Analysis of injection moulding problems

Burner effect /diesel effect

Visible by burn marks at the end of the flow path

Cause of defect:

If the air cannot escape out of the cavity during injection, it is compressed with high pressure by the melt. At the same time, the air is heated to such an extent (> 1000°C) that the air, the plastic and the steel can burn.

Burner/diesel effect error sources:

Material:

• Material releases particles, gases
• Additives, colours outgas

Method:

• Injection speed too high
• No injection profile, fast – slow
• Changeover point too late
• Locking pressure too high
• Dirty ventilation,
• Cleaning interval too long

Mould:

• Ventilation insufficient

Shrink marks

Visible by slots (dents) in the surface

Cause of defect:

Shrink marks are caused by uneven mould shrinkage. They mostly appear at thick sections or at the end of the flow path.

Sources of defect:

Machine:

• Nozzle bore is too small
• Non-return valve worn
• Pressure control incorrect

Material:

• Plastic has too high shrinkage

Method:

• Holding pressure too low
• Holding pressure time too short (seal time)
• Mould temperature too high
• Injection speed too high
• Injection volume too low (change over point=too early)
• Residual melt cushion too small

Mould:

• Dimensions of gate and or gate insert wrong
• Too great differences in wall thickness
• Accumulation of melt (ribs too thick)
• Incorrect cooling configuration

Colour streaks

There are two kinds of colour streaks:

1. Irregular colour streaks

The colour is insufficiently mixed: Visible by irregular coloured streaksEach part has a different hue.

Sources of defect:

Machine:

• Screw without shearing and mixing part.
• No mixing nozzle in use
• Cylinder too small
• Colour dosing appliance defective, not available
• Hopper is too large
• Colour charges itself electrostatically
  

Material:

• Master batch or pigments not compatible with plastic
• Solubility of the pigment insufficient
• Pigments, master batch granulate too large
• Plastic granulate too large

Method:

• Back pressure too low
• Melt temperature too low
• Colour dosing not sufficient

2. Colour streaks by separation

Here the coloured stripes are always in the same place.Colour pigments and plastic are not compatible or too high shear creates separation. Each part has colour streaks in the same place, majority in the area of flow lines.

Sources of defect:

Machine:

• Nozzle bore is too small
• Material
• Master batch or pigments not compatible with plastic
• Pigment thermally too sensitive
• Ratio of pigment too high

Method:

• Injection speed too high (too low)
• Melt temperature too low (too high)
• Mould temperature too low (too high)
• Back pressure, screw speed too high
• Colour dosing too high

Mould:

• Flow cross-section too small
• Flow path too long
• Position of flow lines unfavourable
• Gates too small

Colour difference

Irregular colour tone.

Cause of defect:

Visible by irregular colour tone. From one production to another, very often colour differences can occur.

Sources of defect:

Machine:

• Screw geometry
• Nozzle bore size incorrect, mixing nozzle missing/faulty
• Cylinder too small, too big
• Colour dosing appliance defective, non-existent
• L – D ratio different
• Injection control

Material:

• Batch to batch inconsistency of the material
• Colour pigments
• Drying for too long

Method:

• Back pressure too low.
• Melt temperature too low (or too high), inefficient temperature control
• Colour dosing not sufficient

Mould:

• Hot runner temperature profile is uneven
  

Jetting

Visible strand formation of the injected melt on the moulded part surface.

Cause of defect:

If the melt does not come in contact with the mould wall, or it comes in contact too late, it does not fill the mould in a laminar flow. A melt strand is formed (worm track/snake) which is called “jetting”.

Sources of defect:

Machine:

• Nozzle bore is too small

Material

• Material too high viscosity

Method:

• Injection rate too high or
• no injection profile slow - fast
• Mould temperature too low
• Melt temperature too low

Mould:

• Unfavourable gate position
• Unfavourable gate dimensioning (too small)
• Changeover from gate - moulded part not rounded

Cause of defect:

Sources of defect:

Defective surface gloss

Cause of defect:

Sources of defect:

• Polishing of the mould surface insufficient
• Structuring of the mould surface insufficient
• Coating, material deposits, rust
• Differences in wall thickness
• Incorrect construction of cooling channels

Air bubbles / air streaks

Bubbles inside the moulded part or streaks on the surface Cause of defect:
If there is air in the melt and it is injected into the cavity, air bubbles are formed inside the moulded part, which are visible in transparent parts. If these bubbles reach the mould wall during injection, they will burst open and become visible as streaks on the surface of the moulded part. Air bubbles and air streaks can occur at different places on the part.

Sources of defect:

Machine:

• Nozzle does not seal correctly
• No shut-off nozzle in use

Material:

• Material too high viscosity
• Inconsistent size of granulate

Method:

• Injection speed too high or too low or no injection profile slow – fast
• Mould temperature too low
• Melt temperature too low

Air hooks / tear drops

Visible by U-shaped flow lines after wall thickness changes or lettering

Cause of defect:

In the area of lettering, ribs, domes or sharp wall thickness changes, air which cannot escape quickly enough is swept along, overrun by the melt and embedded. Unexpected flow lines, so-called tear drops, occur.

Sources of defect:

Material:

• Material too high viscosity

Method:

• Injection speed in the critical area too high (too low)
• Mould temperature varies greatly
• Melt temperature too low

Mould:

• Mould venting not optimal
• No radii in critical areas
• Wall thickness changes too great
• Engraving depth too deep
• Cooling not sufficient