KR19980051241A - Manufacturing method of mixture for metal injection molding - Google Patents

Abstract

The present invention relates to a method for producing a mixture used in metal injection molding for producing a metal powder sintered body, wherein the metal powder is preheated to a temperature above the melting point of the organic binder while stirring the blades in the mixing vessel, and then the organic binder is added to the heated metal powder. By adding and mixing to prepare a metal injection molding mixture, by maintaining the viscosity of the mixture at a low shear rate that influences the flow characteristics to improve the flow characteristics, it is possible to produce an injection molded body with a small weight variation.

Description

Manufacturing method of mixture for metal injection molding

The present invention relates to a method for producing an injection molding mixture by mixing a powder and a binder in a metal injection molding method for producing a metal powder sintered body, and more particularly, to prepare a mixture having a low viscosity at low shear rate during flow of the mixture. It is about how to.

Metal injection molding technology generally produces a three-dimensional product using a plastic injection molding method by mixing a fine metal with a diameter of 20 μm or less and then using an injection molding method, and then removing and sintering the added organic binder to manufacture a large amount of metal injection molded products. It is a technique to do. In case of using metal powder, it is possible to obtain a uniform structure, which is the advantage of powder metallurgy, and thus to obtain mechanical properties that are the same as or better than that of cast products. You can do it.

The manufacturing process by the metal injection molding method includes a process for preparing a mixture for uniformly mixing a predetermined amount of metal and a binder, a process for preparing an injection molded product by injection molding the mixture into a mold of a desired shape, a process for removing a binder from the injection molded product, and The degreasing body is finally subjected to high temperature sintering to produce a final product.

The purpose of the mixture manufacturing process of mixing the metal and the binder during the metal injection molding process is to uniformly disperse the fine metal powder and the binder, and to produce components by injection molding, which is a post-process, without defects and to minimize the weight deviation. Molding into parts. If the mixing is uneven, the weight deviation of the injection molded part may exceed 10% and it may be considered good if the part is manufactured with a deviation within ± 0.3%.

K.F.Hens et al., Powder Injection Molding Synposium, 1992, MPIF, R169.

The binder added during the metal injection molding provides fluidity to the powder so that the metal powder can be collected into the mold during injection molding and maintains the shape after cooling. The role of the binder in the metal injection molding is very important. In the preparation of the mixture, the binder is usually added in a volume ratio of about 30-50%, depending on the filling properties of the powder, and is composed of at least two materials. In the binder, a thermoplastic material such as low density polyylene or ethylene vinyl acetate is mainly used as the polymer material which maintains the shape of the molded product until the sintering starts at the final stage of flow and binder removal. The viscosity of the mixture is lowered to allow the metal and the binder to be easily mixed and injection molded, and in the binder removal process by thermal decomposition, the binder is removed at low temperature to form initial pores. In addition to the mixture, a plasticizer for improving the flexibility of the polymer, a surfactant for improving the wettability (wetting) or adhesion properties (adhesion) of the metal and the polymer may be optionally immersed.

As described above, when a large amount of metal powder is laminated on the metal injection-molded mixture made of metal powder and a binder, the viscous flow behavior is sintered and the viscosity decreases as the shear rate increases, and the yield is very high at low shear rate. It is known to have a stress. Yield ability is resistance to flow, so the smaller the value, the better the flow characteristics, which is advantageous for the manufacture of complex shaped parts. The nonuniform mixture has a very high yield stress value, making injection molding difficult.

As a general method of mixing the powder and the binder, a method of mixing all the binders to be heated above the melting point to mix the binders and then gradually loading the powders into the molten binder [R.S. Libb et al .: MPR, Vo1.43, No. 4, 1988, p255] are disclosed. In order to uniformly mix the powder and the binder, mixing is performed for a long time in a sigma type mixer or a Banbury mixer with a large shear force. However, in this case, the composition of the binder is changed due to evaporation or deterioration of the low molecular weight material, and thus, the viscous fluidity is lowered, so that defects may occur during injection molding and it may be filled nonuniformly.

In order to solve this problem, Japanese Patent Application No. Hei 4-337006 discloses a metal powder using a method of adding a metal powder in which a polymer compound is melted first, or charging and mixing a metal powder and a polymer compound at the same time. After dispersing the low-molecular component was added a method for producing a grouped mixture in a short time has been presented. In the case of mixing in this way, a large shear stress is required in the first mixing of the metal powder and the polymer compound, but in this case, the effective mixing area in the container may not be sufficiently filled. Therefore, although the shear rate of the polymer compound is primarily mixed at a high temperature, when the initial mixture is charged, the mixing temperature may be lowered below the melting temperature of the polymer compound, and thus the effect of mixing may be lost. In addition, a method of mixing using a large amount of solvent [J.W.O'Connor et al. Advances in Powder Metallurgy, Vo1.2,1991, MPIF, p85] is used.

