Material Drying
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ABS vacuum void (shrinkage cavity)
Problem Phenomenon
Vacuum-like holes with smooth walls appear inside thick-wall products; the appearance may be accompanied by sink marks, different in form from gas-containing bubbles, often appearing after demolding or during storage.
Cause Analysis
The surface layer of the thick-wall area cools and shrinks first, and the internal melt cannot be compensated due to insufficient injection/holding energy, forming a vacuum void; its core is insufficient filling/holding, which is a completely different mechanism from gas-containing bubbles (moisture, volatiles), and needs to be handled separately.
Solution
①Increase injection pressure, speed, and shot volume, and extend holding time to ensure sufficient melt compensation; ②increase melt temperature and nozzle temperature to improve flow; ③set the gate at the thick-wall area so that the thick-wall area gets compensation first; ④locally increase mold temperature to delay surface solidification; ⑤structurally reduce material in thick walls.
Key Takeaway
For ABS bubbles, first distinguish 'vacuum void' from 'gas-containing bubble': bubbles that exist the moment the mold opens are mostly gas-containing (moisture), those that appear later inside are mostly vacuum voids; vacuum voids rely on compensation, gas-containing bubbles rely on venting; only by distinguishing them can the right remedy be applied.
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