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10/25/2012 | 1 MINUTE READ

Buying Used? Be Careful

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An old extruder is unlikely to have the same capabilities--especially in the area of torque--than a new one will have.

Be careful when buying a used extruder because you might not be getting what you’re paying for. Make sure that the gearbox is in good condition, the cast feed throat doesn’t have excessive wear and that the barrel is also in good condition. Also make sure that the extruder has adequate torque for the application for which it will be used. You need to take these precautions because a 3.5-in. extruder made years ago may not have the capabilities of one made today.


Screw design advancements over the year have raised the output bar in extrusion. But that has required machines with more torque.  Back in the 1960s and early 70s, a typical 3.5-in. 24:1 L/D extruder would be fitted with a 75 hp DC motor and sheaved or geared for a top screw speed of 100 rpm.  This drive configuration is adequate for a flexible PVC profile application even today, and can deliver 700 to 750 lb/hr of output maximum. But this drive configuration has only enough torque available to produce 400 to 450 lb/hr of HDPE.


Today, a good 3.5-in. 24:1 L/D will have at a minimum a 125-hp motor and geared for a top screw speed of about 125 rpm, and a more robust extruder of the same size will have 150 hp drive motor.  With a 150 hp drive motor on a 3.5-in., it is easy to achieve an output rate of 650-700 lb/hr of fractional melt HDPE, if the extruder has been fitted with a screw design of the most up to date screw design technology.


So when looking to buy a used extruder, make sure that you are getting all of the torque that your process requires.

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