JPH08311503A - Method for recycling compound for injection molding of metallic powder - Google Patents

Abstract

PURPOSE: To improve the molding accuracy of the injection molding of metallic powder formed by using waste materials. CONSTITUTION: The waste materials generated at the time of injection molding are reused by mixing the waste materials and virgin materials in such a manner that the weight ratio of the virgin materials and the waste materials once used for molding attains >=75wt.% of the wt.% of recycling materials to be used in the next injection molding at the time of repetitively using the waste materials by mixing the waste materials with the virgin materials in production consisting of the step of injection molding the compound prepd. by mixing metallic powder and an org. binder and forming green parts having desired shapes, the step of degreasing the green parts to form brown parts and the step of firing the brown parts to form sintered compacts. Since the compsn. is approximated to the compsn. of the compd. consisting of only the virgin materials, the molding accuracy is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the production of a metal powder sintered body, and more particularly to a recycling method in which waste materials generated during injection molding are repeatedly used for molding.

[0002]

2. Description of the Related Art A compound used in a metal powder injection molding method (MIM) contains several kinds of organic binders with respect to a metal powder, as disclosed in Japanese Patent Laid-Open No. 4-337006. Each of the organic binders has properties such as "fluidity", "shape retention", and "releasability", and is inevitable for the production of a metal powder sintered body.

By recycling such a compound, the raw materials used are saved. In order to reuse this compound, conventionally, the waste material generated at the time of injection molding was crushed, the crushed compound was put into the injection molding machine again, and the molding was repeated while correcting the change in the physical properties under the molding conditions.

[0004]

However, in the conventional recycling, the size of the sintered body gradually increases with an increase in the number of times of recycling, which may deviate from the dimensional tolerance or significantly reduce the dimensional accuracy. It was This is because the recycling of waste materials lowers the average molecular weight of the binder component in the compound, increasing the fluidity of the compound, changing the filling properties of the compound into the cavity during injection molding, and filling during injection molding. This is a big factor. Incidentally, in the past, the recycled material was used as a mixture with the virgin material, but in this case as well, there was the same problem as described above depending on the mixing ratio of the recycled material and the virgin material.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and there is no dimensional change of the sintered body due to the recycling of the compound, and the dimensional accuracy is formed only by the virgin material. An object of the present invention is to provide a recycling method capable of stably molding a metal powder sintered body equivalent to the above case.

In the present invention, when a waste material generated during injection molding and a virgin material are mixed and repeatedly reused as a recycled material, the weight ratio of the virgin material to the waste material used once for molding is as follows. It is mixed and reused so as to be 75 wt% or more with respect to the recycled material used for injection molding.

FIG. 1 shows changes in the average molecular weight of the binder in the compound due to recycling. according to this,
The average molecular weight of the binder decreases with the number of recycles, and the amount of change is particularly large when the number of recycles is small. This is because when the compound is plasticized in the cylinder of the molding machine during injection molding, heating and shearing force are applied to the compound and the molecular structure of the binder in the compound is cut. Due to the decrease in the molecular weight of the binder, the viscosity of the binder decreases, and along with this, the fluidity of the compound (melt index value: M1 value) increases as shown in FIG.

Injection molding is carried out using these recycled materials to obtain a molded product (green part) as shown in FIG. 3, which is degreased and sintered, and the A dimension is measured, as shown in FIG. As the size increases, the dimensional variation (σ _n-1 ) also increases as shown in FIG. The result of measuring the ratio of the metal powder and the binder in such a green part is shown in FIG. As shown in the figure, it can be seen that the ratio of the metal powder is higher as the number of times of recycling is higher. This is because the viscosity of the binder is reduced, the metal powder and the binder are separated during molding,
This is because the metal powder is filled in the cavity first.

For this reason, when the compound is recycled and used, it is necessary that the viscosity of the binder is not lowered by recycling and the molecular weight is not lowered. However, since the recycle always passes through the cylinder of the molding machine at least once, the molecular weight cannot be lowered, and the virgin material can be used as the recycled material regardless of the operation conditions of the metal powder injection molding method. It is necessary to mix the materials to ensure the same quality as the processing using the virgin material.

In the present invention, the compound which has been used only once in the injection molding and the virgin material are mixed so that the weight ratio of the total to the recycled material is at least 75 wt%, and the mixture is used in the next injection molding. Is.

When processing is performed using only the recycled material without using the virgin material, the dimensions and the dimensional variations become large as shown in FIGS. The change from molding is not so big. For example, as shown in FIG. 4, the change rate is 0.
It is about 25%. However, the average value of the dimensions certainly increases, which is not preferable for the stability of product dimensions.

On the other hand, when the recycled material and the virgin material are mixed and used, the gradient of dimensional change becomes gentle.
This is because the addition of the virgin material can mitigate the decrease in the binder molecular weight. In this case, the total amount of the virgin material and the compound that has only been used once for molding is at least 75 wt% of the recycled material.
Mix so that it becomes%. FIG. 7 shows the change in molecular weight when molded under such conditions. Characteristic curve a in the figure is the case of molding under this condition, characteristic curve b is the case of molding using 100% recycled material, and characteristic curve a was the first time recycled even if the decrease in molecular weight was poor. In the case of recycled materials, the molecular weight (about 80% for virgin materials)
%) Or higher and the molecular weight converges. As a result, the change in the size of the sintered body due to recycling is extremely small, and further, it is extremely stable with the stabilization of the molecular weight.

