Feeder Screws

Feeder Screws "Surging"

Arbo Vibratory Feeder vs Single Screw Feeder Consistency Evaluation

by T.F. Langston, Citsco Incorporated

based on exhibitor presentation at 1998 SPE Antec, April, 1998

It is well-known that screw feeders tend to have a surging discharge which can cause problems for starve-fed extruders. What is not so well known is just how costly the effects of a surging feed system can be, especially in installations with the modern high speed compounding extruders such as the Werner & Pfleiderer Megacompounder or the Berstorff Ultra-Torque. These extruders are capable of operating at speeds of 1,200 rpm and, at such speeds, the effects of feed system surging may be dramatic. Feed system surging may adversely affect the following aspects of starve-fed extrusion:

* Production Efficiency (Output)
* Product Quality (Consistency)
* Product Physical Properties (Consistency)
* Strand Breakage Frequency (Lost Production)
* Broken Screw Elements

A model of a typical "industry standard" single screw auger is illustrated on the right; it is obvious that there are two causes of discharge surging:

1: a cascade of material at the discharge with each rotation of the screw or twice per rotation with double flight augers.

2: interruption of material flow from feeder hopper by the flights rotating through the hopper discharge opening.

Resultant product inconsistency is explained by the graphic illustration of a three component surging feed stream on the right. While the average rate of each component is nearly perfect when evaluated over the industry standard performance sample period of sixty seconds, when evaluated over a meaningful period, say inversely proportional to the rotational period of the extruder screw, the resultant compound ratio shows dramatic variability which is unlikely to be eliminated by the distributive mixing capabilities of the compounding extruder screw. Remember, a compounding extruder only conveys material in one direction--there is no provision for recirculating the compound until the required formulation is achieved(except by using multiple pass compounding which reduces production efficiency and material properties).

Product inconsistency not only affects formulation quality but adversely affects attainable physical properties, strand breakage, breaker plate and screen pack fouling and broken screw elements.

Physical properties may be limited by the processing capacity of the distributive and dispersive mixing elements of the compounding extruder. For example, when the extruder screw is capable of effectively compounding a filler, such as chopped fiberglass, with up to a 60% fill ratio, but a surging feed system creates a short term variability of +/- 10%, the time-critical feed system ratio will be 60% +/- 10% which exceeds the capacity of the mixing elements and causes broken elements, fouled breaker plates, screen packs, plugged die holes and, depending on the specific materials, frequent dropped strands.

The bottom line result is inferior product quality and reduced compounding efficiency

Extrusion efficiency may also be limited more directly by limitations on the operational parameters of the extruder. In these days of extreme pressure to stretch the product and productivity envelope, it is necessary to operate at the limits to remain competitive. In the case of starve-fed compounding extruders, those limits are generally limits of torque, power, pressure or temperature. In almost every case, when significant short term feed system surging is present, those limits will be encountered at the peak of the feed system surge. That's the point at which the screw overloads, the clutch trips or the motor overloads.

In this case, the difference between extruder production capacity is simply the difference between the peak of the feed system surge, where the limit is encountered, and the average system rate--the difference is wasted production efficiency (and lost profits--since the production overhead must be covered by the established production efficiency, an increase of 5-10% may represent an increase in profitability of more than 5-10%).
The charts below graphically illustrate the difference between an Arbo model VS/28 single screw feeder compared with an Arbo model VV/80 vibratory feeder. The difference is due to the relatively high frequency operation of the vibratory feeder--35-70hz, in contrast with the relatively low rotational speed of the industry standard auger feeders.
In an "apples and apples" comparison of feeder volumetric discharge consistency sampled over a very short timebase the difference between single screw auger discharge and vibratory tray discharge surging is graphically obvious. Tests were performed by Citsco with black polypropylene Gala underwater-cut mini-pellets at about 250 pellets/gram. Samples were collected using Hardy 2151 with 1 lb load cell interfaced to Genesis version 3.7

Arbo VS/28 Single Screw Dbl Flight

Average rate = 0.994 lbs/hr

1 second sample time
Arbo VV/80 Vibratory Vee Tray

Average rate = 0.984 lbs/hr

1 second sample time

Arbo VS/28 Single Screw Dbl Flight

Average rate = 0.535 lbs/hr

5 second sample time
Arbo VV/80 Vibratory Vee Tray

Average rate = 0.530 lbs/hr

5 second sample time

Arbo vibratory feeders provide ideal feeding characteristics for starve-fed compounding extruders and are suitable for a wide variety of free flowing materials including pellets, free flowing powders, glass fibers up to 0.250". In addition, the complete absence of wear parts ensures minimum down time for repairs and maintenance--no brushes, no seals, no bearings, no belts, no gears, no lubrication, no adjustments.

A full copy of this paper as presented at the Antec conference is available from Citsco or from Arbo on request. Arbo also offers material testing facilities in Toronto, Canada and Lugano, Switzerland where you may observe for yourself the performance of Arbo vibratory feeders with your materials. Furthermore, for materials which are not suited to vibratory feeders, sticky co-hesive pigments and similar materials, Arbo offers a most diverse line of feeders including single and twin screw, rotary valve, loss-in-weight belt, weighbelt and liquid gravimetric systems.

Citsco Incorporated