JPH0825178B2 - Method of manufacturing injection molded body - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Outline] The present invention relates to a method for producing an injection-molded body, which comprises subjecting a metal powder or a ceramics powder to injection molding to form a molded body, and then firing it at a high temperature to obtain a sintered body. For the purpose of obtaining an injection-molded product satisfying both of them, after coating the powder of the molded product with a polymer resin which is incompatible with polyethylene and has a higher decomposition temperature than the polyethylene, the softening temperature of the polymer resin is not more than And kneading at a temperature above the melting temperature of polyethylene to coat polyethylene,
A powder to be molded having a double coating layer is prepared, and the resin coating powder is injection molded.

[Industrial applications]

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

In order to obtain a molded body with high dimensional accuracy using metal or ceramics, it is customary to carry out cutting, but in some cases 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]

The requirements for the thermoplastic resin (binder for short) used to mold the metal powder in the Whitec process are: In short, the necessary condition of the binder is that the moldability is good and the binder removal property is good.

 However, there is no material sufficiently satisfying this condition.

That is, if the moldability is poor, the fluidity is poor at the time of injection molding, and a short shot (Short-Shot) in which the resin stops in the middle of the mold and a "sinker" that causes a recess in the molded body are likely to occur.

Further, when the amount of binder added is increased to improve the fluidity, a phenomenon that the molded body is collapsed by its own weight during the step of heating and removing the binder is likely to occur.

Conventionally, a thermoplastic resin such as polyethylene or polystyrene has been used as a binder.

All of these resins have a low softening temperature and good injection moldability, and polyethylene is particularly excellent.

However, it is difficult to eliminate the phenomenon that the molded body collapses due to its own weight during the binder removal process.

Therefore, although a resin having a high viscosity even at the thermal decomposition temperature, such as polymethylmethacrylate (abbreviated as PMMA), is used, there is a problem that short shots or "sinks" easily occur due to poor fluidity.

[Problems to be solved by the invention]

As described above, the necessary condition for the binder used to sinter the metal powder or the ceramic powder to produce the sintered body is that the formability and the binder removal property are excellent. The problem is that there is no material that can be fully filled.

[Means for Solving the Problems] The above problem is that the powder to be molded is coated with a polymer resin that is not compatible with polyethylene and has a higher decomposition temperature than the polyethylene, and the softening temperature of the polymer resin is equal to or lower than the softening temperature. It can be solved by adopting a manufacturing method in which polyethylene is coated by kneading at a temperature equal to or higher than the melting temperature of polyethylene to form a molding target powder having a double coating layer, and the resin coating powder is injection molded. .

[Action]

The present invention does not use a single binder, the viscosity is high even at the thermal decomposition temperature of the resin, the binder is less likely to collapse due to its own weight even in the binder removal step, the softening temperature is low, and the viscosity when melted is low, Therefore, the conventional problem is solved by using a binder having a good injection moldability in a two-layer structure.

That is, after coating a resin having a high viscosity even in a molten state and which does not easily collapse due to its own weight on a metal powder or a ceric powder, a double layer coated with a resin having a low viscosity when melted and having good injection moldability All the necessary conditions are satisfied by applying the coating.

Here, the present invention uses a low molecular weight polyethylene, which is conventionally known as a resin having good injection moldability, as a binder used for the upper layer.

Next, the binder used for the lower layer needs to be insoluble in polyethylene during injection molding, has no compatibility,
Moreover, even in the molten state, a high-viscosity resin is required to prevent collapse, and PMMA, acrylonitrile acrylate styrene (abbreviation AAS), polystyrene (abbreviation PS), etc. were selected as resins that meet this condition.

In the present invention, the powder to be molded having such a double coating layer is injection-molded under exactly the same conditions as the conventional method using polyethylene as a binder.

Next, as the binder removal method, the molded body is embedded in a metal powder or ceramics powder having good thermal conductivity, and heated at a temperature not lower than the softening temperature of polyethylene and not higher than the softening temperature of the high-viscosity resin forming the lower layer, After melting polyethylene as much as possible, the temperature is raised to the thermal decomposition temperature of the high-viscosity resin to decompose the high-viscosity resin.

By doing so, since the molded body made of the powder to be molded becomes porous in the melting process of polyethylene, cracks and "blisters" are less likely to occur, and therefore the temperature can be raised faster than before. it can.

Next, in order to carry out the present invention, a method of forming a double coating layer becomes a problem, but in carrying out the present invention, the content of the resin with respect to the total volume is preferably 40 to 50% by volume, and the content of the resin in the coating resin is 40% by volume. Polyethylene with an average molecular weight of 5,000 or less is preferable, and the content of polyethylene in the resin is 60 to 90.
Volume% is good.

 The reason for this will be described below.

