January 2009 Design Tip

Rapid Injection Moulding, an Overview

Over the last several years, these emails have addressed many details of design for rapid injection moulding, but we've never really looked at the overall process. In its simplest form, the injection moulding process works as follows:

  1. Injection moulding resin in pellet form, is loaded into the hopper.
  2. The pellets flow into the heated barrel, where the material is melted.
  3. A ram-driven screw injects the molten material into the closed mould.
  4. After the material cools and solidifies, the mould opens and the part is ejected.

Fig. 1

Fig. 1: The Injection Moulding Process

Add side-actions and the process gets a little more complicated. If you've already received one of our free demo moulds, you've seen how a two-part mould with a side-action cam works. If you haven't received one yet, go to our web site to order one. Meanwhile, here's a 2D version:

Fig. 2

Fig. 2: Protomold's Demo Mould

Figure 2 shows the closed mould. Note that the colors of the mould halves correspond to those in an online ProtoQuote®: green for the cavity or A-side, blue for the core or B-side, and red for side actions.

Fig. 3

Fig. 3: Demo Mould Exploded View

Lift the green A-side mould half (Fig. 3, "a"), and you'll see the yellow injected part with its runner, sprue, and edge gate (all of which will be trimmed off) and the red side-action.

Note the features indicated on the face of the green A-side mould half, particularly the gate, where resin enters the mould cavity, and the drafted sliding shutoffs, which form the outside of the hook on the part (Fig. 3, "b").

Withdraw the red side action (Fig. 3, "c") and note the hole and raised lettering it forms on the side of the part. These are undercuts that could not otherwise be formed in a simple two-part (straight-pull) mould.

Once the mould is fully open the white ejector will rise to push the part off the core on the blue B-side mould half (Fig. 3, "d"). Since the mould halves are normally mounted horizontally in a press, this will cause the part to tumble free from the mould.

Note the features indicated on the face of the blue B-side mould half, particularly the core, which forms the hollow center of the part.

For more information on Protomold's processes, visit our web site.