JPH0823042B2 - Metal injection molding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] A method for producing an injection-molded product, which comprises forming a molded product by injection-molding a metal powder and then firing it at a high temperature to obtain a sintered product, which is obtained by using a small amount of binder. For that purpose, before mixing the binder with the metal powder, the surface of the metal powder is coated with a surfactant to form a material for injection molding.

[Industrial applications]

 The present invention relates to a method for manufacturing an injection molded body.

In order to obtain a molded product with high dimensional accuracy using metal, it is customary to carry out cutting, but there are cases where lathe processing is difficult because the material is hard and brittle.

For example, magnet bases made of soft magnetic alloys such as iron / silicon (Fe / Si) alloys, motor yokes, etc. come in contact with this, and the material is hard and brittle. Is reduced.

On the other hand, after mixing the metal powder with the thermoplastic resin, injection molding into a required shape, placing this in a furnace, gradually raising the temperature to decompose the thermoplastic resin, debinding the binder, and then baking. Witec-process
This method is suitable for processing the above-mentioned materials, and is characterized by high production yield.

[Conventional technology]

When producing a molded product by injection molding a mixture of metal powder and a thermoplastic resin, it is necessary that the amount of the thermoplastic resin (binder for short) be used as small as possible, because the amount of resin added The more there are, the easier the molding is,
This is because the yield of the binder removal step performed after molding is reduced.

That is, a high molecular weight organic compound having a low softening temperature and a low decomposition temperature such as polyethylene or polystyrene is used as a binder, and after removing the binder by a binder removal treatment, the molded body is heated to a high temperature for sintering.

Here, in the binder removal method, the molded body is placed in a furnace as it is, and the binder is heated and decomposed by gradually raising the temperature, and metal powder such as copper (Cu) or alumina (Al). There is a method in which it is immersed in a filler made of ceramic powder such as ₂ O ₃ ) and placed in a furnace, and the temperature is gradually raised so that the filler is decomposed while being absorbed by the filler.

Then, after raising the temperature to near the pyrolysis temperature, several ° C / hou
The decomposition progresses by gradually heating at a rate of about r, but when exposing the molded product, it is necessary to perform it at a low speed of 1-2 ° C / hour, and if it is faster than this, the surface of the molded product The temperature gradient between the interior and the interior of the binder tends to cause cracks, and the gas generated during decomposition of the binder may cause swelling and the viscosity of the binder to cause collapse.

Also, when the molded body is embedded in a metal powder or an oxide powder, the temperature rise rate is moderated and the rate of temperature rise can be increased.However, since the air permeability is poor, the decomposition of the binder is incomplete. There is a problem that carbonization easily occurs.

In this way, a metal molded body is made by the injection molding method,
The binder removal treatment is a major problem in the manufacturing method in which the product is produced by the sintering method.

Next, in order to obtain a sintered body with a high sintering density, it is preferable to use a metal powder with a small particle size, but in that case it is necessary to use a large amount of binder, and it becomes very difficult to remove the binder. There was a problem called, and countermeasures were necessary.

[Problems to be solved by the invention]

As described above, in the process of kneading the metal powder with the binder, injection-molding, debinding and sintering, the binder removal process affects the yield and quality of the product. In order to obtain a sintered body having a high density, it is necessary to use a metal powder having a small particle size, but in this case, a larger amount of binder is required, which makes it difficult to remove the binder.

[Means for solving problems]

The above problem is that a mixture of a binder made of a thermoplastic resin and metal powder is injection-molded to obtain a molded body, and then the molded body is heated to remove the binder and heated to a high temperature to obtain a sintered body. In the method of metal injection molding, the method of metal injection molding involves coating the surface of the metal powder with a surfactant before mixing the metal powder with the binder.

[Action]

It is necessary to use a powder with a small particle size to obtain a sintered compact with a high sintering density, but in this case, the reason why more binder is required than when using a powder with a large particle size Is because the surface area per unit volume is increased by reducing the particle size of the powder, but the inventors can avoid the increase of the binder if the adhesiveness (wettability) of the binder to the powder is improved. I thought it might happen, and conducted the following experiment.

