I got this question and felt it would make a good discussion on the blog…
Where and why is a reverse profile barrel temperature used?
A reverse profile is typically used when processing materials such as nylon and LCP. In fact it is quite common for many semi-crystalline polymers. Basically, such polymers have a melting point. The high rear temperature often helps improve conveyance as well as melt the polymer quickly, Once melted, the polymer can often be cooled a little in the front to improve the overall cycle time and meet the desired melt temperature.
Usually the need for a forward or reverse temperature profile should be approached scientifically. The middle barrel temperatures should be set to the desired melt temperature. The front zones are typically adjusted to obtain the desired melt temperature. The rear temperature should be set to optimize feed and melt consistency.
Historically, low viscosity polymers with high crystallinity tend to process best using a reverse profile… but you should always look to optimize this, since everyone has different screws, barrel, heater bands, thermocouples, and temperature controllers.
Try not to confuse the front temperature with the actual nozzle temperature. The nozzle temperature will be adjusted independently to meet the needs of the specific process, such as avoiding drool or freeze off.