The molded article of the crystalline resin is mixed with a crystalline portion and an amorphous portion. Since the random molecular chains are regularly arranged at the time of crystallization, the resin volume is reduced. This phenomenon is called "contraction".
· Molecular chain conditions when melted
Molecular chains are randomly mixed and motivated
· Curing time
The crystalline portion (i.e., neatly arranged portion) appears in the molecular chain, so the volume is reduced.
Once the resin is cured, its molecular chain will be fixed, and the proportion of the crystalline part / amorphous part in the solid product does not seem to change. However, the actual situation is that the molding material encountered a certain degree of high temperature, the amorphous part of the molecular chain is sometimes rearranged, resulting in crystallization phenomenon. As a result, the molded product volume is reduced. This phenomenon is called "post-contraction". After the contraction easily lead to dimensional changes, dents, warping and other failures.
The use of molded products is highly likely to cause post-shrinkage when the ambient temperature is high. The quenching and curing at the time of molding may cause the crystal to be insufficient, and thus easily cause shrinkage.
To prevent post-contraction, should be in the actual use of molded products before the full crystallization. Specifically, it is allowed to stand for 2 to 3 hours at a temperature of about 20 ° C higher than the ambient temperature when the molded article is used. This is called "annealing". If there is a dimensional tolerance after the annealing process, there is usually no problem.