Skippy: Say, we thought a bit more about the response in the last post, and hadn't realized we didn't ask a couple more 'broad brush stroke" questions -
a) are you using a color concentrate with the PE and PP that is being exuded out from the melt and being deposited/captured/coating the calibration unit or plugging vacuum holes and slots? This could be insulating the calibration unit and preventing efficient heat transfer - may need to be checked on a regular basis if the colorant system and or process can't be modified to eliminating this "plate out" effect. A rubberbanding or thinning/stretching effect in the calibration unit due to this insulating of the calibration could intermittently look like folding at the front of the calibration unit - rather than a constant haul off down the line -
b) we didn't really discuss the travel method, and assumed that the take off unit was capable of overcoming the normal calibration drag and pulling the part at a constant rate of speed - without belt/cleat slippage or obvious motor speed problems. Depending on the drive makeup, one thing to check is that the take off is operating at approximately 70-80 percent of it's rated motor speed in the correct gear reduction range if applicable at the correct haul off rate.
c) is the output of the extruder constant? as we had assumed; - if not could be a host of issues - check by cutting off 10 to 60 second pieces of hot melt at the die -several in a row, cool the mass and weigh on an accurate (gram?) scale to see what if any variation is occuring at the extruder. It is important that the sample size be large enough to overcome the inherent error in your "by hand" cut off of the samples -
Buzz: and d) we were also assuming that the calibration unit had previously been checked for obvious damage, nicks, blocked ports, correct coolant and vacuum hook ups etc. and that the Vacuum tank itself was in correct running order with adequate coolant into and out of the system - sorry for not thinking of those items as well earlier -
Good luck -