PETG tubing question

Skippy: Hey Buzz; Happy 2010!

Buzz: Howdy Skip - same to you. Glad to get 2009 behind us and start moving forward again -

Skippy: - right. To kick off the year - how about a quick processing question - the material? PETG or glycol modified PET for us extruders -

The question goes:
"Can anyone recommend a heat profile for extruding a PETG Tube 3" in Dia with a wall of 1 mm? Experiencing too much sag between the Die and the water tank in a vac sizer."

Buzz: the real answer of course is a heat profile that yields the correct clarity, and the least adhesive version of the material with the most hot guts you can get. This would of course also have to take into account the type of screw, compression, screens, tooling die pressure drop and other items. Not to mention that a heat profile while extruding at rate may or may not be the same as during string up, etc etc etc -

Skippy - well, yes, hopefully a dialog will ensue that will prompt a few more details. Do we have any thoughts that we can share assuming that the heat profile that is being used now is correct (and it just seems too hard to handle?)

Buzz: - you bet - although we can not be sure by the question as asked is whether we are asking in this way due to a surface issue, control of ovality or inability to string up. Will assume the worst -

There are a couple of more important questions for glycol modified PET -

1) What is the DRAW DOWN of the material - draw down orientation can be very useful; we used to produce our tooling approximately 50-80% greater than the size of the drawn down part for 'some' dependable orientation between the pin/shell and the calibration kiss.

2) Do you have an open air passage to the inside of the tube through the pins (perhaps through one or more spider legs) to allow air pressure inside the tube as you string it up?

3) is your tank in a position to give you the best shot at string up and running? Normally, you have the tank on center line in both plan and elevation to balance out "kiss", but this material is ADHESIVE versus just COHESIVE at correct melt temperature, so the KISS has to be at the last possible instant - you can’t just rub it all over and around the entrance of the sizing; it will want to set up too quickly, so you need a fair amount of draw down.

4) What is the nature of your calibration design - wafers, solid sleeve, rifled sleeve etc. as well as the surface texture - smooth, glass beaded, chromed etc.

Skippy: What about this "sag" business?

Buzz: At the same time as you are drawing down, the tube is wanting to "sag" due to gravity; recognizing, planning and depending on the "sag" that you mention for good clarity can be advantageous - so for this reason, the vacuum sizing tank as a whole is inline from a plan and elevation view as well as level for start up but after the string up, you will run it LOWER (still level; drop BOTH ends) than the die and backed away to point that still allows a kiss, but minimal contact prior to going down the "barrel" of the tubing calibration unit - and the tube does some controlled "sagging" as it falls from the die face down to a lower level calibration entrance level, then is pulled in evenly into the calibration unit.

Skippy - Cool; any more pointers?

Buzz: Assuming you are using a vacuum tank, I prefer a vacuum chamber that is 2' or less to get the line up into a tube quickly. Mount a fogging sprayer in the lid of the tank that can be used to spray the material with a mist of water (which will harden it pretty quickly at low speeds). You use this during the vacuum chamber sealing and water fill, then turn it off once a regular vacuum is achieved.

Skippy: What about gasketing?

Buzz: Be sure to have an adequate seal at the down stream exit of the vacuum chamber - one sealing material preferred is a silicone rubber (smooth orange) that is pretty friendly to sticky PETG. Use a few test swatches - you are looking for a supple material that will stretch to seal off as pulled downstream but doesn't stick and release. In general, the gasket material might ben1/8" thick and cut about 1/4-3/8" smaller on center than the OD of the final tube depending on suppleness. It is extremely important to cut the inside circle of the gasket on center with the supports and the tube itself. The seal material should be supported on BOTH sides of the seal with semi rigid to hard supports. The UPSTREAM support ring should be the OD of the tube plus a smidgeon (maybe 1/8" but not more than one thickness of the seal material larger than the final tube OD). Not larger than one times the thickness of the seal material, or the seal can be sucked upstream through the support easily. The DOWNSTREAM support ring should be the OD of the tube Plus 3 times the thickness of the seal material and a smidgeon more, so as not to be pinched too tightly by the tube when it is pulled round and the friction between it and the tube pulls it evenly down stream to minimize drag/release marks.

Skippy - gee the gasket and tube sealing technique described there sounds like a good practice on just about any tubing -

Buzz - Right!

In addition, you will need cooling that is MEASUREABLE in gallons per minute depending on line speed and calibration design for duplication in the future (see another post about water use across a plant), and a water level that has the cooling around the calibration deep enough to prevent air being cavitated into the cooling stream turbulence.

At string up, use a bit of silicone mold release on the shell and pin face that is clean to avoid material sticking to these surfaces and melting into "drool". Try to control your pin/shell temperatures to the extent that you can cool down the shell slightly to below the melt temperature which will result in a duller looking matte surface (melt fracture as the material slows down at the wall) at string up. This will give you a little more in the way of "hot guts" with a material that is very soupy at melt. When the Shell is a bit colder than the Pin, the material will bloom AWAY from the pin face once trimmed off. Spray a bit of silicone mold release on the pin face, then work to string up the line through the calibrator and avoiding touching the die face with the PETG as you pull it into the calibration towards the take off. Some of the sticky material may have stuck to the Shell face during the bloom once you get the line strung, you can scrape the face of the shell as necessary to clean up any material (being careful not to cut the material off the pin etc).

