Plastic/composites are shuttled into and through the barrel with the main goal of heating them to the point of transitioning to a liquid state. Once at melt temperature, how quickly do you have to process it to avoid degrading the resin? We as processors shoot for a shot size approximately 50% of the volume of the barrel. For a part requiring a 12-ounce shot of material, we should be using a press with a 24-ounce barrel capacity. This way we are preparing the next shot during each cycle, we feel this is the ideal residence time.
As a general service provider to numerous customers, we are rarely presented with such evenly matched situations. We generally start with a window of 20% to 80% ratio of shot size to barrel capacity in press selection. We say start because next we assess which material we will be shooting and its sensitivity to heat. A more sensitive resin requires less residence time and needs to be at or above the 50% capacity mark. A more forgiving resin will allow longer residence time and we can use a smaller shot percentage (larger barrel capacity) leaving resin in the barrel longer.
As a total service provider, we provide an overlapping range of presses and barrel capacities to accommodate the range of shot size and materials our various customers require. Working with our customers, we can help tailor designs to optimize the resulting production cycle and reduce overall cost. We prefer to be brought into the loop early, rather than having to react to designs presented to us. Residence time is just one of the variables that can help save money when addressed early.
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