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In the case of pellets, look at the pellet shapes (square, round, flat, cylindrical, etc.). Consistency of the pellet size and shape is a very important factor to ensure repeatability of your feeder. Medical-grade materials generally tend to be the most consistent in terms of pellet size and shape, while lower-cost materials tend to have more fines and varied sizes of pellets. When considering low-rate feeding, processors should make sure their material supplier can supply both clean and consistent material sizes. In the case of powdered materials, particle size and output volume are gener- ally consistent. That said, the flowability of the material can have the greatest impact on accuracy. It is common practice for feeders that handle cohesive or poorly flowing material to incorporate flow aids to overcome material inconsistency. These flow aids can come in both mechanical and software-based solutions. These flow aids are typically defined during the supplier's testing program. WHAT IS YOUR MATERIAL FEED RATE? Most processors define feed rates for additives as a percentage of the overall weight. Consider the example of a small-tonnage med- ical molding application feeding color to produce a translucent product. In this scenario, a shot size of 1.68 g with a color additive of 0.06 g was equal to three pellets. This example should make it crystal clear why it is so important to understand material inconsistency. If the variation of the pellet was too great, the molder could not hold product color consistently. Due to the translucency of the product, feeding two or four pellets would make a difference in the end-product's color shade. The output from the feeder had to be a consistent three pellets. Our lab testing did find some variations of pellet sizes in different batches of colors. After some feeding tests, we were able to overcome these minor variations with some feeder modifications and produce a product with consistent color. Feeding materials at higher rates typi- cally mitigates a number of inconsistencies that standard materials have. During testing of different batches of materials, feeding under 100 g/hr, we have found that small changes in pellet sizes can have a significant impact on the feeder output. Example: Feeding a cylindrical pellet using a hole size of 3/16 in. would feed two to three pellets into the process, but using a 5/32-in. hole would feed one to three pellets. QUESTIONS ABOUT FEEDING? Visit the Blending/Feeding Zone. When considering low-rate feeding, proces- sors should make sure their material supplier can supply both clean and consistent material shapes and sizes. Orbetron designs all minor-additive disc feeders with two major components: a motor and a feeding device. Low-rate feeding usually requires on-the-fly material/ color changes, facilitated by interchangeable hoppers. @plastechmag 45 Plastics Technology M I C R O F E E D I N G