The most critical step in the extrusion process is the proper temperature and heating control of the plastic inside the extrusion barrel, according to ZHYQ Sensor Australia specialists in sensor design and product technology. This applies whether the operator is a profile extruder, wire coating, sheet extruder or involved in any other downstream process.
If the molten plastic does not have the desired properties when it reaches the die head, achieving quality output will often be difficult.
To protect the extruder barrel from over-pressurisation, a shear pin can be installed on a swing gate. Rupture disks also can be installed at the die end of the barrel. If melt pressure is more than the rating of the rupture disk, the disk fails, and the pressure is relieved. A pressure transducer with an alarm is also common as a warning device. The transducer can be set to alert the operator or to shut down the extruder
The pressure inside the extruder barrel during the extrusion process is extremely high. Barrels for most extruders are designed to withstand operating pressures up to 10,000 psi or 700 bar. Normal operating pressures generally range between 1000 and 5000 psi. or 70 and 350 bar. Too much pressure can rupture a barrel.
To regulate barrel temperatures, temperature controllers use barrel heaters and cooling units. Smaller extruders, up to a three-inch diameter, usually have three or four heating and cooling zones. Larger extruders often have six or more zones. Each zone is equipped with temperature sensors and a temperature controller.
To operate, the barrel must be heated. Most barrel heaters are electric. Cast aluminium and mica band heaters are among the most common heaters in the market. Cast heaters are cast aluminium split cylinders with embedded insulated resistance wires.
The simplest type of heater is the mica band heater. Mica band heaters contain coated resistance wires sandwiched between mica insulation with a steel enclosure for support. Ceramic heaters are used in applications with high heat requirements.
As both cast aluminium and mica band heaters must make tight contact with the barrel, periodic inspections and tightening of the heaters should also be part of a regular maintenance routine. Heaters will burn out if they cannot transfer their heat to the barrel.
While barrel heating is essential for operation, also important is barrel cooling for the extrusion process. Extruder barrel cooling is necessary if the plastic gets too hot or if the extruder must be shut down quickly.
Barrels can be cooled using air or liquid. Barrels that are air-cooled have blower units and a plenum chamber under the barrel at each temperature zone. The blower is turned on by the barrel temperature controller when an upper setpoint is reached.
Barrels that are cooled by liquid, circulate the fluid through coils cast into the heater blocks or through separate cooling coils. These can be closed loop, re-using the same liquid, or open loop.
It is critical to maintain temperature control over the melt during the extrusion process. Cooling the barrel is one way to control any overheating of the plastic. However, if an extruder requires a sizable amount of cooling during normal production, it’s an indication of a mismatch between screw design and that particular plastic, or a problem with the extrusion process.