This method can uniformly mix the powder and the binder and reduce the wear roll of the container, but it uses a large amount of solvent that can dissolve all the added binder, so that the powder precipitates uniformly during the segregation and mixing of the binder. Not only are they not mixed, but the process takes a long time.

The present invention has been proposed to solve such a problem, and an object of the present invention is to control the mixing method of powder and binder in the preparation of a metal injection molding mixture by controlling the mixing method of the metal with a low viscosity and uniform weight at a low shear rate as compared with the prior art. It is to provide a method for producing an injection molding mixture.

Furthermore, another object of the present invention is to prepare a metal injection molding mixture composed of a metal and an organic binder, and preheat the metal to a temperature above the melting point of the organic binder, and then add an organic binder to mix the metal and the organic binder to fluidity It is to provide a method for producing this improved metal injection molding mixture.

1 is a schematic view of a part made by injection molding used for injection molding evaluation

In one aspect of the invention, in the preparation of a mixture of a metal used in metal injection molding and an organic binder, the metal powder is preheated to a temperature above the melting point of the organic binder while stirring the bleed in the mixing vessel, and then preheated to the preheated metal. Provided is a method for producing a mixture for metal injection molding, wherein the organic binder is added and mixed.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention relates to a method for preparing a metal injection molding mixture having a low viscosity and a small weight variation at a low shear rate. In order to prepare such a mixture, a metal is first preheated while stirring at a temperature above the melting point of the organic binder. Then, an organic binder is added and mixed to the preheated metal.

That is, the metal powder may be preheated by charging a predetermined amount of metal into the mixer and heating the mixer above the melting point of the organic binder, or by heating the temperature of the mixer to the temperature above the melting point of the organic binder and then preloading the metal into the mixer. have.

Preheating the metal with stirring for at least 5 minutes is preferred.

An organic binder is added to and mixed with the preheated metal to prepare a metal injection molding mixture. The organic binder may be added simultaneously with each component or separately mixed with the organic binder components. The organic binder is composed of a thermoplastic polymer and a low viscosity compound. Examples of the thermoplastic polymer include polystyrene, polypropylene, polyethylene, ethylene vinyl acetate, and examples of the low viscosity compound include carnauba wax, beeswax, amide wax, and paraffin wax. And the like.

Furthermore, to the binder, process aids such as stearic acid and oleic acid may be added as necessary.

The metal powder used as one component of the metal injection molding mixture is not only small but also added in a large amount of 50-70% by volume. Therefore, even if it is not mixed uniformly with the organic binder, even if it is a mixture of the metal and the organic binder of the same composition Variations in viscous flow and non-uniform mixtures lead to poor fluidity during injection molding, resulting in final sinters resulting in defects such as weight deviation, surface cracks, sink marks, etc. It is preferred to stir and mix sufficiently to mix uniformly for at least 30 minutes, preferably less than 30 minutes to less than 120 minutes.

Thus, by preheating the metal having better thermal conductivity than the organic binder, and then immersing and mixing the organic binder, an excellent injection molded product having a small weight deviation can be obtained by maintaining the viscosity of the mixture at a low shear rate that influences the flow characteristics and improving the flow characteristics. It can be manufactured.

Hereinafter, the present invention will be described in detail through examples.

Due Diligence

In the present embodiment, as the metal powder for the metal injection molding mixture, 316L stainless thrill powder having an average particle diameter of 8 μm of an irregular shape manufactured by a high pressure water spray method was used. Injection molding mixtures were prepared by mixing the metal powder and the binder in a sigma mixer in a 53:43 volume ratio under the conditions of the respective comparative and inventive examples described in detail below. Viscosity flow uniformity of each mixture was measured up to 10001 / second at 130 ° C and a slowing rate of 11 / second using a capillary viscometer.The uniformity of mixture mixing was achieved by injection molding the mixture into parts as shown in Figure 1. The weight deviation of was measured and evaluated. The measured physical properties are shown in Table 1 below.

Comparative Example (1)

In the mixer maintained above the melting point of the polymer binder, the edylenevinyl acetate was stirred for 15 minutes with the metal and the polymer binder, and the mixture was further mixed for 20 minutes by adding paraffin wax, which is a low molecular weight binder.

