This invention relates to a process for extruding high viscosity polymeric materials. More specifically, it provides a process for making polymeric extrusions from high viscosity polymeric materials without generating surface roughness on the extrudate, and it permits extruding at higher rates and lower temperatures and reduced energy consumption.
Many polymeric materials are shaped into useful films, rods, tubes or are coated onto wire by extruding them through an orifice of relatively small dimensions. In such extrusion processes, the difficulties in obtaining a smooth extrudate increase as the viscosity of the polymeric material or output increases. The pressure required to force the material through the exit orifice also increases and an unstable flow phenomenon, often called melt fracture, occurs which results in a rough surface on the extrudate. Such rough surfaces can have adverse effects on extruded products, such as polyolefin rubbers, when, for example, the extrudate with a rough surface is pelletized under water. The interstices on the rough surface retain water and this makes drying the polymer difficult or impractical.
It is known that extrusion pressure can be reduced by forming the shaped die from a porous metal material and forcing a gaseous fluid through the pores. However, lubrication with a gas does not provide laminar flow or a smooth surface on the extrudate. It is also known to previously coat shaped extruded materials with various liquids using channels made of porous metal. Such methods have been typically utilized to lubricate the final shaping operation in manufacturing pipe or rods wherein the material is pulled through the die, rather than flowing through by upstream pressure. A method for reducing melt fracture of certain polymeric materials is known in which a small portion of the extruded material is separately heated to a higher temperature than the main mass (to reduce its viscosity) and then is applied as a surface layer in the forming die. This method is difficult to control, limited in viscosity difference, and is not as widely applicable as that of the present invention.