First, regarding the average molecular weight of polyethylene used, since the softening temperature and viscosity increase in proportion to the average molecular weight, it is necessary to use a polyethylene having an average molecular weight of 5000 or less as the polyethylene for the double coating layer according to the present invention. .

That is, when a binder removal experiment was conducted for four kinds of binders having an average molecular weight of 1500, 3500, 5000, 7000, most of polyethylene was eluted in the filler in the sample using polyethylene having an average molecular weight of 5000 or less as the binder. On the other hand, when the average molecular weight is 7,000, a considerable amount remains because of the high viscosity.

From these, polyethylene with an average molecular weight of 3000 was used, iron (Fe) with a particle size of 44 μm or less was used as the powder to be molded, and PMMA was used as a resin with high viscosity and high decomposition temperature. Use 10 to 30% of the amount of binder to the total amount of powder to be molded and binder.
Four kinds were taken in the range of 60% by volume, and the content of polyethylene was changed to 50 to 100% by volume at 10% intervals for each kind, and samples were made to examine the quality of the moldability and the binder removal property.

 Figures 1 and 2 illustrate this result.

That is, FIG. 1 shows the effect of the addition amount of the binder on the total amount of the injection-molded article.
The following compacts have many defects such as short shots, “sinks”, and bubbles. From this, the amount of binder added is not appropriate below 30% by volume.

Samples with 60% by volume added were polyethylene and PMMA.
In all of the samples with different volume ratios, collapse, cracking and swelling of the molded body occurred, and from this point, 60% by volume or more is inappropriate.

From the above, an appropriate binder addition amount is in the range of 40 to 50%.

Next, FIG. 2 shows the sample used in this experiment for the polyethylene added volume%. The result of 60 volume% in the figure means that the binder added amount is 30-60% as shown in FIG.
The result of each sample changed to is shown.

From the results of the experiment, defects such as short shots, “sinks”, and air bubbles frequently occurred in the molded product containing 50% by volume of polyethylene, and therefore it is unsuitable as the amount of polyethylene added.

Next, for 100% by volume, the binder is polyethylene only, but it is unsuitable because the molded body is destroyed by the binder removal treatment.

On the other hand, those having 60 to 90% by volume have good moldability and good binder removal property.

〔Example〕

Using Fe powder having a particle size of 350 mesh or less as a powder to be molded, with respect to 100 parts by weight of this, take 3.0 parts by weight of PMMA,
It was dissolved in methyl ethyl ketone (abbreviated as MEK), added, mixed in a ball mill for 24 hours, and dried by a spray dryer to coat PMMA with Fe powder.

9.5 parts by weight of polyethylene having an average molecular weight of 3000 is added to this, and PMM is added at a polyethylene softening temperature (90 to 100 ° C) or higher.
The mixture was kneaded at 120 ° C, which is lower than the softening temperature (140-150 ° C) of A, to make pellets with a particle size of about 2 mm.

When this composition ratio is converted into volume%, the added amount of binder is 50% by volume, and the ratio of polyethylene in the binder is 80% by volume.

This was injection-molded to make a molded body used for a hammer of a line printer.

Next, the molded body is immersed in a filler made of aluminum nitride (AlN) powder, and is kept at a temperature of 120 ° C. to elute polyethylene, and then at 20 ° C./in a nitrogen (N ₂ ) atmosphere.
The binder was removed by raising the temperature to 450 ° C under the above conditions.
No defects such as collapse, cracking and swelling were observed.

Next, the binder-removed body was subjected to 1400 in a hydrogen (H ₂ ) atmosphere.
It was possible to obtain a good hammer without cracks by performing the sintering treatment at ℃.

〔The invention's effect〕

By implementing the present invention, there is no occurrence of short shots or “sinks” during injection molding, no defects such as breakage, cracks and swelling occur during the binder removal processing, and the binder removal processing takes less than half the time compared with the conventional method. It became possible to do.

[Brief description of drawings]

FIG. 1 is a relational diagram of the addition amount of the binder with respect to the total amount of the injection molded body, and FIG. 2 is a relational diagram of the addition amount of polyethylene in the total amount of the binder.

Claims (2)

[Claims] 1. A temperature not higher than the softening temperature of the polymer resin but not lower than the melting temperature of the polyethylene after coating the powder to be molded with the polymer resin which is incompatible with polyethylene and has a higher decomposition temperature than the polyethylene. 2. A method for producing an injection-molded article, comprising: kneading with polyethylene to coat polyethylene to form a powder to be molded having a double coating layer, and injection-molding the resin-coated powder. 2. A resin-coated powder having a double coating layer made of a polymer resin, wherein the content of the resin is 40 relative to the total volume.
Claims 1 to 50% by volume, wherein polyethylene having an average molecular weight of 5000 or less is used as the polyethylene in the coating resin, and the content in the total resin is 60 to 90% by volume. A method for producing an injection-molded article according to item.