Feed Rate Basics
In the continuous formulation of plastics the primary purpose of feed rate control is to achieve consistently accurate proportioning of recipe components (resin and required additives) prior to pelletization (in compounding) or end-product forming operations.
Feeders produce a precise and controllable discharge rate for the material they are designed to handle. 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 ways feed rate can be expressed and controlled: by volume in units of ft3/hr or cc/min, for instance, or by weight, say in lb/min or kg/hr. The difference between the two 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 near-universally preferred approach.
The Volumetric Concept
Volumetric feeding relies on some form of displacement principle to create a discharge stream. Whether a rotating screw pushing material toward discharge, a rotary feeder dumping its pockets, or a vibratory tray jostling the material forward… all are examples of displacement. Discharge rate is controlled by feeder speed based on prior calibration.
Attributes of the volumetric concept are as follows:
- Open-loop control; feed rate must be inferred from feeder speed
- Cannot automatically compensate for material density variations
- Sensitive to material build-up or other factors affecting calibration
- No direct measurement of material throughput
- Limited turndown; insensitive to system non-linearities
- No innate detection of material supply interruption
- Regular recalibration required
- Limited performance accuracy potential
The Gravimetric Concept
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. While, in practice, there exist several types of gravimetric feeders, a few of which are detailed here (see Core Operating Principles), all employ the same core concept of gravimetric feeding: Continually adjust feeder speed based on direct material weight measurement to result in a precisely controlled discharge rate.
Attributes of the gravimetric concept are as follows:
- 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
The following section (The Gravimetric Feeding Triangle) introduces the three main challenges faced by any gravimetric feeder: material handling, weighing and control. Later sections (Focus on Material Handling, Focus on Weighing, and Focus on Control) present more in-depth treatments of the application issues and equipment solutions available in each of these three areas.