Design Considerations for Metal Injection Molding

Part wall thickness should be as uniform as possible to avoid warping and dimensional changes during processing. Warping can result from variations in filling pressure during injection for cross-sections of different thicknesses, differences in binder removal time during thermal debinding, and differences in thermal mass during sintering. Other issues with large cross-sectional thicknesses include sink marks, potential voids, and bubble defects caused by difficult binder removal. This is especially important in injection molding with metal inserts, where varying thicknesses can create additional stress points during cooling and sintering.

Figure 2.7: Different wall thickness designs (left:不合理/unreasonable, right:合理/reasonable)

Figure 2.8: Wall thickness transition illustration showing the 3H guideline

Maintaining proper wall thickness is crucial for ensuring consistent material flow during the injection phase, which directly affects part quality. In injection molding with metal inserts, designers must pay special attention to the thickness around insert locations to prevent stress concentration and ensure proper material distribution. The 3H rule for transitions helps maintain uniform cooling rates and reduces internal stresses that can lead to cracking, especially when working with the complex geometries often involved in injection molding with metal inserts.

Any injection mold has standard draft requirements to ensure smooth part ejection from the mold. Draft refers to the variation in tool dimensions or angles parallel to the direction of tool movement. Within the allowable design range, draft angles should be as large as possible. However, in metal injection molding processes, because some binders can act as lubricants, draft angles can be minimized.

Nominally, draft angles are specified between 0.5° and 2°. When parts are excessively long or have uneven surfaces, larger draft angles should be used. Typically, the smallest draft angle that has the least impact on part functionality is chosen within the allowable range. This consideration becomes particularly important in injection molding with metal inserts, where the presence of inserts can complicate the ejection process.

External walls of injection-molded parts do not require minimum draft angles because during the cooling phase of metal injection molding, the material shrinks away from the mold walls. This natural shrinkage is beneficial for ejection and can be leveraged in designs involving injection molding with metal inserts to ensure proper release.

Figure 2.9: Internal and external draft angles of a part

The shrinkage characteristics of metal powder injection molded parts enable easy demolding. For example, a core pin with a draft angle of 0.5° to 1° has its largest diameter at the end of the core pin and its smallest diameter at the top of the core pin. This allows the part to slide off the core pin with minimal friction because the part can move freely on all surfaces during initial ejection. This principle applies equally to components created using injection molding with metal inserts, where proper draft angles around inserts prevent binding during ejection.

Injection-molded parts can have draft angles on the stationary mold portion to facilitate easier脱模 from the stationary mold during mold opening, while no draft angles should be present on the moving mold portion to ensure the part remains on the moving mold side. On this basis, ejector pins can be used to eject the part, allowing the molding machine to operate continuously and automatically.

Figure 2.10: Different internal draft configurations for parts

The position of the mold parting line can divide the draft into two different directions to minimize dimensional changes from one side of the part to the other. Different internal draft forms for parts are shown in Figure 2.10. A configuration where two drafts meet in the middle of the part requires smaller dimensional tolerances compared to a configuration with only one draft angle.

In some special applications, reverse draft is also used, where the part is ejected from the stationary mold side to facilitate molding of part features that are difficult to mold in the moving mold. This technique is often employed in injection molding with metal inserts when complex geometries require alternative ejection methods.

When designing draft angles for injection molding with metal inserts, engineers must consider both the part geometry and the insert placement. Inserts can create additional challenges for draft angles, as they often introduce undercuts or require specific orientation. In these cases, creative draft angle solutions or specialized mold actions may be necessary to ensure proper part release while maintaining the integrity of the inserts.

Metal injection molding can also produce threaded features. When molding external threads, the mold parting line is usually distributed along the length of the thread, as shown in Figure 2.11. This parting line can either include each thread or have a 0.13~0.25 mm flat surface along its length, resulting in incomplete threads on both sides of the parting line. When designing threads for injection molding with metal inserts, special consideration must be given to how the insert interacts with the threaded features during both molding and ejection.

Figure 2.11: Typical external threads produced by metal injection molding

Figure 2.12: Mold closure detail for thread formation

If a parting line design that forms complete threads is adopted, flash generated along the parting line when the mold wears can affect thread functionality. When the mold has a good closing surface during clamping, as shown in Figure 2.12, flash or residue can be prevented from affecting thread functionality, but this may result in insufficient thread engagement strength in some applications. This balance between flash prevention and thread strength is particularly critical in injection molding with metal inserts that incorporate threaded features.

Additionally, during the ejection phase of the injection-molded part or before ejection, pneumatic or hydraulic drives can be used to rotate the part, forming external threads. In this type of mold, as part of the injection molding process, pneumatic or hydraulic drives are used to rotate the threaded taps in and out after molding and before or during the ejection phase. Due to the high cost of this processing equipment, it is limited to high-volume production applications. This method can be adapted for injection molding with metal inserts when threads must interface with inserted components.

When designing threads for injection molding with metal inserts, engineers must consider the compatibility between the molded threads and any inserted components. The different thermal expansion rates between the molded metal and the insert material can affect thread fit after sintering. Additionally, the presence of inserts may require modifications to thread geometry or pitch to ensure proper assembly and functionality. Careful consideration of thread design in injection molding with metal inserts can prevent common issues such as cross-threading, insufficient engagement, or stress concentration at the insert-thread interface.

Beyond the fundamental design considerations outlined above, several advanced factors contribute to successful metal injection molding outcomes. These include material selection, surface finish requirements, and post-processing considerations, all of which interact with techniques like injection molding with metal inserts.

Design for manufacturability (DFM) principles are essential in metal injection molding to optimize part design, reduce costs, and ensure consistent quality. This includes designing with uniform wall thicknesses as previously discussed, minimizing undercuts where possible, and considering how parts will be gated and ejected from the mold. When implementing injection molding with metal inserts, DFM becomes even more critical, as the inserts introduce additional variables into the manufacturing process.

Another important consideration is the gate design, which affects material flow, pressure distribution, and part quality. Proper gate placement ensures complete filling of the mold cavity while minimizing weld lines and flow marks. In injection molding with metal inserts, gate placement must also consider the position of inserts to ensure proper material flow around them and prevent air entrapment. The size and type of gate influence cycle time, part aesthetics, and post-processing requirements, making it a key design decision in metal injection molding.