Extrusion 
processors’ problems 
meet their match

The launch of our Extrusion Expert series of webinars in January generated great feedback and a long list of questions. Here we share with you not only the questions your competitors and peers asked, but also the answers to those. It’s an extrusion information overload.

If we’d held it in a lecture hall, it would have been packed to the rafters. Almost 300 processors attended the first webinar in our Extrusion Expert series to learn from our host, Allan Griff, consulting engineer and extrusion expert with more than 40 years of experience in the field. In that webinar, the first of a series of six planned this year, Allan presented on “Data acquisition: Get the numbers.”

The questions poured in, and Allan answered them during the event or in e-mails sent after the event. We looked at the questions, considered how many processors may have the same or similar issues, and knew it made perfect sense to share some of this Q&A with our entire readership.

Some of the questions pertain directly to Allan’s webinar slides whereas others are more general in nature; his answers also go beyond “getting the numbers.” Hope you enjoy the article. Join us for the rest of the Extrusion Expert series, and we look forward to hearing your tales on how you put this information to work in your shop.

Figure 1. The screw-cooling fitting on the rear of the extruder wobbled back and forth with each revolution, showing the count, but the loose fit stressed the connections. Figure 2. A black mark around one of the four holes in the back of the screw can serve as a counting mark.

Q: Is throat cooling necessary if feed is good?
AG: Technically, no, but practically, yes. It depends on the plastic being run and the construction of the feed area—how easily heat conducts up from the barrel into the throat and even the lower portions of the hopper. Water cooling is useful there to keep the metal from getting hot enough for particles to melt and stick to those surfaces. That would make the feeding passage narrower, and in some cases that might limit rate, or almost as bad, make the feed erratic.

Plastics vary a lot as to how easily they stick to a hot surface. Some slippery ones won’t stick at all, while others may be very adherent—obviously, the water cooling would be more useful with these. In addition, particle shape matters—flaky ones (like chopped film or thin bottles) have more surface and less mass, and are more likely to stick.

Seldom is throat cooling confined to the throat, which is the vertical passage from the hopper down into the screw. Most extruders are made with a feed casting—a metal casting that includes the feed opening and the throat above it, as well as the surrounding for the first few flights of the screw, and the casting has water passages that cool all these places.

This presents a problem in controlling the rear barrel, where sticking to the barrel is necessary for good solids conveying (my Key Principle #5) (Ed. note: Allan’s Key Principles of Extrusion are available at www.griffex.com/tenkeys.pdf). The typical extruder sacrifices some of this control via this cooled feed section. It can be argued that the cooled zone prevents melted material from leaking backward around the shank of the screw; that is possible with low-viscosity melts and large clearances, but is seldom a problem. At best, we can have some thermal isolation of the feed casting from the actual barrel.

Part of maintenance is to check the circuit to see if water is really flowing and how much is flowing. This becomes a baseline to compare in the future. (If you know what good is, you’ll know what fishy is.) Also see if the valve that adjusts and shuts off the flow is operative. I remember one case where the water was permanently connected, no valves were visible, and no one knew how much was flowing or how to turn it off!

The best way to find out if such cooling is needed is to turn it off and watch what happens over time. It may take many hours for the throat and lower hopper to heat up to trouble temperature, but if you are experimenting, you can follow it with an infrared gun and see when you have equilibrium. You can do this easily with small machines. For larger machines, production people may not want to do experiments, so it may make sense to leave it alone because the water doesn’t cost much and the machine comes prepared to do it. It will be most useful for adhesion-prone materials (ethylene copolymers, plasticized PVC), but for pelletized resins like nylons and HDPE it may be worth looking into the cooling, after figuring out how much it really does cost. PET is a special case, as it easily sticks to itself if not completely recrystallized, especially in flake form.

Sometimes you have to wait for a problem to occur that can be traced to erratic feed, or else see actual material stuck to the throat on inspection.

