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3. Hump: This pattern has the highest temperature settings (typically 20+ degrees higher than desired melt temperature) in the middle of the screw to correspond with the transition section where the majority of the melting takes place. Settings near the feed throat are typically at the softening point of the resin while the settings at the front of the barrel and the nozzle should be at the desired melt temperature. This profile is recommended when SBCR is 25-50% and overall residence time is 2 to 4 minutes.
4. Flat: This profile uses the desired melt temperature as the settings for all of the barrel zones except for the feed throat, which should be set at or below the softening point of the resin. This profile is typical of processes where the SBCR is 20 to 40%.
In actual practice, the specific screw design also plays an important part in obtaining the desired melt. It is possible that different screw designs may require different profiles to achieve similar melt characteristics even if they have the same or an equivalent SBCR.
By varying the screw speed and back pressure, shear heating is, for the most part, an easily controlled source of heat to the material. The best parts are typically produced when there is a balance of shear heating and heat from the heater bands. Once a temperature profile is chosen, it is recommended that the processor monitor current flow into the heater bands. The proper balance of shear heating and thermal heat is achieved when the current cycles regularly (typically several times a minute). This is of particular importance in the transition zone of the barrel; if the barrel is pided into quarters along the length, the transition zone is typically the middle two quarters of the barrel. Typically the heater bands in the feed zone will cycle regularly or be on nearly all the time. The heater bands in the metering zone and nozzle should cycle regularly but less frequently than the feed zones. Because accurate temperature control is so important, it is always a good practice to routinely check the calibration of the controllers.
Figure 36. Energy needed to bring PP to melt temperature
Figure 37. Various barrel temperature profiles
If a heater band is on all the time, either the set point is too high to be reached or there is a problem with the thermocouple or heater band (it is working but not reading the actual temperature). If the thermocouple is fine and the desired setting for the zone is not out of line, the processor can increase the temperature settings upstream (closer to the feed throat) of the zone in question. Should this not reduce the time that the heater band is on, the temperature setting on the zone should be lowered until the band cycles regularly. This will prolong the life of the heater band and reduce the energy usage.
If a heater band does not draw current, or does so infrequently, there are two possible problems. Either most of the heat going into the plastic at this barrel zone is via uncontrolled shear heating or the thermocouple is broken, both of which should be corrected. A broken thermocouple will typically read out the maximum permissible temperature. If all of the cavities are filling with acceptable cycle times, the heater band set points in the zones upstream of the zone in question should be reduced. If this fails to get the heater band cycling, reset the upstream zone(s) to their original temperature(s) and increase the temperature set point.
It may appear that the procedures above are only serving to increase the overall melt temperature. While this is true to a small extent, the benefit is that they aid in providing a more homogeneous and controllable melt temperature that will improve the molded parts.
Now that we have a homogeneous melt stream in the barrel or accumulator, we need to examine the introduction of the plastic into the mold. The viscosity, or resistance to flow, of the resin is affected by temperature and shear rate. Increasing the melt temperature reduces the viscosity of the resin making it easier to fill the mold. Increasing the pressure or injection rate increases the shear rate, which decreases the viscosity making it easier to fill the mold. Therefore, given a resin, machine and mold, there are three variables that can be used to fill out the mold: