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Screw design for polyolefin processing was covered in a previous section. A worn barrel and/or screw creates an increased gap between the two resulting in resin moving backward or staying in place instead of being conveyed forward by the screw. A worn screw and/or barrel can lead to poor melt consistency, degradation of the resin, and shot inconsistency. Screw speed should be set so that the screw consistently recovers 1-2 seconds before the mold opens.
Backpressure is typically set at the minimum level that delivers a homogeneous melt (no unmelted pellets leaving the nozzle). However, backpressure may need to be increased to improve temperature consistency in the melt and to minimize or eliminate streaks due to poor dispersion of colorant. During plastication, most of the energy provided for melting the resin pellets comes from shear heating due to friction between the pellets, screw and barrel. As the back pressure is increased, the screw works the material more in order to convey it forward, thereby raising the temperature of the melt more quickly. The increased work by the screw also increases the mixing of the molten plastic resulting in better temperature and homogeneity of the melt. However, too much back pressure can result in degradation of the plastic, an increase in the screw recovery time, increased energy costs, and more wear on the screw and barrel. Shear heating is also dependent on the viscosity of the plastic, screw design, screw speed, and back pressure. The latter two can be varied to some extent by the processor to control the shear heating and melt temperature.
The shot-to-barrel capacity ratio (SBCR) can also have an effect on melt, and therefore part quality. The ideal range for the shot to barrel capacity ratio is 30-60%. If the SBCR is less than 30%, too many shots of material reside in the barrel under the influence of heat from the heater bands and shear from the screw. This may lead to overheating and degradation of the resin. If the SBCR is greater than 60%, less than 2 shots of material are in the barrel, which typically does not allow the melt temperature to equilibrate. A high SBCR will also mean that the screw may recover (develop the next shot) just before the mold opens which can lead to cycle alarms due to inadequate shot size. A SBCR range of 30-60% will provide adequate time for the melt temperature to equilibrate.
In order to achieve proper melt homogeneity, all of the pellets should be melted by the time they reach the middle of the transition zone on the screw. Figure 36 depicts the amount of energy needed to process a polypropylene impact copolymer. In order for the pellets to be fully melted halfway through the transition zone, 71% of the energy to reach the desired melt temperature (in this case, 450°F) must be transferred to the polymer.
The heater bands on the barrel provide only a small amount of the energy needed to melt the plastic. Most of the energy from the heater bands maintains the barrel temperature during processing and raises the temperature of the solid pellets in the feed zone. There are four typical temperature-setting patterns for injection molding on a barrel with five heater zones (Figure 37):
Figure 34. Injection molding cycle for a 16 oz. stadium cup (4 cavity, HDPE, 31g each, 7.8 sec. cycle)
Figure 35. Injection molding cycle for automotive fascia (12 lb. shot, 0.125 in. thickness, 107 sec. cycle)
1.Increasing: This pattern has the lowest temperature setting at the feed throat and the highest at the front of the screw with a steady increase of the temperature settings in between. The nozzle is typically set at the same temperature as the front zone. This pattern is the one most commonly used and is particularly recommended for lower melting point materials (such as EVA or EMA) to prevent bridging at the feed throat of the extruder. It is also recommended when the SBCR is low, typically <30%.
2. Decreasing: This pattern has the desired melt temperature at the front of the screw and the highest temperature at the back of the screw. The nozzle is usually set at the same temperature as the front zone. This temperature profile is recommended when the SBCR is >50% and screw recovery and residence times (time from resin entering the extruder to leaving the nozzle) are short. Sufficient feed throat cooling must be provided to prevent bridging. Otherwise, a low temperature set point should be used at the feed throat. In addition, this profile can increase the chance of air being entrapped in the melt instead of venting back through the hopper.