Extrusion, definition

The Extrusion Process:

An extruder is somewhat like a long, hot, motor-driven meat grinder… This is an unsophisticated definition, but it is helpful to the novice.

The Extrusion Process:

The process for making extruded products, such as medical tubing, profiles, tape, filament, hose, pipe, wire jacketing, stainless steel braided catheters, multi-lumens, etc., starts with an extruder. Solid plastic pellets are poured into the extruder hopper. The extruder melts the plastic (resin or polymer) and pumps the melt (molten plastic) through a die orifice that yields the desired shape. The die orifice is generally a percentage larger than the actual size of the finished product. This percentage is based on a number of factors: size, shape, and material. The melt enters a vacuum sizer or a water-cooling tank where the correct size and shape are developed and cooled. The shape might be helped along by use of profile shaping tools or plates that are water or air-cooled. Next, the newly formed product enters a puller, which is often a pair of motor-driven, urethane-covered rolls or belts. It is the puller (take-off, haul-off, pinch rollers, cat) which pulls the molten resign from the end of the die through the vacuum sizer, water tank, etc. At the end of the line a cutting machine, coiler, traveling saw, punch press, etc., does the final processing of the product.

The Extrusion Line:

All the equipment lined up, ready to process extruded components; extruder, cooling tank, puller, cutter, etc., is termed the ‘Extrusion Line’

Basic Extruder
Screw & Barrel:

The extruder screw is the moving component, which melts and pumps plastic. The screw is turned in the extruder barrel (cylinder) with power supplied by a motor operating through a gear reducer.
The screw fits within the barrel with only a few thousandths of an inch clearance. It is machined from a solid steel rod and surfaces almost touching the barrel are hardened to resist wear. The screw is polished and may be chrome-plated to resist the corrosive action of some resins, such as PVC: Poly-Vinyl Chloride.
The shank of the screw (rear) fits (keyed) into a driving mechanism, supported within a thrust bearing. To pump molten plastic through a die located at the front of the extruder. It is necessary to develop pressure in the melt. An extruder screw is designed to do this. The pressure in the melt acts against the screw, pushing it against the thrust bearing. In many typical extrusion operations, pressures ranging from 1,000 to 7,000 pounds per square inch (PSI) are developed in the melt. In a two-inch extruder, 2,000 pounds pressure would act on an area of 3.14 square inches. Therefore, the rear-ward thrust of the screw would be about 6,280 pounds.
At first glance, the extruder barrel resembles a cannon, like the ones used during our Civil War. Extruder barrels are made of steel (4140) and have thick walls to withstand the high internal pressures. The inside surfaces of the barrel are almost always made of a special hard steel alloy, such as having a Xaloy liner. This provides the barrel with wear resistance and some corrosion resistance.