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| Author | Topic: Extruding HDPE |
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satha Member Posts: 4 |
posted March 04, 2005 04:26 AM
  
I would like to know the recommended head pressures at which to run HDPE? I am trying to make HDPE sheet and find that the gauge control and surface finish of the sheet is not good. Could this be due to a lack of shear in the material/ poor mixing? By the way the extruder has a melt pump attached. thank for the help. regards IP: Logged |
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Steve H Moderator Posts: 348 |
posted March 04, 2005 05:00 AM
Welcome to the single screw Forum Satha, what size, L/D is your extruder. What sort of sheet die are you using, what thickness sheet are you trying to produce. How long are the lands on this die, what temps are you setting/getting. ------------------ If at first you don't succeed, skydiving is not for you. IP: Logged |
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satha Member Posts: 4 |
posted March 07, 2005 03:21 AM
Hi Steven, The line we have is co-ex, A=120mm L/D=32:1 ; B=85mm L/D=28:1. Both melt streams combine in a feed block. Due to a design oversight there is no pressure gauge at the head of both extruders. We are only able to measure the pressure at the channel after the screen changer going into the melt pump. The pressure there is typically 10 to 15 bars. (147~220psi). The die is a T type with the cooling rolls in horizontal arrangement. Sheet thickness we are trying to get is 1.57mm (0.060”). One other thing that might be worth mentioning, the die has a full width of 1500mm. It has internal deckles but the surface becomes worst if we deckle it down to make one roll at a time. This means we are forced to make 2 rolls once. (590mm width x 2). The land is pretty narrow 3 to 4 “and I know that the die is not a coat hanger design. Thank you for the feedback I appreciate all the help I can get. IP: Logged |
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Tom C Moderator Posts: 479 |
posted March 07, 2005 08:12 AM
Satha, Do both extruders have gear pumps? Typically I think gear pumps need suction pressures of 500-1000 PSI to function correctly. Since application of the deckels worsens the surface problem it is likely you are experiencing melt fracture. A typical solution for melt fracture is using a fluoropolymer processing aid. ------------------
Tom Cunningham
Extrusion Technical Services www.ExtrusionTechnicalServices.com IP: Logged |
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Steve H Moderator Posts: 348 |
posted March 10, 2005 02:15 AM
Hi Satha Normally with a melt pump you would monitor inlet and discharge pressure, with the inlet pressure controller increasing/decreasing screw speed to maintain feed to the pump, is this the case with your line? It would be really helpful to give us a full list of all readings/settings of your line, temps, amps, screw speed (the sets and gets), screw design, etc etc, otherwise it's a bit like telling us your car won't go and asking us to tell you what's wrong with it. My guess is that the low discharge pressure is causing uneven material flow within the die and that raising the discharge pressure to 750 -1500 PSI will see the problem disappear. ------------------ If at first you don't succeed, skydiving is not for you. IP: Logged |
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satha Member Posts: 4 |
posted March 10, 2005 02:43 AM
Both extruders have gear pumps. the material is from chevron, it's a blowmolding grade 5502 MI 0.35g/10min. Temps range is 175 to 250 C and the gets are close. I measured the melt temp as it exits the die i a got an avg of 247C. Not sure of the amps right now i need to check as i am off site this week. I agree with you on the measurring of pump inlet and discharge pressure, but alas i have no way of measuring the discharge pressure. I was also wandering if this problem effects the oriantation of the sheet in any way?? Any thoughts would be helpfull. will get back on the amp loads ASAP. Thanks and bye, IP: Logged |
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satha Member Posts: 4 |
posted March 11, 2005 05:42 AM
you are right the extruder is controlled by setting the head pressure and the screw speed increases/ decreases to compensate for any deviation from set point. The set point we used when making the HDPE was 30 BAR or 440 psi.dont know if this means anything but the melt pump inlet pressure was 10 bar or 150 psi and although there is no transducer after the melt pump the transducer at the die (die pressure) read 31 bar or 450 psi. We dont currently record amps so i dont have that data. IP: Logged |
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Steve H Moderator Posts: 348 |
posted March 15, 2005 12:57 AM
quote: I'm wondering why you would choose this grade over the sheet grade offered by the same maker? How many chill rollers is the sheet coming into contact with? How much nip pressure at the first nip? Orientation in sheet extrusion is a function of the draw down (haul off speed). Steve IP: Logged |
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