[0013]

【Example】

(Example 1) SUS316L of water atomized powder having an average particle diameter of 8 μm was used as the metal powder, and polystyrene (PS), acrylic (PMMA), wax, and EVA were used as the organic binder. This organic binder is a metal powder 1
11 parts by weight was mixed with 00 parts by weight to prepare a compound for injection molding, and the green part shown in FIG. 3 was injection molded. Then, the waste material generated at this time was pulverized, the same amount of virgin material was added and mixed, and the injection molding using this compound was repeated 5 times. less than,
The weight ratio in this operation will be described.

The waste material generated by the first recycling includes the material used twice for injection molding and the material used for injection molding.
The material that has been reused is mixed, and the ratio is 1: by weight.
It is 1. The same amount of virgin material as this is added to this waste material, mixed and used for the second recycle molding. At this time, the weight ratio of the material used twice for molding, the material used once for molding, and the virgin material is 1: 1: 2.
Therefore, the total of the material used once for molding and the virgin material is 75% by weight of the entire molding material used for the second recycling molding.

The waste materials generated by the second recycling include those used three times for molding, those used twice for molding, and those used once for molding. Is 1: 1: 2. The same amount of virgin material as this is added to this waste material, mixed and used for the third recycling molding. At this time, the weight ratio of the material used three times for molding, the material used twice for molding, the material used once for molding, and the virgin material is 1: 1: 2: 4. Therefore, the total of the material used once for the molding and the virgin material is 75% by weight of the entire molding material used for the third recycling molding.

The waste materials generated by the third recycling include those used four times for molding, those used three times for molding, those used twice for molding, and those used once for molding. They are mixed, and their weight ratio is 1: 1:
2: 4. The same amount of virgin material as this is added to this waste material, mixed and used for the fourth recycling molding. At this time, the weight ratio of the material used four times for molding, the material used three times for molding, the material used twice for molding, the material used once for molding, and the virgin material was 1: 1:
It is 2: 4: 8. Therefore, the total of the material used once for molding and the virgin material is 75% by weight of the entire molding material used for the fourth recycling molding.

After performing recycle molding 5 times under the above conditions, all green parts were degreased in the atmosphere, and the obtained brown parts were heated to 1000 ° C. for 10 ⁻⁴ to 10 ^−5.
Sintering was performed in a vacuum of about Torr up to 1320 ° C. under 5 Torr in an Ar atmosphere. This sintered body was subjected to burr treatment by barrel polishing, and then its dimensions were measured.
The measurement result is shown in FIG.

(Comparative Example 1) In Example 1, the ratio of the virgin material used for the second and subsequent recycling moldings to the waste material used for the first molding was 6% of the total recycled material for that cycle.
5% by weight, processed under the same conditions, and measured for dimensions. The result at this time is shown in FIG.

(Example 2) Metal powder having an average particle size of 8 μm
m, water atomized powder SUS630, polystyrene (PS), acrylic (PMM) as organic binder
A), wax and EVA were used. 8 parts by weight of this organic binder was mixed with 100 parts by weight of metal powder to prepare an injection molding compound, and the green part shown in FIG. 3 was injection molded. Then, the waste material generated at this time was pulverized, the same amount of virgin material was added and mixed, and the injection molding using this compound was repeated 5 times. In each recycle molding, the total amount of the waste material and the virgin material used in the molding once was adjusted to be 75% by weight of the whole recycled material used in the molding in that time.

All the green parts molded as described above were degreased in the air, and the obtained brown parts were
In a vacuum of about 10 ^-4 to 10 ^-5 Torr up to 000 ° C,
Thereafter, it was sintered up to 1320 ° C. under 5 Torr in an Ar atmosphere. This sintered body is subjected to burr treatment by barrel polishing,
After that, the dimensions were measured. The measurement result is shown in FIG.

(Comparative Example 2) In Example 2, the ratio of the virgin material used for the second and subsequent recycling moldings to the waste material used for the first molding was 7% of the total recycled material for that cycle.
The amount was set to 0% by weight, processing was performed under the same conditions, and the dimensions were measured. The result at this time is shown in FIG.

(Embodiment 3) Metal powder having an average particle size of 8 μm
m, water atomized powder SUS316L, polystyrene (PS), acrylic (PMM) as organic binder
A), wax, EVA and polypropylene (PP) were used. 11 parts by weight of this organic binder was mixed with 100 parts by weight of metal powder to prepare an injection molding compound, and the green part shown in FIG. 3 was injection molded. Then, after pulverizing the waste material generated at this time, 1.5 times the weight of the waste material was added and mixed, and the operation of injection molding using this compound was repeated 5 times. In each recycle molding in this example, the total of the waste material and the virgin material used for molding once is 84% by weight of the whole recycled material used for molding at that time.

