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| Author | Topic: L/D ratio question |
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dullkid Member Posts: 2 |
posted June 26, 2007 06:05 AM
  
Hi, I have been a regular visitor here for some time but this is my first time posting. We are planning on getting an extruder to make sheet for thermoforming. The material we intend to make is mainly HIPS with some flexibility to make HDPE and PP if need be. The line is a co-ex machine. The machine manufacturers I spoke to are all recommending extruders with L/D of 33:1for both extruders. All the sheet extruders I have seen thus far (very limited experience) has been 30:1 or less. I was hoping to get some info on what effect this would have on the process / materials. The ones I can think of are longer residence time, higher sheer and better melting. Any other pointers or thoughts on the subject would be of great help. kid IP: Logged |
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Tom C Moderator Posts: 860 |
posted June 26, 2007 07:16 AM
Screw L :D was first increased to help with melting and pressure stability. Then it was thought that additional L :D could help improve specific output; Lbs/revolution by being able to melt more resin through added barrel contact area. Higher speciic output also helps to lower the melt temperature. However, after a certain L :D there can be diminished returns depending on the screw type. Often I see long L :D screws where much of the screw is set up as metering. This works to even out flow from the bad design applied prior to metering, but it is certainly less than optimal. Extruder design and screw design, when done properly, should give strong consideration to the process you are running, the type of melt quality you need to generate, and the economics of the product being produced.
------------------ Tom Cunningham [This message has been edited by Tom C (edited June 26, 2007).] IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 67 |
posted June 26, 2007 04:04 PM
For a given polymer, extruder length is a key factor for determining the optimum screw design. Others key factors are motor power, drive speed, and diameter. ------------------ IP: Logged |
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dullkid Member Posts: 2 |
posted June 27, 2007 10:04 PM
Tom and Steve, Thank you for the replies, looking at the specification that I have with me, one thing that stands out is that different manufactures use motors of different power to drive screws of the same size. Is there any rule of thumb that I can use as a guide? Also when you say drive speed is that the same as the screw speed measured in RPM? Lastly, some of the people I talk to mention torque but in rather vague terms, some explanation on this would help me understand all a lot better. Regards, kid IP: Logged |
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Tom C Moderator Posts: 860 |
posted June 28, 2007 11:41 AM
Dullkid, These are complex issues that you are asking about. Much of the knowledge comes from expereince. However expereince can not always predict the outcome if you are doing something that is not standard. For myself I use computer simulation of the extrusion process to help with design. I find that it give me excellent results. ------------------ Tom Cunningham www.ExtrusionTechnicalServices.com IP: Logged |
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