It’s simple, really: the primary purpose of feed rate control in regard to plastic injection molding is to achieve consistently accurate proportions of recipe components (resin and required additives) prior to pelletization (in compounding) or end-product manufacturing.
Enter feeders: feeders produce a precise and controllable discharge rate for the material they are designed to work with. When operating in concert, multiple feeders automatically adjust their respective discharge rates to maintain desired recipe proportions regardless of total system throughput.
There are two principal ways feed rate can be expressed and controlled: by volume — in units of cubic feet/hour or cubic centimeters per minute, for example — or by weight, perhaps in pounds per minute or kilograms per hour. The difference between the two approaches is crucial to the quality of the proportioning operation. Since virtually all plastics feeding and proportioning applications are weight-based for reasons of both cost and quality, gravimetric (weight-based) feeding is, today, the overwhelming favorite method.
The Gravimetric Approach
A gravimetric feeder adds a weigh system and new control scheme to what would otherwise remain a basic volumetric feeder. In doing so, direct measurement and control of discharge rate becomes possible for the first time. While, in practice, there exist several types of gravimetric feeders, all employ the same core concepts: continually adjusting feeder speed based on direct material weight measurement resulting in a precisely-controlled discharge rate.
Advantages of the gravimetric concept include:
- Closed-loop control; direct measurement and control of feed rate
- Within broad limits, automatically compensates for material density variations
- Within broad limits, insensitive to material build-up on metering element
- Direct measurement of material rate and throughput
- High turndown, highly linear
- Automatic detection of material supply interruption
- No calibration required (supplier dependent)
- High performance accuracy potential
Feeders have, and will remain, an integral part of the plastic injection molding process. While simple in concept, they are nevertheless vital in producing quality plastic injection molded parts.