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| Author | Topic: compression ratio |
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charles Senior Member Posts: 7 |
posted March 31, 2006 08:55 PM
   
How can you determine compression ratio for a single screw extruder that uses a 6" screw and has a L/D of 30:1. The channel depth of the flights in the feed zone are 9/16". IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 27 |
posted March 31, 2006 09:16 PM
The compression ratio is the depth of the solids conveying channel divided by the depth of the metering channel. Normally, it is about 3 to 4, but it does depend on the polymer. The proper compression ratio promotes proper melting. Proper compression ratios are often given for a specific polymers. Compression section length is another factor that is specific to the polymer type. Tables of polymers often give these data over a suggested range. ------------------ IP: Logged |
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charles Senior Member Posts: 7 |
posted March 31, 2006 10:11 PM
Thanks for the info. We are presently running LDPE. But have been looking at changing to a LLDPE/LDPE blend 50/50. IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 27 |
posted April 01, 2006 02:00 PM
Charles- For general LDPE purposes, a cr of 2.4 is recommended*. However, the cr is calculated on a basis of cross-sectional area of the solids conveying section/the cross sectional area of the metering section. Almost the same as channel depth ratio, but could be significant for small screws when cylindrical coordinates are assumed for the calculation. A 10 L/D is recommended for the transition between solids and metering. *"Extruder Principles and Operation," M.J. Stevens and J.A. Covas, 1995, pg 294. ------------------ IP: Logged |
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the plastic surgeon Senior Member Posts: 5 |
posted April 03, 2006 12:39 PM
Is there a copy of this compression ratio chart on the net anywhere by chance. I am just a simple pvc compounder but I am finding out the longer I do this the more I need to know about my extruders. Currently I run a semi rigid PVC powder (60 - 70D Shore) on three different 4.5" single screws. My compression ratios were just calculated last week to be roughly 2.64, 2.72, and 2.85. I was told that I had to run PP last week and heard that I needed at least a 3 compression ratio. Sadly we proved that to be correct as we had to torque the pellets like crazy to get them to melt. Thanks, Chris IP: Logged |
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Stephen J. Derezinski Senior Member Posts: 27 |
posted April 04, 2006 08:04 PM
The issue of proper compression ratio is one of matching up the flow rate capacity of the solids conveying to that of the metering section. Each of these flow rate capapcities depends on their section channel depth, so the ratio of the depths becomes a common scaling factor known as the compression ratio. The length of the transition section between the solids conveying section and metering section should be long enough to accomodate melting of the polymer and short enough to maintain rubbing or close proximity of the unmelted solids to the barrel wall. Close proximity to the wall will promote good melting through friction and through high viscous shear between solids and barrel. Accurately determining these factors is the job of the screw designer. ------------------ IP: Logged |
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