Why difference in dimension ?

Question

If a mould design with four cavity per shot and each cavity having the same dimension and design. By principle the moulded plastic parts should also have equal dimenion. However you will notice one of the cavity will have under size. can any menbers in this group give some comment. Thanks Regards

Rolf Wissmann · Accepted Answer

There could be several reasons for that.If you are really certain and have re-checked, that the cavity dimensions ofall 4 cavities are identical (and you don't just believe what the toolmakeris claiming), then I suggest you compare the spru and runner system from themachine nozzle to the cavity gate. If, for example, the dimensions of thegate, spru and/or the runners to the "smaller part cavity" were smalleryou'd end up with less injection pressure and consequently with smallerdimensions due to more shrinkage in that cavity. I am assuming your spru andrunner system is exactly symmetrical from the machine nozzle to the cavitygate! If not, that could be another resason ...     wrote:

Ernie · Answer

Rolf pointed out the most likely causes: differences in cavity sizes or uneven filling due to unsymmetrical runners. Another possible cause is different freeze-off time for runners or gates.

One possible way to diagnose the cause is to increase the screw forward time (the time the mold is filling). If the problem is caused by differences in runner size/length/no. of turns, increasing screw forward time should minimize the differences (the lighter parts should become proportionately heavier). But, if the problem is in gate or runner freeze-offs, increasing screw forward time is not likely to cause much difference; in this case look for differences in cavity cooling.

Ernie

Why difference in dimension ?

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