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MIM parts manufacturability design (process design & sintered post processing)-(2)

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1.7 holes and slots

In addition to reducing part quality and forming uniform wall thickness, holes and slots are a useful functional structural feature of MIM parts, and generally not increase part prices.

However, adding holes and slots will increase complexity of mold, which will increase mold cost. Hole perpendicular to parting line is easiest to form and with lowest cost. But hole parallel to parting line, although easy to form, but need to increase slider or hydraulic cylinder, which will most increase cost of mold manufacturing.

Internal connecting holes can be shaped, and this design must be carefully considered to prevent sealing and burring.

If possible, a hole should be made into a D shape hole to make a plane on core rod, so as to enhance mold sealing. Otherwise, matching parts should be made into arc surface, and its thin edge will abnormal wear.

1.8 root cutting

It is easy to form with external root cutting parting line on open mould. Making this shape requires increasing mould parts, increasing mould cost and reducing productivity. Some internal root cuts can be made with sliders, others with movable cores. In most MIM part’s design, designer may decide to eliminate internal root cuts due to add costs and possible flash edges.

1.9 casting system

Injection material enters mold cavity through gate, which is generally much larger than plastic injection molding due to high metal content of MIM injection material.

Because gate usually leaves a trace where finished part is removed from molding cavity, gate setting needs to balance required craftsmanship, function, dimension control and aesthetics.

Sprue is preferably arranged on parting line of mold so that injection material flowing path impinges on cavity wall or core rod.

In addition, for parts with different wall thickness, gate is usually set at thickest section so that injection material flows from thick section to thin section. In this way, sprue can eliminate holes, grooves, stress concentration and streamline on surface of parts.

If you want to produce parts with multiple cavities, you must also take into account size and setting of gate, so as to ensure that injection material supplied to each cavity is same with a balanced filling rate.

1.10 parting line

If possible, all structural features should be oriented perpendicular to parting line to allow forming part to be removed from mold.

Generally, parting line is changed into demonstration line of part surface, which is inevitable result of combination of two half mold.

Geometry of whole part is formed in upper half of mold. In this case, parting line can only be along bottom edge of part, and no parting line is generated. Typically, a model can be designed to be unnoticeable edge, thereby "hiding" parting line.

Parting line is best located on a flat surface, but sometimes in order to form desired structural features simple shapes must be changed.

Increasing complexity of parts may increase cost of making and maintaining mold, but cost of molding may be reduced, otherwise, machining or assembly operations are required.

1.11 Support of sintering part

In process of de-binding and sintering, MIM green parts shrinks by about 20%. In order to minimize distortion, MIM parts must be properly supported during sintering. Typically, MIM parts are placed on flat ceramic plates or trays. It is desirable to design sintered plate or tray to have a large plane or common to several structural features so that standard brackets can be used. Long-span, cantilevered or vulnerable MIM parts may need to be supported by special brackets or fixtures. These production costs are very high.

Keywords in the article: MIM parts injection molding
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