All the green parts molded as described above were degreased in the atmosphere, and the obtained brown parts were
In a vacuum of about 10 ^-4 to 10 ^-5 Torr up to 000 ° C,
Thereafter, it was sintered up to 1320 ° C. under 5 Torr in an Ar atmosphere. This sintered body is subjected to burr treatment by barrel polishing,
After that, the dimensions were measured. FIG. 10 shows the measurement result.

(Comparative Example 3) In Example 3, the ratio of the virgin material used for the second and subsequent recycling moldings to the waste material used for the first molding was 6% of the total recycled material for that cycle.
The amount was set to 0% by weight, processing was performed under the same conditions, and the dimensions were measured. The result at this time is shown in FIG.

(Embodiment 4) Average particle size of 10 as metal powder
μm, gas atomized powder SUS630 was used, and polystyrene (PS), acrylic (PM
MA), wax and EVA were used. 6 parts by weight of this organic binder was mixed with 100 parts by weight of metal powder to prepare an injection molding compound, and a green part shown in FIG. 3 was injection molded. Then, after crushing the waste material generated at this time, 1.2 times the weight of the virgin material is added and mixed, and the injection molding is performed using this compound.
Repeated times. In each recycle molding in this example, the total of the waste material and the virgin material used for the molding once is about 80% by weight of the whole recycled material used for the molding for that time.
Has become.

All the green parts molded as described above were degreased in the air, and the obtained brown parts were
In a vacuum of about 10 ^-4 to 10 ^-5 Torr up to 000 ° C,
Thereafter, it was sintered up to 1320 ° C. under 5 Torr in an Ar atmosphere. This sintered body is subjected to burr treatment by barrel polishing,
After that, the dimensions were measured. FIG. 11 shows the measurement result.

(Comparative Example 4) In Example 4, the ratio of the virgin material used for the second and subsequent recycling moldings to the waste material used for the first molding was set to 5 of the entire recycled material for that time.
5% by weight, processed under the same conditions, and measured for dimensions. The result at this time is shown in FIG.

In the above FIGS. 9 to 11, the characteristic curve "-○-" indicates the change in the A dimension in each embodiment and the characteristic curve "-○-"
◇ ─ ”shows the change in A dimension in each comparative example. The characteristic curve indicated by the broken line is the primary approximation line of the dimensions in each example, and the characteristic curve indicated by the alternate long and short dash line is the primary approximation line of the dimensions in each comparative example. From these figures, by making the total ratio of the virgin material contained in the recycled material after the second time and the recycled material used in the molding once more than 75% by weight, the primary approximation of the size of each recycling However, it is clear that the dimensions are almost horizontal and the dimensions do not change. On the other hand, this is 75 wt
It is clear that below%, the dimensions gradually increase. Therefore, when reusing the compound, 2
By setting the ratio of the virgin material contained in the recycled material after the first time and the waste material used in the molding only once to be at least 75% by weight, it is possible to prevent changes in size and accuracy. This becomes more remarkable as the blending ratio of the binder component in the compound is higher.

[0029]

According to the present invention, the total ratio of the virgin material contained in the recycled material after the second time and the waste material used for the molding only once is used at least 75 times of the recycled material.
By adjusting the weight percentage, it is possible to prevent changes in dimensions and precision. Therefore, the compound can be reused while the processing conditions such as molding, degreasing, and sintering are kept constant, and a metal powder sintered body having the same size and accuracy as the case of processing with a virgin material can be molded.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing changes in the average molecular weight of a binder with respect to the number of times of recycling.

FIG. 2 is a characteristic diagram showing changes in the fluidity of the compound depending on the number of times of recycling.

FIG. 3 is a perspective view of an example of a green part.

FIG. 4 is a characteristic diagram of changes in the A dimension with the number of times of recycling.

FIG. 5 is a characteristic diagram showing the relationship between the number of times of recycling and variation in dimensions.

FIG. 6 is a characteristic diagram in which the ratio of metal powder / binder is plotted for each number of recyclings.

FIG. 7 is a characteristic diagram in which the composition at the time of recycling is plotted for each number of times of recycling.

FIG. 8 is a characteristic diagram of Example 1 and Comparative Example 1.

9 is a characteristic diagram of Example 2 and Comparative Example 2. FIG.

FIG. 10 is a characteristic diagram of Example 3 and Comparative Example 3.

FIG. 11 is a characteristic diagram of Example 4 and Comparative Example 4.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tadahiro Nakano 2-43-2 Hatagaya, Shibuya-ku, Tokyo Olympus Optical Co., Ltd.

Claims (1)

[Claims] 1. A step of injection-molding a compound obtained by kneading a metal powder and an organic binder into a green part having a desired shape, a step of degreasing the green part into a brown part, and a firing of the brown part. In the method for producing a metal powder sintered body, which comprises a step of forming a sintered body, the waste material generated at the time of injection molding and the virgin material are mixed and repeatedly reused as a recycled material, the virgin material and The weight ratio to the waste material used once for molding is 7 for the recycled material used for the next injection molding.
A method for recycling a compound for metal powder injection molding, which comprises mixing and reusing so as to be 5 wt% or more.