Q: What is the best way to reduce gel levels in HDPE?
AG: First, make sure you are talking about gels by examining them in a low-power microscope. Gels are uncolored, even if the resin is colored, and roundish but not perfectly round in shape. They won’t dissolve in solvents for the base resin, but that’s academic for HDPE as anything that dissolves HDPE is something you won’t want to work with.

If you are sure these are “classic” gels, the next question is whether they are coming in with the resin or are created in the process. The best way to find this out is to run the virgin resin through a clean, small extruder. It’s virtually impossible for gels to form from clean resin in just a few minutes. Thus, if the product comes out with gels from the beginning, and the gel content stays more or less the same for a while, they are in the resin and you have a good case for rebate or refund if it was sold to you as clean. There is even a machine on the market that blows film and counts “discontinuities per square meter of surface” to quantify gel content.

You can also get the same answer on production machinery, but it is less precise. If the gels don’t appear for a while, maybe a few hours or even a few shifts, and the material and conditions stay the same, it’s likely that they are forming in the system, most likely the adapter and die in places where the melt is moving most slowly (large diameters, sharp bends). Formulation matters: Some processing aids coat the inside of the flow paths and discourage gel formation, as well as reduce backpressure and thus allow lower melt temperature.

Resins vary in thermal stability, too, and less-stable materials will degrade faster into gels and (eventually) black or brown particles. Running a small product (low mass rate) in a large machine will contribute to this, as the melt must move more slowly and stay hot longer. If the gels start as soon as the suspected resin is used and stay relatively constant, they are in the feed. You can, of course, have both things going on at once, where there are plenty of gels in the incoming resin, but after a while you are adding to them by degrading materials in the head/die.

Q: Which of the screw segments or zones is very important?
AG: They are all important, of course, but the one I was referring to as special was the rear (first) barrel zone. In this zone, unlike the others, the barrel wall is not fully coated with melt, but is still usually well above the melt temperature of the plastic, so the particles can stick to the barrel as needed for good “inpush.” Grooved-barrel extruders, typically used for HDPE film, are an exception; the particles slip on the barrel but only in the forward direction, but that zone is still independently important.

I try to separate this rear zone from the others so that people will consider it alone, or perhaps combined with #2 in a long extruder with five or six zones, rather than move all the settings up or down in unison. There is an optimum setting of zone #1, not necessarily the hottest or the coldest, where the sticking is best—any hotter and the particles melt on contact and slide around self-lubricated, and inpush starts to fall off. This optimum will vary with the resin, with the feed temperature, and with the screw speed, so it really is best found by “trial and success,” and I am thus skeptical of any proposed settings that are alleged to apply to any one material in all cases.

The rear barrel temperature is also the “aspirin” of the troubleshooter—it’s what you change if you don’t really know what else to do, especially when processing semicrystalline polymers. I have seen it resolve problems of surging, excess pressure, or melt temperature and air entrapment, and wouldn’t be surprised to hear other stories, including some in which it made things worse.

Q: How and why does overheating HDPE make it stronger?
AG: When any polymer is heated, there is some chain breakage. As might be imagined, the hotter the temperature and the longer the time at that temperature, the more the degradation. However, there is also a cross-linking and chain-growing reaction that takes place where the loose, broken ends of a molecule look for and find places on some other chain to attach. This makes that molecule larger and stronger and compensates for the weakening effect of chain breakage. In the case of some HDPE grades, the viscosity actually increases (equivalent of melt index decrease). But this can’t go on indefinitely, because the breakage is normally inhibited by antioxidant in the formulation, which is consumed as the melt remains at the high temperature, and eventually is used up. After that, the breakage occurs faster than the cross-linking and the viscosity starts to fall again, representing a weakening effect.

I was involved in these experiments many years ago, when we were trying to show that the use of “scrap” HDPE pipe did not necessarily weaken the pressure pipe made from such a mixture. We ran several HDPEs in a torque rheometer for up to an hour, far longer than the usual residence time in an extruder, and sure enough, the torque (measure of viscosity) that had slowly increased for the first half hour or so started to fall off as expected from the above explanation.

