Monday, March 23, 2009

Plastic Thermal Expansion & Contraction

So, today I spent some quality time at a potential customer's factory. They would like to replace some plastic and metal panels with higher performing plastic sheet material. They have a great business and a line of great products that are designed to be used on pickup trucks. You can imagine that the temperature extremes the product encounters are extreme - potentially from -50F to well over 130F (surface temp of the plastic could be 175F).

Everything that exists has a "coefficient of thermal expansion". Materials like glass have a very low rate - metals have a larger rate - and plastics have many different rates.

The application is a 3 ply laminate of plastic, metal, and EPS foam. All three materials expand and contract at different rates. That said, over the length of the pickup truck bed, which can be 8ft, materials can grow and shrink from 1/8" to 1/4". The fact is, even a change of 1/32" can deform a panel in a frame.

It looks like from initial discussion that a thermoset plastic, such as FRP, or fiberglass reinforced plastic, will not only reduce the amount of movement, but offer a higher impact resistance and make for a heavier duty product.

The point is: when you are using Plastics in an environment with large temperature swings, you will have a plastic part that will grow, and shrink, with the the temperature changes. Some typical coefficients of thermal expansion can be found on www.eplastics.com and a cool chart to figure this at www.ridoutplastics.com. You can see that Plexiglas move about 10 times more than glass. For a list of a range of materials, check this link.

3 comments:

  1. So... If I wanted to design a covered walk way with plexiglas in an environment with 100 and -20 degree weather, will I get a 1/8" shrinkage?

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  2. Yes - Depends on how long/big the piece is. You have a 120F degree change. The movement is approximately .00984" per foot length for each 20 degrees of temperature change.

    The bigger the piece, the more change. A 96" long piece at 100F would only be 95.75" long at -20F. That's only a 1/8" movement from each end. Now, this is an extreme so you need to allow for this IF the environment warrants it. Using a flexible weather stripping or silicone sealant that will move with the Plexiglas is important - and most glass installers and contractors can figure this out with this information in hand.

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  3. Thanks. Very nice post. Your web site is very beautiful.
    Job Provider Contractor

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