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Designing for Performance and Economy

Everyone appreciates when things work the way they are supposed to. If several decades of extruding has taught us anything, it's that small changes to part designs will determine whether or not your part works for you. So we thought we'd share a few tips we've picked up along the way. After all, what are websites good for if not to share information?

 

Wall Thickness

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

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.

   
Adding a void can reduce the part weight,   but it also adds to the tooling cost.  

 

Avoid Kinking

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” thick, consider a full radius:

   
Even if the print specifies the part on left,   it will look like the one on the right!

 

Design Flexibility

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.

   

 

Compression Ratio

The ideal compression of a gasket is 25-50%, although this depends on many factors.

 

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.

 

Degradation

Be sure to consider the potential for degradation, such as the ability of the elastomer to resist chemicals and UV radiation.

 

Attachment Strategies

Here are some common attachment strategies to consider:

   
Slide in

 

Push in

 

   
Clip on

 

   
Adhesive tape

 

Packaging

How is it to be packaged? Our most common forms of packaging are spooling (typically on a cardboard spool with a 12” ID) or, if spooling would damage the part, packed straight, in boxes up to 10 ft long.

 

Thermal Linear Expansion

TPV has a coefficient of thermal expansion that is 90 x 10-6/°C, which is 3-4 times greater than that of aluminum, but this is still very small, and is almost never a consideration. For example: an aluminum extrusion that is 48” long will expand about .023” when heated by 20°C (36°F). A TPV extrusion of the same length will expand .086”. 

 

Hopefully this is helpful. Our team here at Reed Rubber Products really believes in examining all of the details. When the details are right, the part will work the way it's supposed to. That's the reassurance we provide to our customers.

Reed Rubber is here to help! We have produced more than 3600 profiles for applications in automobiles, trailers, windows, doors, air conditioners, picnic coolers, and so on. How can we assist you? 

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