Injection-molding defects and analysis of their causes

1. Incomplete filling (short shots, insufficient material).
2. Flash (burrs).
3. Silver streaks (splay).
4. Sink marks.
5. Warpage (deformation, twisting).
6. Parts sticking in the mold.
7. Discoloration and burning.

4.1 Incomplete filling

1. Barrel, nozzle and mold temperatures are too low.
2. Injection pressure is too low or injection speed is too slow.
3. Feed adjustment is improper.
4. Injection time is too short.
5. Runners or gates are too small, too few or poorly positioned, or the feed system is blocked.
6. Poor nozzle fit.

4.2 Flash (burrs)

1. Barrel, nozzle and mold temperatures are too high.
2. Poor precision at the mold parting surface.
3. Excessive injection pressure or insufficient clamping force.
4. Unsuitable mold design or feed arrangement.
5. Poor cavity venting.
6. Inadequate mold sealing, debris, or bent/deformed mold plates.

4.3 Silver streaks (flow marks)

Silver streaks are defects appearing on the part surface in the melt-flow direction when gas interferes with cavity filling.

1. Insufficient drying of raw materials.
2. Excessive material temperature or poor plastic quality causes decomposition gases.
3. Excessive injection pressure or speed subjects the plastic to high shear and decomposition.
4. Back pressure is too low, screw speed is too high, or screw retraction is too fast, creating a vacuum.
5. Inadequate venting; add vent grooves.

4.4 Sink marks

1. Mold temperature is too low for thin-walled parts or too high for thick-walled parts.
2. Injection and holding times are too short, or injection pressure is insufficient.
3. Unsuitable barrel/nozzle orifice size or temperature.
4. Severe screw or plunger wear in the barrel causes backflow.
5. Increase gate/runner dimensions.

4.5 Warpage (deformation, twisting)

1. Mold temperature is too high or cooling time is insufficient.
2. Large differences in part wall thickness.
3. Improper ejector positioning causes uneven forces.
4. Improper gate location.
5. Excessive injection or holding time creates stress.

4.6 Parts sticking in the mold (drag marks)

1. Injection pressure is too high or injection time is too long.
2. Mold temperature is too high.
3. Gates are too large or improperly positioned.
4. Inadequate cavity surface finish.
5. Insufficient draft makes demolding difficult.
6. Unsuitable ejector positioning or structure.

4.7 Discoloration and burning

Discoloration occurs when plastic decomposes excessively or remains too long in the barrel, becoming charred and then entering the cavity with the melt.

1. Loss of temperature control causes overheating and decomposition.
2. Poor mold venting.
3. Excessive injection pressure or injection time.