Consistent melt temperature is necessary for consistent part quality, but achieving this requires balancing multiple factors including barrel usage, barrel temperature, screw speed, back pressure, dwell time, and more.
During injection molding process, plastic injected into mold must be molten, homogeneous, and have desired melt density. Under action of heating belt and screw, melt is prepared in barrel. Temperature setting of heating belt, screw speed and back pressure all have a direct effect on melt quality.

 

Each of these settings has an independent or interactive effect on melt quality. Therefore, it is important to understand basic theory and apply these concepts when establishing a molding process.

Attention needs to be paid to maximum residence time of material in barrel. At a given melting temperature, every plastic degrades over a certain amount of time. Take polyetherimide (PEI) as an example: if melt temperature is 370℃, hold it at 370℃ for more than 12 minutes and material will degrade. Therefore, maximum residence time of PEI at 370℃ is 12 minutes. If processing temperature is increased to 410℃, maximum dwell time will be reduced to 6 minutes. That is, the higher melt temperature, the shorter maximum residence time of material. Goal set by process should be to keep maximum residence time of plastic in barrel less than allowable maximum residence time of material at set process temperature. Relevant parameters can be found in TDS provided by material supplier.

Molten plastic has two sources of heat: a heating belt outside barrel and shear heat from rotation of screw. Heating tape wraps barrel and provides external heat to plastic inside barrel. At the same time, screw rotates inside barrel, agitating material forward and using shear to provide energy to plastic. (Extended reading on importance of melt temperature and mold temperature)
Assume a cycle time of 30 seconds for a set of molds. If barrel usage percentage is 20%, plastic in barrel can support five shots (1/0.20). Therefore, depending on cycle time, residence time of material in barrel is 30 x 5 = 150 seconds. If barrel usage percentage is high, say 75%, then material in barrel can support 1.3 (1/0.75) shots, so dwell time becomes 30x1.3=39 seconds. Above two calculations show that the lower barrel usage percentage, the longer dwell time of material.

 

Figure 1. Setting example of barrel usage rate affecting barrel temperature.
Since longer material residence times can lead to material degradation, it is desirable to set barrel temperature as low as possible within specified range (Figure 1), with a target melt temperature of 490°F. In the case of low barrel usage, barrel front area should be kept near or slightly above target melt temperature, while when barrel usage is high, most areas of barrel can be kept close to or slightly above target temperature in order to get as much heat as possible into plastic as quickly as possible. Temperature must be set according to bucket usage percentage and maximum dwell time. (Extended reading: Setting barrel temperature of scientific injection molding)

Once barrel temperature setting is determined, screw rotation can be optimized. Heating tape transfers heat to plastic close to barrel wall, but shear force selected by screw provides heat that melts plastic inside. Screw also serves to homogenize melt (Figure 2). At lower screw speeds, screw selection does not provide enough shear, so resulting melt temperature will be lower than target temperature. Conversely, at higher screw speeds, shear heat generated can be very high and can result in melt temperatures well above target temperature.

 

Fig. 2 Screw speed has a direct effect on melt temperature.
Therefore, research to find optimum screw speed is to find functional relationship between melt temperature and screw speed. Figure 3 shows that melt temperature increases with increasing screw rotation. In this case, screw speed was set to 11 inches/sec. Another advantage of performing this test is ability to determine range of screw speeds that can be used.
If you need to increase screw speed and reduce screw recovery time, but want to minimize cooling time, you can try to operate based on this data. Figure 3 shows that if acceptable change in melt temperature is 5°F, screw speed can be set from 9 to 12 inches per second.

 

Figure 3 By conducting experiments, it was possible to determine what range of screw speeds would produce a homogeneous melt at acceptable material temperatures.