Why does sprue tighten when you heat it "just right"?
Am working on the CA P-6E and did the rigging fairly slack and then brought out the soldering iron and waved it like a magic wand at just the right distance. Snapped them good and tight. I always thought that heating made things expand.
Not sure why this works, but sure glad it does.
It's because the polymers that make up styrene plastic contract when heated. This contraction manifests itself by making the styrene string shorter, and "tightening" the rigging.
F Marion
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brought out the soldering iron and waved it like a magic wand at
that heating made things expand.
I can partly concur with the previous poster's explanation. The occurence is called "relaxation" I believe. What I understand of it is the following. When you stretch a hot sprue you not only permanently deform the material, but you also introduce an elastic deformation that causes an internal material stress and make the entangled polymer molecules unentangle. The elastic deformation and the stress "freezes" during the quick cooling of the thread by leaving the polymer molecules unentangled. When reheating the thread the stress is released due to the softening of the material and makes the thread regain a part of its original shorter shape.
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