Optimal process settings are critical to the ultimate productivity level, part quality and final cost of injection moulding. The Troubleshooting Guide covers basic problems which may be encountered and possible first option solutions for the user.

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Injection Moulding Troubleshooting Guide

Please note
The Injection Moulding Troubleshooting Guide covers some of the common problems encountered with this process, but is by no means complete. Conditions are variable and include: machine used, machine setup, mould type and complexity, materials and manufacturing environment. Specific conditions cannot be anticipated. If problems persist, please contact HMC Polymers Technical Service for direct consultation and advice.

The following technical information is provided for convenience only, without any warranty or guarantee of any kind. Users are encouraged to verify all information provided independently to their reasonable satisfaction. This includes any technical information, specific recommendations, testing, or analysis, including, without limitation, any information regarding moulding processes, or the selection of a product for a specific use.

Short shots
Short shots appear when an injection mould has not been properly filled. As a result, parts appear incomplete. This effect usually appears far from the gate, especially where long flow distances are involved, thin walls (or a combination), or thin ribs to be filled by the polymer.

The effect of short shots may also appear in other areas of the moulding and often the reason can be traced to gate design.

(Left) Incompletely filled part   (Middle) Solidified flow front   (Right) Incompletely demoulded lattice 

Common causes for short shots can be:
  • Insufficient amount of material injected
  • Insufficient injection fill speed
  • Insufficient pack pressure
  • Insufficient injection time
  • Unbalanced multiple cavity mould
  • Foreign material clogging nozzle
  • Stock temperature too low
  • Runners, gates, sprue or vents too small
  • Mould temperature too low
  • Undesired barrel heating capacity
  • Excessive wear of screw, barrel or check value

Mould flash
Parts exhibiting mould flashes are characterized by film-like plastic edges attached to the molded parts. The flashes often appear near commissures, sealing faces, venting channels or ejectors.

(Left) Large area over-spraying     (Middle) Flash on turning knob    (Right) Flashes on venting channel

Common reasons for mould flashes can be:
  • Excessively high injection pressure (fill or pack)
  • Foreign material on mould surface (contamination, dirt, etc.)
  • Low clamping force pressure
  • Stock temperature too high
  • Resin melt viscosity too low
  • Shut-off faces of mould mismatched

Sink marks
Sink marks are aesthetic defects at the surface of the moulding. They appear as depressions and typically appear at thicker sections of the moulding. The root cause is usually shrinkage of the PP material during the crystallization process.

(Left) Sinks near ribs    (Middle) Sinks due to wall thickness    (Right) Sinks on cylindrical core with poor temp. control

Possible causes for this excessive shrinkage can be:
  • Insufficient injection pressure
  • Insufficient dwell or hold time
  • Insufficient amount of material
  • Insufficient cooling or injection time
  • Excessively high injection speed
  • Melt or mold temperature too high
  • Poor part design, non-uniformed walls and/or excessive wall thickness
  • Improper gate location or design

Voids appear as small bubbles or blisters within the moulding and are typically created by entrapped air, gas, or perhaps a vacuum. Voids become more apparent if the item is highly transparent.

(Left) Blisters on part surface   (Middle) Blisters due to injected air   (Right) Air bubbles solidified during flow

Possible causes can be:
  • Failure to fill mould completely
  • Poor venting of mould, particularly around projections
  • Improper gate loctaion
  • Fill rate too rapid (trapped air produces short shots)
  • Mould temperature mismatched
  • Excessive part thickness (greater than 6.3 mm (1/4”)
  • Entrapped moisture
  • Entrapped air through porous or very fine additive powders absorbing air

Flow marks and weld lines
Weld lines represent an optical as well as mechanical defect in a moulded part. Weld lines typically appear in the area where the polymer flows come together during the injection process. Flow marks appear due to flow turbulences during injection. Those effects are particularly visible in colored or metallic pigmented parts in the form of colour changes.

(Left) Colour change at weld line    (Middle) Colour change at weld line   (Right) Visible notch on transparent part

Possible causes can be:
  • Stock temperature too low
  • Mould temperature non-uniform or too low
  • Mould fill too fast or too slow
  • Excess mould lubricant
  • Scratched or dirty mould surface
  • Fill speed and/or packing time too low
  • Inadequate venting
  • Improper gate location or design

Sometimes following the injection process, parts may be produced which show crack formations when low stress or flexing is applied.

This phenomenon can be caused by:
  • Degraded material from barrel (excessive melt temperature)
  • Contamination by another material(s)
  • Improper design (Inadequate radii at corners, notch or thread)
  • Use of improper colour concentrates (non-compatible carrier resin)
  • Voids
  • Stock and mould temperature too low
  • Excessive amount of regrind
  • Moulded stress too high

Grooves are a surface defect where “rings” appear at the surface of moulded parts mainly around pin point gates and concentrically spreading over the moulding.

(Left) Concentric record grooves   (Middle) Concentric record grooves    (Right) Record grooves starting at gate

Reason for this type of defect can be:
  • Insufficient stock temperature
  • Insufficient injection speed
  • Mould temperature too low
  • Improper gate location or design

Jetting is a similar defect to grooves. Also in this case rough or matt lines appear at the surface of the moulding starting at the gate and spreading over the entire part. The lines will appear different in gloss and colour and are more visible with coloured parts.