That is, one sample has a particle size of 4 to 4 as powder.
Using 5 μm iron (Fe) powder and polyethylene as a binder, polyethylene having a volume ratio of 50:50 was added to the Fe powder and kneaded with a kneader (kneader) for 2 hours.

In the other sample, stearic acid was used as the surfactant, and the volume ratio of Fe: polyethylene: surfactant was 50: 4.
At 9: 1, stearic acid was dissolved in ethanol and mixed with Fe powder, and then ethanol was evaporated to coat Fe powder with stearic acid.

 Then, it was kneaded with polyethylene in the same manner as above.

When comparing the two powders, it was observed that the former had more than half of the Fe powder and polyethylene separated, while the latter had agglomerated granular Fe powder coated with polyethylene.

On the other hand, in order to agglomerate the former into a state similar to the latter, it is necessary to increase the amount of the binder to be added such that the composition ratio of the powder and the binder is 30:70 or more.

That is, since the surface tension of polyethylene is reduced and the wettability is improved by adding the surfactant, the powder is coated and the lubricity is improved even by adding a small amount of the binder.

Here, the necessary conditions for the surfactant are that the melting point is in a temperature range where workability is good, the structure is simple and thermal decomposition easily occurs, and elements other than carbon (C), oxygen (O) and hydrogen (H) are added. It is necessary to exclude the saturated fatty acid [CH
₃ (CH ₂ ) _n COOH] and unsaturated fatty acids.

For example, palmitic acid C ₁₅ H ₃₁ COOH melting point 63-64 ℃ heptadecyl acid C ₁₆ H ₃₃ COOH 〃 60-61 ℃ stearic acid C ₁₇ H ₃₅ COOH 〃 71-72 ℃ nonadecanoic acid C ₁₈ H ₃₇ COOH 〃 68.7 ℃ pehenic acid C ₂₁ H ₄₃ COOH 〃 81-82 ℃, etc.

As described above, according to the present invention, even when a metal powder having a small particle size is used, a surfactant is used to obtain a molded article having good injection moldability by adding a smaller amount of binder than before, and the binder amount is small. Therefore, the binder removal property is also good.

〔Example〕

Example 1: Fe / Si alloy metal powder having a particle size of 4 to 5 μm, polyethylene having an average molecular weight of 3000 as a binder, and stearic acid as a surfactant were used.
With a volume ratio of 4: 1, stearic acid was dissolved in ethanol and mixed with Fe.Si alloy powder, and then ethanol was evaporated to coat the surface of Fe.Si alloy powder with stearic acid.

Next, polyethylene was added thereto, and the mixture was kneaded with a kneader for 2 hours.

When injection molding was performed using this material, a good molded product of a hammer for a line printer could be obtained.

Next, the molded body was embedded in a filler made of AlN powder, kept at a temperature of 120 ° C. to elute the binder made of polyethylene and stearic acid, and then in a N ₂ atmosphere.
When the binder was removed by raising the temperature from 0 to 400 ° C at a rate of 20 ° C / h, it was possible to obtain a binder-removed molded product that did not collapse, crack, or swell.

Comparative Example 1: The same metal powder and polyethylene as in Example 1 in volume ratio
It was mixed at 55:45 and kneaded in the same manner as in Example 1.

When this was injection-molded, the gate was clogged, and a good molded product could not be obtained.

A good molded product can be obtained when the capacity ratio is 45:55 or more, but all of the molded products obtained under these conditions collapsed due to the binder removal treatment.

〔The invention's effect〕

By carrying out the present invention, even if a powder having a small average particle size is used, injection molding can be performed with a smaller amount of binder added than in the conventional case, and since the amount of binder is small, binder removal processing can be performed with good manufacturing yield.

Claims (1)

[Claims] 1. A mixture of a binder made of a thermoplastic resin and a metal powder is injection-molded to obtain a molded body, which is then heated to remove the binder and heated to a high temperature to obtain a sintered body. In the metal injection molding method, the surface of the metal powder is coated with a surfactant before the metal powder is mixed with the binder.