JPH05320707A - Composition for metal powder injection-molding - Google Patents

Abstract

PURPOSE:To provide a metal powder injection-molding composition which is highly degreased, having a high formed body retaining strength and without causing cracks and shrinkage holes in the formed body when cooled. CONSTITUTION:The composition consists of a sintering metal powder and an org. binder, and the binder is obtained by mixing 100 pts.wt. of the metal powder, 8-21 pts.wt. of a thermoplastic resin, 0.5-3 pts.wt. of a plasticizer and 1-4 pts.wt. of a lubricant. Concretely, 0.5-7 pts.wt. of polystyrene (PS) as the thermoplastic resin, 0.5-7 pts.wt. of polybutyl methacrylate (PBMA), 0.5-7 pts.wt. of amorphous polyolefin (APO), 0.5-3 pts.wt. of dibutyl phthalate (DBP) as the plasticizer and 1.4 pts.wt. of stearic acid and paraffin wax as the lubricant are used.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a composition obtained by injection-molding a composition obtained by adding a thermoplastic resin, a plasticizer, and a lubricant to a metal powder and kneading the mixture, followed by debinding and sintering. TECHNICAL FIELD The present invention relates to a composition for metal injection molding for producing a precision machined component having a shape.

[0002]

2. Description of the Related Art In a powder injection molding method, an injection molding composition prepared by mixing a metal powder with a thermoplastic binder is heated to impart fluidity and then injected into a mold. It has an extremely excellent feature that it can be molded without causing deformation due to chipping or distortion in a thin object, a thin object, or a sharp edge. The injection molded body thus obtained is heated to remove the binder, and then sintered to obtain a sintered metal body in which the original molded body isotropically contracts.

[0003]

However, the conventional metal powder injection molding compositions have the following serious drawbacks. (1) The main purpose is to knead, mold, and remove the binder at a low temperature, and therefore polyethylene and paraffin wax as a binder and a small amount of an auxiliary agent are added to the metal powder. A molded body obtained from a binder system mainly composed of such a low melting point compound is likely to be deformed during degreasing as the metal powder becomes closer to a spherical shape and the particle size becomes larger, and moreover, polyethylene or paraffin wax is used as a main component. Therefore, there is a big drawback that the degreasing rate is low. (2) Polyethylene or paraffin wax is a typical non-polar polymer or low molecule, as is clear from the structure, and the intermolecular force is also a weak binding force such as van der Waals force, so intermolecular When they are used as a binder, they have a weak cohesive action, and therefore, they have weaknesses such as weak holding strength of the molded body and brittleness. Moreover, this effect increases as the powder particles become coarser. (3) Since polyethylene and paraffin wax are crystalline polymers, they have the property that their volume increases (or decreases) drastically at the melting point (or freezing point), and the rate of increase thereof also changes rapidly. As a result, when used as a binder for an injection molding composition, there is a drawback that cracks and sink marks are likely to occur in the molded body during cooling. (4) Since polyethylene is a linear polymer and has crystallinity, molecular orientation is likely to occur during molding, and as a result, the shrinkage rate differs between the flow direction and the linear direction (2.5 in the flow direction).
%, 2% in the right-angled direction), and when used as a binder, deformation and sink are likely to occur, and there is a problem in terms of dimensional stability. Further, the internal stress caused by the molecular orientation may cause residual strain in the molded body and significantly reduce the strength.

To compensate for these drawbacks, in some cases, a copolymer of ethylene and vinyl acetate (EVA resin) may be added. EVA resin contains a large atomic group such as acetic acid group in the polyethylene chain, which hinders the crystallinity and lowers the degree of crystallinity, and also has good compatibility with paraffin and has a polar group, resulting in polyethylene. Another advantage is that paraffin wax is added to an injection molding composition containing a binder as a main component to improve the brittleness of a molded product. However, on the other hand, the adhesiveness with the mold becomes stronger and the mold releasability of the molded product remarkably deteriorates, resulting in deformation and damage of the molded product due to excessive protrusion, damage to the mold and molding machine, and reduced workability due to these. It was causing a lot of problems.

The present invention eliminates the above-mentioned brittleness of molded products, poor releasability, anisotropy of molding shrinkage, deformation during degreasing, and insufficient degreasing, and metal powder injection with good sinterability. The object is to provide a molding composition.

[0006]

[Means for Solving the Problems] In order to solve the above problems,
When I searched for the optimum composition for injection molding, I found 100
With respect to parts by weight, a thermoplastic resin 8 to 21 is used as a binder.
By using a composition in which 1 part by weight, a plasticizer of 0.5 to 3 parts by weight, and a lubricant of 1 to 4 parts by weight are used, markedly superior performance can be exhibited as compared with conventional injection molding compositions. It became so. Here, the composition for injection molding contains 0.5 to 7 parts by weight of polystyrene, 0.5 to 7 parts by weight of polybutyl methacrylate, and amorphous polyolefin based on 100 parts by weight of metal powder as the thermoplastic resin. 0.5 to 7 parts by weight, similarly the plasticizer was 0.5 to 3 parts by weight of dibutyl phthalate, and the lubricant was 1 to 4 parts by weight of paraffin wax and stearic acid.

