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| Author | Topic: sticks to surface |
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aivar Member Posts: 4 |
posted August 27, 2006 03:33 AM
   
Have been running flex PVC on a new single screw extruder for 6 months 24/5 with no mayor problems. After 3 weeks vacations the line was started and what occured: the material sticks to screw and die head surfaces, making it necessary to disassemble the die for cleaning at least twice a week. All the parameters ( temps., back pressure) are the same than earlier. Material supplier states that no changes have been done. The extruder is 38mm, 25 L/D, screw has helical fluted section, cooling bore, but no cooling used during 6 months period. Screw rpm 45, output 12 kg/h. Any idea, what could be wrong? IP: Logged |
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Tom C Moderator Posts: 769 |
posted August 27, 2006 09:21 AM
In plastics processing the material is always sticking to the metal surfaces, or we would have lots of other problems. We depends on the force of the material moving above the surface interface to keep the material at the surface interface moving along fast enough so it does not burn and degrade. I checked your output rate and it is within normal range. You say the motor draw is the same as before, so the viscosity of the material should be the same. This implies the self cleaning effects should be the same. These are some possibilities; The metal surface has corroded somewhat, even microscopically, increase the force required to keep the surface clean. The raw materials have absorbed some moisture in the summer and during the downtime. This causes some minor foaming and lowering of cleaning forces. Moisture absorbtion causes some chemical reaction which increases the speed of degradation. Degraded material sticks harder at the interface. Your material supplier has lowered or changed stabilizer content either knowingly or unknowingly (the operators added the wrong amount) which casues the material to degrade faster. Do you know the melt temperature before or after the change? Soemtimes during a long shutdown an extruder heater can get moisture inside of it, and shorts out on full power startup. If this occured on a heater pair, and the were wired in parallel, and the thermocouple was mounted near the dead heater half, it would casue the live heater to run too hot, causing degradation problems. Good luck. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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aivar Member Posts: 4 |
posted August 27, 2006 12:19 PM
Thanks,Tom, for the reply. the moisture problem came to my mind, but as far I know, flex PVC is not a hygroscopic material. Or is it possible that some moisture was included in the compounding stage? Unfortunately we have no dryer for continous run at the moment. I try to measure the moisture content somehow. Melt temp. probe is not installed. You mentioned the corrosion of the screw. There were some signs of wear on the screw root in metering zone, not on flight. the screw is made of high tensile steel, nitride hardened. What material and surface treatment is suggested for the screw, running FPVC? How to increase the force to keep surface clean - lower temps on barrel? Some people say it is always necessary to use screw cooling when running FPVC. We tried it at first, but the result was nonhomogenous melt and twice as much RPM required to achive the same output. On the other hand, keeping screw cooler probably increases its lifetime. Perhaps we should get a larger machine and run it at low rpm, but screw cooled?! IP: Logged |
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Tom C Moderator Posts: 769 |
posted August 27, 2006 01:39 PM
Most people would not recommend running PVC against steel. Corrosion starts on a microscopic level which will cause polymer to stick. I like 17-4PH stainless for these applications because of it's strength. Be careful of galling against other stainless surfaces. Chome plating is used, but porosity casues issues. Triple chrome plating is used to solve that issue. Chrome seems to be a nice release surface. High polish on these surfaces seems to help. Screw cooling with hot water would seems to be a good technique to control melt temperature. You want to take some heat out that way, not solidify the polymer. I use computer simulations to weight the options of melting, mixing, output rate, die resistance, degradation, machine time, screw cooling, screw surface stress, and so on. As you can see the options anre amyn and complextiy increases when different materials are included. The simulations help to narrow doen the choices. Let me know if you are interested in having some work done along those lines. It is a lot cheaper than cutting metal, running tials and buying equipemnt. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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aivar Member Posts: 4 |
posted August 28, 2006 03:36 AM
Thanks again, Tom. It would be nice to get familiar with simulation program, of course. But I have no idea at the moment, how and when. I keep working on the problem and let you of the findings. Kind regards, IP: Logged |
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aivar Member Posts: 4 |
posted September 03, 2006 04:54 PM
There is one more issue different than earlier. A lot of white dust among the material. Suppose it is PVC powder. FPVC pellets (transparent, slightly yellow) are supplied in octabins. The filter of vacuum hopper loader gets all covered with the dust very fast. I used to clean it once a month earlier. But now I have to do it every 3-4 days, otherwise it drops to hopper. Could this possibly cause the sticking issue? regards, IP: Logged |
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Tom C Moderator Posts: 769 |
posted September 04, 2006 09:29 AM
Seems odd that flex PVC should generate a lot of dust in handling. Suppose your supplier is now dusting the outside of the pellets with stabilizer instead of compounding it into the pellets. Potentially the stabilizer could be sucked off the pellets. I would do some tests on the powder to see what it is. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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xavier_0909 Member Posts: 1 |
posted September 12, 2006 01:31 AM
Hi all I am staying in singapore, currently, i have feedback that PVC, EPDM, SBR are the problematic rubbers that cause sticky issue to moulds. I have tried asking a few rubber manufacturer for the rubber samples, but none know of their rubber content. I would like to buy these types of material for testing, where and what kind of product are these material made for? Else i can just get the end product. IP: Logged |
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