1973 Volume 1 Issue 1 Pages 27-36
The hydrostatic extrusion of several polymer samples, i. e., polyethylene, polypropylene, nylon 6, polycarbonate, polytetrafluoroethylene, polyoxymethylene and poly (4-methyl-1-pentene), was studied employing a 500 ton hydrostatic extrusion press whose maximum extrusion pressure is 15000kg/cm2. Preliminary attempts were made to extrude polymer melt by this hydrostatic extruder. A billet of polyethylene (15mm in diameter) and the pressure medium (castor oil) were pre-heated to 110°C to facilitate the melting of polymer at extrusion. It could be assumed that the preheated polyethylene would become molten due to the heat evolved during extrusion. The billet was extruded under a pressure of 7500kg/cm2 through a die hole 1mm in diameter. The extrusion ratio was as large as 225, and the shear stress was about 2×109 dynes/cm2. The extrudate was granular and no continuous product was obtained. It was postulated that the extrudate was kept in a molten state for a considerable long period of time after extrusion because of the depression of the melting point due to extrusion. Extrusion under a pressure of 3000kg/cm2, using a die with a hole 2mm in diameter, gave a melt-fractured product. Extrusion experiments are being continued to eliminate the factors hindering stable extrusion.
The relation between the extrusion pressure P and the extrusion ratio R in the hydrostatic extrusion of polymer solids can be given in a manner similar to metal extrusion by
where a is the slope of the P vs. lnR plot and thus gives a measure of the increase in pressure required for an increase in extrusion ratio and b is a constant characteristic of the sample. Values of a for the polymer samples were compared with those for metal samples. The a-value of polyethylene is near that of aluminum, while the a-value of nylon 6 is similar to that of copper. However, the polymer samples have only limited ductilities, and there is no definite relation between a-value and ductility. Nylon 6, having a comparatively large a-value (3.77 tons/cm2), is much more ductile than polytetrafluoroethylene with a smaller a-value (0.85 ton/cm2). The production of cracked or fractured extrudates of polymers would be due to the swelling in radial direction and various irregular deformations during extrusion. In other words, the viscoelastic behavior of polymers is responsible for the abnormal extrusion. The P vs. ΔV/V plots of polymers are generally time-dependent and it was found that the compression curve in the equilibrium state is markedly different from those obtained at any finite rates of compression. On the contrary, no time-dependent effect was observed in the compression of molten n-paraffin. The plot in the equilibrium state and the plots at any rates of compression overlapped perfectly with each other.
The polyethylene sample obtained by hydrostatic extrusion displayed a uniaxial orientation, but no indication of selective uniplanar orientation was found at the surface of the extrudate. This is very different from the structure of samples made by the cold-extrusion method which, like metal extrusion, gives extrudates with uniplanar orientation.
To prevent radial expansion on extrusion, the polymer was enclosed in a thick-walled sheath of metal and then subjected to hydrostatic extrusion. The metal-clad billet of polymer thus prepared yielded a satisfactory extrudate at an extrusion ratio at which the bare polymer billet gave a fractured product. The sample of polypropylene made from aluminum-clad billet at an extrusion ratio of 6.83 under a hydrostatic pressure of 3700kg/cm2 showed a higher transparency and a higher density than the ordinary sample.