SHOT SIZE, SHOT INVENTORY, RESIDENCE TIME
Sizing your mold to the proper molding equipment may seem obvious but many people don’t do it. If you want to get the best out of your tryout, the clamp tonnage must be enough to keep the mold closed and the shot capacity of the machine sized properly to the amount of plastic it will inject. Each of these three items including a density calculation are outputs of your Setup sheet.
SHOT SIZE:
Shot Size is the volume (usually expressed in grams or ounces) of the amount of plastic injected into the mold. The general Rule of Thumb is to use 50% of the shot capacity + 20%. Machines were originally quoted in Ounces or Grams. However, you had to understand this was in Ounces of General Purpose Styrene. So you had to go back and calculate the density to see if your material could be used in the machine. Today many machines are sized in maximum theoretical injection volume (cubic inches or cubic centimeters). This makes the conversions easier because you simply multiply the density of your plastic (grams per cc) time the volume of the machine of the machine and you get the machine’s capacity (in this case) in grams. Using less than 20% of the machine’s capacity increases the time the material will remain in the barrel (residence time). While some materials are very heat resistant, others will quickly degrade and substantially loose both their physical and chemical properties. Using more than 70% of the machine’s shot capacity overcomes the problem of overheating the material. However, we now encounter the problem of not completely melting all the material we intend to inject.
SHOT INVENTORY
Shot Inventory is the number of shots residing in the barrel. It is used to calculate residence time. Here is how it works: The shot begins in the material hopper. As the screw turns, it is fed into the screw flights. As the molding cycle continues, the screw continues to turn pushing our shot either forward into the barrel, where it is finally pushed into a hot runner system or directly into the mold where it is cooled. Our calculation for shot inventory is how many shots are in the screw flights, barrel, and hot runner system.
RESIDENCE TIME
Residence Time is the amount of time (expressed in minutes or seconds) it takes a theoretical pellet of plastic from the time it enters the barrel to the time it enters the mold. It is probably the most important variable when considering a properly sized machine.
When the material drops from the hopper onto the screw flights, it begins to see heat. The screw then turns mashing the material up against the heated barrel walls, crushing the pellets and pushing it forward. This causes the screw to be pushed backward to where it finally stops. Some of this material remains in the screw flights, merrily cooking. The screw moves forward, injecting the shot. The screw turns again and our shot goes beyond the check ring into the barrel cavity. Again it cooks. As the molding cycle repeats itself, our sample shot is finally injected into the mold. Here it may be directly injected into a conventional sprue and runner system OR it may move into an extension nozzle or hot runner system. If it is residing in either of these heated systems, again it is cooking. Residence time is the time a particular pellet (for our calculations it will be a shot) takes to get from the raw material hopper to be cooled in the mold. Its calculation would be the number of shots residing in a heated system (the shot inventory) divided by the cycle time. Obviously, the two determining factors for this calculation will be the cycle time and shot volume.
SAMPLING:
Random sampling plans tell you about how consistent (precise) the process is. Sequential samples minimize the process component and give an excellent picture of the variation on the mold.