Post-Extrusion Shrinkage

Volume changes due to diffusion of butene from, and air into, a 5 mm thick foam sheet of density 28 kg m, with 

When the crystallinity of polyethylenes is increased, the gas permeability through the film decreases. The factors involved are the tortuosity of the gas path through the amorphous phase, and the effect of the crystals in restricting the mobility of the amorphous polymer chains (chain immobilisation factor). The logarithm of the permeability of nitrogen, argon and carbon dioxide decreased almost linearly with increased crystallinity of PE, with the ratio of the gas values remaining almost constant for a particular PE. 

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The effect of the type of impact modifier on the melt flow of a PVC window profile formulation as a function of shear rates encountered during extrusion was investigated and the relationship between the melt flow and mechanical properties of the profiles evaluated. A Rheoplast Capillary Rheometer with a pre-shearing device was employed to investigate the melt viscoelastic properties of the formulations and the performance of the formulations in terms of post-extrusion shrinkage, surface gloss and enthalpy relaxation discussed. 2 refs. 

From the above analysis on the elongation and hoop stresses, it was clear that Tooling B with the lower DDR had substantial reduetion in the elongation stress and hoop stress compared with Tooling A. The stress reduction was beheved to reduce the post-extrusion shrinkage. Therefore, tooling B was recommended for the extrusion process of this cable product. 

In summary, numerical analysis showed that all four stress conqxtnents have been reduced by using a lower draw down ratio tooling. The reduced stress state favors forming a more relaxed jacket sheath, which would result in less post-extrusion shrinkage. This was confirmed by the experiment results from cable tensile tests. 

Die extmsion, draw down ratio, viscoelastic fluid model, PVDF, post-extrusion shrinkage, tensile yam buckling... 

Small production runs are processed by compression or transfer molding at 800 to 3,000 psi and 104 to 188°C mold shrinkage is 2 to 4 percent. Long production runs are more economical by injection molding at 5,000 to 20,000 psi, 188 to 252°C, and a 25 to 90 sec cycle. Extrusion requires post-cure in a 316 to 427°C hot-air oven, typically 60 ft/min steam post-cure can run 1200 ft/min. Calendering typically runs 5 to 10 ft/ min. 

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