Brittleness

Possible Cause

Suggested Remedy

1.  Melt temperature too low

2.  Material overheated, resulting in molecular breakdown

3.   Contamination by foreign material or excessive pigment usage

4.   Excessive amounts of regrind

5.   Injection rate too slow

6.   Improper gate location or size

7.    Moisture in material during processing

8.    Dry-as-moulded properties

Increase melt temperature (weak weld lines)

a.         Decrease melt temperature

b.        Residence time in cylinder excessive – use smaller barrel

c.         Decrease overall cycle

d.        Reduce back pressure

a.         Inspect resin for contamination (replace if contaminated)

b.        Purge injection cylinder thoroughly

c.         Keep hopper covered

d.        Review material handling procedures for regrind usage

e.         Reduce filler or pigment loading

a.       Reduce regrind % mixed with virgin

b.      Regrind level dependent upon application: general rule 25-30%

c.       Keep hopper covered

d.      Review material handling procedures for regrind usage

e.       Reduce filler or pigment loading

a.       Increase injection or first stage pressure

b.      Increase boost time

a.       Relocate gate away from potential stress area

b.      Increase gate size to obtain optimum filling

a.       Review material handling to eliminate moisture pick up

b.      Dry material prior to moulding

c.       Utilize hopper dryers

Moisture condition parts to increase toughness

Bubbles, Voids

Possible Cause

Suggested Remedy

1.  Excessive internal shrinkage

2.  Melt temperature too high

3.  Moisture in material

4.  Air entrapment

5. Condensation on mould surface

a.   Increase packing pressure

b.   Increase injection forward time

c.   Increase gate thickness

d.   Minimize, or core out, heavy sections in part design

e.   Increase feed, ensure cushion

f.   Replace check valve if cushion cannot be held

a.   Decrease melt temperature

b.   Lower back pressure

c.   Lower screw RPM

a.   Review material handling to eliminate moisture pick up

b.   Dry material prior to moulding

c.   Utilize hopper dryers

d.   Review percent of regrind

a.   Add mould venting

b.   Relocate gate

c.   reduce clamp pressure to allow parting line vents to work

a.   Wipe mould surface thoroughly with solvent

b.   Increase mould temperature

Burn Marks

Possible Cause

Suggested Remedy

1.  Melt temperature too high

2.  Air entrapped in mould

3.  Injection rate too fast

4.  Moisture in material

a.   Decrease melt temperature

b.   Lower back pressure

c.   Lower screw RPM

a.   Vent cavity at final point of fill

b.   Decrease first stage pressure or injection speed

c.   Relocate gate

d.   Clean vents and/or enlarge vents

e.   Reduce clamp pressure to allow parting line vents to work

a.   Decrease injection rate

b.   Decrease first stage pressure

c.   Decrease boost time

d.   Enlarge gates

a.   Review material handling to eliminate moisture pick up

b.   Dry material prior to moulding

c.   Utilize hopper dryers

Cracking, Crazing

Possible Cause

Suggested Remedy

1.  Packing excessive material into the mould

2.  Non-uniform or too cold a mould temperature

3.  Knockout system poorly designed

4.  Inadequate draft angles or excessive undercuts

a.   Decrease packing pressure

b.   Decrease shot size

c.   Increase transfer position (to lower injection peak pressure)

d.   Decrease injection time

a.   Increase mould temperature

b.   Supply uniform cooling to the cavity

Redesign knockout system for balanced ejection forces

Rework mould

Dimensional Variations

Possible Cause

Suggested Remedy

1.  Non-uniform feeding of material

2.  Large variation in cylinder temperature due to inadequate or defective controllers

3.  Unbalanced runner system, resulting in non-uniform cavity pressure

4.  Insufficient packing of part

5.  Regrind not uniformly mixed with virgin

6.  Moulding conditions varied from previous run

7.  Part distortion upon ejection

a.   Adjust temperature profile for optimum feeding

b.   Increase shot size to maintain uniform cushion

c.   Replace check valve if cushion cannot be held

Replace or calibrate controllers

a.   Increase holding pressure to maximum

b.   Increase injection rate

c.   Balance/increase runner and gate sizes to provide uniform filling

Increase injection forward time and/or pressure to ensure gate “freeze off”

a.   Review regrind blending procedure

b.   Decrease percentage of regrind

Check moulding records to ensure duplication of process conditions

See Part Sticking in Mould

Discoloration, Contamination

Possible Cause

Suggested Remedy

1.   Material overheated in injection cylinder

2.  Burned material hanging up in cylinder, nozzle (black specks), or check ring

3.  Material oxidized by drying at too high temperature

4.  Contamination by foreign material

a.   Decrease melt temperature

b.   Decrease overall cycle

c.   Residence time in cylinder excessive for shot size – use        smaller barrel

