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Factors affecting dimensional accuracy of metal injection molding

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-07-08 08:44:00
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1.Effect of feeding on dimensional accuracy of metal powder injection molding products 

When designing feeding system suitable for metal injection molding, it is often necessary to consider not only shape conformation of green part, but also fluidity and formability of feeding material, which are usually contradictory to each other. The binder with good fluidity is not necessarily suitable for shape conformation of green part, and it is same. Therefore, design of feeding system must take both into consideration.

Generally speaking, since loading of feed directly determines shrinkage rate of metal powder injection molding products, the higher loading of feed, the smaller shrinkage of products in process of de-binding and sintering, and the higher dimensional accuracy of products. However, too high loading will also lead to decrease of feeding fluidity or formability, which will lead to bubble during injection. In process of de-binding and sintering, increase of gas pressure in these voids will lead to cracks in part. Low loading capacity will also lead to deformations such as collapse during de-binding or sintering. Therefore, when selecting powders and adhesives, loading capacity of feeding materials should be increased to maximum extent without reducing fluidity and formability. The preparation of uniform feed is a key to obtain high-density powder injection molding products. If feeding mixture is not uniform, some defects such as deformation and shrinkage will occur, which will increase size deviation of final parts. 

2.Injection Influence on precision of metal injection molding products

Injection is a key process of metal injection molding, it directly determines part size, has an important influence on precision control of metal injection molding product. Melt feeding maintain steady flow is the key to success of injection, and others like injection temperature, pressure and holding time has important effects on quality. Improper control of these parameters will lead to inner hole, crack, flow grain, black grain, low shrinkage and density defects.

The influence of injection pressure is mainly manifested in the mold filling rate and residual stress inside the blank after injection, while small influence on viscosity of feed. High injection pressure and low injection temperature can improve product dimensional accuracy.

The increase of injection temperature can reduce viscosity of feed, which is beneficial to steady mold filling, thus improving feed molding. However, the increase of the injection temperature will prolong the cooling time of the injection blank and increase injection time. At the same time, when the injection temperature is too high, the feed will flow out automatically, which will reduce the injection feed quantity and affect the blank density. In addition, high injection temperature will lead to more residual stress in the green part, which will lead to uncontrollable crack during debinding and increase the difficulty of subsequent process control.

3.De-binding effect on metal injection molding product precision

In de-binding process of metal injection molding, adhesive that accounts for 30 ~ 45% of part volume will be removed. If not properly controlled, product will crack, warping and have other deformation relative to sintering process. The dimensional deformation generated in de-binding process cannot be eliminated by later process. So how to control debinding products deformation becomes an important factor affect final size precision, de-binding methods have solvent de-binding, thermal de-binding, such as catalysis, siphon de-binding, catalytic debinding is the most commonly used, catalytic debinding shrinkage rate is less than 1%.  Different debinding way affect precision of de-binding, but after sintering, product size difference is small.

4.Sintering influence on dimensional accuracy of metal injection molding products

Sintering is the last step of powder injection molding and the main stage of solid or liquid phase moving shrinkage densification of metal powder. During this process, product will have a shrinkage rate of about 13-18%, so sintering is particularly important for control accuracy. In sintering process, another factor that affects size accuracy is uneven size change.

Keywords in the article: metal injection molding
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