How Engineering Plastics Expand With Temperature

A drawing lands on your bench with a pretty tight tolerance and the material specified is black Nylon. We know this happens because after decades of looking after engineering plastic customers, we’ve helped with many phone calls requesting assistance. Now the problem with tight tolerances is, for instance, Nylon will expand and contract perhaps more than some may think and knowing the different rates or at least having a rough idea is going to add to your armoury.

How it’s worked out
I think we found the reason why formulas weren’t our most popular subject at school, but in the name of plastics we’ve decided to roll up our sleeves and push through the pain barrier. I have to warn you that what follows could be classed as a cure for insomnia or maybe put in the same category as a trip to the dentist.

The data sheet
This is how it looks on the Nylon 6 data sheet;

What the ‘unit’ bit means is; ten to the power of minus five based on the ‘value’ of 12 is 0.00012 (you move the decimal point 5 places)

The next part of the ‘unit’ formula apparently means; per 10°C and also, for every 100mm of dimension. (Allegedly) The ‘parameter’ bit is based on the temperature range the manufacturers have chosen as a standard. There are a few plastics such as Nylon and Acetal where their actual expansion rate increases ever so slightly at temperatures over 60 degrees C.

So for example, 100dia nylon6 rod will expand or contract by 0.12mm per 10°C of temperature increase or decrease. (10 degrees x 100mm x 0.00012 = 0.12mm) So for 20° its 0.24mm and if it was 200mm diameter @ 20° that would be 0.48mm, you can see how the numbers start to stack up. The size 100mm is also used in terms of calculating length expansion as well, so nylon will increase at 0.12mm per 100mm of length, which means a 1000mm length could increase by 1.2mm per 10°.

So that you don’t have to work them all out, we’ve done a table for you which gives you the rate of expansion already worked out based on; per 10° and per 100mm to make it a little bit easier.

Crazy Numbers
What we’ve got in the table are some real differences, Tufnol moves very little at all, PEEK (Jack of all trades) as always is typically very good and then we jump to HDPE and UHMWPE, these two and Polypropylene are pretty much the ones to keep your eye on, because if you installed a UHMWPE wear strip in a food production area that was in a 3mtr strip length and the temperature change was around 15°C, that strip would want to grow by an amazing 9mm

Can anything be done?
A category of materials we’ve mentioned in an earlier “tech tip” article is the (GF) glass filled grades, if you look at the table, you may spot that it has a decent reduction, pretty much 50% less in contraction and expansion, its still not getting close though to Tufnol but it’s often a reason why glass filled grades are specified.

Comparisons with metals
We then dug into some comparisons with just a few other materials commonly found on drawings just to see what you may be up against

Based on the info, the comparisons with metals and engineering plastics reveal that as a round figure you can basically suggest that Nylon expands and contracts at ten times the rate of steel, now we’re not saying that it may be the most important thing you’re likely to discover but we’re sure you’ll agree that one day knowing this stuff may just come in handy.

Extreme experience
Although not strictly an engineering application, we had a signmaker who constructed an enormous sign earlier this year, he had butted the panels of Acrylic and enclosed the sign in an aluminium frame, needless to say on one those hot days this summer, it finally pushed the end of the frame completely out and the material buckled severely, when we calculated for him the potential distance this huge sign expanded by, it could have potentially been close to 80mm on that day.

In summary
The basics then are that expansion rates aren’t going to score too high on most people’s list and not really a deal breaker but remember a 3mtr strip of UHMWPE or HDPE can grow as much as 9mm for a 15° temperature change and your other quick reference to keep in mind is that on average Nylon and Acetal could grow as much as ten times that of your typical metals.