Plastic Injection Molding Tips

Tips to possible solution on some common plastc injection moulding problems.
The conditions for moulding will of course depend on the design of the article, the ratio of the longest glow path to section thickness, and the machine on which it is moulded.
The following brief summary of the range of conditions typically met although deviation are possible depending on circumstances.
The Value of temperature differentials in adingin the production of distortion free moulding should ne remembered.
Cylindle temperature can be in the range of : 200~280℃.
Injection pressure normally in the range of 550~1,000 kgf/cm2
Moulding temperature 20~60.

The above factors may give some guidance in the interpretation of the troubleshoting guide.

Short Mouldings
Inadequate shot capacity
  • Increase feed settings.
  • Confirm plasticizing rate is adequate.
  • Transfer to larger capacity machine.
Material freezing off before completemould filling
  • Increase injection speed.
  • Raise mould temperature.
  • Increase injection pressure.
  • Increase injection time.
  • Raise material temperature.
  • Ensure adequate venting.
    Purge gates, runners or vents
    Enlarge gates, runners or sprue and balance where necessary.
  • Inadequate melt flow rate, Use higher MFI grade.

Incompatible material

Skin of material freezing off prematurely

Melt fracture
  • Check for contamination
  • Purge the machine.
  • Dry material.
  • Raise material temperature.
  • Raise mould temperature.
  • Lower injection speed...
    Adjust injection speed
    Modify gate geometry
    (eg. gate to small, land to long)
    Add Cold Slug wells in runners.
    Increase melt / mold temperature.


Shut - off faces of mould mismatched
  • Reface and realign where necessary.
  • Check mold surface for flatness.
  • Check tool for deformation
  • Check integrity of Mold shutoff
  • Change gate location
  • Clean mold surface, and ensure no foreign material on mould faces.
Mould opening up under pressure
  • Increase clamp pressure if possible.
  • Use larger press.
  • Decrease peak cavity pressure,
    decrease fill rate and/or use profile projection.
  • Enlarge gates to enable the filling pressure to be reduced.
  • Balance gates and runners where applicable.

Melt viscosity too low

  • Decrease injection speed and lower material temperatures

Poor Weld Lines

Converging streams of materials not welding properly at interface

  • Increase material temperature
  • Increase mould temperature
  • Increase injection speed
  • Rearrange gating to avoid converging flows particularly over long distances or move weld lines into more acceptable areas e.g. vented zones. Use higher MFI grade where possible.

Foreign material at interface of converging streams

  • Use less mould lubricant.
  • Ensure that pigments are well dispersed.
  • Possible contamination with other plastics in feed orbarrel, moisture,oil, etc.

Ejection Diffculties
Insufficient cooling
  • Increase cooling time.
  • Decrease mould temperature.
  • Decrease material temperature.
Excess pressure in cavity
  • Decrease injection pressure.
  • Decrease packing pressure and hold-on time.
Insufficient taper
  • Increase as required.
Poor surface finish
  • Clean and polish preferably in line of draw.
Area or position of ejectors unsatisfactory
  • Add larger or additional ejectors.
Cores out of alignment
  • Improve core location and/or alter gate position.
Sprue offset relative to nozzle aperture.
  • Realign spruce.

Sinking and Voilding
Moulding conditions causing excessive localized shrinkage
  • Increase feed setting
  • Increase pressure hold-on time.
  • Increase injection pressure.
  • Maintain adequate cushion.
  • Lower temperatures.
  • Check machine capacity is adequate.
Incorrect cooling arrangements
  • Increase cooling time.
  • Reduce mould temperature (sinking).
  • Increase mould temperature (voiding)
Design of mould or product
  • Use higher MFI grade, preferably a copolymer.
    Where suitable in other respects, use a filled grade or blowing additive/agent.
Unsuitable grade of material
  • Ensure that gating is into thickest section of moulding.
  • Increase gate size.
  • Eliminate thick sections.

Warping and Bowing
Differential shrinkage caused by: High residual stress from flow conditions
  • Decrease injection pressure.
  • Optimise pressure hold-on time.
  • Increase injection speed.
  • Increase material or mould temperature.
  • Use higher MFI grade.
  • Relocate gates or increase number.
High residual stress from unbalanced cooling
  • Arrange water connections to
    give opposing temperature gradient to that of material e.g. connect water inlets to gate area and outlets from edges.
  • Rearrange cooling circuit layout to give correct gradient.
Wall thickness variations
  • Re-design to give even wall thickness throughout.
  • If thickness variations are due to core deflection, improve core location or alter gate position.
Inadequate cooling
  • Increase cooling time.
  • Lower mould temperature.
  • Lower material temperature.
Unsatisfactory ejection
  • Check ejection system and improve as necessary.

