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Repeatability Timescales

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An issue of growing importance in the specification and measurement of feeder repeatability relates to catch sample duration. Traditionally, as explained earlier, feeder repeatability has been measured over a duration sufficient to reflect ongoing, ‘steady state’ machine performance.

However, many of today’s precision plastics processes require knowledge of repeatability performance based on shorter intervals and thus smaller catch sample sizes. This is especially relevant to high-speed compounding or extrusion applications where the timescale of mixing within the extruder is typically much shorter than the one-minute catch sample employed as the traditional standard of feeder repeatability measurement.

Factors affecting repeatability performance at very short timescales include material characteristics such as particle size and flow properties, feeder selection and sizing, discharge stream characteristics, weighing resolution, control dynamics and precision, and environmental factors such as vibration and shock. As illustrated, at shorter and shorter timescales these factors will combine to progressively hamper momentary repeatability performance.

Complicating any prospect of in-plant measurement of feeder repeatability at very short timescales is the need to obtain precisely timed samples of the same brief duration. At such timescales human error prevents any attempt at manual sampling, and without an automated on-line sampling system, processors must rely on pre-purchase supplier testing.

Finally, with specific regard to loss-in-weight feeder applications, performance during the necessary refill phase of the feeding cycle has, historically, often been excluded from repeatability performance measurement. However, especially where repeatability performance timescales are short, precision processors should require supplier testing that reflects all phases of machine operation.

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