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| Author | Topic: Surging |
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extman Member Posts: 1 |
posted June 07, 2001 05:57 AM
  
When one has a surging problem, what are the first things that they sould be checking ? Thanks. IP: Logged |
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Peter P Senior Member Posts: 61 |
posted June 07, 2001 11:20 AM
Check the screw design. In a standard screw, while the solid is being converted to melt, it is being transported along the barrel. The channel depth changes in the compression zone - it gets shallower, so the downstream velocity increases (it accelerates). The melt can cope with this acceleration - it is held together by cohesive forces. However the solid bed has no cohesion and breaks-up - it is this bed break-up that contributes to surging. Although it is not only bed break-up that is responsible for surging, you need to ensure, using your screw design, that the acceleration is controlled. Regards, Peter P. IP: Logged |
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louis33 Senior Member Posts: 146 |
posted June 07, 2001 05:59 PM
quote: The key to solving any extrusion problem is to first identify the problem. Surging generally refers to a periodic size change. That could be caused by the extruder, as Pewter suggested - with an inappropriate screw design, or tooling, as in a "sizing " surge, or some outside influence. Were you able to run without the surge and then it developed? What changed? If you never had a surge free process, is the surge coming from tooling drawdown? That kind of surge would normally be a short surge, occurring over a few feet or less of finished product. Is the puller takeoff speed holding steady or is it drifting up and down with time? Is the extruder drive motor holding steady or is it drifting up and down with time? By eliminating all those factors and a few more like did someone leave the overhead door open or turn on a floor fan, you can zero in on the screw design as the problem. Another way to look at it is to ask yourself "if I wanted to create this kind of surge, how would I do it?" Louis IP: Logged |
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DAN CRAWFORD Member Posts: 6 |
posted June 20, 2001 09:41 AM
ALL THESE ANSWERS ARE VERY GOOD STARTING POINTS. THE FIRST THING I LOOK AT IS THE HEAD PRESSURE AND MELT TEMP. OF MY MATERIAL THEN ALL THE OUTSIDE ITEMS THAT EFFFECT THE EXTRUDATE AS IT EXITS THE HEAD. IT NEVER HURTS TO CHECK YOUR MATERIAL FLOW AT THE FEED BOX. MELT TEMP. CAN CAUSE SURGING IF THE MATERIAL IS FLOWING TOO FREELY AND NOT MAINTAINING A STEADY PRESSURE. HOPE THIS HELPS IP: Logged |
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2beeps Moderator Posts: 111 |
posted July 01, 2001 05:34 PM
All the above replies have excellent points to examine for reducing surge.  My perspective (single screw vented 24:1-32:1L/Ds with hopper crammer for rigid PVC powder) says that EVEN if all other factors are fixed, and a good-apparently even- die flow occurs, the inherent nature of single screws will still create a slight surge or pulsation with each turn of the screw. Thus, if dimensional tolerance is critical, it might be time to consider a gear pump? Or a twin screw?? Regards, Skip [This message has been edited by 2beeps (edited 07-01-2001).] IP: Logged |
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