Buying a Used Extruder
Be careful when buying a used extruder because you might be buying a “pig in a poke”. For example, a 3.5” extruder is not always a 3.5” extruder. Besides making 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, it is most important that the extruder has adequate torque for the application that it will be used.
Over the years, as screw design technology has advanced, higher throughput rates have been achieved. But in doing so, more torque is required. Back in the 60’s and early 70’s, a typical 3.5” x 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 FPVC profile application even today, and can deliver 700 to 750 lb/hr maximum. But, this drive configuration has only enough torque available to produce 400 to 450 lb/hr of HDPE.
Today, a good 3.5” x 24:1 L/D will have at a minimum a 125hp more and geared for a top screw speed of about 125 rpm and a healthy 3.5” extruder will have 150 hp drive motor. With a 150 hp drive motor on a 3.5”, it is easy to achieve 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.