hi,
i am facing a typical problem of barrel temp overshoot, we process polycarbonate, abs and pp. machine is 130 ton g2 series toyo. when we turn on heaters parameter cut off at right temperature ie set @220 cut off at 220 but once the machine starts it's cycle the temp starts overshooting i.e. set@ 220 actual is 235 in mid 3 zones of the five. as if barrel is generating the heat on it's own
to further rule out screw bend problem 1. we formed lumps and free refilling (ie. at the time of refill we allowed material drooling from barrel) with different backpressure and injection rate there was no overshooting temperature remained stable. we tried this on pp and abs at least 20 cycles each and no overshooting at all even of 1deg at any zone
2.next step we formed lumps and this time we made a nozzle touch with mold to stop drooling and the temperature started overshooting again with almost 1 deg rise at 2-3 shot gaps and by 15 - 20 shots we were overshooting by almost 4-7deg in middle 3 zones

what we have tried to solve all this is
1 checked all the thermocouples
2 checked all heaters

found nothing amiss.... now before opening screw set i want to double check if i am missing onto something

if anyone can guide me on this that would be great

thanks

Try playing with your back pressure and screw speed. You may be getting "shear temperature" which is caused by friction. Reduce back pressure and speed until the problem goes away.

...oh, and the mold gating design can also increase your back pressure. If you aren't having this problem while cycling without filling the mold, it may have something to do with the gating. However, a process problem is hard to solve remotely.

I'm with Kevin, sounds like shear heat some where. Process,or worn screw and or barrel?

As per your description it must be the shear heat,
to be sure you can check one more thing.

This temperature increase would be quicker and higher in case of more viscous material.

The Easiest way to solve this problem is by cooling the barrel with the help of fan air during operation,
so temperature may get down quickly
but this way your electrical power consumption may increase.

Do you know what Screw Type is in the Barrel ? Is it an "All Purpose" Screw or maybe another type that is specific use with PP. Since the Shearing action within the barrel creates most of the resin heat, I would suspect that the Barrel Type and/or your Barrel Speed is contributing to the temperature rise.

I have to agree it sounds like a worn screw and barrel. remove screw and measure the clearance between the two. You will probally find that there is more 0.016 clearance between the barrel and screw. We replace ours when we have this much clearance.
Although some smaller sized screw and barrels you have to run with a tighter clearance
such as 22mm and below. These have to run with less clearence 0.006. This is becauce the checkrings are soo small they actually expand during injection and too much clearance will cause the check ring to crack.

In the past I have seen this with screw and barrel wear issues, or screw application problems as others have suggested but consider this. You may not have enough energy in the chip when it gets to the compression zone so all the work is being done in the zone of the override. I have raised earlier zones to add energy. Be careful not to get the zones too high in the feed zone as your chip will get sticky and stick to the screw. This will reduce area between the flights and effect the volume of throughput. I am sure you know this. Also, I have also increased the process temp in the dryer to introduce more energy there.

Also check your residence time in the dryer to ensure the temp of the chip is all the way through the chip, not just on the surface. Do the cycle time and part weight calculation to see what the hourly throughput is. Get the bulk density of the residence and based on height in the dryer and hopper volume make sure you are getting the minimum the resin mfg'er recommends. If it is marginal raise your dryer hopper level. We always tried to get as much energy into the chip as we could before the compression zone because sheer caused us other problems.

Good luck!