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Manufacturability design of MIM parts (process design & post-sinter process)(1)

Categories: ZCMIM NewsStars: 3StarsVisit: -Release time: 2019-04-26 08:36:00
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The simplest MIM parts are produced with a cavity formed by the combination of two half mold. Among them, half mold is composed of a core with uniform gap installed in another half mold, and uniform gap is for forming a uniform wall thickness. Mold core is molded inside structural features of part, while cavity is shaped outside structural features t. All structural features must be able to remove from cavity and core by ram.

As complexity of MIM parts increases, sliders, cores, and other tools commonly used in plastic injection molding can be used. When designing MIM parts, following key points must be considered: uniform wall thickness, thickness transition section, core removal hole, de-mould slope, rib and web, chamfering and rounding, thread, hole and groove, root cutting , gating system, parting line, decorative features, sintering support, etc.

1.1 Uniform wall thickness

If possible, wall thickness of entire MIM part should be same. Different thicknesses could cause distortion, internal stress, holes, cracks, and dents. It also causes uneven shrinkage, which affects dimension and control.

Thickness of part is preferably in range of 1.3 to 6.3 mm.

1.2 Thickness transition section

 In some cases, uniform wall thickness cannot be satisfied. A gradual transition between thicknesses should be designed. As figure 1: 

                                                                                                                                                                                       Figure1- Thickness transition

 

1.3 Core hole

The use of core hole reduces cross-section within standard to achieve a uniform wall thickness, reduce material consumption and reduce or eliminate cutting operations. Preferential direction is parallel to the direction of mold, in other words, perpendicular to parting line. Since core rod is supported at both ends, it is preferable to use a through hole without a blind hole, and blind hole uses a cantilever rod.

1.4 Stripping slope

Draft angle is a small angle on surface that is parallel to direction of part’s movement. For core rod, it is particularly precise. Draft angle is used to ejection. It is generally 0.5 to 2 degrees. The actual draft angle increases with depth of hole or recess and part ‘s complexity or number of cores.

1.5 Reinforced ribs and webs

Reinforcing ribs and webs are used to reinforce thinner walls and avoid thick sections. In addition to increasing strength and stiffness of wall thickness, it also improves material flow and limits distortion. Thickness of reinforcing ribs shall not exceed thickness of adjacent wall. Where thicker reinforcing ribs are required, multiple strengthening ribs should be used instead.

                                                                                                                                                                              Figure 2 shows proportion of recommended ribs.


1.6 chamfering and rounding

Chamfering and rounding can reduce stress at intersection of structural features; eliminate sharp corners that may cause cracking and corrosion of structural features, facilitate material flow into model and help parts to draft from cavity, which is beneficial to molding operation. 


Keywords in the article: MIM metal injection molding
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