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I was looking at a spec sheet for a photopolymer and saw these two different subsections under the "Thermal" section. I tried Googling around as I usually do, but no site could give me the clear difference between the two and some simply defined the two as the same. I have a hard time believing that since there are two distinct test methods indicated by the ASTM. Could someone please clarify what the difference is?

PS - To further clarify, I mean the difference between the two results, not the ASTM test methods.

Spec Sheet

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  • $\begingroup$ Welcome to Engineering! To be clear, what "two results" confuse you? The heat deflection at different pressures or the different values in the second and third columns? $\endgroup$
    – Wasabi ♦
    Apr 28, 2021 at 21:25
  • $\begingroup$ Thank you! The differences between the second and third rows and essentially if these are also describing temperature dependent, yield points too? @Wasabi $\endgroup$
    – Sophia
    Apr 28, 2021 at 22:07

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  • CTE characterizes dimensional change, with no load. It corresponds to a relatively fundamental physical principle, making it easy to use in a design calculation
  • Heat Deflection test characterizes the deformation under heat AND load, which includes a variety of phenomena happening at the same time. The result is dependent on the particular geometry and loading conditions specified by the test, in a way that makes it hard to extrapolate for design purposes. On the other hand, it is more useful when "shopping" for a material, by making it possible to compare them in standard conditions for applications that are limited by both heat and load at the same time.
  • For the two different lines of the heat deflection test, they correspond to different standard loads.
  • Annoyingly, the actual standards are not made freely available, but a general description can usually be found with a little searching. (e.g. here is one for ASTM-D648 )
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  • $\begingroup$ Thank you for the information! Could you further clarify something about this detail; If I wanted to meet a design requirement that the material cannot have a scorching/melting point (And w/o any loads) below 80 C would the CTE meet that req? The part is supposed to endure these temperatures without permanently deforming (Reaching its yield point). So, essentially does CTE describe permanent deformation or temporary deformation at/above the provided temperature? $\endgroup$
    – Sophia
    Apr 28, 2021 at 22:03
  • $\begingroup$ CTE won't be good for that. It describes thermal expansion at a one or perhaps a handful of specific temperatures. Generally speaking it is reversible. It might result in a stress if something stops the material from expanding, including temperature gradients within the material, but generally it assumes no stress. CTE says nothing about strength, nor material degradation, nor any of the weird things polymers do when overheated ... a general "service temperature rating" with a good sized margin is often good for what I think you are describing. $\endgroup$
    – Pete W
    Apr 28, 2021 at 22:09
  • $\begingroup$ I see, then is there a specific spec I should look out for on the data sheet then for that sort of info or is that something I need to test myself? $\endgroup$
    – Sophia
    Apr 28, 2021 at 22:11
  • $\begingroup$ "maximum service temperature" is a good one, to keep it simple. If you are concerned about some other material property, sometimes you can find them listed also at one or two elevated temperatures. Also there is often data on flammability, glass-transition-temperature, melting, and so on. http://matweb.com/ can be a resource to explore $\endgroup$
    – Pete W
    Apr 28, 2021 at 22:14
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    $\begingroup$ Very informative. I'll take a closer look. Thank you so much! $\endgroup$
    – Sophia
    Apr 28, 2021 at 22:25
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The coefficient of thermal expansion is the change in size of the material as the temperature changes, which is obvious when looking at the units: a length change per metre per degree.

Heat deflection is not about a change in length.

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    $\begingroup$ Ahh, but what is "heat deflection" describing? Your answer is a bit lacking :-( $\endgroup$
    – Carl Witthoft
    Apr 29, 2021 at 12:19

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