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• Injection/packing pressures
• Injection rate
• Melt temperature
The curves in Figure 38 indicate the relative temperature-pressure relationships for PE resins of varying melt indices. The higher the MFR or MI, the lower the injection pressure and/or the temperature required to fill a mold. Assuming the same mold filling characteristics (fill speed and fill time), cycle time and injection temperature, a high flow resin:
1. Will allow pressures to be reduced about 25% when the resin MI or MFR is doubled.
2. Will allow a decrease its melt temperature of about 70°F (40°C) when the resin MI or MFR is doubled.
The effect of a higher flow PE resin on temperature and pressure can be seen in Figure 39. Note that as the MI or MFR of the resin increases the possible reduction in temperature and/or pressure will become less.
However, the switch to a polyolefin with higher flow characteristics usually results in a loss of other properties such as resistance to stress cracking and impact strength, especially at lower temperatures.
Injection and/or packing pressures are typically the first settings adjusted by the processor because they have a quick response on mold fill. Increasing the pressures will help fill out the mold correcting for short shots and reducing or eliminating surface defects such as sink marks and ripples near the gate. The downside of increasing pressure is the chance of trapping air in the cavity resulting in burn marks or of increasing the flash on the parting line due to the mold opening. Increasing injection pressures also pack resin more tightly into the cavity, which may reduce shrinkage, increase the gate temperature(s), and increase molded-in stress. The reduced shrinkage can lead to a part sticking in the cavity and also post-mold dimensional differences.
Figure 38. Temperature-pressure relationships for polyethylene resins of several melt indices
Increasing the injection rate(s) reduces the viscosity of the resin, which may reduce the amount of molded-in stress in the part. In addition, an increased injection rate may also yield a more uniform part temperature (due to faster introduction of material into the mold) which can reduce differential shrinkage (i.e. warpage) due to temperature variation. Increasing the injection rate(s) without a decrease in injection pressure can lead to flashing of the part. Changing the injection rate(s) also has a fast effect on part quality although it may take time to finetune the rate(s) for optimum quality. Excessive molded-in stress can lead to an increase in warpage and a decrease in impact strength and environmental stress cracking resistance.
Sometimes the injection rate and injection pressure are not independent variables; i.e. the machine is set with a maximum pressure and runs on injection rate settings which are set on screw position. This setup will allow the machine to vary the injection pressure based on the pressure needed to meet the rate set points. Conversely, the injection pressure can be specified based on screw position and the rate is allowed to vary. Some processors are now utilizing pressure sensors within the cavity to control the operation of the machine via cavity pressure. This is a new approach that is gaining acceptance for molding parts with critical tolerances. It is also applicable to molds (such as syringes) where core shift is of concern.
The final way to control the viscosity of the resin is to adjust the melt temperature. An increase in temperature will decrease viscosity. Changing temperature settings yields a slower response than pressure or injection rate. High resin temperature can lead to degradation and require longer cooling time while low temperatures can lead to shot inconsistency, higher injection pressures, and excessive wear/damage to the screw and barrel.
When setting an injection rate or injection and packing pressure profile, the aim of the processor should be to provide a smooth delivery of material into the mold. A momentary slowing of the screw due to either the transfer from one step to another or too large of a step can result in a hesitation of the plastic flow front. Hesitation of the melt front can cause surface defects such as flow lines or tiger stripes, which may lead to poor weld and/or knitline strength. Therefore it is necessary to reduce the rates or pressures in a consistent manner to prevent flashing of the tool, potential core shift(s), and bottoming out of the screw.