The thermal expansions of the specimens injection-molded from nylon 6, 66, their copolymer, polyacetal and polycarbonate were measured, and the relationship between thermal expansion and dimensional changes caused by the relief of internal residual stress and crystallisation was discussed.
The results obtained were as follows:
1) The thermal expansion of the specimens injection-molded from the crystalline high polymers depends on the thermal history which influences on the crystallinity and the relief of the internal residual stress.
2) The apparent second order transition points are about 80°C for the unannealed molded specimens from nylon 6 and 66, about 100°C for their annealed molded specimens and 160°C for unannealed specimens from polycarbonate.
3) Above the apparent second order transition points, the relief of internal residual stresses and its velocity become larger and the apparent thermal expansion varies remarkably. Therefore, the coefficient of thermal expansion plotted against temperature reach a maximum at about 80°C for unannealed nylon 6 and 66 and at about 100°C for their annealed specimens, and at about 160°C for polycarbonate, and then these curves also reach a minimum at about 170°C for nylon 6 and 66, and above 200°C for polycarbonate, nearing to their melting regions.
As for nylon copolymer and polyacetal, no similar change to the above was found.
4) The coefficients of thermal expansion of the specimens with high crystallinity are smaller at lower temperatures than those having low crystallinity, but larger at higher temperatures.
5) The ratio of the coefficient of cubic expansion to that of linear expansion for unannealed specimen of nylon 6 is about 3 only when the temperature is below 100°C, and for annealed one the same value is obtained below 150°C.
6) The relationship between the coefficient of thermal expansion and temperature is similar to that between the dielectric loss tangent and temperature.