Residence times can be too short and too long, but too long is the more common problem. The barrel residence time may be critical so as to avoid thermal degradation of the molding resin. If the units are wrong we will have a numerical answer that is wrong for the intended calculation. Sometimes it is easy to forget the units, but this can result in incorrect conversions and incorrect answers if we do not perform all necessary steps to make units come out right. It is useful to reexamine the above calculation to review how we cancel units. We can now divide 228 lbs by 52.47 lbs/hr to get dryer residence time in hours: We convert 5.7 ft 3 dryer capacity to pounds:
We must first determine the molding press thruput rate or total pounds per hour processed.
The molding cycle is 28.5 seconds and the shot size is 188.4 gr. We can weigh a sample or consult the resin supplier to determine the resin's bulk density in this example it will be 40 lbs/ft 3. First we must gather the knowns so as to calculate the unknowns: Knowns: Dryer hopper is 5.7 cu feet of capacity. A given dryer has an internal volume, but will hold different weights of resin depending on that resin's bulk density. In order to calculate residence time in the dryer we must calculate the dryer weight capacity. Many resins require drying prior to molding to remove either surface moisture or absorbed moisture from the pellets. 6 Clamp Tonnage and Cavity Pressure Calculator.