Processor Tips



Get Better Mixing From Your Injection Molding Screw

By:
23. March 2012 10:41

In injection molding, operators running a machine using a standard, general-purpose, single-stage screw will typically increase the back pressure whenever they need better mixing.  Their thinking is, by increasing the backpressure on the screw, the screw will work harder to pump the material forward, which in turn will result in a high mixing action in the metering section of the screw.  But there are two big flaws to this approach: the resulting high melt temperature and longer recovery time. Since the screw has to work harder to convey the resin forward, this additional work results in higher melt temperature. Higher melt temperature results in extended part-cooling time after molding. And since the screw has to work harder to pump the resin, it takes long for  it to recover. 

 
Instead of tweaking with back pressure, look into screws designed chiefly for better mixing. These can be designed for either shear sensitive materials or non-shear sensitive resins line PP, PE, PS, etc).  Once these screws are installed  you’ll need to set up a new operating procedure because you won’t need high backpressure to get the mixing quality you need; the screw is now doing the mixing dynamically.  The operator should now reduce the back pressure to the minimal amount that the machine will operate under (35 to 50 psi) and start from there to find the best operating condition. 
 

Not only will the achieve better mixing with the new screw design, but they also will have fast recovery times, and lower melt temperature which will translate into faster overall cycles and in turn more savings to the customers bottom line.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

 

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