This was not my actual question. The question was how do I stop the spiking of oversize material through a screen on a grinder? It sounded just like most any mulch company in the Country reducing and regrinding.
Even though you are using a specific size screen, doesn’t necessarily mean all material will be less than that screen size. With pallet material or any other kiln dried or dried wood material, each piece needs to be cut and pushed through the screen. If you were using some type of green waste for example, because of the moisture in it, you would get a much finer product and more of a shredded product with that same size screen.
Those large spears getting through the screen are the same as any other Company will get.
Here are a few ideas of how to reduce them:
· Make the initial grind smaller- This will keep the rotor area more full, and with less open space there is less of a chance of larger pieces being able to fall through the screen.
· Make sure there is no way any materials can get from the infeed hopper onto the discharge belt without going through the screen. Make sure there are no gaps between the screens or between the screen and anvil/cutting edge.
· You will get larger sizes when there is the least amount of wood in the grinding chamber. So if the infeed belt of the grinder is stopping and starting a lot it means the grinding chamber is emptying out so the hammermill can pick up speed. With little wood in the grinding chamber, the pieces have all kinds of open screen to fall right out. Slow the infeed enough so that it does not stop, or stops as little as possible and by keeping the chamber full and you will see a major reduction in the larger pieces.
· Purchase or make Baffled screens. These are normal screens with flat stock welded to the back of the screen at the bottom of each hole row on an angle to stop any long pieces from shooting right through the hole. The spears hit the baffle and hold it so the hammermill can cut it again.
· Keep the tips as tight as you can to the cutting edge and the screens. If you need to build up the anvil or cutting edge with hard surface more often that is what you need to do. If the tip is as close as ½” to the cutting edges you will get a clean cut. As you wear away the tips and the cutting edge the gap determines your initial size and you will get larger pieces that need cut again. A bead of hard surface across the screens in front of the row of holes will help also. This will make the screens more brittle so use a welder’s knowledge about heat on metals and only do a few inches at a time then move to a different part of the screen.
· You will hear a lot of people say “Use round hole screens, they make the best product” when the fact is, the difference between a round and square hole screen is the amount of open area on the screen available for wood to go through. It is much less with a comparable round hole versus a square hole.
· Rotate your screens if possible. The leading hole edge in the screen start to round as they wear. If you can flip them 180 degrees, you will be able to use the other end of the hole with a much cleaner sharper edge. Not all screens have the ability to do this.
· Grind once instead of twice with a 2” or so screen. Then use about ¾” screens in a trommel or other screen to screen and produce your product. This has its own set of How- To’s but it can be an option.
· Grind as normal and then screen the product to scalp off any of the oversize spears.
· Use a smaller screen in the grinder- Not ideal. This would be the last resort.
That should be a good start and you should be able to find something that works for you or a combination that may work for you.
Questions? Dave Whitelaw firstname.lastname@example.org