The injection unit consisted of following major elements:

Barrel

It is a cylinder where raw material changes from pellet to molten stage. To well control the temperature inside the barrel is really important during the process. This is determined by the machine manufacturer but following principle shall be followed: the temperature control should provide at least three independent zones, with thermocouples placed near the center of each zone. Burn-out of one or more heater bands within a zone may not be readily apparent from the temperature controllers, so some molders have used ammeters in each zone to detect heater band malfunctions.

Screw

It is a part inside barrel to transform the plastic, which including:

• Rotates and Move Backward: while it rotates, it moves plastic from back area to front area and creates shear heat to help plastic pellets melts. While screw moves backward, it creates space in front of barrel for melt plastic ready for injection into mold.
• Move forward: Push the melt plastic into mold.

Screw design is a key parameter for productivity because the screw rotation time is an inherent part of the cycle time. Exceeding the output capability of an inadequately designed screw will cause wide temperature variations and unmelted particles (sometimes unmelt and degraded material have been observed at the same time). The result is loss of toughness, variability in shrinkage and dimensions, warping, surface defects, plugged gates (leading to short shots) or other molding problems. The length of the screw will also affect the melt quality (an insulating material needs some time to get the thermal energy transferred even if the shear contributes to the heating process).

The ideal screw size is determined by the volume of the current shot. Optimum productivity will be achieved when the shot size requires a screw travel during plasticization equal to or lower than 50% of the capacity of the injection unit. Otherwise, screw rotation speed will have to be decreased at the end of the travel to guarantee a homogeneous melt, leading to a loss in productivity. Practically, optimum productivity is achieved with a screw travel of between 1 and 2 diameters of the screw. Thermal settings of the injection unit will be dependent on the residence time (HUT) and hence dependent on the cycle time. Rules will be presented under “Molding Process.”

Check Ring

It let plastic move from back area of screw to front area while screw rotates and rove backward, and it does not let plastic move from front area of screw to back area while screw move forward.

Malfunctioning that allows polymer backflow is also a common experience and is caused by poor design or maintenance. A leaking Check ring will add to screw re-filling time, which can increase cycle tiom, and it will also cause poor control of holding and dimensional tolerances. The Check ring must meet the following requirements:

• No holdup spots
• No flow restrictions
• Good seal
• Control of wear

Nozzle

It is a part of injection unit that contact to mold during molding process. The following should be considered:

• The heater band should extend as close to the nozzle tip as possible and cover as much of the exposed surface as practical. This counteracts any heat loss, especially heat loss to the sprue bushing.
• The appropriate thermocouple location is important.
• Adequate temperature uniformity is required so that local overheating or premature freezing is avoided.
• The nozzle heater should have its own independent temperature controller.