KR100302564B1 - Powder injection molding method - Google Patents

Abstract

It is an object of the present invention to provide a powder injection molding method which can produce a product having excellent dimensional accuracy without environmental pollution by using a binder that is formulated so as not to reduce injection fluidity and by removing the binder by a solvent extraction method.

The present invention for achieving the above object, the raw material powder consisting of 30 to 70% by volume ratio, candelilla wax and polystyrene (60) contained in a weight percentage of 90: 10 to 10: 90 60 95% by weight and the remainder of the binder consisting of ethylenevinyl acetate (EVA) are mixed in the remaining volume ratio. Then, the mixture mixed in the mixing step is prepared into injection moldable pellets, and injection molded into a mold having a desired shape to prepare an injection molded product, and the candilla wax and polystyrene in the injection molded product are extracted with a solvent and removed, and the binder is removed. The degreasing body is sintered at a high temperature. In addition, as the raw material powder used in the present invention, it is preferable to use any one or two or more of iron, iron-nickel and stainless powder, or to use any one of oxide, nitride, cemented carbide, and cermet powder. In addition, it is preferable to use any one of a ketone solvent group, such as acetone, benzene, and alcohol, as a solvent.

Description

Powder injection molding method

The present invention relates to a powder injection molding method, and more particularly, to a powder injection molding method capable of minimizing deformation during manufacture of a sintered body of metal, cemented carbide or ceramic powder.

Powder injection molding technology is a production technology capable of manufacturing a large amount of precision parts having a three-dimensional shape, and is a part manufacturing technology using powder materials such as metal, cemented carbide, and ceramics. The main difference between the powder injection molding technology and the plastic injection molding technology is that in the powder injection molding process, the final product is manufactured by removing the binder after injection molding and hot sintering of the powder.

The general manufacturing process of such powder injection molding is as follows.

First, a mixture preparation process is performed in which the powder and the binder are mixed in a constant ratio and made into injection moldable pellets. Thereafter, the mixture is injection molded into a mold having a desired shape to prepare an injection molded product, and the binder is removed from the injection molded product to perform a removal process for preparing a degreasing body. And finally, the sintered body is sintered at a high temperature to perform a sintering process for producing a final product.

The binder used in the powder injection molding process imparts fluidity to the mixture mixed with the powder, thereby making it possible to produce a desired shape by injection molding. The binder used for this purpose must be removed prior to final sintering, so the ease of removal must also be excellent. The binder component most commonly used with such a binder is a thermoplastic polymer.

Typical configurations of such binders include a main binder that maintains its shape until the final point of binder removal and is the main agent of flow, a subbinder that promotes reduced viscosity of the mixture and lubrication of the mold, a plasticizer that improves the flexibility of the polymer, It is composed of a surfactant for improving the wetting (wetting) or adhesive properties (adlhesion) of the powder and the polymer.

Polypropylene, polystyrene, polyethylene, ethylene vinyl acetate, etc. are mainly used as the main binder, and as the sub-binder, waxes such as carnauba wax, paraffin wax, montan wax, beeswax, edible oil, peanut oil, etc. Oil is used. As the plasticizer, solutions such as DBP and DOP are mainly used, and stearic acid and oleic acid are used as the surfactant.

In the conventional powder injection molding binder and its removal method, the binder is removed only by heat by a circulating gas (US Pat. No. 4,404,166), and only the low melting point material in the binder is selectively extracted by a solvent. The binder was extracted using a solvent extraction method (US Pat. No. 4,765,950), a gel-forming binder (US Pat. No. 4,734,237), a thermoplastic binder (US Pat. No. 5,059,387), and a catalytic reaction to remove the remaining main binder as heat. Removal method (Power Injection Molding Symposium, 1992, NPIF, p. 99.).

First, as the composition of the binder used in U.S. Patent No. 4,404,166, 5 to 50% of polypropylene having a melting point of 170 ° C is added as a high melting point material mainly for shape maintenance, but preferably 20% is added, and as a low melting point material 10% carnauba wax and the remainder consist of paraffin wax. Here, polypropylene serves to maintain the shape of the injection molded body while the low melting point material is removed.

However, the metal powder mixture described in US Pat. No. 4,404,166 is composed of 20% polypropylene, 10% carnauba wax, 69% paraffin wax and 1% stearic acid by weight as a composition of the binder. The metal powder mixture thus constructed has a low compatibility between polypropylene and paraffin wax, so that even if a high melting point polypropylene is added, a softening point appears at the melting point of the paraffin wax. It is difficult to remove the binder only by heat, which leads to a long process time.

