It’s easy to think of shredders and granulators as “brute force” equipment, with all brawn and no brains, but the opposite is true. There are few pieces of plastics processing equipment that allow for so many variables, so much configurability, as a shredder or granulator. Thus, the application information that you provide—whether for purposes of selection and purchase or repair/refurbishing is very important to considering the options and making a sound decision.
Below are the seven criteria that I use to recommend size reduction equipment for shredders or granulators. These factors are especially important for those whose applications aren’t just “run of the mill.”
What’s your process?
- Injection molding
- Blow molding
- Profile extrusion
- Film/sheet extrusion
What type of material is being reduced? Which additives?
Knowing the base material, as well as the key additives, is another essential, leading to a series of questions: Is the base material easy or hard to grind? Abrasive or non-abrasive? Hot or cool material? Are there any additives? Will the granulate be dusty?
These factors not only have influence on the cutting chamber, rotor, and horsepower requirements, but also on the evacuation requirements. If material is light (film or thin-wall parts) or tends to shatter and create dust, you’ve got to think about how you’re going to keep it flowing, how you’re going to evacuate it, and how you’re going to manage the dust.
The material type, additives, and part dimensions will also have a huge impact on throughput.
How are you going to do the infeed?
The size and configuration of your hopper must match up not only with the infeed method, but with the anticipated volume of scrap, including temporary surges of scrap that can occur at process startup:
What are the characteristics of the scrap to be reduced?
- Molded parts
- Thickness or density
What are the dimensions of the scrap being reduced?
- Length and width are very important, but don’t forget about the depth of the part. That is vital because the part must not only fit into the hopper, but have sufficient space in the cutting chamber to maintain contact with the rotor.
- Sometimes, you can get away with a smaller infeed and hopper if you feed it manually, since the part will always be properly oriented. Robotic or conveyor-fed equipment requires additional attention to determine the optimum hopper and cutting chamber dimensions, but these configurations offer greater flexibility in the long term, allowing either manual or automated feeding.
What throughput do you expect to achieve?
All grinders have estimated throughput ratings, but this throughputs can vary greatly based on many different factors, including material type, additives, and part configurations (i.e., size of part, density/bulkiness, wall thickness, etc.) To get an optimal size reduction solution, requires attention to a range of other factors including:
- Rotor and bed knife configuration
- Rotor speed
- Material temperature
- Evacuation method
What screen size should be used in the size-reduction equipment?
Regrind must be properly and consistently sized if it is to be reprocessed successfully in a mix with virgin material. But screen size must be balanced with throughput, since small screen sizes can reduce throughput. Screen size is the final consideration in grinder selection.