Grinders

Grinders are how we reprocess and reclaim our scrap.  They are an excellent way to recycle material at the press site and reduce the amount of lost material.  However there are usually the on-going problems of the grind chamber overheating and ‘fines’ clogging up the system.

To understand grinders we have to understand basketball.  If you compare the diameter of the basketball to the diameter of the basket you’ll see the basket is twice as large as the ball.  However, even professional players rarely exceed putting the ball in the basket 50% of the time with out a ‘slam dunk’.  In a grinder the blade impacts the plastic reducing its size then it flies around the chamber until it can fall through the holes in the screen.

The key to keep the grinder from over heating AND to reduce fines is to minimize the residence time in the chamber.  Using the basketball analogy if you want your average particle size to be approx .150 inches, you could easily use a screen with 3/8th inch diameter holes in the screen.  Further if you used a vacuum loader that had a constant flow of air through the grinder and dumped the regrind into a separate container before it was loaded into the machine, you’d keep the grinder cooler.

Grinder blades work on momentum, but unless you are putting large amounts of plastic into the grinder at one time you can slow the speed of the blades down and improve the pounds(kilos)/hour through the machine because there will be less time bouncing around in the chamber.

You don’t have to use huge noise grinders.  Examine the use the auger or ‘crow’s feet’ grinders.  These run on a slow RPM but a constant feed:  One cut equals one pellet without residence time in the chamber.  If in doubt, bring in two grinders on consignment and have a ‘grind off’ test for a few weeks.  The winner gets the PO; the loser takes back his machine.

It goes without saying dull knives in your grinder will make excellent fines as well as chuck a few metal chips into your machine to plug everything up.  Good maintenance and common sense can improve the quality of your regrind for a small expense.

Think about it.

Contact me with questions put in the subject line QUESTIONS.
Bill Tobin, WJT Associates, E-mail: bill4012@hotmail.com