However, before we all go out and buy recycled milk bottles to strengthen our HDPE film and pipe, we must remember two things:
a) Recyclate already has some of the antioxidant used up (two meltings if it’s been pelletized, one if flake).
b) There is contamination inherent in reuse of scrap, and particles may act as stress concentrators, which lead to failure even if the base resin is still strong. What’s important here is fine filtration and selection of resins with good resistance to crack propagation. People who run pressure pipe will know what I mean, or if they don’t, they should.

Q: Could you please comment on measuring thickness and feeding information back to the extruder to get uniform 
linear flow?
AG: The surest way to stabilize linear flow is with a gear pump. It isn’t a 100% guarantee as changes can still be caused by puller variations or “hot-lips disease” (see box above), but it is good for ironing out the cyclical variation from surging or the more erratic changes due to feeding problems.

Direct feedback to screw speed can be done from a thickness signal. If response isn’t fast enough, these devices can also feed forward, adjusting the puller, but the usefulness of this method depends on the elasticity/plasticity of the melt as it leaves the die.

Q: We use an infrared sensor to measure melt temperature as the extrudate comes out of the die. What else can we use not only to monitor the melt temperature, but also to collect data for analysis?
AG: Melt thermocouples work just like the metal-sensing thermocouples, but their tip is immersed in the melt (slides 24-27 from the presentation). On the most useful ones you can adjust the probe depth, but these also are the least rugged and require added attention in use. Pay attention to where the probe is placed—it should be somewhere in the adapter after the screens. There is a temptation to combine it with the pressure gauge and put it into the hole at the screw tip. Pressure reading will still be OK, but temperature won’t be representative. It’s better than nothing, as you can still compare data, but you can’t trust their absolute values.

Regarding data collection, check with the maker of the controllers as well as the extruder itself. There are a few specialized companies who can do this, too.

Q: We produce HDPE profiles, and after a short time we begin to see material collecting on the die face that eventually drips off on the product, or makes a streak on the product when it hangs up in the sizing device. We have had some success by lowering the temperatures in the extruder and the die face. Do you have any further suggestions?
AG: This is a case of die drool or die deposits. There are three approaches:
a) Use a processing aid that coats the inside of the die lands. This will not only reduce drool, but also reduce the chances of melt fracture at the surface when it is coming out very fast, and reduce backpressure so that melt temperature can be lower if that is desired.
b) Direct a thin stream of air at the exit line, which will cool the skin and keep it from curling up and away from the mass of the melt. You can test it by mounting an air blower in a temporary position.
c) Analyze the drool to see what it is. It could be an additive such as antioxidant, in which case you could change the antioxidant (not so easy as it comes with the resin) or run at a cooler temperature, which you already are doing. Slip agents and antistatics can do the same thing. It could also be a low-molecular-weight fraction of the resin (oligomers), which again requires resin change or cooler melt as remedy.

Q: We currently produce several profiles with a TPO and we are having consistency problems with a TPO for profiles. We have contacted the supplier, but we have always been told, “everything is in spec.” What can we try at our end, and what questions should we be asking the supplier?
AG: The unanswered question here is whether the supplier’s spec is really too wide a spec. Get the material spec sheet or find it in one of the online resin databases. For a TPO, they should at least have melt index (specify test conditions) and density, and probably tensile and heat-resistance data. It would be nice if they could guarantee a melt index with a ±5% variation and test every lot they send. This is a tight spec, and the supplier may either refuse or ask a higher price.

This now becomes a purchasing issue rather than a technical one. Ask them what limits they will guarantee. You can also test melt index on other materials to see if they are held more closely than this one. Other tests can be useful, too, but this is one of the easiest. It is also possible, of course, that something is changing in your plant, but unlikely that it would happen with this resin and no others.

