JPH0941004A - Injection molding method of metallic powder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain sintered compacts having the decreased differences in the density of sintered compacts according to their sections and having good dimensional accuracy by preventing the thermal oxidation decomposition of an org. binder, thereby preventing the deterioration of a compd. SOLUTION: A sample 1 is formed to a quadrangular prism shape having a hole vertically penetrating the sample. The molding of the sample 1 is executed by setting the cylinder temp. of an injection molding machine at 160 deg.C at the highest. The degreasing of the sample is executed under a condition of the max. 350 deg.C in an atm. atmosphere and the sintering thereof is executed under a condition of the max. 1320 deg.C under 5Torr of an Ar atmosphere. Gaseous nitrogen is made to flow continuously at a flow rate of 2 liters per minute from the lower part of the material feed port of the injection molding machine at the time of molding and the occupying ratio of the nitrogen volume in the space part between a heating cylinder and a material supplying section is set at >=90vol.%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention supplies a compound for metal powder injection molding (hereinafter referred to as compound) to an injection molding machine, heats and melts it in a heating cylinder, and injects it into a mold to produce a green body. The present invention relates to a metal powder injection molding method for producing a sintered body through a step of degreasing the green body after obtained and a step of sintering the obtained brown body.

[0002]

2. Description of the Related Art Recently, as a method for mass-producing high-density metal parts with complicated small shapes, for example, Japanese Patent Laid-Open No. 59-22.
9403 (Metal Injection Molding).
Hereinafter, this is referred to as MIM).

The above MIM is obtained by first kneading and pulverizing metal powder and binder, or pelletizing them and supplying them to the injection molding machine as a compound, and heating and kneading in a heating cylinder to form the compound. Obtain a molded body. After that, a degreasing step of removing the organic binder from the molded body is performed, and a sintering step is further performed to obtain a sintered metal product.

Usually, when a compound is heated and melted in a heating cylinder of an injection molding machine, it is carried out in an air atmosphere. Further, even when the compound is recycled and used, the recycled material obtained by crushing the spool runner portion obtained by injection molding is heated and melted again in the air atmosphere and injection molded.

[0005]

However, the above-mentioned prior art has the following drawbacks. That is, the metal powder and the organic binder are contained in the compound as described above. This organic binder undergoes thermal oxidative decomposition when heated and melted to a high temperature in a heating cylinder in an air atmosphere. Along with this, the molecular weight of the organic binder decreases, and the viscosity of the compound decreases. In other words, the compound has deteriorated.

Since the viscosity of the compound is lowered, the metal powder and the organic binder are easily separated when the compound passes through the thin portion when the molded product having the thin portion is obtained. That is, when passing through the thin portion, the organic binder whose viscosity is lower than that of the metal powder is more likely to pass through, and the thin portion or the anti-gate portion passing through the thin portion is more likely to be filled with the organic binder. . In other words, it becomes difficult for the thin portion and the anti-gate portion to be filled with the metal powder.

Therefore, the filling amount of the powder in the green body tends to be different for each part. Due to this, a difference in shrinkage amount at the time of sintering occurs depending on the site, resulting in a difference in wall thickness, width dimension, etc. between the thin portion and the non-gate portion and the gate side portion,
This will affect the dimensions of the sintered body. Further, the relative density of the sintered body also differs due to the difference in the site. From the above, when injection molding is performed by heating and melting in an air atmosphere,
A large difference in the density of the sintered body depending on the site results in a sintered body having poor dimensional accuracy.

Further, when the compound is recycled and used, it is heated and melted at a high temperature at least once in a heating cylinder in an air atmosphere, and this is further heated and melted in an air atmosphere to be molded. The thermal oxidative decomposition of the binder will proceed further. In other words, the compound is more deteriorated. Therefore, as compared with the case where the virgin material is used, the difference in the density of the sintered body depending on the site is large, and a sintered body having poor dimensional accuracy can be obtained.

The object of claim 1 is to prevent the thermal oxidative decomposition of the organic binder to prevent the deterioration of the compound, so that the difference in the density of the sintered body depending on the site is small and a sintered body with good dimensional accuracy can be obtained. A metal powder injection molding method is provided.

[0010]

The invention according to claim 1 heats and melts a metal powder injection molding compound comprising a kneaded product of metal powder and an organic binder in an inert gas atmosphere containing 90% by volume or more of an inert gas. By doing so, injection molding is performed while suppressing thermal oxidative decomposition of the organic binder to obtain a molded body.

The inert gas is supplied to the cylinder of the molding machine and the material supply section to replace the internal atmosphere of these with the inert gas atmosphere. 90% by volume of inert gas
The effect can be expected even when the content is% by volume, but it is more preferably 99% by volume or more. 90% by volume of inert gas
If it is less than the above range, the effect of suppressing the thermal oxidative decomposition of the organic binder cannot be obtained. As the inert gas to be used, nitrogen, helium, carbon dioxide, argon and the like can be considered, but nitrogen is preferable from the viewpoint of cost.

