Short shots, flash, sink marks, streaks, warpage and brittle parts - each with its real cause ranked by likelihood, and the fix in the order to try it.
Injection moulding defects read differently from blow moulding ones - the pressures are ten times higher, the cycles are shorter, and the material is pushed through a closed mould instead of formed by air. But the diagnostic discipline is identical: the defect tells you which system failed, and the fix is one change at a time.
This is the injection companion to our blow moulding troubleshooting guide, written for the machines on our injection moulding page. Eight defects, causes ranked by how often we see them, fixes in the order that works.
The MethodEvery injection defect lives in one of three systems. Pressure defects show as short shots, flash and sink marks. Temperature defects show as streaks, burns and brittleness. Time defects show as warpage, sink marks and inconsistent weight. Identify which system the defect points at, change that one variable, and check three parts before touching anything else.
Causes: injection speed or pressure too low, melt too cold, blocked nozzle or gate, poor venting.
Fix in order: raise injection speed in steps - raise melt temperature 10°C - check nozzle and gate for blockage - clean mould vents.
Causes: clamp force too low, second-stage (packing) pressure too high, damaged parting line, melt too hot.
Fix in order: verify clamp tonnage vs part area - reduce packing pressure - inspect and clean the parting surface.
Causes: holding pressure or time too short, thick sections, melt too hot, cooling too brief.
Fix in order: extend holding time before pressure - raise holding pressure - core out thick sections - reduce melt temperature.
Causes: wet material, air whipped in by high screw speed, degraded material.
Fix in order: verify dryer output and drying time - reduce back pressure and screw speed - full purge of the barrel.
Causes: unbalanced mould temperatures, cooling too short, ejection stress.
Fix: balance both mould halves within 5°C - extend cooling time - check ejector pins for uneven stress.
Causes: flow fronts meeting cold, low melt temperature, poor venting at the merge point.
Fix: raise melt and mould temperatures - reposition the gate if cosmetic-critical - add venting where fronts meet.
Causes: trapped air compressed and ignited at fill end, injection speed too high at the end of fill, blocked vents.
Fix: clean and deepen vents - reduce second-stage injection speed - check for a flow trap in the cavity.
Causes: wet hygroscopic material, degraded or burnt material, regrind ratio too high, wrong material in the hopper.
Fix: dry to spec and verify with a moisture meter - full purge - cap regrind at 20-25 percent - confirm the material grade.
Dryer temperature and residence time, mould temperature controllers at setpoint, hydraulic oil level, first-shot weight logged.
Weigh five parts, log the average, inspect the last part against the golden sample. Drift is the earliest warning of every defect on this page.
Clean mould vents, check the non-return valve for wear, verify heater zones against the controller readout.
The check ring (non-return valve) is the most-worn part on any injection machine - and the hidden cause of inconsistent shot weights and short shots. Keep a spare on the shelf from day one, per the spare parts checklist.
Short shots mean the mould is not filling completely: injection pressure or speed too low, melt temperature too low, a blocked nozzle or gate, or insufficient venting. Raise speed and pressure in steps, verify melt temperature, and clean the vents.
Sink marks come from insufficient packing: holding pressure or time too short for thick sections to be compensated. Extend holding time and pressure, reduce melt temperature slightly, and core out thick sections where the design allows.
Silver streaks are moisture or trapped gas: material not dried to specification, excessive screw speed whipping air in, or degraded material. Dry the material properly, reduce back pressure, and purge the barrel.
Flash means material escaping the mould: clamp force too low for the projected area, second-stage pressure too high, a damaged parting line, or melt temperature too high. Check clamp tonnage first, then packing pressure, then the parting surface.
Warpage comes from uneven or differential shrinkage: unbalanced mould temperatures, cooling time too short, or ejection too early. Balance the mould halves' temperatures, extend cooling, and check for uneven ejection stress.
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