thermal expansion of mylar

According to my material selection guide, polyester CTE is 4 E-5 /F, cast
iron is 6 E-6 /F. Most unreinforced plastics as quite similar in expansion
to polyester.
The difference in expansion works out to 3.4 E-5 /F, or 6 E-6 /C.

A 1 C change in temperature will cause a 0.001 inch relative movement in 16
inches between polyester and cast iron.

A usual shop environment is not controlled anywhere close to 1 C, your
milage may vary.

Polyester also has a tensile modulus of around 500 kpsi. Thus a piece 0.004
x 0.5 inch (0.002 sq in) will stretch 0.001 inch in 10 inches with a 0.1
pound tension.

The other side of this stretchyness is that if held by its ends under a
little tension in a metal frame, its length will equal that of the metal
frame, as long as it remains under tension at the highest temperature of
use.

Moral of story: keep it under tension.

Elliot Burke
HighTide Instruments