Design Optimization 101

An extruder pushes the thermoplastic rubber through a shaping die while at the same time downstream equipment pulls it at a steady rate. If the flow through the die is not uniform across the die, it can bow and distort.

A part with uniform walls is easier, quicker, and therefore cheaper to balance. If possible, keep the wall thickness of a rib or protrusion to within 50% of the adjacent wall.

The ribs on this part are 50% as thick as the adjacent wall.

Voids can reduce the part cost and often serve a functional purpose. However, each void typically adds at least $500-$1500 to the cost of the tool. Large voids with thin walls may also require calibration blocks, which can add another $300-$1500 to the tooling charge.

The void is created in the extrusion die by means of a hollow pin that blows air gently to keep the void from collapsing. A small void requires a small pin which, with its air bleed hole, can be very delicate. Therefore, the minimum diameter void we can make in a part is 0.08” diameter.

A bulb seal that will be installed in a tight bend radius, or even packaged in a certain way can become kinked. In general, a thicker, softer wall is less likely to kink than a thinner one.

Allow for Minimum Radii

As a practical matter, the minimum radius for sharp corners is .015”. For ribs and protrusions less than 1/16”, consider a full radius. Even if the print specifies the part on the left, it will look like the one on the right!

The flipper on the left, below, can very easily be lengthened, if desired. The bulbous end of the flipper on the right, however, performs a very limited function (or possibly none at all) but makes it more difficult for us to extend the length.

Allow for Temperature Changes

When designing and testing a seal, make sure that you allow for the full range of performance temperatures. Fortunately, unlike PVC, thermoplastic rubber retains its compressibility and elasticity consistently from -60F to +250F.