Dull and glossy patchiness
Poor mould surface
  • Ensure mould finish is smooth/even.
  • Repolish if necessary
  • Poor pigment dispersion.
  • Improper gate location or design
Flow behavior of melt

Tiger Strips
  • Raise material temperature.
  • Decrease injection speed.
  • Increase injection pressure.
  • Raise mould temperature.
  • Use higher MFI grade. Eliminate section variations.
  • Eliminate converging flows around apertures by design modification or relocate gates.
  • Reduce injection speed, increase material temperature.
Material temperature variation
  • Nozzle temperature too low.
  • Sprue or extension nozzle too long.
  • Improve nozzle heating or insulation.

Poor Impact Strength
Crack propagation from notches
  • Radius all sharp corners and intersections that could act as notches.
Moulded- in stress
  • Reduce injection temperature by increasing material and mould temperature.
  • Increase injection speed
    Arrange correct mould temperature gradient
  • Enlarge gate to handle temperature.
    Improve venting.
Incorrect choice of grade
  • Change grade as necessary and where suitable in other respects
Degrade material or contamination
  • Check for contamination.
  • Decrease material temperature or residence time.
  • Use less regrind

Material jetting into thick section cavity Position gate to feed material against a wall or break-up flow by other means e.g. tab gate or use preform.

Surface defects around gates
Mould too hot
  • Reduce material temperature.
  • Reduce mold temperature if possible.
  • Water inlet to be in gate area.
  • Provide extra cooling circuits.
Splash marking
  • Reduce decompression - Suckback'

Excessive odour from overheated material
  • Reduce material temperature.
  • Hold-up in barrel and hot runner system.
  • Use less regrind
  • Allow cooling with free air circulation before nesting or packing
Unsuitable grade
  • Use higher MR or more lightly stabilized grade.

Burn Marks
Air trap in mould cavity

Compress air in mold degrade resins
  • Decrease injection speed, speed cavity pressure.
  • Reduce melt temperature
  • Ensure gate venting
  • Ensure correct gate location
  • Check for core deflection is not resulting in trap air.

Splash Marking
Release of moisture or volatiles from contamination
  • Dry material
    Check storage of material and avoid damp or cold conditions.
  • Ensure masterbatches are dry.
  • Decrease decompression ('suckback').
Overheating of material
  • Reduce material temperature.
  • Decrease injection speed.
  • Increase gate size.
  • Decrease screw back pressure.
Chilling of material in mould
  • Increase mould temperature.
  • Increase material temperature.
  • Increase injection speed.
Mould finish
  • Ensure no condensation due to overcooling.
  • Ensure mould finish is smooth/even.
    Repolish if necessary.

Streaking and specks
Poor pigment or masterbatches dispersion

Streaks on surface caused by volatiles such as moisture or degraded
  • Improve mixing procedure.
  • Increase screw backpressure.
  • Increase screw speed.
  • Increase temperature gradient in cylinder, with hopper end temperature lower.
  • Volatiles created by hot spot in manifold.
  • Dry resins.
  • Decrease Melt temperature
Degraded material or contaminate
  • Use less regrind.
  • Decrease material temperature.
  • Changing to smaller machine by
    reduce dwell time in cylinder.
  • Ensure that screw, cylinder and nozzle are clean.

Sticking on Mold
Over packing Reduce injection pressure.
Under packing See -  short molding
Excessive shrinkage,
tool design causes physical attachment to the core or cavity
  • Improperly balanced mold temperatures (colder on movable half)
  • Reduce cycle time (sticking on cores)
  • Increase cycle time (sticking in cavities)
  • Insufficient knockouts
  • Remove undercuts
  • Increase draft angles
  • Surface irregularities in the mold (polish cavity surfaces)
  • Highly polished core surface (vacuum lock), polish to a coarser finish, apply a surface coating eg. NanoMouldRelease, or increase venting.
Sticking on sprue
  • Check sprue bushing for wear,
  • undercut or any abrasion
    Check size, alignment of the hole in nozzle or spruce bushing.
  • Decrease injection pressure.
  • Decrease hold time.
  • Increase mold close timing.
  • Increase nozzle temperature.

Volume decreases as plastic cools and crystallizes or part is not fully packed out due to gates freezing off too soon or
insufficient cooling time
  • Excessive shrinkage – Increase cavity pressure and hold time.
  • Part oversized or not enough shrinkage – Decrease cavity pressure.
  • Maintain adequate cushion
    Increase hold time.
  • Delay gate sealing to allow pack out (increase melt temperature)
  • Mold or melt temperature too high (gates not freezing off)
  • Improperly balanced cavity and core temperatures.
  • Runners or gates too small
  • Wall thickness variation

The injection moulding guide is based on our current knowledge and experience. In view of the many factors that may affect processing and application, this data does not relieve processors from the responsibility of carrying out their own tests and experiments; neither does it imply any legally binding assurance of certain properties or suitability for a specific purpose. We expressly disclaim liability for any loss, damage, or injury directly or indirectly suffered or incurred as a result of or related to anyone using or relying on any of the information in this documents. We expressly disclaim any contrary implication.

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