Comparative Example (2)

The metal powder and the binder were mixed in the same manner as in Comparative Example (1) except that low density polyethylene was used as the polymer binder.

Comparative Example (3)

The metal was first charged into a mixer kept at a melting point of silver of a high molecular binder and stirred for 5 minutes, followed by mixing for 15 minutes by adding ethylene vane acetate as a high molecular binder and paraffin wax as a low molecular weight.

Comparative Example (4)

The metal was first charged into a mixer maintained at a melting point of the polymer binder at a temperature higher than that and stirred for 15 minutes, followed by mixing for 105 minutes by adding the same polymer binder and the low molecular binder as in Comparative Example (3).

Inventive Example (1)

The metal was first charged into a mixer maintained at a melting point of the polymer binder at a temperature higher than that and stirred for 15 minutes, followed by mixing for 20 minutes by adding the same polymer binder and the low molecular binder as in Comparative Example (3).

Inventive Example (2)

The metal powder and the binder were mixed in the same manner as in Example 1 except that low density polyethylene was used as the polymer binder.

Inventive Example (3)

The metal was first charged into a mixer maintained at a melting point of the polymer binder at a temperature higher than that, followed by stirring for 15 minutes, and then the same polymer binder and the low molecular binder as in Example 1 were added and mixed for 20 minutes. The polymer binder and the low molecular weight binder were premixed separately before charging.

Inventive Example (4)

The metal was first charged into a mixer maintained at a temperature above the melting point of the polymer binder and stirred for 5 minutes, and the same polymer binder and the low molecular binder as in Example 1 were added and mixed for 30 minutes.

Inventive Example (6)

The metal was first charged into a mixer maintained at a temperature above the melting point of the polymer binder and stirred for 5 minutes, followed by mixing for 55 minutes by adding the same polymer binder and the low molecular binder as in Example 1.

As shown in Table 1, Inventive Examples 1 and 2 were charged with a polymer binder and a low molecular weight binder for 20 minutes after preheating the powder for 15 minutes, and compared with the same mixing method and time as the conventional method (Comparative Examples 1 and 2). It can be seen that the viscosity decreases by more than 160 Pa-sec at a shear rate of 10 l / sec, depending on the type of polymer binder. In addition, Inventive Example 3 used the same binder as Inventive Example 1 but separately used only by preheating and mixing the metal, which showed a lower viscosity than Comparative Example 1. Inventive Example 4, which preheated the metal powder for 5 minutes, mixed the polymer and the low molecular weight binder for 30 minutes, and Inventive Example 5, which preheated the metal powder for 5 minutes and mixed the polymer and low molecular binder for 55 minutes, showed a lower viscosity than Comparative Example 1. It was. However, in Comparative Example 3, which preheated the metal powder for 5 minutes and mixed the polymer and the low molecular binder for 20 minutes, the viscosity was about twice that of Comparative Example 1, and the metal powder was preheated for 15 minutes and the polymer and the low molecular binder were 105 minutes. In the case of the mixed Comparative Example 4, the viscosity is also high, which is not suitable.

In addition, the parts molded with the mixture of Comparative Examples 1-4 were all molded with the mixture of Comparative Example 3, which had the highest standard viscosity of 0.5% or more and the weight of the injection molded product was uneven and the viscosity was the highest at the shear rate of 10 1 / sec. The largest standard deviation was 1.2%. On the contrary, the parts molded into the mixture according to the present invention showed excellent injection molding with a standard deviation of 0.2-0.3% of the weight of the injection molded body.

Thus, by preheating the metal having better thermal conductivity than the organic binder, and adding and mixing the organic binder, the viscosity of the mixture is kept lower than the mixture prepared by the conventional mixing method to improve the flow characteristics at a low shear rate that influences the flow characteristics. It is possible to produce an excellent injection molding with a small weight variation.

Claims (4)

In preparing a mixture of metal and organic binder used in metal injection molding, the metal powder is preheated to a temperature above the melting point of the organic binder while stirring the blades in the mixing vessel, and then the organic binder is added and mixed to the preheated metal powder. Method for producing a mixture for metal injection molding, characterized in that. The method of claim 1 wherein the preheating is performed with stirring for at least 5 minutes. The method according to claim 1 or 2, characterized in that the stirring and mixing is performed for 30 minutes to less than 120 minutes. The method of claim 1, wherein the binder is a thermoplastic polymer selected from the group consisting of polystyrene, polypropylene, polyethylene, ethylene-vinyl acetate, and low viscosity selected from the group consisting of carnauba wax, beeswax, amide wax, pyraffin wax Compound, comprising