The heating and melting temperature raises the temperature of all parts in the cylinder including the nozzle part of the cylinder. However,
In the vicinity of the material supply portion, the material may be set to a low value in order to prevent the materials from sticking to each other and solidifying to form a bridge. In addition, the range of the inert gas atmosphere is most preferably within the heating cylinder, which is the range of heating and melting, but it is also possible to set the inert gas atmosphere in the hopper that holds the compound until molding, in order to prevent oxidation in the air. It is desirable to reduce it.

In the first aspect of the present invention, as described above, the compound for metal powder injection molding comprising a kneaded product of the metal powder and the organic binder is heated and melted in an inert gas atmosphere containing 90% by volume or more of the inert gas. Thereby, the thermal oxidative decomposition of the organic binder can be suppressed even in the heating cylinder under high temperature. This can prevent a decrease in the molecular weight of the organic binder and prevent a decrease in the viscosity of the compound. That is, deterioration of the compound can be suppressed.

By suppressing the decrease in the viscosity of the compound, when a molded product having a thin portion is obtained, it becomes difficult for the metal powder and the organic binder to separate when the compound passes through the thin portion. As a result, it is possible to suppress the variation in the powder filling amount due to the difference in the parts in the green body. Therefore, since the sintering shrinkage occurs evenly in each part, the dimensional difference does not occur not only in the parts such as the gate side and the non-gate side, but also the deformation of the thin portion can be prevented. Also, with respect to the density of the sintered body, it is possible to obtain a uniform sintered body that does not vary depending on the site.

Even when the compound is recycled and used, the thermal oxidative decomposition of the organic binder can be suppressed both when the virgin material is used and when the recycled material is used, so that the deterioration of the compound is further reduced. Can be suppressed.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION

(First Embodiment of the Invention) FIGS. 1 and 2 show a sample of an embodiment of the present invention, FIG. 1 is a plan view, and FIG. 2 is a side view. The compound used in the embodiment of the present invention was obtained by kneading metal powder of SUS316L and paraffin wax, acrylic, polystyrene, ethylene-vinyl acetate copolymer, dibutyl phthalate, stearic acid as an organic binder in a total of 11 parts by weight. Was used.

Sample 1 used in the embodiment of the present invention
The shape of the green body is a quadrangular prism shape (W5 mm x L12 mm x t2.5 m) having a hole (hollow part) penetrating vertically.
m, hollow portion φ3.5 mm). Sample 1 above
Cylinder temperature of injection molding machine can be up to 1 when molding
The setting was performed at 60 ° C. The degreasing condition was performed in the air atmosphere at a maximum of 350 ° C. The maximum sintering condition is 1320 ℃, A
It was conducted in an atmosphere of 5 Torr.

At the time of molding, nitrogen gas was continuously flowed from the lower part of the material injection port of the injection molding machine at a flow rate of 2 liters per minute, and the nitrogen volume occupation ratio of the space portion in the heating cylinder and the material supply part was set to 99% by volume. Table 1 shows the measurement results of the wall thickness dimensions of the parts A and B and the relative densities of the parts A and B of the sintered body obtained under the above conditions.

[0019]

[Table 1]

From the results shown in Table 1, the difference in wall thickness between the portion A and the portion B was as small as 0.003 mm. Also, A
The difference in relative density between the B and B parts was 0.1%, which was very small.

According to the embodiment of the present invention, a uniform sintered body having no difference in the sintered body size and the sintered body density depending on the site was obtained.

(Embodiment 2) The compound used in the embodiment of the present invention is a metal powder of SUS316L and paraffin wax, acrylic, polystyrene, ethylene-vinyl acetate copolymer as an organic binder,
A mixture obtained by kneading a total of 7 parts by weight of dibutyl phthalate and stearic acid was used.

The shape of the sample used in the embodiment of the present invention is the same as that of the first embodiment of the invention.
When molding the sample, the cylinder temperature of the injection molding machine was set to 160 ° C. at the maximum. The degreasing condition was performed in the air atmosphere at a maximum of 350 ° C. The maximum sintering condition is 1
It was conducted at 320 ° C. in an Ar atmosphere of 5 Torr.

At the time of molding, nitrogen gas was continuously flowed from the lower part of the material injection port of the injection molding machine at a flow rate of 2 liters per minute, and the nitrogen volume occupation ratio of the heating cylinder and the space in the material supply part was set to 99% by volume. Table 1 shows the measurement results of the wall thickness dimensions of the parts A and B and the relative densities of the parts A and B of the sintered body obtained under the above conditions.

From the results shown in Table 1, the difference in wall thickness between the portion A and the portion B was 0.002 mm, which was very small. Also, A
The difference in relative density between the parts B and B was 0%, which was not observed.

According to the embodiment of the present invention, as in Embodiment 1 of the present invention, a uniform sintered body having no difference in the sintered body size and the sintered body density depending on the site was obtained.

