JPH0445202A - Powder injection molding - Google Patents

Abstract

PURPOSE:To prevent the deterioration of dimensional accuracy caused by shrinkage by using a jig having small contacting area with a work and sintering the work under condition of hanging the work in the air. CONSTITUTION:In the sintering treatment of the work 10 subjected to injection- compacting of powder and debinder treatment, by utilizing bolt hole 12, etc., in the work 10, jigs 14 are inserted. At one end of the jig 14, an extending diameter part 16 for supporting the lower end part of a bolt hole 12 in the work 10 is arranged and at the other end thereof, the through hole 20 detaining a wire 18 for hanging is arranged. The work 10 is sintered under condition of hanging down from a bar 22 in a furnace through the jigs 14 inserted in the bolt holes 12 and the wires 18 for hanging. The work 10 itself can maintain good roundness without receiving friction force from furnace floor surface at the time of shrinking.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a powder injection molding method in which powder is mixed with a binder, injection molded, and then processed into a product through binder removal treatment and sintering treatment.

[Conventional technology]

Metal powder injection molding is attracting attention as a so-called fifth-generation manufacturing method, following m-machining, tie casting, powder metallurgy, and lost wax. Conventional machining methods require too many machining processes when the shape of the part is complex, and methods using powder metallurgy are not suitable for mass production, but are suitable for parts with complex shapes. It has the disadvantage of being stiff. Furthermore, the die casting method has the disadvantage that only low melting point metals such as aluminum alloys and zinc alloys can be used as raw materials, and the lost wax method has the disadvantage of being inferior in mass productivity. Injection molding using metal powder was introduced to compensate for these conventional drawbacks. This metal powder injection molding method uses metal powder (average particle size 10
AJ, n degree) and an organic binder are kneaded (heated or at room temperature) and homogenized, then granulated using a beretizer etc. to form a molding material, and this molding material is injection molded using an injection molding machine. That is what it is. A molded body formed by an injection molding machine is generally called a green part (green molded body). Clean parts can be heated, depressurized, pressurized,
The binder is removed (degreased) by extraction and supercritical methods. The degreased body from which the binder has been removed is generally called brown parts. Brown parts have a scum atmosphere (N2, N2, N H3
, vacuum). Sintered bodies are generally called white parts. The white parts are manufactured after undergoing post-processing as necessary and undergoing predetermined inspections. This manufacturing method using injection molding of metal powder is especially suitable for manufacturing small, complex-shaped mass-produced products! It is said to be a promising method, and has attracted much attention in recent years with the aim of putting it into full-scale practical use.

[Problem to be solved by the invention]

However, in this injection molding method, since the metal powder is injection molded, it is necessary to give the metal powder appropriate fluidity and moldability, and for this reason, as mentioned above, an organic resin is used as a binder. There are circumstances in which it is necessary to contain a large amount of. Since this binder is unnecessary as a component of the final product, it is removed by a binder removal process, but as a result, gaps are formed there due to the removal of the binder. Since this gap shrinks during the next sintering process, the outer diameter of the workpiece ends up shrinking considerably (approximately 15 to 25%). As a result, the floor on which the work is placed during the contraction (fixed)
Frictional force is generated between the workpiece and the shrinking workpiece, and this causes slight variations in the direction of shrinkage, resulting in uneven shrinkage and a problem of poor dimensional accuracy (particularly roundness). The present invention was made in view of such conventional problems, and prevents deterioration of dimensional accuracy due to shrinkage in the sintering process, thereby producing sintered crystals (injected crystals) with higher dimensional accuracy. The object of the present invention is to provide a powder injection molding method that can produce molded products.

[Means to resolve division conflicts]

The present invention is a powder injection molding method in which powder is mixed with a binder and injection molded, and the product is manufactured through a binder removal process and a sintering process. The above object is achieved by carrying out the work with the work suspended in the air.

[Effect]

In the sintering process of the injection molding method, the shrinkage rate of the workpiece is very large because the binder is removed in the previous stage.As a result, as shown in Figure 3, the shrinkage rate of the workpiece is very large.
When contracting, frictional forces F1 and F2 are generated between the workpiece 4 and the surface 2A of the floor 2 in the furnace, which is in a fixed state. This frictional force F+ (or F2) poses no particular problem if it is generated completely uniformly in the contraction direction, but in reality, the vertical load on the floor 2 is applied at each position on the contact surface with the floor 2. This does not necessarily occur uniformly because the coefficient of friction between the floor 2 and the workpiece 4 at the contact surface varies slightly. Therefore, during the sintering process, variations in the frictional force cause variations in the shrinkage rate, which manifest as dimensional errors. This effect is particularly noticeable in the case of cylinders or cylindrical products that require roundness. Therefore, in the present invention, a jig with a small contact area with the workpiece 4 is used to separate the workpiece 4 itself from the floor 2 of the furnace and suspend it in the air for sintering. As a result, it becomes possible to suppress as much as possible the variation in shrinkage that occurs due to variation in the frictional force, which mostly acts on the workpiece 4 during sintering, and it is possible to obtain a product with extremely high dimensional accuracy.

