Cyclic gob weight and loading variations from stirred glass delivery systems

Abstract

With stirred glass delivery Systems cyelic variations in gob weights and gob throws (the angles at which free falling gobs land in the molds) are caused by wobbly stirrers which are displaced (offset) from the delivery system's center line and rotating at speeds differing from the gobbing rate. This loss of Statistical process control disappears if the stirrer's rotational speed is synchronized with the gobbing rate. However, even in a perfect setup, if the stirrer is offset from its central location, a new steady weight and a constant hooking of the gobs away from the center and toward the smaller gap side result. Three equations are derived. One equation predicts the projected horizontal angle for the orientation of a gob as it lands in a mold. Another predicts the number of gobs before equal or almost equal weights repeat. A third equation relates weight Variation to wobble and to the radial location of the rotating stirrer or needle. This equation shows the symmetry that may be expected in weight Variation curves. It is used to generate a normalized theoretical weight curve by setting the averaged percent differences between the maximum and minimum weights divided by the average weight to 1 %. This curve may also be used to disclose the number of gobs per cycle. Both loading and weight variations resulted when a non-centered, wobbling stirrer was modelled. The correlation between an experimental and a calculated weight Variation curve, due to the stirrer's rotation and normalized to 1%, was excellent. This investigation also showed that modelling could be extended to disclose changes in the weight (flow) of molten glass exiting from an orifice associated with a stirrer's rotation and alignment.