EXTRUSION PROCESSORS’ PROBLEMS MEET THEIR MATCH By MPW Staff 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.” Allan Griff 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. 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. <strong>Plastics</strong> 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 plasticstoday.com/mpw EXTRUSION 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 MODERN PLASTICS WORLDWIDE • MARCH <strong>2010</strong> 33
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