Injection moulding defects: causes and remedies

Dents in the surface, usually above thick sections or opposite ribs and bosses. The melt shrinks as it cools and no further material flows in.

Possible causes

• Holding pressure or holding time too low, gate freezes too early
• Melt or mould temperature too high
• Material accumulations, ribs that are too thick or abrupt wall thickness changes
• Cooling time too short: the wall is still hot inside and keeps shrinking
• Melt cushion missing or too small, so the holding pressure cannot act

Remedies

• Raise holding pressure and holding time until the part weight stops increasing
• Enlarge the gate or move it to the thick section
• Even out wall thicknesses, design ribs clearly thinner than the wall
• Lower the melt temperature
• Lengthen the cooling time and set a constant melt cushion

Brown or black discolouration, usually at the end of the flow path or in corners. Usually trapped air is compressed so strongly during filling that it heats up and burns the melt (diesel effect). Burn marks can also come from overheated or degraded melt.

Possible causes

• Mould venting too small, dirty or blocked
• Injection speed too high
• Shear heating of the melt at narrow gates or nozzles
• Degradation through too high a melt temperature or too long a residence time in the barrel (then also without trapped air)
• Clamping force so high that the vent gaps are squeezed shut

Remedies

• Clean or add vents, especially at the end of the flow path
• Reduce injection speed towards the end of the flow path (profile)
• Lower the melt temperature, shorten the residence time
• Enlarge the gate or nozzle if the shear is too high
• Do not set the clamping force higher than necessary

Silvery or dull streaks in the flow direction, often starting at the gate. Usually moisture or gas in the melt evaporates and bursts open at the flow front.

Possible causes

• Pellets too moist, not dried or dried too briefly
• Condensation on cold pellets or contamination by foreign matter, release agent or additives
• Air in the melt: back pressure too low or decompression too large
• Shear or degradation streaks: melt temperature, back pressure or screw speed too high, the material starts to degrade
• Melt fracture at too high an injection speed through narrow gates

Remedies

• Dry the material according to the datasheet, check the dryer and dew point, let cold pellets acclimatise
• Check the material path and barrel for contamination, purge if necessary
• Raise the back pressure and reduce decompression if air is drawn in
• Lower melt temperature, back pressure and screw speed if the melt is being damaged
• Reduce the injection speed or enlarge the gate, check the residence time in the barrel and in the hot runner nozzle

Thin protrusions at the parting line or at slides. The melt pushes the mould open slightly or the mould does not seal.

Possible causes

• Clamping force too low for the projected area
• Injection or holding pressure too high, switchover too late, overdosing
• Melt too hot and too fluid
• Parting line worn or dirty, mould not parallel

Remedies

• Recalculate the clamping force and set it high enough
• Reduce injection pressure and speed as well as holding pressure, switch over earlier
• Lower the melt temperature
• Clean the parting line, repair the mould

A visible line where two flow fronts meet, for example behind openings or with several gates. The weld line is often also the mechanical weak spot.

Possible causes

• The flow fronts are already too cold when they meet
• Melt temperature, mould temperature or injection speed too low
• Unfavourable gate positions, no venting at the weld line

Remedies

• Raise melt and mould temperature and injection speed
• Move the gate so the weld line ends up in a non-critical area
• Vent at the weld line

The part is not completely filled; corners, rib ends or thin walls at the end of the flow path are missing.

Possible causes

• Too little dosed or switchover too early
• Melt or mould temperature too low, injection pressure or speed too low
• Flow path too long for the wall thickness, gate too small, vent blocked
• Non-return valve leaking, nozzle blocked or too cold, feeding problems during dosing
• The machine reaches its pressure limit

Remedies

• Check the metering volume and switchover point and recalculate the shot volume
• Raise temperatures, injection pressure and speed
• Enlarge the gate, clean the vents
• Check the non-return valve, nozzle and feed; check whether the machine runs into its pressure limit

The part deforms after ejection, surfaces become wavy, frames twist. The cause is uneven shrinkage and residual stress.

Possible causes

• Uneven mould cooling, different wall thicknesses
• Temperature difference between the two mould halves
• Cooling time too short, the part is ejected too warm
• Orientation of the melt or of fibres, holding pressure too high or too low

Remedies

• Lengthen the cooling time and temper the mould evenly
• Reduce the temperature difference between the mould halves
• Adjust holding pressure and its profile
• Review geometry and gate position, even out wall thicknesses

Change only one setting per trial and document it. The points above are general experience; the actual cause depends on mould, material and machine. For mould sampling and defect analysis on site, see the expertise page.

Mould sampling and defect analysis on site

• Clamping force calculator (avoiding flash)
• Shot volume calculator (short shots)
• Cooling time calculation (warpage)
• Material guide: drying and processing window
• Plastics lexicon