March 2004 Design Tip

Be open to shut-offs.

This month's Design Tip combines elements of the prior July and November tips to address the important relationship between the draft designed into the part, the resulting parting line of the mould and the final quality of the parts.

As we reviewed in prior Design Tips, draft is a critical feature of almost all part designs to be injection moulded because it helps the part eject from the mould as easily as possible. But when you are deciding exactly how to design the draft, it is helpful to understand how your decisions will effect the mould's parting line geometry and shutoff surfaces.

Figure 1 illustrates how the decision to draft the walls of a simple part can have a major effect on how the mould will be designed. In "Design #1" the walls of the part are drafted so that they can be ejected from within deep, thin mould cavities. The issue with this approach is that deep, narrow slots are a challenge to mill and polish, so you may not be able to achieve the desired geometry or final surface quality.

Figure 1 : Draft design affects mould design.

Figure 2 : Mould design affects part quality.

On the other hand, if the walls of the part are drafted as shown in part "Design #2" the mould becomes a core/cavity design that is much easier to mill and polish. And the end result is a part that can have significantly improved surface finishes as shown in the photographs in Figure 2.

You can visit the Protomold Design Guide for other helpful Rapid Injection Moulding design information.