Hot-lips disease (pulsing)
If the end of a die is separately controlled by a conventional proportioning controller, it will be pulsed every 10 seconds or so, and this may give a thicker product when the heater is on (thinner for blown film). This is easily spotted if the thick areas are in time with the heater cycle.

If heaters are very close to the die lips, such a variation is inevitable, and they must be controlled by a nonpulsing device (expensive) or a variable-resistance power source, preset and left that way with no control loop (most common).

Circulating fluid will work, too, but is rare. One of the best alternates is insulation (attached by magnets or stainless 
steel Velcro), which keeps the head/die hotter, no cost and 
no pulsing. The best heater is a sweater.

Q: What’s the best screw design for highly filled materials?
AG: Screw design is a controversial topic, as so many people have different designs that they developed and promote. A lot depends on the extruder, as a long machine allows more to happen in the screw. A lot also depends on how much filler is “high,” and on whether you have fillers already dispersed in a compatible resin, or are adding them in powder form and need a lot more mixing.

If I already have an extruder, I would avoid the question (and the need to buy a new screw) by trying the desired mixture on a system I have, selecting a line with some past history of good mixing if possible. I would also pay attention to:
a) Power requirements: Fillers will increase viscosity and maybe get too close to maximum power available. Some remedies for that include running hotter feed and/or hotter barrel (less heat needed from the motor = less power draw, and even a hotter die might help if it didn’t overheat the melt and slow down production rate), or changing speed range with retrofit pulleys to get more power from the existing system.
b) Mixing quality: Better dispersion may mean less filler needed to get desired properties. Water cooling of the screw, or installation of a static mixer after the screens, or special mixing breaker plates, or a tighter screen pack, or changing the carrier and loading level of a concentrate are all ways to improve mixing.

If I can’t make adequate product from the existing system, the problems I encounter would indicate for me what I could do next. I can accept the idea of a new screw, but only if I clearly see why that screw would solve these problems, and what past experience that design had with high filler loadings. A computer simulation using viscosity data at extrusion shear rates (in the clearances as well as channels) would be helpful, too, and to me is essential for advance planning of large (hence expensive) screws.

Q: How can you determine temperature offset due to shear?
AG: Shear heating is the increase in a melt thermocouple reading because of the friction of the melt against the thermocouple, which can raise the displayed reading by as much as 10ºC (18ºF). Usually it is not as great, and usually it is ignored, as what matters most is the consistency in melt temperature, rather than the absolute value. However, where chemical reactions (foaming, cross-linking) are involved, or where the material is in serious danger of degradation, a more accurate reading of melt temperature is desired. This is done by measuring the rise at a given set of conditions (known resin, flow rate, and viscosity) and subtracting it from the displayed value.

You can measure the rise by stopping the machine suddenly, and watching the melt temperature quickly drop to the real value as the shear heating stops suddenly, but the mass does not move and the thermocouple can record its true temperature at the point of measurement. This isn’t as easy as it sounds, because sudden stoppage requires management of what is coming out of the die at full speed (if it is slowed down before stopping, you won’t get a true reading of the shear heating). Further, it may be dangerous to start up again without bringing the screw speed down to zero and raising it as usual during a startup. Most modern machines have controls that prevent such a restart “in gear,” but it may have been disabled or not have been there in the first place. It may be most convenient to run this test at the end of a run, when you have to shut down anyway.

Q: What is the value of flush-mounted thermocouples for measuring melt temperature?
AG: Extruders don’t normally have flush-mounted melt thermocouples in the adapter or die. The only ones I’ve seen are the rather common and rather useless combination gauges, where a thermocouple is inside the pressure gauge at the screw tip. There was an infrared gauge sold 20-30 years ago (Vanzetti), but it quietly disappeared.

The problem is usually PVC, where there is fear that material caught behind the probe will initiate degradation. That is technically possible, but most systems have other places where degradation is more likely—bends, long spaces after the breaker plate, and the like. If the probe is the main concern, you are doing a lot of other things right. I am more comfortable with knowing the melt temperature, even for PVC, and suggest a variable-depth device with minimal penetration in its “rest” position, but with the ability to be adjusted to move away from the wall and toward the center of the stream.

