Extrusion of PETG - more thoughts

Skippy - hey Buzz - we never did actually give a "heat profile" to start up the PETG in the last discussion -

Buzz - yes, correct. Why? Well, mostly to see if these comments were helpful in the event that they did in fact have a start profile capable of

a) getting melt to the tooling and
b) keeping a steady state melt coming

No mention of the current melt temperature or current heat profiles or equipment, rates, tooling etc was given, (we are also assuming that adequate drying was in fact occuring) and PETG doesn't have too much in the way of "hot guts" anyway -

Skippy: so we pulled up a little short in the actual "heat profile" part of the answer on purpose since they didn't mention the equipment, screw/barrel etc being used. Suffice it to say that achieving the recommended melt temperature on heated (and controlled heating) screws and tooling is necessary; more information is required to make a more specific recommendation.

Buzz: Yes, much will depend on the screw length/diameter and screw design and intended rates and draw-down. The first couple of zones can be used to keep the melt cooler (but this again depends on screw design) and it is generally wise to keep barrel cooling OFF during start up to avoid freezing material along the screw and creating melt blockages.

Skippy: Typically, processors successful at extruding tubing and profiles from PETG use an "efficient" screw design to keep the melt and unmelt seperate until near the end of the transition area - particularly with thin re-granulated clean dry scrap. And generally (as indicated by Eastman) high compression screws like those designed for HDPE will generally not be suitable for PETG copolyester since excessive melt temperature and poor melt strength usually result -

Just our two cents
Skippy and Buzz

Worn screws/barrels and extruder surging

Buzz: Oh yes, Skippy; another thought on the discussion so far about extrusion screw and barrel wear and "surging".

As you know, we have been involved with extrusion companies in two broad camps -

a) those that religiously measure and rebuild screws and replace barrels when they expect wear to be too much of an impact on reliable extrusion, and

b) those that have machines that you can rattle the screws around in worn barrels to a good degree

and yet because they can continue with process updates and changes produce product profitably, these companies couldn't care less about the thought of "measuring the wear" and moving directly to repair/replacement.

Skippy: - but how can this be? If both can achieve steady state extrusion with a wide variety of virgin and regrind materials in new and worn equipment, and both can exhibit machine conditions where "surging" exists - there must as Paul Harvey says be a "the rest of the story" -

Buzz: well yes. We have some thoughts on this that we would like to share with our readers and use to develop some discussion on solutions as well as discuss why this TYPE of question is so difficult - once we've had a little tryptophan er um TURKEY to help marshall our thoughts, we shall continue.

Skippy: - what direction is the discussion likely to go?

Buzz: well, I'd like to see us confirm that there is essentially no right or wrong answer here, but due to the system or environment in which it is answered, it could go either way.

BECAUSE of that, perhaps it is time for us to spend a few sessions in a much more compressed time span talking about a number of the other extrusion variables that can contribute to "surging" as well, how to spot them, how to eliminate or at least control them, and certainly the importance of being aware of them. In this way, our readers hopefully will jump in with additional thoughts and our dialog will be that much more productive -

Skippy - can't wait - see you in a day or two . . . Happy Thanksgiving to all!

(to be continued)