In addition, U.S. Patent No. 4,765,950 uses non-polar polypropylene, polyethylene, and paraffin wax as binders to separate binders and powders during injection molding, which may cause density deviation of injection molded parts, and as a solvent to remove low melting point materials. The use of methylene chloride (methylene chloride) is a environmental problem.

Therefore, the present invention has been made to solve the problems of the prior art as described above, by using a binder formulated so as not to lower the injection fluidity, and by removing the binder by a solvent extraction method, products excellent in dimensional accuracy without environmental pollution The purpose is to provide a powder injection molding method that can be prepared.

1 is a flow chart showing a powder injection molding method according to an embodiment of the present invention,

2 is a view showing an injection molded body used in the present invention.

The present invention for achieving the object as described above, the raw material powder composed of 30 to 70% by volume ratio, candelilla wax (polylilla wax) and polystyrene (90: 10 to 10: 90% by weight) contained polystyrene) 60-95% by weight and the remainder is mixed with the remaining volume ratio of the binder consisting of ethylene vinyl acetate (EVA). Then, the mixture mixed in the mixing step is prepared into an injection moldable pellet and injection molded into a mold of a desired shape to prepare an injection molded product, and candelilla wax and polystyrene in the injection molded product are extracted and removed by a solvent, The degreased body from which the binder has been removed is sintered at a high temperature.

In addition, as the raw material powder used in the present invention, it is preferable to use any one or two or more of iron, iron-nickel and stainless powder, or to use any one of oxide, nitride, cemented carbide, and cermet powder. In addition, it is preferable to use any one of ketone solvent groups, such as acetone, benzene, and alcohol, as a solvent.

Ethylene vinyl acetate used as a polymer in the present invention has a polar group in the structure, the adhesion property with the powder is superior to the non-polar polypropylene or polyethylene to minimize the separation of the powder and the binder during injection molding, polystyrene and paraffin The compatibility with the wax also prevents the separation between the binder during injection molding. In addition, ethylene vinyl acetate has a property that does not react with the solvent during solvent extraction, the addition amount is suitable 5 to 40%. At this time, if the added amount is less than 5%, the solvent-extracted injection molded product may be damaged by handling during the transfer to the post-decomposition pyrolysis process, and in the case of 40% or more, rapid heating when removing the binder in the post-decomposition pyrolysis process It is difficult to maintain the shape at the speed and the process is long, so it is difficult to obtain the advantages by solvent extraction.

In addition, the composition ratio of candelilla wax and polystyrene is suitably 90:10 to 10:90 based on the weight%, and when the binder is dissolved only with polystyrene, the mixing and injection molding viscosity is very high, and only the candelilla wax is composed of In this case, there is a disadvantage in that the strength of the molded body is lowered.

The candelilla wax used in the present invention has a density of 0.95 to 0.99 g / cm 3 and a melting point of 68 to 70 ° C., and acetone, benzene, carbon disulfide, decalin, Gasoline, O'Neill, carbon tetrachloride, hot chloroform, turpentine, alcohol and the like are used.

In the following, with reference to the accompanying drawings a preferred embodiment of the powder injection molding method according to the present invention will be described in detail.

EXAMPLE

1 is a flowchart illustrating a powder injection molding method according to an embodiment of the present invention.

First, the experimental process using the metal powder will be described.

The metal powder used in the present invention is a stainless powder having an average particle diameter of 8 µm, and the injection molding mixture mixes the metal powder and the binder in a sigma-type mixer at a volume ratio of 55 to 45 for 1 hour at 150 ° C. In this way, the mixed metal powder mixture is cooled and crushed to prepare an injection molded product as shown in FIG. 2. At this time, solvent extraction is carried out by dipping the injection molded product in acetone, which is a ketone-based solvent, and rotating the solvent at a constant speed with a stirrer to remove polystyrene and candelilla wax in the added binder, terminating the reaction, and removing the remaining binder component by pyrolysis. . At this time, the temperature increase rate was raised to the final temperature of 500 ° C. within 5 ° C./min and maintained for about 30 minutes to terminate the pyrolysis process. Sintering is carried out between 1300 and 1370 ° C and held for 1 to 4 hours in a hydrogen atmosphere.

