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Extrusion Q&A: The answers to your tough questions

Almost 1000 processors registered for our latest Extrusion Expert webinar, "Between the screw and the die," which we held on April 14. Naturally the participants brought their toughest questions with them to the event.

MPW Staff

May 17, 2010

6 Min Read
Extrusion Q&A: The answers to your tough questions

Allan Griff, the star of this webinar series, a consulting engineer and extrusion expert with more than 40 years of experience in helping processors extrude better parts, fielded the questions with aplomb. Below are a few of the questions as well as Allan's answers. Enjoy the article, read the longer version of it in our June issue, and make sure to join us for the next Extrusion Expert webinar, entitled "Do monkey with the formula," on the ins and outs of improving your recipes with fillers and additives. That event is scheduled for June 16 at 11:00 EST.

Q: How do you calculate the time to melt the cold 'blob' in front of the screen before you can start?
Griff: There are two such masses (blobs), one between the screw tip and the screens, and another after the screens in the adapter. With no screen changer, there should be very little space between the screw tip and the screens. With a screen changer, however, there may be a substantial space, and this is a prime location for degradation if left hot and not moving for a long time. 

It is possible to calculate the times to melt these spaces, but quite complicated, as it involves heat transfer through the breaker plate as well as through the gate and adapter, requires knowledge of heat conductivity and viscosity of the plastic as a function of temperature, and is also affected by the last barrel zone temperature and the radiation from and the insulation of the gate and adapter. Even if I had results of such calculations, I'd be hesitant to rely on them without seeing their confirmation by actual extrusions using the resins for which they were calculated.

Fortunately, it is usually unnecessary to do this, as the mass, exposure and content of most dies will be the limiting factors, and these masses of plastic on either side of the screens melt more quickly. With a small die on a large extruder, however, this may be the limit, and in such cases we typically rely on experience rather than calculation. I can give you a rough guide: on a 2.5" (60mm) extruder with adequate heating of the metal in this area, 20 minutes may be enough, and on a 4.5" you may need up to an hour. If you are "soaking" much more than this and there is no other component that requires it, you may try to reduce the soak time, but do it very cautiously - cut it 5 or 10 minutes at a time, watch the ammeter as you start the machine, and be familiar with its behavior under the prior soaking time. Application of insulation, either temporary or permanent, will also help reduce this time.

Q: What is the best way to clean a breaker plate?
Griff: The first rule is to clean it hot, if possible, as some resins stick tenaciously to the metal and cannot easily be removed cold. The exception is when it is cleaned in a solvent (rarely done), which may boil or even ignite if a hot plate is immersed in it. Most resins, including polyolefins, are not easily dissolved, so the plates are cleaned either by hand or in an oven. Hand cleaning is the simplest, cheapest and slowest method. Use brass or other non-scratch tools, as the plate is a relatively soft steel to allow drilling. Scratches may not impair its mechanical function, but may be starting places for hangup and degradation in slow-moving areas.

The pliers-and-airhose technique is useful, especially for resins that shrink a lot when they harden (polyethylene, for example, shrinks close to 20%). Grab the melt with the ends of long-nose pliers, direct the air stream to where the melt contacts the metal, and pull gently. The air cools the metal and that shrinks away the plastic, and if pulled just the right amount, it comes out of holes and crevices like pizza cheese. It takes a little time to learn to do it right, and is quite rewarding for few minutes, but then it becomes work like anything else.

The next step "up" is the use of a cleaning oven. Conventional ovens work, but may leave adhesive char that may have to be scraped, brushed, or even drilled out. Remember to set the plate horizontal so the resin can drip downward out of the holes. Use brass, not steel, tools.

There are commercial fluid-bed cleaners that suspend the plate in a basket surrounded by swirling particles of very hot alumina or other abrasive, which leave a much cleaner surface than possible with hand cleaning. A similar device doesn't use particles but heats the plate in a vacuum, which melts and then vaporizes the plastic on the plate. Still another does without the vacuum, but uses the same vaporizing high temperatures, much like the cleaning cycle in some home ovens.

I'm often asked about cleaning with a flame (torch), as this is quick and easy. The accepted answer is to avoid this if possible, as the intense heat might distort the metal and thus cause leakage later. However, burning out a plate is a good example of my "proverb": If you do something wrong, then do it right. If the temptation to burn out the plate is too strong to resist and there is no efficient alternative, do it slowly and horizontally. If done slowly and carefully, with just enough heat to melt out the resin, you can avoid metal distortion. As for horizontal, the process will go more quickly if the molten resin can drip downward out of the holes, as noted above for conventional ovens. Furthermore, if the plate is clamped horizontally in a vise, you won't be marring the sealing surfaces on the plate, which need to be smooth and true for a good, leak-resistant metal-to-metal seal.

The need for speed can be reduced by having two or more plates for each machine. That way, you are not under time pressure to clean the plate quickly to get back into production again. Finally, where there is no screen changer and the system must be shut down and the head taken apart, the consequent time and trouble may sometimes be avoided by using an effective purge—one which removes enough of the prior material to avoid shutdown. And even if the head is to be dismantled, a purge may simplify the eventual cleaning process.

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