Most extrusion operations generate scrap, and in the case of sheet for thermoforming it can exceed 70% of total output. How can you handle all this stuff?
Most screw designs can accommodate small amounts of regrind, and small percentages often increase output. The amount of regrind an extruder can handle is a function of the design of the hopper, feed throat, and screw, as well as the characteristics of the regrind itself. Not only does regrind impact output, but it can affect stability. In our February column we covered the importance of the design of the hopper and feed throat and how they can influence output. These same characteristics are even more important with regrind because it is generally not as free-flowing as virgin pellets or powder.
Naturally the screw design plays an important part, as it must be able to accept a lower bulk density feed material and still maintain output. Fortunately, most screws are over-designed for feeding capacity of virgin polymer. That’s mostly because feed-section design is inexact, and feed sections are over-engineered to be on the safe side.
Processing problems often start to develop when regrind content reaches 20% to 25%. That’s mostly because of the lower bulk density of regrind. As a result, you won’t get as many lb/hr through the hopper, feed throat, and screw. Additionally, the flow of the virgin/regrind mixture may not be steady, due to instantaneous bridging as the mix moves from the hopper entrance to the screw flights. This feed disruption can pass through the screw as instability or surging.
Even though the screw may be over-designed for feeding capacity, it quickly reaches its limitation as the bulk density drops. So, when running a high percentage of regrind, you should test the regrind’s bulk density compared with that of virgin. Fill a container with virgin and then with regrind and compare the weights after subtracting the container weight. If your regrind weighs less than 70% of the virgin you may not be able to maintain output and stability with more than a low level of regrind.
I learned some shop-floor tricks of the trade from George Kruder of HPM, a pioneer who developed many practical approaches for extrusion. He would take a sample of the regrind/virgin mix in a 24-oz container and slowly pour it out on a flat surface. It would mound up in the shape of a peak. The angle of the peak from the flat surface is the “angle of repose” and is an indicator of the internal friction characteristics of the mixture. The higher the angle of repose, the more difficult it will be to feed the mix through the hopper and feed throat and into the initial feed channels of the screw. Regrind such as film scrap or foam that “stacks” rather than “mounds” often cannot be fed without a crammer or stuffer. Try comparing your virgin polymer to your virgin/regrind mixture by this test to see the difference.
A lot can be done to improve regrind characteristics for higher output and stability. The primary thing is to experiment with your granulators to get the highest bulk density and lowest angle of repose possible. Experiment with screen size. Ensure that rotor and bed knives are sharp and side-plate clearances are tight. If you can eliminate stringers and random large particles that slip through worn granulators, you’ll improve bulk density and flow.
Optimizing particle size from the scrap grinders will have a pronounced effect on your ability to feed higher percentages of regrind. It’s not necessarily smaller or larger particles that provide the highest bulk density and best free-flow characteristics. Regrind from different processes and shapes will act differently. This is particularly true if the regrind has a lot of curved particles that do not pack well or slip on each other, such as from blow molded or thermoformed containers.
If you still are seeing a significant loss of output and stability after taking these steps, a review of the design of your extrusion equipment is in order. Don’t underestimate the importance of the design of the equipment sitting above and around the screw. Obstructions, changes in geometry and column height become more important with regrind. And screw design changes may be the answer when all else fails.
Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact firstname.lastname@example.org or (724) 651-9196.