder[2] PMMA[ll] is dissolved and removed from the injection moulded green part[6]. Then the partially debound injection moulded part is taken out from the acetone bath and dried[15] in a oven at 40° to 55 °C for 2-to 5 hours. Then the partially debound injection moulded part is placed inside a low temperature furnace in nitrogen atmosphere for thermal debinding[16] so that other component of binder[2] that is commercial wax[12] is removed. The specimen is heated slowly from room temperature to 75° to 90°C at the rate of 3° to 5 °C/min and kept at that temperature for 50 minutes to 2 hours. Again the part is heated to 390° to 450°C at the rate of 1° to 5°C/min and kept at that temperature for 45 to 60 hours and then allowed to cool inside the furnace to room temperature. With this debinding technique around 98% of the total binder is removed and the debound part[8] is very much fragile and rest of the binder is removed at the presintering stage. Sintering is carried out conventionally. The following examples are given by way of illustration and therefore should not be construed to limit the scope of the present invention. EXAMPLE-1 (1) Preparation of feedstock: Hundred (100) parts by volume of feedstock was prepared at solids loading ratio (the ratio of powder volume to the total volume of powder and binder mixture) 50 volume percent with average 10 micron particle size SUS304L stainless steel powder (7.9 gm/cc) and the invented binder for MIM. As per table 1.0 & 2.0 90.26 parts by weight percentage powder was added with 9.74 parts by weight percentage invented binder for MIM. The resulting mixture was kneaded in a Z-blade mixer at 120 °C for one hour to thereby thoroughly fluidized the same. Then the fluidized material was palletized with a pelletizer and obtained pellets were cooled to room temperature. Table 1.0 (Table Removed) Table 2.0 (Table Removed) (2) Moulding: The pelletised feedstock was fed to the injection moulding machine through the hopper. The barrel temperature and nozzle temperature both were set to the required temperature as mentioned below. In the barrel feedstock got fluidized and a certain quantity of feedstock was injected to the standard tensile specimen (flat) die cavity through the nozzle. Then it was allowed to cool so that it became solid and ejected out of the die cavity. The following were the moulding parameters. Nozzle temperature 120 °C Die temperature 30 °C Screw displacement 25.6 mm Injection pressure 162 Kgf. Holding time 45 Sec. Screw rotation 60 rpm. (3) Debinding: The moulded product as obtained above was immersed into a acetone bath for 24 hours. After taking out of the acetone bath the moulded product was dried at 50 °C for 3 hours. After drying the product was placed inside a furnace in nitrogen atmosphere and heated at the rate of 5 °C/min. from room temperature to 80 °C and maintained at this temperature for 1 hour and again it was heated to 400 °C at the rate 1 °C/min and maintained at this temperature for 50 hours thereby debinding the same. The results were as follows: Weight of the product before debinding 24.38 gram Weight of the product after debinding 22.05 gram Weight loss 9.55 % Binder removed by Debinding 98.2 % Each weight is the average of five moulded products. (4) Sintering: The debound product was sintered in a conventional manner. The results were as follows: Relative density 95.7 % Tensile strength 41.5 Kgf/mm2 Shrinkage Length before sintering: 122.36mm ; Length after sintering: 102.37mm Lengthwise shrinkage : 16.34 % Width before sintering: 7.9mm; Width after sintering: 6.47mm Widthwise shrinkage: 18.1 % Thickness before sintering: 2.99 mm; Thickness after sintering: 2.47 mm Thickness-wise shrinkage: 17.39% Shrinkage values are the average of five sintered products. EXAMPLE-2 (1) Feedstock preparation: Hundred (100) parts by volume of feedstock was prepared at solids loading ratio (the ratio of powder volume to the total volume of powder and binder mixture) 50 volume percent with average 20 micron particle size Titanium-Aluminium intermetallic alloy powder ( 3.87 gm/cc) and the invented binder for MIM. As per table 3.0 & 2.0, 81.95 parts by weight percentage powder was added with 18.05 parts by weight percentage invented binder for MIM. The resulting mixture was kneaded in a Z-blade mixer at 120 °C for one hour to thereby thoroughly fluidized the same. Then the fluidized material was palletized with a pelletizer and obtained pellets were cooled to room temperature. Table 3.0 (Table Removed) (2) Moulding: The pelletised feedstock was moulded in the same manner as described in Example-1. The following were the moulding parameters. Nozzle temperature 120 °C Die temperature 30 °C Screw displacement 25.6 mm Injection pressure 88 Kgf. Holding time 40 Sec. Screw rotation 60 rpm. (3) Debinding: The moulded product as obtained above was debound in the same manner as described in Example-1. Binder removed by debinding was 98.1% 1/0. (4) Sintering: The debound product was sintered and maximum 96.8 % relative density was achieved. The main advantages of the present invention are: 1. The novel binder of the present invention consists of a multi-component which provides ease of processing in MTM- 2. The novel binder of the present invention generates viscosity high enough to minimize risk of metal-binder separation without the requirement of high moulding pressure. 