February 2004 Design Tip

Expect to eject

The Protomold Rapid Injection Moulding process uses ejector pins of various sizes to push the plastic part out of the mould after it has solidified. The sizes and arrangement of these pins are selected to minimize the impact on your part design.

Figure 1 is an example of the illustration Protomold provides early in the process of designing the mould so that the location and size of both the gate(s) and ejector pins can be approved.

Figure 1 : A typical Protomold-
supplied gate and ejector pin layout
submitted for customer approval.

Figure 2 : The wall of this part
has been increased to support the
full impact of the ejector pin
without damaging the part.

Sometimes it is appropriate to adjust the design of the part to accommodate the need for ejection pins to push the part out of the mould. For example, Figure 2 illustrates how an injector pin "landing pad" has been integrated into the wall of a part design in order to provide sufficient material for the full diameter of the pin to meet the part. Landing pads may also be used to provide additional support for the ejection of thin curved walls, and in some cases the pins themselves can be contoured to fit the shape of the part.

Of course, always remember to provide as much draft as possible so that the ejector pins can do their job, especially for applications where it isn't possible to use mould-release lubricants to help the part eject more easily.

Each situation is different, but a good understanding of the use of ejector pins is important when designing a part to be Rapid Injection Moulded.

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