The reason why the present composition is particularly excellent is as follows. Polystyrene is a nonpolar polymer having a structure in which a benzene nucleus is added to a vinyl group. Therefore, the crystallinity is low and the molding shrinkage ratio is 0.4%, which is extremely small. Polybutyl methacrylate has a tenacious property because it has a polar group but is relatively weak, and when used as a binder for injection molding, there is no anisotropy of the shrinkage of the molded product due to the orientation and the strength of the molded product. Will also be excellent.

Further, polystyrene and polybutyl methacrylate have good thermal decomposition characteristics and show extremely high binder removal properties when used as a binder for injection molding. Amorphous polyolefin (APO) is a highly flexible material and has a property of further improving the stickiness of a molded body.

The metal powder used in the present invention is pure iron, carbonyl iron, carbonyl nickel, cobalt, high-speed steel,
The thermoplastic resin includes commercially available polystyrene, polybutyl methacrylate and amorphous polyolefin, and the lubricant includes commercially available paraffin wax and Stearic acid may be mentioned, and the plasticizer may be dibutyl phthalate.

The metal powder and the binder are preferably mixed in a volume ratio of 35 to 65% for the former and 65 to 35% for the latter, whereby the kneadability and molding of the metal powder and the binder are performed. It is possible to obtain an injection molding composition having excellent properties, releasability and degreasing property. That is, the production of a metal powder injection molded article using the composition of the present invention is performed as follows.

First, the above composition is kneaded by heating at 140 to 180 ° C., and then pelletized for easy handling.
The pelletized kneaded product is molded into a predetermined shape by an injection molding machine. The obtained injection-molded body was heated in the atmosphere or an inert gas atmosphere at a temperature rising rate of about 5 to 50 ° C./hr for 2
By heating to 80 to 600 ° C., the binder contained therein is removed, and thereafter sintering is performed at a temperature of 1200 to 1450 ° C.

[0012]

As a result of using the above means to solve the problem,
Moldability, releasability, and degreasing property have been significantly improved compared to conventional products.
It was revealed that a product having very excellent sintering characteristics was obtained. The reason is shown below. (1) The thermoplastic resin is added in a total amount of 8 to 21 parts by weight with respect to 100 parts by weight of the metal powder. The reason is that if the total amount of the thermoplastic resin exceeds 21 parts by weight, the density of the molded product does not increase and the molded product is deformed during degreasing, or the dimensional shrinkage rate at the time of sintering becomes large and the dimensional accuracy is deteriorated. This is because a sintered body cannot be obtained, and conversely, if it is less than 8 parts by weight, the fluidity is poor and molding cannot be performed, or even if it is possible, the molded body becomes brittle. As the binder, in addition to these thermoplastic resins, a plasticizer and a lubricant are added. These impart elongation, flexibility and fluidity to the kneaded product, and promote the action of the kneaded product flowing into every corner of a die having a complicated three-dimensional shape during molding. (2) By using polystyrene, polybutyl methacrylate, or amorphous polyolefin instead of the sticky EVA resin as the thermoplastic resin, the releasability is remarkably improved. (3) FIG. 1 shows the thermal decomposition curves of stearic acid, wax, polybutyl methacrylate, amorphous polyolefin, and polystyrene, respectively, and the thermal decomposition curves of the kneaded product containing the above compound. The thermal decomposition curve of the kneaded product of the present invention is gentler than the thermal decomposition curve of the above compound alone. Therefore, the thermal decomposition of the binder gradually progresses during the heating and heating process, so that swelling or cracking does not occur during debinding and a good degreased body can be obtained.

As can be seen from FIG. 1, stearic acid, paraffin wax (melting point 46 ° C.), polybutyl methacrylate.
When used alone, amorphous polyolefin and polystyrene show relatively rapid decomposition and evaporation in a narrow temperature range. This is because, in the case of a single composition, when the bond peculiar to the molecule reaches a specific temperature, decomposition is preferentially started.

On the other hand, the composition for injection molding according to the present invention (solid line in FIG. 1) is a multi-component system in which a plurality of organic compounds having different decomposition initiation temperatures are mixed at an arbitrary ratio as described above. Therefore, the molecules with weak chemical bonding or the components with low molecular weight are preferentially decomposed or melted and evaporated, and the binder removal proceeds. At this time, if a strong interaction (eg, cohesive action between different molecules or repulsive force) does not work between the components, the total amount of decrease of the binder due to decomposition and evaporation at a predetermined temperature depends on the temperature of the compounding amount of each component. It is expected to be multiplied by each decomposition rate of and given by the sum of them. That is, it is considered that additivity is established. An example is shown in FIG. The dotted line in the figure shows the prediction of the thermal decomposition pattern of the present injection molding composition from the thermal decomposition pattern of each component used in the present injection molding composition in the atmosphere. This predicted decomposition evaporation curve is 150-350.
In the temperature range of ° C, it is suggested that the leaching due to the melting of the binder and the release of the gas generated by the thermal decomposition proceed slowly, and such a pattern can be removed from the binder-removed body. It is very preferable because it eliminates swelling and deformation that tend to occur.

