Troubleshooting Guide for Injection Molding I

Troubleshooting Guide for Injection Molding I

In most cases, the surface quality of thermoplastic injection moulded parts is the main criterion for their quality. Due to the complex interrelationship between the moulded part and the mould, the moulding compound and the processing, it is often very hard to recognise the origin of problems and thus to take immediate action.

This troubleshooting guide is designed to help in analysing surface defects in the injection moulding and to provide hints on avoiding and or reducing defects. It consists of descriptions, pictures and notes about the different defects, which helps in classifying the problem. It provides a short explanation of possible physical causes for the defect.

Furthermore, important notes on general faults are supplied, as well as notes on boundary conditions which should be considered. There are also flow charts containing hints on avoiding or reducing defects. Remedy and hints are given, concerning the process, the moulded part, the mould design and the moulding compound. A blank Data Acquisition Record can also be found here to help in evaluating the optimisation process. Finally some case studies of real-life problems are included.

Detection and Classification of Defects

Injection moulding defects are classified into seventeen sections:

1. Sink marks
2. Streaks
2.1 Burnt streaks
2.2 Moisture streaks
2.3 Colour streaks
2.4 Air streaks and air hooks
2.5 Glass fibre streaks
3. Gloss/gloss differences
4. Weld line
5. Jetting
6. Diesel effect (burns)
7. Record grooves effect
8. Stress whitening/stress cracks
9. Incompletely filled parts
10. Oversprayed parts (flashes)
11. Visible ejector marks
12. Deformation during demoulding
13. Flaking of the surface layer
14. Cold slugs/cold flow lines
15. Entrapped air (blister formation)
16. Dark spots
17. Dull spots near the sprue

Sink Marks

Physical cause
Sink marks occur during the cooling process, if the thermal contraction (shrinkage) of the plastic cannot be compensated in certain areas. If the outside walls of the moulded part are not stable enough, due to insufficient cooling, the outer layer is drawn inside by cooling stresses as shown.
There are three fundamental cases:

  • Solidification too slow
  • Effective holding pressure time too short
  • Not enough holding pressure transfer; because flow resistances in the mould are too high.

Note: For optimum holding pressure transfer the moulded part should be gated to the largest cross-section. In order to avoid premature solidification of the sprue and gate system, sufficient dimensioning is necessary.

Moulded part with sink marks (gating at the thin wall)
Moulded part with sink marks (gating at the thin wall)

Sink marks appear for example near material accumulations as depressions on the surface of the moulded part, if the thermal contraction (shrinkage) cannot be compensated as illustrated.

Sink marks due to wall thickness variations
Sink marks due to wall thickness variations
Sink marks on the cylindrical core whose temperature was not controlled correctly
Sink marks on the cylindrical core whose temperature was not controlled correctly

Correcting sink marks: Check and/or change machine settings. Change mould or moulding compound.

Streaks

Streaks caused by burning, moisture or air can look very similar making classification difficult if not impossible. The signs listed here do not have to appear, they only give reason to suspect a certain type of streak.
Signs for burnt streaks

  • The streak appears periodically
  • The streak appears behind narrow cross-sections (shear points) or sharp edges in the mould
  • The melt temperature is near the upper processing limit
  • Lowering the screw advance speed has a positive impact on the defect
  • Lowering the melt temperature has a positive impact on the defect
  • Long residence time in the plasticising unit or the space in front of the screw (due to, e.g., cycle breaks or low shot volumes)
  • High reclaim content, or a part of the material has already been melted several times before
  • The mould is equipped with a hot runner
  • The mould is equipped with a shut-off nozzle.

Examples of mouldings with burnt streaks

Burnt-streaks-due-to-excessive-residence-time-in-the-plasticising-cylinder
Burnt-streaks-due-to-excessive-residence-time-in-the-plasticising-cylinder
Burnt streaks due to high shearing heat in the gate
Burnt streaks due to high shearing heat in the gate
Burnt streaks due to excessive residence time in the plasticising cylinder 1
Burnt streaks due to excessive residence time in the plasticising cylinder 1

Signs for moisture streaks

  • The material tends to absorb moisture (e.g., PA, ABS, CA, PBT, PC, PMMA, SAN)
  • When slowly injecting into the air, the melt shows blisters and/or is steaming
  • The solidified flow front of a partial filling shows crater-like structures
  • The moisture content of the material before the processing is very high.
Streaks due to moist granules
Streaks due to moist granules
Streaks due to moisture on the mould surface
Streaks due to moisture on the mould surface

Signs for air streaks

  • The moisture content in the environment is very high (especially in combination with cold moulds and cold granules)
  • The defect becomes smaller with lower decompression
  • The defect becomes smaller with lower screw advance speed
  • Blisters are visible in the injected material
  • The solidified front flow of a partial filling shows crater-like structures.
Air streak behind a wall thickness variation
Air streak behind a wall thickness variation
Air streak (near the sprue) due to sucked in air during decompression
Air streak (near the sprue) due to sucked in air during decompression
Air streak due to entrained and stretched air near rib
Air streak due to entrained and stretched air near rib

Burnt Streaks (Brown or Silver)

Physical cause
Burnt streaks are caused by thermal damage to the melt. The result can be a decrease of the length of the molecule chain (silvery discoloration) or a change of the macromolecules (brownish discoloration).

Possible causes for thermal damage:

  • Temperature too high or residence time too long during predrying
  • Melt temperature too high
  • Shearing in the plasticising unit too high (e.g., screw speed too high)
  • Residence time in the plasticising unit too long
  • Shearing in the mould too high (e.g., injection rate too high).

Note: Inject ‘into the air’ in order to check the melt temperature. Measure temperature with a needle thermometer.
Thermal degradation of the plastic has a negative impact on its mechanical properties, even if no damage is visible on the surface.
Correcting burnt streaks (brown or silver)
Check and/or change machine settings, change mould or moulding compound, start new cycle reducing melt temperature.

Scroll to Top