(Left) Jetting starting at gate    (Middle) Jetting near gate   (Right) Jetting - melt strand squeezed

The possible causes can be:
  • Undeveloped front flow
  • Excessive injection speed
  • Programme injection (distance – pressure – speed)

Air streaks
Air streaks in moulded parts appear as matt, silvery or white lines (streaks) at the surface of the moulded parts. They can usually be found near domes, ribs and where the wall thickness of the moulded part may vary. They also can appear near the sprue or near engraving and depressions.

(Left) Air streak due to trapped air near a rib   (Right) Air streak due to air sucked in during decompression

Reasons for this effect can be:
  • Inadequate venting
  • Excessive injection speed
  • Air creates suck back
  • Excessive moisture in material
  • Mould temperature too low

Moisture streaks
Moisture streaks can appear on the surface of moulded parts as a U-shaped profile open against the direction of the flow. They usually appear as silvery streaks and their surface is rough or porous. Moisture streaks caused by the moisture on the mould surface appear as large and dull lamellar structures.

(Left) Due to moisture in granules  (Centre) L: Moist granules - R: Mosture on mould   (Right) Due to moisture in granules

Moisture streaks are typically caused by:
  • Moisture in resin
  • Moisture condensation on surface of mould

Colour streaks
Colour streaks can appear on colored parts as a result of uneven distribution of the colour pigments in the item or different orientation of isotropic pigments in the moulded item. Thermal effects (degradation of pigments) can also cause different colour shading visible on the item.

(Left) Orientation of metal effects pigments caused by flow   (Right) Colour streaks due to incompatible masterbatch

Colour streaks can be created through:
  • Inadequate colour dispersion or distribution
  • Improper masterbatch
  • Improper design
  • Lack of lubricant for dispersion
  • Stock temperature too high
  • Excessive injection pressure

Stress whitening
Stress whitening is caused by stress cracks in the polymer material due to internal as well as external stress. In PP, stress whitening typically appears with copolymers (HECOs) as micro cracks between the matrix and rubber phase). Typical appearance locations are de-moulding points. Stress cracks caused by internal stress can often appear days or even weeks after production during the relaxation process of the moulded part.

(Left) Whitening during demoulding   (Centre) Stress cracks after purchase  (Right) Stress whitening on integral hinge

Stress cracks can be due to:
  • Characteristic of individual plastic (Copolymer, impact resistance, etc.)
  • Residual stress due to moulding condition (e.g.: melt temperature, part design, resin flow behaviour etc.)
  • Chemical resistance (ESCR)

The distortion of moulded parts is called warpage. Warpage can appear directly after remolding of the part, but also appears days later after the recrystallisation of PP is complete. Typically warpage appears in large, flat items with long flow lengths and items with narrow ribs.

Warpage can be caused by:
  • Part ejected too hot
  • Improperly balanced core and cavity temperatures
  • Inadequate or poor location of knockout mechanism
  • Overpacking in gate area because of high injection pressure
  • Moulded-in stresses due to low stock temperature or mould too cold
  • Improper part design, non-uniformed walls
  • Improperly balanced multiple gates
  • Flow too long, insufficient gates

Burnt streaks
Burnt streaks are defects caused by degradation of the polymer during the injection process and appear as brownish or silvery discolourations at the surface of the moulded article.

(Left) Too long in plasicizing cylinder   (Centre) High shearing heat at gate   (Right) Too long in plasicizing cylinder

Burnt streaks can be caused by:
  • Injection pressure too high
  • Injection speed too high
  • Back pressure too high
  • Stock temperature too high
  • Inadequate venting
  • Improper gate location or design
  • Other material contamination

Excessive shrinkage
Excessive shrinkage is a characterized by moulded parts which appear smaller than the intended dimensions or show warpage.

Excessive shrinkage can be caused by:
  • Cure time too short
  • Pack pressure too low
  • Mould or stock temperature too high
  • Insufficient injection pressure
  • Runners or gates too small
  • Poor part design, varying wall thickness

Gloss difference
Gloss differences typically appear due to wall thickness differences of the moulded parts and subsequently different rates of cooling at different areas.

(Left) Gloss differences near ribs   (Centre) Due to wall thickness variations   (Right) Gloss differences near weld lines

Gloss differences can be caused by:
  • Flowability at surface of mould
  • Processing temperature
  • Variable cooling conditions
  • Stress residue in mould

Sticking in mould
Molded parts may stick in the mould and cannot be ejected properly. This typically occurs if the shrinkage level is too low or too high to allow easy ejection, or the shape and surface finish of moulded articles creates a “sticking” effect in the mould itself.

Sticking in the mould can be due to:
  • Overpacking – injection pressure too high
  • Underpacking – excessive shrinkage
  • Insufficient cooling
  • Highly polished core surface > draw polish
  • Insufficient knockout action
  • Surface irregularities in the mould
  • Insufficient core and wall tapers
  • Undercuts