3. The novel binder of the present invention does not contain readily vapourisable liquids, such as water, this condition is good for recycling. Hence, the runners and green scrap generated can be recycled many times. 4. The novel binder of the present invention is easy to mould on any injection moulding machine and can be moulded at temperatures of the order of only 110° to 120 °C barrel temperature and moulds at only 30° to 35 °C. 5. The novel binder of the present invention allows gradual viscosity change which makes processing condition less demanding and does not suffer from sudden property changes. 6. The novel binder of the present invention does not require specific type of debinding equipment and can use first stage solvent debinding followed by thermal debinding. 7. Post-sintering shrinkage is very uniform in all directions, thus making the mould design an easy task. We Claim: 1. A novel binder composition for metal injection moulding (MIM), which comprises polymethyl methacrylate (PMMA) in the weight percentage of 30 to 40 and commercial wax in the weight percentage of 60 to 70. 2. A novel binder composition as claimed in claim 1, wherein the polymethyl methacrylate (PMMA) is polymer made by free vinyl polymerisation from monomer methyl methacrylate. 3. A process for the preparation of novel binder for metal injection moulding (MIM) from the novel binder composition as claimed in claim 1-2, which comprises heating the synergistic mixture, with the components polymethyl methacrylate (PMMA) in the weight percentage of 30 to 40 and commercial wax in the weight percentage of 60 to 70, at a temperature range of 90° to 130° C to obtain a homogeneous melt, allowing the melt to cool to room temperature to solidify to obtain binder. 4. A process as claimed in claim 3, wherein the binder obtained is solid at room temperature and the pour point is in the range of 90° to 130° Centigrade. 5. A metal injection moulding (MIM) process using the novel binder as claimed in claim 1-4, which comprises adding the binder to metal powder and subjecting to kneading at a temperature in the range of 90° to 130 °C to obtain injection moulding feedstock, effecting injection moulding by conventional methods to obtain green moulded part with binder, subjecting the said part with binder to two stage debinding consisting of solvent debinding followed by thermal debinding, sintering the debound green part by conventional methods. 6. A MIM process as claimed in claim 5, wherein the ratio of binder to metal powder is dependent on particle size distribution of the metal powder employed and is controlled in conventional manner. 7. A MIM process as claimed in claim 5-6, wherein the metal powder is selected from iron or iron alloy powder, such as stainless steel powder, hard metal powder such as titanium, tungsten, boron, magnetic material powder. 8. A MIM process as claimed in claim 5-7, wherein the two stage debinding process comprises immersing the green part into a solvent bath such as acetone and allowing to stand for a period of 23 to 26 hours, taking out the part from the solvent and drying by conventional methods at a temperature in the range of 40° to 55 °C for a time period of 2 to 5 hours, subjecting the dried part to nitrogen atmosphere inside a conventional furnace, raising the furnace temperature to 75 to 90 °C at a step of 3 to 5 °C per minute, maintaining this temperature for a period of 50 to 120 minutes, raising the furnace temperature to 390 to 450 °C in steps of 1 to 5 °C per minute, maintaining this temperature for a period of 45 to 60 hours, allowing the furnace to cool to room temperature at a normal rate to obtain debound green part. 9. A novel binder composition for metal injection moulding (MIM) substantially as herein described with reference to the examples and figures 2 and 3 of the drawings accompanying this specification. 10. A process for the preparation of novel binder from the composition substantially as herein described with reference to the examples and figures 2 and 3 of the drawings accompanying this specification. 11.A metal injection moulding (MIM) process substantially as herein described with reference to the examples and figures 1,2 and 3 of the drawings accompanying this specification.

Documents:

996-del-2002-abstract.pdf

996-del-2002-claims.pdf

996-del-2002-correspondence others.pdf

996-del-2002-correspondence po.pdf

996-del-2002-description (complete).pdf

996-del-2002-drawings.pdf

996-del-2002-form-1.pdf

996-del-2002-form-18.pdf

996-del-2002-form-2.pdf

996-del-2002-form-3.pdf