Mould Ideas

As illustrated in Figure, draft is the angle between the direction of ejection of a part from the mold and the surface of the part. Its traditional function is to facilitate the removal of the part from the mold, but in Protomold's Rapid Injection Molding process, it also permits deeper geometries to be milled while reducing cost and insuring trouble free molding.

Draft is not always necessary and may be unimportant in shallow features. However, when rib-like features are deeper than 3x their width, draft of at least 0.5 degrees may be required. And drafts of 1.0 degrees and larger may result in lower costs with the Rapid Injection Molding process. 

It is important that textured surfaces have adequate draft to prevent the part from sticking in the mold and to prevent "drag" marks. Protomold recommends that highly textured surfaces (T-1) have a minimum of 2-3 degrees of draft and that more heavily textured surfaces (T-2) have a minimum of 5 degrees of draft. 

Another example of the importance of draft in the manufacturability of your design is illustrated in Figure.

In this case the draft not only helps the part eject from the mold more easily, but it also minimizes the amount of sliding required between the mold's telescoping shutoff surfaces. A minimum of 3.0 degrees of draft is required for telescoping shutoff features. 

Whenever possible, a part to be injection molded should be designed with generously radiused corners to enhance its quality and moldability. 

As illustrated in Figure, corners designed without radii can cause stress concentrations. These in turn may reduce the ability of the part to withstand load and/or cause warping in its geometry. 

Figure illustrates how sharp corners might adversely affect the flow of resin during molding, potentially causing incomplete fill. They also tend to cause the part to stick to the mold during ejection, which can cause a variety of problems. 

And in Figure we show how the judicious use of fillets can also help to improve mold life by helping to minimize corner stresses at the bottom of tall, thin cores in the mold (at the entrance to deep thin holes in the plastic part). These fillets also help to enhance the ability of the mold to fill and further reduce internal part stresses.