By David Pietig, General Manager/Certified Equipment Appraiser, Arlington Plastics Machinery, Inc. (MDNA Member Firm)
The normal practice used to be to recycle your plastic products in a shredder only if the size, density or type of product required it. Now more and more processors are using a shredder before the granulator if there is any risk of metal contamination.
By using a complete Shredder/Granulator System that conveys the plastic from the shredder through a metal separator to the granulator, you can maintain fewer work stoppages when dealing with impure recyclables.
A shredder can handle metal contaminates better than a granulator and costs less to repair if something does damage one of the teeth on the shredder. You can usually replace the tooth or teeth that are damaged for a small cost and much quicker than replacing the entire set of blades or rotor of a granulator. And, you can usually operate the shredder for a longer period of time with a single damaged tooth.
By shredding the product first you can actually free the metal from large parts of the plastic making it easier to be separated. The smaller pieces from the shredder that feed into the granulator create less wear on the granulator components.
If you put metal contaminated recyclables directly into your granulator, you can easily damage your granulator blades. You then need to replace the entire set of blades or have them sent out to be sharpened. A large enough piece of metal can cause damage to the rotor or shaft which will be costly in both time and money. A new rotor is expensive and it takes time for even the best repair shops to repair.
A shredding/granulating system is a little more costly upfront and a little more involved to set up but if continual output is the main concern, it definitely needs to be considered.