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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

Sophia
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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 )
Pete W
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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.

Solar Mike
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