In the following, the experimental procedure using the ceramic powder will be described.

The powder for powder injection molding used in the present invention uses a zirconia powder having a particle diameter of 0.3 μm. In the injection molding mixture, the powder and the binder are mixed in a sigma mixer at a volume ratio of 43 to 57 for 1 hour at 150 ° C. The mixed mixture is crushed after cooling to prepare an injection molded product as shown in FIG. 2. Solvent extraction and pyrolysis are performed in the same way as metal powder. Then, sintering is carried out between 1400 and 1500 占 폚 and maintained for 1 to 4 hours in an atmospheric atmosphere.

In the following, the experimental procedure using cemented carbide powder will be described.

The cemented carbide powder for powder injection molding used in the present invention uses WC-Co-TiC-TaC mixed powder. In addition, the injection molding mixture mixes the powder and the binder in a sigma mixer at a volume ratio of 48 to 52 for 1 hour at 150 ° C. The mixture thus mixed is crushed after cooling to produce an injection molded body as shown in FIG. Solvent extraction and pyrolysis are performed in the same way as metal powder. Then, sintering is performed between 1400 and 1500 ° C. and maintained for 1 to 4 hours in a vacuum atmosphere.

The experimental results as described above are shown in Table 1. The strains after pyrolysis and sintering shown in Table 1 are average values of the initial diameter of the injection molded product and the dimensional change after pyrolysis and sintering four times at different positions in a non-contact manner in a two-dimensional projector.

TABLE 1

Note 1 EVA: Ethylenevinyl Acetate, PS: Polystyrene, CW: Candelilla Wax

2. Strain (%) = Deflection (mm) / Specimen Diameter (mm) × 100

As can be seen from Table 1, Comparative Examples 1 and 2, although the binder composition is within the scope of the present invention, the strain after pyrolysis using only the pyrolysis process exceeded 2% and the height was partially reduced as well as cracks were observed on the surface. In general, the powder injection molding technique is a process for manufacturing a final shape product with a dimensional accuracy of 0.5% or less, and sintered products having excellent dimensions can be obtained in the final sintering process only when the dimensional precision in the pyrolysis process is secured.

In the case of Comparative Examples 3 and 4, the binder removal step was carried out by pyrolysis treatment after solvent extraction, but the ethylene vinyl acetate remained in a large amount, which was disadvantageous during pyrolysis.

However, Inventive Examples 1 to 4 used stainless steel, zirconia, and cemented carbide injection molding. The more binder components removed by solvent extraction, the smaller the thermal decomposition deformation and the higher the sintering precision.

Among the Inventive Examples 1 to 4, as in Inventive Examples 1, 2, and 4, the ratio of the composition of candelilla wax and polystyrene to 90:10 to 10:90 is higher than the strain, which is beyond the range as in Inventive Example 3, after deformation. And it was confirmed that the sintered strain is small (see Table 1). In conclusion, it was confirmed that the composition ratio of candelilla wax and polystyrene in the present invention is preferably 90:10 to 10:90 based on the weight%.

As described in detail above, the powder injection molding method of the present invention may minimize the strain by using a binder formed so as not to lower the injection fluidity and using a solvent extraction and pyrolysis process.

The technical idea of the powder injection molding method of the present invention has been described above with the accompanying drawings, but this is only illustrative of the best embodiment of the present invention and is not intended to limit the present invention.

Claims (4)

In the powder injection molding method for producing a metal, cemented carbide or ceramic particles into a sintered body, Raw powder consisting of 30 to 70% by volume, 60 to 95% by weight of candelilla wax and polystyrene in 90% by weight of 10 to 10:90, and the remainder of ethylene vinyl acetate Mixing the binder consisting of (EVA) in the remaining volume ratio, Preparing an injection molded body by preparing the mixture mixed in the mixing step into an injection moldable pellet and injection molding into a mold having a desired shape; Extracting and removing candelilla wax and polystyrene in the injection molded product with a solvent; Powder binder molding method comprising the step of sintering the degreasing body the binder is removed at a high temperature. The powder injection molding method according to claim 1, wherein the raw material powder is any one or two or more of iron, iron-nickel and stainless powder. The powder injection molding method according to claim 1, wherein the raw material powder is any one of an oxide, nitride, cemented carbide, and cermet powder. The method of claim 1, wherein the solvent is any one of a group of ketone solvents such as acetone, benzene, and alcohol.