What You Have to Know Manufacturability Design of MIM Parts

The design of MIM parts is similar to plastic injection molding. Free from the limitations of traditional metal-forming processes, part designers can start by re-imagining new parts from a new point of view, how production processes can reduce material weight, combine multiple parts into a single part, or shape functional and decorative features.

　　1. Technological design

The simplest MIM parts are produced with a cavity formed by a flat combination of two half moulds.(reference: MIM mold structure) 

Among them, the half mold is installed in the other half of the mold with a uniform gap of the core, the remaining uniform gap is in order to form a uniform wall thickness of the parts.

Core forming is the structural characteristics of the parts inside, while cavity forming is the structural characteristics of the parts outside.

All structural features of the design must be formed parts that can be extruded from the cavity and solidified from the core by means of ejectors.

As the complexity of MIM parts increases, sliders, cores, and other tools commonly used in plastic injection molding can be added.

When the structural features of the parts are added, the complexity of the parts increases. At this time, the MIM parts can gain economic benefits by eliminating the operating costs of tools and technical equipment related to the subsequent processing or assembly operations.

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

When designing MIM parts, the following key points must be considered to fully obtain all the benefits of this process: uniform wall thickness, thickness transition section, core-removal hole, stripped-off Angle, stiffening rib and spoke plate, chamfering and rounding, thread, hole and groove, root cutting, casting system, parting line, decorative features, sintered support, etc.

These are described below.

1.1 uniform wall thickness

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

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

1.2 thickness limited

In some cases, the wall thickness cannot be uniform, the different thickness should be designed to gradually transition.

1.3 cored hole

The use of core removal can reduce the cross section to the criterion limit, reach uniform wall thickness, reduce material consumption and reduce or eliminate machining operations.

The preferred direction is parallel to the opening direction, in other words, perpendicular to the parting line. Because the core rod is supported at both ends, so it is best to use through hole, not blind hole, blind hole is used by the cantilever bar.

1.4 stripping slope

The stripper slope is a small Angle on the surface that is parallel to the direction of movement of the model part. For the core bar, to be particularly accurate. The demoulding inclination is for the convenience of the demoulding and ejecting molding parts. The release Angle is usually 0.5 degrees to 2 degrees. The actual demodulation inclination increases with the depth of forming hole or concave and the complexity of parts or the number of cores.

1.5 reinforcing ribs and spokes

Reinforcing ribs and spokes are used to reinforce thinner walls and avoid thick sections.

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

The thickness of the reinforcing ribs shall not exceed that of the adjoining walls. Thick stiffening ribs are required structurally and should be replaced by multiple stiffening ribs.

1.6 chamfering and rounding

Chamfering and rounding can reduce the stress at the intersection of structural features .Eliminate the sharp Angle that may lead to the characteristic cracking and corrosion of the model structure, facilitate the flow of injection material into the model and help the parts to come out of the mold cavity, and facilitate the molding operation.

1.7 thread

Both internal and external threads can be formed by MIM process, but compared with unscrewing the core, tapping threads are more precise and cost effective.

In order to get rid of the model components that screw out the molded thread, the model part of the molded thread, the external thread is best located in the parting line of the model construction.

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

1.8 holes and slots

Holes and grooves, in addition to reducing part quality and forming uniform wall thickness, are a useful functional structural feature of MIM parts, and generally do not increase part prices.

However, adding holes and slots will increase the complexity of the mold, as shown in figure 8-a, which will increase the cost of the mold.Holes perpendicular to the parting line are easiest to form and cost the least.Holes parallel to the parting line, although easy to form, need to add sliders or hydraulic cylinders, which will increase the cost of early mold manufacturing.

The internal connecting hole can be formed, as shown in figure 8-b. In order to prevent the problem of sealing and burr, this design must be carefully considered.

If possible, a hole should be made into a d-hole to make a plane on the core rod, so as to enhance the sealing of the mold, otherwise, it is necessary to make an arc surface with the parts, and its thin edge will cause abnormal wear.

1.9 root cutting

With the open die, the external root cut as shown in FIG. 9 is easy to form on the parting line. Making this shape requires increasing the die parts, increasing the cost of the die and reducing the productivity.

Some internal root cuts can be made with sliders, others with movable cores.

In most MIM part designs, designers may decide to eliminate internal root cutting due to increased costs and possible flash problems.

1.10 casting system

The injection material enters the mold cavity through the sprue. Due to the high metal content of the MIM injection material, these sprue of MIM are generally much larger than those of plastic injection molding.

Since the gate usually leaves a mark where the finished part comes out of the molding cavity, the setting of the gate needs to balance the required craftsmanship, function, size control and aesthetics.

The gate is best set on the mold parting line, as shown in FIG. 10, so that the path of the injection material flow can impact the mold cavity wall or core bar.

In addition, for parts with different wall thicknesses, the sprue is usually set at the thickest cross section so that the injection material flows from the thick section to the thin section.This setting of the gate eliminates holes, grooves, stress concentrations and streamlines on the surface of the part.

If you want to produce parts with multiple cavities, you must also consider the size and configuration of the sprue to ensure that the same amount of injection material is supplied to each cavity with a uniform filling rate.

1.12 decorative features

Figure 12 shows the marking, embossing, number of parts and identification marks of die number and hole number, all of which can be easily formed in the proper position of the part without increasing the cost of the part.

These features can be highlighted or recessed, and the MIM process can produce high levels of feature detail, including sharper diamond embossing.

1.13 sintered parts support

In the process of degreasing and sintering, the green MIM parts shrink by about 20%. In order to minimize the possible distortion, the MIM parts must be properly supported during sintering.

Typically, MIM parts are placed on flat ceramic plates or trays.

As shown in FIG. 13, it is best to design the sintering plate or tray into a plane with large surface or common structural features of several parts, so that standard supports can be used.MIM parts with long spans, cantilevers or vulnerable areas may need to be supported by special brackets or fixtures for the parts.These production costs are very high.