In its most basic form a combination weighed/scale is a mass flow device that gathers multiple weighed charges to meet an intended target weight. There are multiple philosophies with regard to exact method (as controlled by operational programming, and operator set up both have positive and negative results), but most systems operate on the same basic principles.
Packaging System Design
One of the most important considerations to the peak operation, and the functional efficiency of a combination weigher is the initial overall packaging system design. Truly, the way in which a scale is fed is the first consideration of the most efficient operational success.
There are some product specific derivations to the next, but product fed slightly off center, at a rate which creates almost a continual flow onto the top cone/dispersion plate, is optimal. The first most common error that is not recognized when evaluating a scales operating efficiency is flooding or overfeeding a scale. Too much product on the top of a scale is a bigger detriment to operation and accuracy, than is too low a volume of product. Simply stated, flooded scales over feed, with less control, and adversely effect accuracy by throwing overweights more often. Top cone call and satisfied settings are critical, and should be tailored to product and weighment vales in each operational recipe.
Combination Weigh Scale Accuracy
The next important consideration for accuracy is that a well tuned scale should have varying amplitudes and run time on the linear feeders. The linear feeders, supply statistical varied volume of product required in the collection buckets to allow the weigh buckets the optimal number of varied weights available from which to choose.
The wrong feed rate either too fast, or too slow, will flood or starve a scale just as readily as poor product supply as described The ideal weighment draws from four buckets on a fourteen head scale, minimally three buckets (this does vary depending upon bucket size versus product size and weighment volume). This variation further increases the efficiency and timing of the scale. The more choices the scale has the better it is able to match the target weight.
Properly Size the Combination Weigh Scale
The most common mistake when specifying a scale for an application is sizing. A scale with buckets too large is very inefficient and “clunky” in it’s operation, requiring more settling and cycle time than might be necessary for a properly sized scale. Likewise a scale with buckets too small experience product bridging and/ or product hanging in buckets and through bucket doors.
The second largest mistake is to ask a scale to do too much. It goes hand in hand, that if scale sizing is important, product and weighment volume variations through the same system would be just as detrimental to operational efficiencies. Large swings in product densities and weighment volumes are compromises for accuracy, and efficiency.
There is a simple rule; that is not so simple..more buckets equals to more weigh buckets for the scale to choose from, equals, more available combinations, which in turn equates to higher accuracy. Negatively, more buckets equate to wider geometry, and more distance from the center discharge, and a higher system elevation to accommodate the correct trajectory of the product, and likely hood of product travel issues, affecting packaging machinery efficiency and throughput.
So there is a diminishing return, and it goes both ways. For most scale/bucket sizes the 14 head scale is optimal. As you reduce the number of buckets, and therefore the number of combinations available, you gain nothing in geometry. So obviously the 12, 10, and 8 head models are purely economic compromises, which if you analyze the lost efficiencies, product, and throughput, will continually cost you operating capital over the life of the scale when compared to the 14 head model.