Suck back

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Suck Back 1) When the pressure on the sprue is not held long enough for the resin to cool before the screw returns, some of the hot resin in the cavities or runner system may expand back into the nozzle and cause sink marks on the finished part. 2) Also called melt decompression and screw pull-back. A processing technique where the screw or ram is drawn rearward to relieve the plastic pressure in the nozzle. Used to control drool, stringing and to improve sliding ring nonreturn valve performance. Note: This technique often causes splay when air is drawn into the polymer melt as the screw or ram is drawn rearward.