I got this question and felt it would make a good discussion on the blog...
Larry
Where and why is a reverse profile barrel temperature used?
My Response
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.
Additional Thoughts
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.
-Andy
Hello Andy.
When is it good to adjust a reverse profile?
Milan
Good Question Milan,
Basically, the temps should be adjusted based on one of three factors.
Rear - To better optimize the feed
Front - To ensure the melt temperature matches the middle temp
All Zones - To make a significant change in the melt temperature
-Andy
Andy,
When I will use reverse profile, could I have problem with trapped air between pellets? Cause when material will melt too soon, there can create seal of molten material and air trapped between pallets can't go out.
MJ
Andy.
What should I do first? Optimize barrel temperatures or feed throat temperature?
Milan
Milan,
The easiest way to do it is as follows:
1) Set all the temperatures to the desired temp (straight profile)
2) Determine the optimal feed zone temperature
3) Adjust the front zone up or down to obtain the desired melt temp.
-Andy
Andy,
I saw in your free video, that I should adjust all temperatures to desired melt tempereture, and when front zone doesn't equal melt temeperature I should change just middle and rear zone temperature.
Can I do it in this way?
Milan
Hello Milan,
I believe it states it a little differently... but the most important aspect is to have structured approach which works to optimize feed while obtaining the desired melt temperature.
-Andy