Measurements on-site showed that the high speed filling of this powder into sacks was leading to the generation of considerable electrostatic charge on the powder. Figure 1 illustrates ideal close packing of uncharged spheres, maximizing the packing density, while figure 2 shows the effect of charge on packing. Here, the force of repulsion prevents the closest packing of the powder, leading to a reduction in bulk density. On-site trials had already demonstrated high levels of charging, so the laboratory results confirmed the hypothesis that the packing problems were due to electrostatic charge. The case study demonstrates how, in a sack filling operation, unwanted static charge can lead to excessive spillage and underweight sacks.
Powder can even back up in the filling machine as it tries to pack the set weight in the sack.
This may stall the machine requiring it to be stripped down and cleaned before resuming packing, giving rise to considerable packaging down-time. Processing problems can be reduced or eliminated by the design and implementation of appropriate charge control techniques. The solution in this particular case lay in eliminating the charge on the powder. A specially-designed static eliminating system was constructed for installation on the filling machine. It is seldom possible to prevent charging without significantly altering the material or the process in question.