The extrusion process is a key component of many of our daily activities. It provides the tubing that distributes our drinking water, heats or cools our homes and could deliver life saving medication to a person in need. The process involves the use of polymers (plastics) that when heated change from a solid to a vicious liquid. Once the polymer is in its liquid state it is extruded (pushed) out a die that provides a specific shape. The process is contained in an extruder that consists of a round barrel with a screw inside. The polymer is placed into a hopper throat. As the screw rotates the polymer moves from the feed section up and through the die and takes shape of the orifice in the die.
Once the part is extruded out of the die in the molten state it passes through a sizer to control the shape of the part as it cools. This can be done in many methods depending upon the material and line rate (feet per minute) in the process. Typically with tubing products a vacuum tank is used. The tank creates a vacuum chamber in which the part is passed thru. This forces the tube to push out against a sizer that is shaped and controls the size of the polymer until it is cooled below its melting point (Tg – glass transition temperature). Similar methods to sizing are vacuum calibration, air racks and open water tanks.
The polymer is cooled and then pulled thru a take off puller. The take off puller is a key component to maintaining and controlling the size of the extrusion. The puller must take the material being exited from the die thru the sizer at the same continuous rate. The puller is typically set to a constant speed that is relational to the extruder output. The puller speed is typically given feet per minute (fpm). The fpm is key to determining the amount of sizer required to control the polymer and provide the right size product required by the customer.
Once the polymer is cooled and thru the take off puller then it can be cut to length, punched or formed to provide the finished product. Flexible products can be wound on spools or coils.
Key components of the extrusion process:
- Motor Drive – consistent screw rotation critical for controlling output of polymer
- Land length – critical to control final memory of polymer as it exits die
- Draw down – oversize of die in relation to finished part
- Swell factor – takes into consideration shear rate of material at differing fpm (feet per minute)
Sizing and equipment
- Vacuum calibration
- Non-contact sizing
- Air Rack - typically used for prototyping
- Water tank
- Atmospheric sizing
- Take-off pullers: Catepuller & Two-belt puller
- In-line knife cutter
- End forming