Design for manufacturability of MIM parts (process design&post-sintering processing) - Part 1

Design for manufacturability of MIM parts (process design&post-sintering processing) - Part 1

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Design for manufacturability of MIM parts

The design of MIM parts is similar to that of plastic injection molding.

Earlier, we introduced the criteria for selecting MIM process. Because it is not limited by the traditional metal forming process, part designers can use new ideas to re-imagine new parts from the beginning, imagine how the production process can reduce the weight of materials, how to combine multiple parts into one part, or how to form functional and decorative features.

In order to make MIM parts (whether newly designed or replacing parts previously produced by other processes) give full play to the advantages of MIM process and enhance the processability in the design process, the following design criteria are proposed and published on CNPIM.COM powder injection molding website.

In this section, some information about the subsequent processing of sintering is also included.

I Technological design

The simplest MIM part is produced with a cavity formed by the plane combination of two half-molds. (Refer to reading: MIM mold structure)

Among them, the half mold is composed of the core with uniform clearance installed in the other half mold, and the uniform clearance is used to form parts with uniform wall thickness.

The core molding is the internal structural features of the part, while the cavity molding is the external structural features of the part.

All structural features of the design must be molded parts that can be released from the mold cavity and solidified from the core with the ejector pin.

When the complexity of MIM parts increases, the slider, core and other tools commonly used in plastic injection molding can be added to form.

While increasing the structural features of the parts, the complexity of the parts increases. At this time, the operation costs of tools and technical equipment related to general and subsequent processing or assembly operations can be eliminated, so that the MIM parts can obtain economic benefits.

At each stage of design, these benefits and costs must be carefully weighed against each other.

When designing MIM parts, in order to fully obtain all the benefits of this process, the following key points must be taken into account: uniform wall thickness, thickness transition section, core removal hole, demoulding slope, reinforcing rib and spoke plate, chamfer and rounding, thread, hole and groove, undercut, pouring system, parting line, decorative features, sintering support, etc.

The following will be described separately.

1.1 Uniform wall thickness

If possible, the wall thickness of the entire MIM part should be the same. Different thickness will cause distortion, internal stress, holes, cracks and dents. In addition, it will lead to uneven shrinkage and affect the dimensional tolerance and control.

The thickness of parts should be within 1.3~6.3mm.

In order to make the wall thickness of MIM parts uniform, Figure 1 shows several commonly used methods to change the shape.

Design for manufacturability of MIM parts (process design&post-sintering processing) - wall thickness uniformity

Figure 1 - Several common methods of changing shape

1.2 Thickness transition section

In some cases, the wall thickness uniformity cannot be met, so the transition between different thicknesses should be designed gradually. As shown in Figure 2

Design for manufacturability of MIM parts (process design&post-sintering processing)

Figure 2 - Thickness transition

1.3 Core removal hole

The use of core-removing holes can reduce the cross section to the limit of the criterion, achieve uniform wall thickness, reduce material consumption and reduce or eliminate cutting operations.

As shown in Figure 3, the preferred direction is parallel to the mold opening direction, in other words, perpendicular to the parting line.

Because the core rod is supported at both ends, it is better to use through hole instead of blind hole. The blind hole uses cantilever rod.

Design for manufacturability of MIM parts (process design&post-sintering processing)

Figure 3 - Core removal hole

1.4 Demoulding slope

The release angle is a small angle on the surface, which should be parallel to the moving direction of the model part.

For mandrels, be particularly precise.

The demoulding slope is for the convenience of demoulding and ejecting the preformed parts. The demoulding slope is generally 0.5~2 degrees. The actual release angle increases with the depth of the formed hole or concave, the complexity of the part, or the number of cores.

Figure 4 shows some cases where the release angle is required.

Design for manufacturability of MIM parts (technological design&post-sintering processing) - draft angle

Figure 4 - Stripping angle

1.5 Stiffening rib and web

Stiffening ribs and webs are used to reinforce thinner walls and avoid thick sections.

In addition to increasing the strength and rigidity of the wall thickness, it can also improve the material flow and limit the distortion.

The thickness of the reinforcing rib shall not exceed the thickness of the adjacent wall. Where thicker stiffeners are needed structurally, multiple stiffeners should be used instead.

Figure 5-1 shows the recommended ratio of stiffeners. Figure 5-2 shows how to use reinforcing ribs and cored holes to reduce weight while maintaining the functional strength of parts.

Design for manufacturability of MIM parts (process design&post-sintering processing)

Design for manufacturability of MIM parts (process design&post-sintering processing)

Figure 5 - Stiffening rib and web

1.6 Chamfer and rounding

Figure 6 shows chamfering and rounding.

Chamfering and rounding can reduce the stress at the intersection of structural features; It eliminates the sharp corners that may lead to the cracking and corrosion of the structural features of the model, facilitates the injection material flowing into the model and helps the parts to come out of the mold cavity, which is conducive to the molding operation.

Design for manufacturability of MIM parts (process design&post-sintering processing)

Figure 6 - Rounding and chamfering

1.7 Thread

Internal and external threads can be formed by MIM process. However, compared with screwing out the core, the thread tapped by tap is more precise and cost-effective.

In order to remove the model components that screw out the formed thread, the external thread of the model part of the formed thread should preferably be located on the parting line of the model construction.

In order to maintain the thread tolerance of thread diameter, it is generally specified that there is a 0.127mm small plane on the parting line, as shown in Figure 7. This can ensure that the model is properly sealed, reduce the trace of the parting line, avoid the burr at the root of the thread, and thus reduce the maintenance of the model.