On the other hand, the thick solid line in the figure shows the actual thermal decomposition curve of the composition for injection molding of the present invention. In the first half of the thermal decomposition rate up to 50%, the predicted value agrees very well. It can be seen that it is gradually decreasing. The latter half (decomposition rate 50 to 100%) deviates from the predicted value, but it is the deviation toward the faster decomposition, and some interaction occurs between organic compounds or between organic compounds and metal near this temperature. It is thought to be because However, in general, in the case of an injection molding composition, after 40 to 60% of all binders are removed, sufficient passages (open pores) are formed for the thermal decomposition components to escape, so the remaining components With respect to the above, it is rather preferable that the binder removal rate be high, and therefore the present composition is very preferable as an injection molding composition.

The composition for injection molding according to the present invention has very good heat fluidity, moldability and releasability, and a molded article molded using the composition has strain, swelling, Defects such as cracks no longer occur. In addition, the binder in the molded body
However, since it was gradually decomposed by heat and removed, it swelled due to degreasing and no cracking occurred. Therefore, it has become possible to obtain a product having no defects in the sintered body and having excellent dimensional accuracy.

[0017]

EXAMPLES Next, preferred examples of the present invention will be described. To the stainless powder A having an average particle size of 8 μm, the stainless powder B having an average particle size of 12 μm, the carbonyl iron powder A having an average particle size of 4 to 5 μm, and the carbonyl iron powder B having an average particle size of 4 to 6 μm, which were obtained by the water atomizing method. Binders having the compositions shown in Table 1 were combined to prepare the composition of the present invention, and using this, samples of several shapes (Examples 1 to 9) were prepared under the conditions shown in Table 2. In addition, Tables 1 and 2 show examples using stainless steel powder A as comparative examples other than the present invention (Comparative Examples 1 to 3).

[0018]

[Table 1]

[0019]

[Table 2]

As can be seen from Table 2, the molded product is brittle by the conventional method, cracks are generated at the time of mold release and the molded product collapses (Comparative Examples 1 and 3), or conversely, the tackiness is too strong depending on the blending. Therefore, the releasability was remarkably reduced, and as a result, the molded body was sometimes attached to the mold and deformed during ejection (Comparative Example 2). On the other hand, by using the composition according to the present invention, the mold releasability from the mold and the strength of the molded body were remarkably improved, and a molded body having a small mold shrinkage and excellent mold transferability was obtained.

Next, Table 3 shows the appearance and debinding ratio of the degreased product after the binder was removed by heating the injection molded product in air or in a nitrogen gas atmosphere. In the conventional method (Comparative Examples 1, 2, and 3), the degreasing rate was as low as 85 to 95%, and the shape was deformed due to crushing, whereas when the composition of the present invention was used, The degreasing rate of 97% or more for the binder removed in the atmosphere and 99% or more for the binder removed in nitrogen (Examples 1 to 9) while maintaining the shape of
Was obtained, and it was very excellent in degreasing property. Further, the sintered body obtained by sintering the degreased body thus obtained,
There were no surface roughnesses, the relative density was as high as 95 to 98%, and the dimensional variation was ± 0.6 to 0.3% or less, which is a very good result (Examples 1 to 9 in Table 4).

[0022]

[Table 3]

[0023]

[Table 4]

[0024]

The following effects are exhibited by the present invention. (1) Since the kneaded product has good flowability and good formability, it is possible to form a product having a three-dimensionally complicated shape. (2) Compared with the conventional products (Comparative Examples 1 to 3), the shrinkage rate of the molded product is small and the dimensional stability is excellent. (3) Since the mold releasability from the mold is far better than that of the conventional product, the yield of the molded product is high and the workability is also good. (4) The degreasing property of the molded body is extremely good, and the swelling of the degreased body
Since there is no cracking or disintegration, the density of the sintered body is high, and an excellent product with small dimensional variation can be obtained.

[Brief description of drawings]

FIG. 1 is a thermal decomposition curve diagram of each binder composition used in the present invention and a thermal decomposition curve of the binder of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsuyoshi Saito 36 Shimogamo-Kifunecho, Sakyo-ku, Kyoto

Claims (1)

[Claims] 1. A composition comprising a sintering metal powder and an organic compound-based binder, wherein the composition is the metal powder 10.
0 to 2 parts by weight of the thermoplastic resin 8 to 2 as a binder
1 part by weight, 0.5 to 3 parts by weight of a plasticizer, and 1 to 4 parts by weight of a lubricant are blended, and the thermoplastic resin is 0.5 to 7 parts by weight of polystyrene (PS) and polybutyryl methacrylate. (PBMA) 0.5 to 7 parts by weight, amorphous polyolefin (APO) 0.5 to 7 parts by weight, plasticizer as dibutyl phthalate (DBP) 0.5 to 3 parts by weight, lubricant as stearic acid and paraffin wax A metal powder injection molding composition comprising 1 to 4 parts by weight.