Skippy - is it hard to handle down line towards the take off?

Buzz: Once you are pulling the material down line, twist the material as you pull it into a 'rope" in water that has been brought up to the bottom edge of the calibration. The transition point is so abrupt that it will harden very quickly at this point; the twisting into the rope keeps it straight -

This next part is a little tricky - it takes a time or two to get the hang of it; read and visualize it a couple times before trials:

Once in the take off, you need to seal up the vacuum chamber -

a) move the tank forward to an adequate "kiss point", and
b) will open the drain to the chamber slightly (only a crack - this will help prevent pulling too much vacuum too quickly) and you
c) will apply vacuum to the calibration chamber, while you
d) turn the water misting/fogger lightly on (don't blow it out the face of the die) to set the tubing up as a "tube" while you simultaneously use a wet rag in the exit side seal area to "seal" the vacuum chamber around the rope until the "tube" arrives and is sealed with the gasketing. You do turn on the general fill water to be bring up the water level as well - just enough to fill but not flood out the front and hit the die - the light vacuum being maintained with the wet rag at the back and the drain cracked should allow you to make a "tube" in the sizer and keep water from going forward to the die -

Skippy: So . . . you want to be pulling enough vacuum to encourage the air pressure in the tube to expand it to fill the calibration unit and be raising the general water level to coincide with covering the tube when the mostly filled tube gets to the rear seal. Once you have good vacuum and the tube covered, close off the drain, bring your water level on up and level out your vacuum in the chamber?

Buzz: Perfect! Get things up to rate, drop your tank slightly and move the kiss back to a sweet spot and run with it.

These thoughts should get you up and running - good luck.

Just our two cents -
Skippy and Buzz

Worn Screw/Barrel - Extruder Surging - more thoughts

Skippy: Our first stab on this question was put out there and a question came back on our preliminary thoughts below - concerning whether this was seen on single or twin screws. Others have observed more wear at the start of the life cycle on singles -

Buzz: Oops, engaged fingers faster than brain. The intent is not to be aggregating total screw and barrel wear as being a one side or the other phenomenon; regret it appearing to be so one sided - to continue;

In singles, we did see wear in a brief spike at the beginning of each screw and barrel rebuild for a short duration while all of the items "wore in" so to speak (remember, we were looking at situations where multiple screws and materials went into each machine).

Skippy: What happened after this initial break in period?

Buzz: We would observe a long period of wear which would be fairly predictable when on lines committed to more or less one material, and then accelerate nearer the end as the wear began to accumulate enough to cause significantly greater changes to process 'output' and we applied more and more resultant torsion to the screw and barrel interfacial region with other process variables. Our feeling was that since the screws were captive at the hub and near the gate, the torsion resulted in a greater arc in the "bend" to the screws and opened up the clearances at an accelerating rate in the 'middle' of the processing area.

Skippy: Did machine size or rate expectations play into this at all?

Buzz: Other factors coming into play did include the over-all rate as a percentage of "top end" capability since the observations were made on 2 1/2 and 3 1/2" machines and our goals did include increasing rates to "gain contribution per hour." I guess the anecdotal way to communicate this concept is the same as saying that when painting, using an 18" roller versus a 9" doesn't double the output at the same intended quality with out other factors being affected.

The other major difficulty area for our data may have been the constant change from one material to another, the various screw designs with and without various mixing elements in differing locations down the screw. Rather than having to overcome "wear" in one part of the screw and barrel with ONE screw and barrel, things were changing up and down the inter-facial area probably from just after the feed section until well into the metering sections from a number of the combinations.

Skippy: so in the end, wear begins to accumulate and . . .

Buzz: Well, once we began to be confronted with the results of leakage back over the flights, and the corresponding reduced outputs etc, the natural tendency was to try to put a toe or two out over the edge without leaping into the abyss - increase screw speed.

Skippy: Hmm. Most find that dealing with the resultant increased shear, and "residence" time for a portion of the material along with the general increase in melt temperature requires rethinking how to pump the material down a screw and barrel (with increasing wear) in differing manners versus suffering the resultant degradation, adhesive versus cohesive, flow related and sometimes aesthetic issues related to a hotter melt.

Buzz: Right. This tug of war over the abyss was generally continued until we would be delivering sufficiently high rate, continual output in terms of rate but dealing with stratified material coming out of the metering section in the 'plug flow' transformation at the breaker plate after the screw tip with a lower level of melt homogeneity. Instead of relying on the breaker plate and screens to simply help move ribbon to plug flow, we found ourselves relying on these to play an increasing role in melt mix distribution as well.

Sorry, all this once again is beyond the scope of this discussion. The short course was that it was (well past?) time for a screw and or barrel rebuild or replacement; and remember first that as processors, we need to look holistically at all the the interelated variables and manage them as just that - a whole.

Skippy: What about twin screws?

Buzz: A couple of things; on twins - yes we saw the most amount of wear at the end of the process since the forces exerted in the conveying mechanism were significantly different and other control options like screw tempering and differing feeds like starve versus flood feeding in combination with screw rpm and fusion rate modifications with the raw material supplier on formulations gave us a good many more options over the life of the screws and barrels -

Just our two cents