【Example】

Embodiments of the present invention will be described in detail below based on the drawings. First, the general process of injection molding of metal powder will be explained using FIG. A binder (for example, a resin such as polystyrene or polyethylene) is added to the metal powder (e.g., iron) to give it fluidity and moldability, and it is usually kneaded after being heated to a temperature higher than the melting temperature of the binder. . After the kneading process, the kneaded material is frozen, and then crushed into a size that can be injection molded using a crusher or pelletizer, etc. to form fine pellets (granulation process), and the pellets are injection molded using an injection molding machine. , to obtain green parts (green molded bodies). Since green parts contain unnecessary binder, the included binder is removed (degreased) in a binder removal process. - For boat fishing, green parts are placed in a furnace called a binder removal furnace, and the temperature is gradually raised to the binder decomposition temperature in an oxidizing or non-oxidizing atmosphere to decompose and volatilize the binder. A brown part (degreased body) is obtained by the binder removal process. Brown parts have many gaps where the binder is removed, and the existence of these gaps is the main cause of shrinkage during sintering. The brown parts are placed in a sintering furnace (firing furnace) and sintered (
The shrinkage rate at this time can reach as high as 15% to 25%, although it varies depending on the binder composition. Conventionally, this shrinkage causes frictional force to be generated between the workpiece and the hearth surface, and variations in this frictional force cause variations in the shrinkage rate, which induces dimensional errors. Therefore, in this embodiment, the occurrence of this problem is prevented by performing sintering using the method shown in FIGS. 1 and 2. FIGS. 1 and 2 are perspective views showing an embodiment of the present invention. In FIG. It has a thick cylindrical shape with a low wall thickness t, and if it is sintered while being placed on the floor of the furnace as in the past, the frictional force generated between it and the hearth surface will be reduced. It has a shape that is very susceptible to uneven shrinkage due to influences. Therefore, the jig 14 is inserted from below using the bolt holes 12 and the like of this work 10. This jig 14 has an enlarged diameter part 16 at one end (the lower end in the figure) that can support the lower end of the bolt hole 12 of the workpiece 10, and the other end (the upper end in the figure) for hanging. wire 18
A through hole 20 for hanging is provided. The workpiece 10 is inserted into the bolt hole 12 using a jig 14.
The material is sintered while being suspended from a crowbar 22 in a furnace via a hanging wire 18. Therefore, when the workpiece 10 itself contracts, it does not receive any frictional force from the hearth surface.
Good roundness can be maintained. In addition, in the above embodiment, the bolt hole 1 of the workpiece 10
The jig 14 was inserted from below using a second or the like, and the workpiece 10 was suspended via the hanging wire 18.
In the present invention, the method of hanging the workpiece itself is not limited to this, and it is of course free to change the hanging method as appropriate, taking into account the shape of the workpiece or the surfaces that are important for the product and the surfaces that are not. There is. The present invention is applicable not only to powder injection molding of metals but also to powder injection molding of ceramics.

【Effect of the invention】

As explained above, according to the present invention, it is possible to effectively prevent the problem of uneven shrinkage caused by the frictional force generated due to shrinkage of the workpiece during sintering, which reduces the dimensional accuracy of the product. You can get the relaxing effect of being able to do things.

[Brief explanation of drawings]

Fig. 1 is a perspective view showing an embodiment of the present invention, Fig. 2 is a perspective view showing the jig shown in Fig. 1 taken out, and Fig. 3 is a perspective view showing a workpiece and a floor surface due to shrinkage during sintering. FIG. 4 is a schematic cross-sectional view for explaining that frictional force is generated between the two. FIG. 10... Work (degreased Nibrown parts), 14.
... Jig, 18... Hanging wire, 22... Crowbar. Sub-agents Tsuyoshi Makinoyama and Satoshi Hiroshi Keisuke

Claims (1)

[Claims] (1) In a powder injection molding method in which powder is mixed with a binder and injection molded, and the product is manufactured through a binder removal process and a sintering process, the sintering process is performed using a jig with a small contact area with the workpiece, A powder injection molding method characterized in that the workpiece is suspended in the air.