A granulator is machine that granulates scrap plastic into a smaller more manageable size. These granules can be later used in other plastic processes or sold as a commodity in the open market. Granulation and size reduction is continuing to grow in importance. Customers are increasingly demanding recycled content in their products and material cost remains on the rise. This guide aims to help you understand your own scrap plastic stream along with the features of today’s granulators. We will also discuss considerations for machine set up so that you can have an efficient granulating system. Granulators (sometimes referred to as plastics grinder) are no longer an after though in the corner of your plant. Choosing the correct machine can be an effective way to manage your material cost, help raise recycled content and most important help the bottom line.
The first step in choosing the right granulator is to actually understand what your application is. Ask yourself, “ What does my material look like?”.
Start out by describing your material. How much do we need to granulate? Is it large bulky parts? Long continues pipe? It is important to identify physical size and shape of your parts. Another important consideration is the material itself. PVC and glass filled nylon react much different than polypropylene. If you have multiple feed steams, it may make sense to put percentages on them. For example, you run 95% spurs and runners and the occasional purge it may make sense to buy a machine to handle your sprues and runners and find a different solution for those purgings. There is no such thing as the all in one granulator and by trying to do it all on one machine can become very inefficient and over time very costly. Having answers for these questions before you search for a granulator will save help you decide what type of rotor, chamber size and horsepower you will need to tackle the job.
Now that you have a better understanding of your plastic feed stream, it’s time to look at the specific granulator components. It starts with the rotor and there are 3 basic types of rotor design; open, closed, staggered. An open rotor is designed to handle thin walled parts. Typically machines can be running 3 to 5 knife configurations throughput is maximized. The material is allowed to flow effectively with an open rotor design. A closed rotor machine is used for large thick parts. Staggered rotor designs act like a hybrid between open and closed rotors. The staggered rotor offers more cuts per revolution, however maintains a solid rotor for strength for tough applications.
Along with rotor design it’s important to consider how the fly knife will engage the bed knife. A scissor cut is produced by offsetting the knives and bed knives the fly knife doesn’t fully engage at once. This helps with horsepower requirements. A bed knife is the other half to the cutting action in a granulator. Most granulators use two bed knives or stationary knives; however it is possible to have three or four bed knives to increase cutting action.
The chamber size and shape is also important when choosing a granulator. The chamber size is typically identified by length and width and if the chamber is tangential or standard. A tangential feed section will allow the fly knives to take bigger bites out of the parts on the downward strokes. The rotor is offset from the hopper which is typically used for large bottles, buckets and other bulky material. A standard chamber will let parts fall on the rotor directly. This prevents the fly knives from taking on too much plastic at once. This is most beneficial with a thick wall part.
After the part has been cut it comes to the screen. Screen size is very important as it will dictate the final partial size. Typical screen sizes can range from .25” up to 2”. Screens can usually be changed easily, however the size must be considered when estimating the throughput your machine will produce. Having smaller holes, will lower the throughput, conversely larger holes will allow much more plastic to pass through faster. Having a throughput goal in mind will help decide how you want to set up your machine to run it efficiently.
Now that we have matched our feed stream with the correct granulator we know need to decide how to set up the machine. Granulators can be fed in many ways. Belt conveyors can be set up to feed these machines typically on an incline. In some cases power rolls can be used to feed sheets and film. Both of these can use an amp feedback to speed up or slow down the feeding based on how hard the machine is working. Hand feeding or using robots are also used to feed parts as well.
After the granulator has done its work, it’s time to evacuate the granules. There are many ways to do this as well. Airveyor discharge is a very common way. A blower evacuates the particles and then passed through a cyclone where dust can be collected and the newly sized scrap can be dumped in a Gaylord box. Another similar method includes dumping on a belt conveyor or directly into a box. Smaller runs can be handled with a bin discharge that needs to be emptied when full
Now that you have a better idea of your own feed stream, a basic understanding of granulator features and some ideas of granulator set up, hopefully you can search for a machine that will work for you.