Since thermoplastics shrink as they cool, they grip to cores or male forms in the mold. This makes ejection difficult and may lead to scuff marks on the part. To facilitate ejection, part surfaces parallel to the draw direction of the tool require taper or draft. This draft allows the part to break free by creating a clearance as soon as the mold starts to open. The amount of draft needed depends on several factors:
Taking into account the above factors, the guideline for draft angle dimensions is as follows (see Figure 1):
Draft for shut-off
The most straightforward mold construction is where the parting plane of the mold halves is a flat surface perpendicular to the draw direction (see Figure 2). This setup also offers the easiest way to make sure that there is no gap between the mold halves when they’re closed. After all, the clamping force of the injection molding machine acts perpendicular to the parting plane. However, it’s not always possible or even beneficial to keep the parting plane flat.
In case of such a stepped parting line, the parting line jumps from one level to another. The surface needed to bridge these levels is called a shut-off because it’s where the mold halves shut against each other. The shut-off should never be exactly parallel to the draw direction. This leads to drag, causing the tool to wear over time. In a worst case scenario, misalignment during mold closing can cause significant damage. Instead, draft should be applied to guarantee proper shut-off.
On stepped parting surfaces, apply a draft of seven or more degrees. Five degrees should be considered as a minimum.
Figure 2 - Stepped versus flat parting line.