The guide that should hang on the wall of every injection molder: the seven most common molding defects, their root causes and the solutions to try — in the order process, mold and material.
1. Sink Marks
The surface pulling inward opposite thick sections. Cause: the thick area cools late and cannot be fed as it shrinks.
Solutions: Increase the holding pressure and holding time · position the gate close to the thick section · equalize the wall thicknesses as far as possible and core out thick areas · try lowering the mold temperature. For a root-and-branch solution, MuCell foam injection molding technology physically eliminates sink marks — the gas pressure does the packing from the inside.
2. Warpage
The part bending/twisting after it comes out of the mold. Cause: regions within the part cool at different rates and shrink differently.
Solutions: Keep the two halves of the mold at separate and correct temperatures with a temperature control unit · increase the cooling time · smooth out wall thickness transitions · manage the flow direction by design in fiber-reinforced materials · stage the holding profile.
3. Silver Streaks
Silvery streaks on the surface in the direction of flow. Cause #1: damp raw material — the evaporating water leaves gas traces in the melt. Cause #2: degradation of the material at excessive temperature.
Solutions: Dry the material to the datasheet values (a dry air dryer is essential for hygroscopic materials — details in our drying guide) · check the barrel temperatures · reduce frictional heat by lowering back pressure and screw speed · round off sharp corners at the gate.
4. Short Shots
The melt freezes before the mold is full. Cause: insufficient pressure/speed, low temperature, inadequate venting or insufficient capacity.
Solutions: Increase the injection speed and pressure · raise the melt and mold temperatures · check the vents · if the part weight exceeds 80% of the machine's capacity, move to a larger injection unit — we explained the 20–80% rule in our machine selection guide. Thin-walled parts may require high-speed series.
5. Flash
A thin film spilling out of the parting surface. Cause: insufficient clamping force, a worn mold surface or excessive injection pressure.
Solutions: Verify the tonnage calculation (projected area × factor) · switch to holding earlier · put the mold parting surfaces into maintenance · check the force distribution across the four tie bars — if the mold is off-center, one side opens.
6. Burn Marks
Black/brown spots at the end of flow. Cause: trapped air burns the material through adiabatic heating (diesel effect).
Solutions: Open up/deepen the vents · profile the injection speed (slow it down toward the end) · consider vacuum venting of the mold.
7. Weld Lines
A weak line where two melt fronts meet. Cause: at holes and with multiple gates, the fronts meet when they have already cooled.
Solutions: Increase the melt and mold temperatures · add venting at the weld area · move the line to an invisible/unloaded area by changing the gate position · advance the fronts in one direction with hot runner sequential injection (cascade).
A Systematic Approach
Order matters in troubleshooting: first the material (is it dry, is it the right one?), then the process (temperature, speed, pressure, time), and last the mold (venting, gate, cooling). Change one parameter at a time and record it.
For chronic quality problems, contact our technical team for a process audit and equipment recommendations — we analyze on site whether the problem comes from the machine, the mold or the material.