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| Author | Topic: Screw length |
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Vic Senior Member Posts: 9 |
posted May 26, 2003 09:39 PM
  
The screw on our SSE sticks out from the barrel about 220mm from the barrel. It is surrounded by an adapter section with two heater bands controlled by 1 thermocouple. Is this a normal setup? Considering the shear heat buildup in this section shouldn't we have some form of cooling in this area. Also the adapter surface is not chrome plated, only nitrided. Should it be chromed? We extrude LDPE and mixtures of LDPE/LLDPE and LDPE/EVA sheet and are experiencing gauge control problems across the width. Thanks IP: Logged |
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Steve H Moderator Posts: 184 |
posted May 26, 2003 10:11 PM
Hi Vic quote:A little unusual, what L/D and screw diameter is it? Are you extruding sheet, thin sheet or cast film? Steve H IP: Logged |
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felix Senior Member Posts: 12 |
posted May 26, 2003 11:51 PM
The screw may stick out of the barrel because it is extending into a screen changer? The screw tip should be very close to the breaker plate (less than 1/8th), so if the breaker plate is not close to the barrel exit, the screw should extend past the barrel end. If there is excessive shear heat is being generated, it is most likely coming from the screw geometry, earlier on, not the screw extension. IP: Logged |
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louis33 Senior Member Posts: 63 |
posted May 27, 2003 07:56 AM
Random thoughts here The front zone always has heaters on it and I have never seen a chrome plated barrel. Having said that, it sounds like someone modified your machine. Perhaps to accomodate a torpedo type tip on your screw for flow control. You did not say if it was flat or pointed. I'd stick with the heaters and not worry about the lack of plating unless of course it is giving you problems with buildup or burning. Somehow I doubt that a moving screw in an extended barrel would cause that kind of problem. IP: Logged |
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zabielski Senior Member Posts: 159 |
posted May 27, 2003 08:10 AM
I would be very concerned if the extended barrel is not supported properly. I believe that The BERLYN Extruders had many law suites due to poor design of "sagging" barrels. This caused severe galling in the metering section, and any flange "added" is a disaster waitnig to happen. IP: Logged |
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Vic Senior Member Posts: 9 |
posted May 27, 2003 07:14 PM
Felix, you are correct the screw does extend to a screen changer. The screw tip is unusual aswell in that it is shaped like a drill bit with the pointed tip cut off, a bit hard to explain. Could someone explain the logic behind this design? PS the previous problem of poor vaccum seal was solved by putting a triple vacuum seal rather than the single one that was on. IP: Logged |
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louis33 Senior Member Posts: 63 |
posted May 28, 2003 10:34 AM
There are some that believe that the blunt face of a feed screw allows material to build up and burn as it is a "dead spot" in the flow. A tapered tip could help eliminate that problem - if indeed it is a problem. In any event, it sounds like someone has been experimenting. Hope that helps! IP: Logged |
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TomBlack Moderator Posts: 20 |
posted May 28, 2003 04:26 PM
Hi Louis, Most OEM's prefer a blunt, 12degree or 15degree nose angle as the long tapered nose angles have too much unwiped surface area resulting in polymer degradation. The best I've used is a 12degree noze with offcenter nose angle so it wipes itself with each revolution... ------------------ IP: Logged |
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louis33 Senior Member Posts: 63 |
posted May 28, 2003 06:44 PM
Tom You are talking OEM's here and I agree with your statement. But this sounds like an after market modification. I've seen torpedo ends on feed screws with special adapters that match the angle perfectly with a minimum gap between the two. They promote very good flow with no dead spots and minimum residence time. Keep in mind that there is no wiping action in the typical straight delivery head or crosshead, so that is not all that important as long as the flow channels are streamlined and the volume of material is kept to a minimum to promote plug flow. Ahh - plug flow, that ever elusive goal. It's all about the flow channels. IP: Logged |
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