The regrind you produce should be free-flowing, cleanly cut, uniform in size, and free of dust and fines. That’s because there’s not one processing machine or control out there—injection molding, blow molding, extrusion, or the rest—that can consistently convert poor-quality regrind into high-quality finished product. And that’s why, if you’re going to use regrind in your own plant, or sell it to other processors, you’ve got to be sure your granulator is well-maintained.
If it’s not, you’re going to know it because the conditions that cause poor regrind quality also cause other problems, like:
• Reduced throughput—Material flow through a well-maintained granulator ought to be rapid and relatively quiet. The less time that it takes to grind a part, the better, since that indicates that the part is being cut up quickly, cleanly, and with a minimum of rotor revolutions. If it starts to take more time/more revolutions to granulate the same parts, you’re sure to see throughput drop.
• High noise levels—Granulators that aren’t working properly are going to make more noise as parts bounce around and rotors tend to smash rather than cut parts.
• Increased heat and power draw—Frictional heat will build within the cutting chamber and motor amps will rise if the cutting process isn’t working right. In extreme cases, rotors can stall or jam completely.
All of these symptoms point back to the #1 source of plastics granulator problems: Dull or misaligned knives and screens. Let’s take a closer look:
Plastics granulators process material by cutting it between two sharp surfaces—a fixed knife and a rotating knife. Ideally, both knives are hardened and machined to manufacturer specifications so that the knife edges, which are separated by a precisely-defined gap, complement each other in making smooth, uniform cuts. As the regrind is reduced in size by repeated cutting, it becomes small enough to fall through the screen at the bottom of the cutting chamber.
This process is essential for cost-efficient material use, but stressful and demanding on the equipment. Wear on knives and screens is constant, and is accelerated when processing high material volumes, tougher materials (glass-, calcium-carbonate-filled), or dirty, gritty materials (recycled scrap). As knife edges wear, the critical gap between the rotating and fixed knives expands, so that dulling knives are taking bigger bites of material on each pass. It doesn’t take long to see the results in the regrind—fewer clean cuts, more impact marks, increased chipping and shattering, and an increase in particles and fines.
At the same time, the gap between the rotating knife edges and the screen increases too. Instead of pushing properly-sized granulate through the screen to the catch bin, some regrind (particularly low-bulk-density regrind like films) can begin to accumulate on top of screens where it reduces throughput or results in clogs that require shutdown and cleaning.
Mechanical wear doesn’t just affect knives, however: all of that fast-moving material also has an effect on screens. Circular holes tend to elongate with wear, gradually allowing larger, chunkier and non-uniform granulate to drop into the bin.
What can you do? Regular, preventive maintenance is the key to producing consistent, high-quality granulate. The routine is straightforward:
1) Remove and replace dulled bed/rotor knives with properly-sharpened knives.
2) Adjust gap between revolving and fixed knives to manufacturer recommendations.
3) Clean and inspect, and when necessary, replace screens to ensure consistent granulate sizing.
Some regrind processors, such as recyclers, perform granulator cleanings, knife changes, and adjustments daily or with each shift. Others adopt a weekly, monthly, or hours-based maintenance program.
This maintenance can be more challenging on older granulators, particularly when it comes to reinstalling sharpened knives. Many older designs allow knives to be adjusted only after installation, or limit adjustment to one set of knives (e.g. rotor knives are fixed, bed knives only are adjustable).
Today, some granulator makers (including Conair) are adopting designs that utilize workbench fixtures and improved knife designs (e.g., equipped with adjustable positioning screws) that enable precise pre-adjustment and gapping of knives before installation in the granulator. This approach combines exceptional consistency, improved safety, and far greater simplicity and ease in the knife-replacement and gapping proces