One of the goals of rapid injection molding is to rapidly produce parts. The correct design helps to ensure that good parts are produced in the first run. It is important to determine how the part will be placed in the mold. The most important consideration is that the part must remain in the mold half containing the ejection system.
Cavity and core
In a typical injection molding machine, one half (a side) of the mold is connected to the fixed side of the press, and the other half (B side) of the mold is connected to the moving jig side of the press. The clamp (or b) side contains an ejector actuator that controls the ejector pin. The clamp presses side a and side B together, the molten plastic is injected into the mold and cooled, the clamp pulls side B of the mold apart, the ejection pin is started, and the parts are released from the mold.
Let's take the mold of plastic drinking cup as an example. In order to ensure that the parts and the ejection system are kept in half of the mold, we will design the mold so that the outer part of the glass is formed in the mold cavity (side a) and the inner part is formed by the mold core (side B). As the plastic cools, the part will shrink from side a of the mold and onto the core on side B. When the mold is opened, the glass will be released from side a and stay at side B, where the glass can be pushed out of the core through the ejection system.
The a side (cavity) and B side (core) of the mold are represented by ejector plates and pins placed on the B side.
If the mold design is reversed, the outside of the glass will shrink from the cavity on side B to the core on side a. The glass will release from side B and adhere to side a without ejector pins. At this point, we have a serious problem.
Rectangle example
Let's consider a rectangular shell with four through holes. The outer part of the shell is the cavity on the side a of the mold, and the inner part is the core on the side B. However, the design of holes can be handled in two different ways: they can be drawn toward side a, requiring a core on side a of the mold, but this may cause parts to stick to side a of the mold.
A part with four through holes and a tab leading out to side B.
A better method is to draft the core to side B to ensure that the parts adhere to side B of the mold. Similarly, any lug or strip from the part or across the internal hole should be pulled to side B to prevent sticking to side a and bending or tearing when the mold is opened. Of course, the design should also avoid the appearance of heavy texture on the outside of the part without sufficient draught, as this may cause the part to stick to side a.