You may be surprised at the variation from center to wall. There were two classic SPE papers written on this topic in the 1980s; they discovered as much as 30ºC (55ºF) variation in some cases, and concluded that if there is as little as 6ºC (10ºF) variation, you’re doing very well.

Indirect methods may be useful, too—an infrared sensor on the extrudate as it leaves the die, or manipulation of an adapter heater control to find the temperature that it just turns on and quickly off again.

Q: Why do some extruders have screw cooling and others don’t?
AG: Many extruder screws are bored for temperature control, but most of them don’t use this feature (see Figure 1, p. 34; slide 28). Reasons for its use might be:
a) to improve mixing in the screw. This works quite well, but also reduces the output per revolution from 5%-30%. This doesn’t mean less output; in fact, the improved mixing may allow faster production of good product than before.
b) to control the very end of a single screw running rigid PVC, to keep it below degradation point and thus perhaps allow faster operation or a longer time between shutdowns. In this case, the medium is not water, but heat transfer oil, so that temperatures of 150°C can be reached without high pressure.
c) to avoid sticking to the screw root with certain plastics that are susceptible to this problem, such as flake PET. In such a case, we need only cool the first third of the screw. For (a) and (b) we need to control all the way to the end.
Some twin-screws also have hollow screws that transfer heat back from the output end to the feed end, or take it out entirely via a heat exchanger. The result is a possible faster screw speed without excessive heat and consequent degradation.

Q: You mentioned a black mark on the back of a screw. What is it there for?
AG: A black mark is put there to make it easier to count rpm, to make sure the rpm display is working OK, and to get rpm in case it isn’t.

In Figure 2 (p. 36, slide 13), there is such a black area around one of the four holes in the back of the screw. This could serve as a counting mark, but it should really be sharper and darker to be more distinct, especially if the screw is turning fast.
I didn’t mention the four holes, as they didn’t relate to my topic of numbers, but they are important, too—they are the place where the screw pusher is fixed. Not all screws have this feature, but it is quite useful to help push out the screw with minimal damage.

Another image showing the rear of the extruder is Figure 1, with the screw-cooling fitting. There may be a mark on that screw as well, but it can’t be seen. It wasn’t necessary on this machine, as the fitting wobbled back and forth with each revolution and was easy to see. (That’s not a good thing, as it stresses the connections; a proper installation would have the fittings immobilized in a way that allows rotation but no lateral movement.)

Q: Most of this information can be used for extrusion blowmolding, can’t it?
AG: That’s quite true. Extrusion blowmolding (EBM) is fed by extruders, and the same needs apply—especially to wheel systems, which are basically pipe extruders with a specialized takeoff. There are more numbers to be gotten, of course, in addition to the ones I mentioned, such as blowing air and details of parison programming and mold cooling, but this is like any other extrusion (film, sheet, etc.) with specialized needs for the cooling phase of the operation.

There is an interesting “marriage” of these processes—twin-sheet thermoforming of hollow objects. This was done more than 50 years ago with collapsible cubical containers used for battery fluids, then used for hollow cases for tools with a smooth outside and formed “nests” inside, and even automotive gas tanks, using multilayer sheets.

Q: Who are the best providers of computerized data acquisition systems for extrusion?
AG: Most extruder and controller makers will help their customers, and sometimes will help others that don’t yet have their equipment but are good prospects for sales. The missing link is the person on the plant floor to inventory what there is and how well it is working, and eventually do the actual wiring from the instruments to the processor. This should preferably be people from your own factory, as they need someone to understand what is being done. Plus, if an outside source does all the wiring and goes away, who do you call when things go wrong?

I do know some companies and individuals who might do such a service and remain on call for problems, but can’t express such preferences in public, to maintain my position of independence. With private clients, we have a relation of mutual confidence and I can tell it more like it is.  —mpweditorial@cancom.com

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