Basic Principles of Plastic Extrusion

Pipes are made by plastic extrusion.

Plastic extrusion is the process in which a portion of raw plastic, which is generally in bead form and precolored, is melted and shaped into a continuous form by an extrusion machine. This process produces many common household items such as adhesive tape, pipes, window frames and wire insulation.

The Mechanical Principle

A barrel is turned by a screw, which pushes a piece of plastic forward. The intended outcome is to multiply the force to overcome one of three great resistances. The first resistance is solid particles, or feed, rubbing against each other and a barrel wall within the initial few turns of the screw, or feed zone. The second resistance is the melt adhering to the barrel wall, and the third resistance is flowing within the melt while it is being pushed forward.

The Thermal Principle

Extrudable plastics will melt when they are heated and solidify when they are cooled, a characteristic of all thermoplastics. The most important source of plastic melting heat is generated inside the barrel by friction. When the screw is turned by the motor against the resistance of the viscous melt, the generated friction is the source of the heat. This is true for most systems and applications with the exception of slow-moving screws, small systems, extrusion-coating applications and high-melt-temperature plastics. The source of heat for all other operations impacts the effect on extrusion. Rear barrel temperature is important as it affects the rate of solids that convey in the feed as well as the bite. Head and die temperatures must stay around or at the melting temperature if they are not specifically used for gloss, pressure control or flow distribution.

The Speed Reduction Principle

Generally in extruders, modifying the motor speed changes the speed of a screw. At full speed, most motors turn around too fast for an extruder screw. Turning too fast generates excessive frictional heat, impacting the uniformity of the melt. A ratio which falls between 10:1 and 20:1 is typical for a reduction. There are two stages to a reduction: the use of a pulley set or gears and the use of gears with a center set screw in the last and largest gear. In a small number of machines which are slow moving, three stages of reduction may be utilized.

References

• Feed Screws: The 10 (11) Key Principles of Extrusion