d.   Decrease nozzle temperature

e.   Decrease screw RPM

f.   Decrease back pressure

g.   Check calibration of cylinder controllers

h.   Check barrel and nozzle heater bands and thermocouples

a.   Purge injection cylinder

b.   Remove and clean nozzle

c.   Remove and inspect non-return valve for wear

d.   Inspect barrel for cracks or gouges

e.   Decrease injection rate

Reduce drying temperature to 180°F (82°C)

a.   Keep hopper covered

b.   review material handling procedures for virgin and regrind

c.   Purge injection cylinder

Flow Lines

Possible Cause

Suggested Remedy

1.  Melt temperature too low

2.  Mould temperature too cold

3.  Gate size too restrictive, causing jetting

4.  Material impinging on cavity wall or core

5.  Non-uniform wall thickness

6.  Insufficient mould venting

Increase melt temperature

Increase mould temperature

a.   Increase gate size

b.   Decrease injection rate

a.   Decrease injection rate

b.   Relocate gate

Redesign part to obtain a more uniform wall thickness to provide for optimum filling

Improve mould venting

Lamination

Possible Cause

Suggested Remedy

1.  Melt temperature too low

2.  Mould temperature too cold

3.  Injection rate too low

4.  Holding pressure too low

5.  Gate size too small

6.  Contamination

Increase melt temperature

Increase mould temperature

a.   Increase first stage pressure

b.   Increase boost time

Increase packing pressure

Increase gate size for improved filling

See Discoloration, Contamination

Splay (Silver Streaking)

Possible Cause

Suggested Remedy

1.  Excess moisture in material during processing

2.  Melt temperature too high

3.  Excessive shear heat from injection

4.  Air entrapment

5.  Condensation and/or excessive lubricant on mould surface

6.  Moisture condensing in feed section of barrel

a.   Review material handling to eliminate moisture pick up

b.   Dry material prior to moulding

c.   Utilize hopper dryers

a.   Decrease barrel temperatures

b.   Decrease nozzle temperature

a.   Decrease injection rate

b.   Reduce screw RPM

c.   Increase runner size and/or gates

d.   Check for nozzle obstruction

a.   reduce screw decompression

b.   Improve mould venting

a.   Increase mould temperature

b.   Clean mould surface with solvent

c.   use mould release sparingly

a.   Decrease throat cooling

b.   Increase rear zone temperature setting

Surface Imperfections (Glass On Surface, Mineral Bloom)

Possible Cause

Suggested Remedy

1.  Melt temperature too low

2.  Mould temperature too cold

3.  Insufficient packing pressure

4.  Injection rate too slow

5.  Insufficient material in mould

6.  Water on mould surface

7.  Excessive lubricant on mould surface

8.  Moisture in material

9.  Insufficient venting

Increase melt temperature

Increase mould temperature

Increase packing pressure

a.   Increase first stage pressure

b.   Increase boost time

c.   Increase injection speed

a.   Increase shot size and maintain constant cushion

b.   Inspect non-return valve for wear

c.   Decrease injection transfer position (thereby increasing the peak pressure)

a.   Increase  mould temperature

b.   Repair any water leaks

a.  Clean mould surface with solvent

b.   Use mould release sparingly

a.   Review material handling to eliminate moisture pick up

b.   Dry material prior to moulding

c.   Use hopper dryers

Provide adequate vents

Warpage

Possible Cause

Suggested Remedy

1.  Moulded part ejected too hot

2.  Differential shrinkage due to non-uniform filling

3.  Differential shrinkage due to non-uniform wall thickness

4.  Knockout system poorly designed

5.  Melt temperature too low

6.  Glass fibre orientation

a.   Decrease melt temperature

b.   Decrease mould temperature

c.   Increase cooling time

d.   Cool part in warm water after ejection

e.   Utilize shrink fixture

a.   Increase injection rate

b.   Increase packing pressure

c.   Balance gates and runners

d.   Increase/decrease injection time

e.   Increase runner and gate size

a.   Provide increased cooling to thicker sections

b.   Increase cooling time

c.   Operate mould halves at different temperatures

d.   Redesign part with uniform wall sections

Redesign knockout system for balanced ejection forces

Increase melt temperature to pack out part better

Relocate gate

Weld Lines (Knit Lines)

Possible Cause

Suggested Remedy

1.  Melt temperature too low

2.  Mould temperature too cold

3.  Insufficient pressure at the weld

4.  Entrapped air unable to escape from mould fast enough

5.  Excessive lubricant on mould surface plugging vents

6.  Distance from gate to weld line too far

7.  Injection rate too slow

Increase melt temperature

Increase mould temperature

a.   Increase first stage injection pressure

b.   Increase boost time

c.   Increase packing pressure

d.   Increase pack time

e.   Increase injection speed

a.   Increase or provide adequate vents at the weld area

b.   Decrease injection rate

a.   Clean mould surface with solvent

b.   Use mould release sparingly

a.   Relocate or use multiple balanced gates

b.   Cut overflow tab in mould to improve weld line strength

a.   Increase injection speed

b.   Increase first stage injection pressure

c.   Increase boost time