By John Whaley
Product Manager - Extrusion & Orientation
In a previous article, I discussed pressure change vs. timed movements when setting up the control mode on a continuous belt screen changer. It was determined that timed cycle should be used as the primary screen advance command with a pressure override as a backup. The ideal settings are such that the timed sequence is long enough to generate an occasional pressure override but short enough that the screen doesn’t get pinned to the breaker plate. However, since the screen is advancing more frequently under this control scheme compared to a simple pressure setting, is it possible to use too much screen?
The short answer mentioned in the previous article is that screen cost is a pretty low number so screen usage shouldn’t be a concern. Here is why:
We frequently justify the screen roll cost against the standard square mesh filter packs used in slide plate machines. When a roll of screen can cost $1,000 or more depending on the size, it inevitably attracts the attention of purchasing departments. But when you break down the screen cost as a function of the pounds of product processed through the filter, it becomes clear that screen cost is insignificant.
Production: 4.5 in. production line extruding 800 lbs./hr. with a timed screen advance cycle setting of 0.5 in./hr.
Screen Roll Cost: 120 linear foot roll = $1,050, ($1,050/120 ft.) = $8.75/ft. or $0.73/in. or on an hourly basis $0.36/hr.
Calculations: (800 lbs./hr.)/(0.5 in./hr.) = 1,600 lbs./in. of screen. At a screen cost of $0.73/in. yields a screen cost per pound of production of $0.000456/lb. ($0.73/1,600 Ibs.) or $0.91 ($0.000456 x 2,000 Ibs.) per ton of production.
Total: 2,194 lbs. (2,000 Ibs./$0.91) of product for every $1.00 of screen consumed.
No matter which accounting model is used - conversion cost/hr. or conversion cost/lb. - screen cost is a pretty low number. The cost associated with the added risk of unplanned production shutdowns resulting from attempts to minimize screen consumption are much greater. Screen usage, therefore, should not be the primary concern when determining screen advance command settings.