Rheological Study on Cold and Warm Processings of Polycarbonate

The forces required to compress billets of crystalline polymers below their melting points were determined and related to the initial billet dimensions and amount of deformation. The studies involved the use of three high‐density polyethylenes and an acetal copolymer. The results were found to obey an equation derived from plasticity theory using, at each temperature and for each material, a single parameter very nearly equal to the yield strength determined in ordinary tensile tests, and having the same temperature dependence. An empirical equation that describes the pressure required to form cups by backward extrusion of metal billets is also successful for the same process with polymers, using the yield parameter from the compression experiments. Cold working causes a high degree of orientation, increased clarity, and greatly increased tensile strength.

Force requirements in forging of crystalline polymers

Wissbrun

1971

Cold forming of plastics part I. Draw forming of thermoplastic sheets

Koch

Prevoršek

et al. 1971

An experimental procedure is outlined to examine the potential of thermoplastic sheets in draw forming. Experiments carried out on a variety of materials indicate that the following requirements must be fulfilled for a thermoplastic sheet to be cold formable: (1) The glass transition of polymer should be above ambient temperature and above the temperature of forming, (2) tensile elongation at break should equal or exceed 30%, (3) ratio of tensile to compressive yield stress should equal or exceed 1.6 and (4) sheet must not yield locally (neck) when strained in tension. An experimental method has been developed to determine the compressive, friction and bending forces which oppose the drawing force exerted by the punch. It is shown that the compressive force is, in most cases, largest. A stress analysis is carried out leading to an expression correlating the maximum depth of draw as a function of basic properties of sheets such as tensile strength (St*) and compressive yield stress (Sc). The effect of rolling on drawability is examined and interpreted in terms of the ratio St*/Sc. The cold formed items have a lower heat distortion temperature than their thermoformed counterparts.

Dimensional recovery of cold‐rolled polycarbonate

Rusch

1972

The dimensional recovery of cold-rolled polycarbonate was measured between 100 and 147"C, and compared to stressrelaxation data. The strain-recovery isotherms were superimosed to produce a master curve which could be represented gy a distribution function, U ( T ) , termed the strain recovery spectrum. V ( T ) is found to be nearly identical to the relaxation spectrum, H ( 7 ) , calculated from the stress-relaxation master curve; and the shift in V ( T ) as a function of temperature, WLF shift factor, is similar to that obtained from other viscoelastic measurements on polycarbonate. For the deformations studied, 25 and 50% reduction in thickness, the recovery behavior is found to be independent of strain (linear), suggesting that nonlinearity in a polymer glass results from large stresses, and not from large strains.