Draft Angles

Thermo Pressure Formed parts are usually formed in female dies. These dies do not have cores that restrict the normal shrinkage of the part as it cools. This normal shrinkage of a three-dimensional part actually pulls the outer walls of the formed part away from the side walls of the die that are perpendicular to the open face or deck of the die. This very useful condition allows some Thermo Pressure Formed parts to be designed with little or no draft on their side walls. This is a major advantage over injection molded parts that are prevented from shrinking away from the cavity by the presence of the core.

Zero draft angles are possible with added manufacturing difficulties. However, Thermo Pressure Formed parts will be easier to produce if they can be designed with 1/2, 1 or even 5 degrees per side draft angles.

Inside surfaces such as the frontal recess or the depression in the top of the part shown in Figure 11 are a different matter.

As the plastic material in these areas cools and shrinks, it draws down tightly onto those portions of the die that form these recesses. These inside surfaces should preferably be smooth and provided with draft angles of at least 1 degree per side, and preferably 5 degrees per side.

The corners on inside surfaces such as these should be provided with the largest possible radii to avoid the stress concentrator effect and the molded-in stress that will be developed as the shrinking material draws down tightly against the edges of these positive portions of the die.

Points to remember are that (1) any draft is better than no draft at all; and (2) the larger the draft angle, the better, in keeping with the end-use requirements of the product.

The designer should also recognize that self-lubricating, tough materials with high shrinkage factors such as nylon or polyethylene will be easier to remove from female dies. Brittle, non self-lubricating materials with low shrinkage characteristics such as acrylic or styrene, will be more difficult to eject from dies which have minimum draft angles. It is always good to remember that no two plastic materials are the same and each material has its own design requirements.