(Embodiment 3 of the Invention) The compound used in the embodiment of the present invention is the same as that of Embodiment 1 of the invention.
It is the same material as, but it is recycled. The maximum cylinder temperature of the injection molding machine is 1 for both virgin material and recycled material.
The temperature was set to 60 ° C., and nitrogen gas continuously flowed from the lower part of the material injection port of the injection molding machine at a flow rate of 2 liters per minute to make the nitrogen volume occupation ratio of the heating cylinder and the space part in the material supply part 99% by volume. .

The sample used has the same shape as in the first embodiment of the invention. The degreasing condition was performed in the air atmosphere at a maximum of 350 ° C. Sintering conditions up to 1320 ℃, Ar
It was performed in an atmosphere of 5 Torr. Table 1 shows the measurement results of the wall thickness dimensions of the parts A and B and the relative densities of the parts A and B of the sintered body obtained under the above conditions.

From the results shown in Table 1, the difference in wall thickness between the portion A and the portion B was 0.002 mm, which was very small. Also, A
The difference in relative density between the B and B parts was 0.1%, which was very small.

According to the embodiments of the present invention, similar to the above-mentioned embodiments of the present invention, a uniform sintered body having no difference in sintered body size and sintered body density depending on the site was obtained. It was also confirmed that the effect was even greater when recycled materials were used.

Comparative Example 1 In this comparative example, the compound used in the first embodiment of the invention was used, and the compound was heated and melted in an air atmosphere without using nitrogen gas at the time of molding. Other injection molding, degreasing and sintering are performed according to the first embodiment of the invention.
The same conditions were used. Table 1 shows the results of the measurement of the wall thickness dimensions of parts A and B and the relative densities of parts A and B of the obtained sintered body.

From the results shown in Table 1, the difference in thickness between the A part and the B part was as large as 0.018 mm. The difference in relative density between the A part and the B part was as large as 0.5%. From the above, in this comparative example, a non-uniform sintered body having different sintered body dimensions and sintered body densities was found.

Comparative Example 2 In this comparative example, the compound used in the second embodiment of the present invention was used, and the compound was heated and melted in an air atmosphere without using nitrogen gas at the time of molding. Other injection molding, degreasing and sintering are performed according to the second embodiment of the invention.
The same conditions were used. Table 1 shows the results of the measurement of the wall thickness dimensions of parts A and B and the relative densities of parts A and B of the obtained sintered body.

From the results shown in Table 1, the difference in wall thickness between the A part and the B part was as large as 0.017 mm. The difference in relative density between the A part and the B part was as large as 0.7%. From the above, in this comparative example, a non-uniform sintered body having different sintered body dimensions and sintered body densities was found.

(Comparative Example 3) In this comparative example, the compound used in the first embodiment of the present invention is once recycled. During molding with a virgin material and molding with a recycled material, heating and melting were performed in an air atmosphere without using nitrogen gas. Other injection molding, degreasing and sintering were performed under the same conditions as in the third embodiment of the invention. Table 1 shows the results of the measurement of the wall thickness dimensions of parts A and B and the relative densities of parts A and B of the obtained sintered body.

From the results shown in Table 1, the difference in thickness between the A part and the B part was 0.026 mm, which was very large. The difference in relative density between the A part and the B part was as large as 1.0%. From the above, in this comparative example, a non-uniform sintered body having different sintered body dimensions and sintered body densities was found. Further, when the recycled material is used, the difference in the size of the sintered body and the density of the sintered body is further increased.

[0037]

The effect of claim 1 is that since the organic binder can be prevented from thermal oxidative decomposition and the deterioration of the compound can be prevented, the difference in the density of the sintered body depending on the site is small,
It is possible to obtain a sintered body with good dimensional accuracy.

As another effect, by preventing the metal powder from being oxidized at the time of molding, the inclusion of oxides and porosity in the structure of the sintered body can be reduced, so that the density of the sintered body is improved. In addition, since a proper reaction between oxygen and carbon occurs during sintering, the growth of crystal grains is properly performed, and as described above, the density of the sintered body is also improved, so that Mechanical strength is improved. Further, since a sintered body having few porous portions and proper growth of crystal grains can be obtained, corrosion resistance is also improved. As described above, the present invention also has the effect of improving the mechanical properties of the sintered body.

[Brief description of drawings]

FIG. 1 is a plan view showing a first embodiment of the invention.

FIG. 2 is a side view showing the first embodiment of the invention.

[Explanation of symbols]

 1 sample

Claims (1)

[Claims] 1. A green body prepared by supplying a metal powder injection molding compound comprising a kneaded material of metal powder and an organic binder as a material to an injection molding machine, heating and melting in a heating cylinder, and injecting it into a mold. In a metal powder injection molding method for producing a sintered body through a step of degreasing the green body after obtaining the green body and a step of sintering the obtained brown body, an inert gas containing 90% by volume or more of an inert gas is used. A metal powder injection molding method, characterized in that the material is heated and melted in an active gas atmosphere to perform injection molding.