Burner effect /diesel effect
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:
Shrink marks
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
Colour streaks
There are two kinds of colour streaks:
1. Irregular colour streaks
The colour is insufficiently mixed: Visible by irregular coloured streaks Each 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
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
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:
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
Incomplete filled parts
Incomplete filled parts
Occasionally non-injected parts at the end of flow length or in thin sections.
Cause of defect:
Non-completely-injected parts at the end of flow length, at thin-walled section or at the venting of ribs This error often happens irregularly.
Sources of defect:
Machine:
- Nozzle does not seal against the mould.
- Non-return valve worn out
- Dosing inaccurate
- Specific injection pressure too low
- Draught on the machine
- Ambient air temperature fluctuates
- Heating power of the temperature control unit too low
- Regrind is not added homogeneously.
Material:
- Material too high viscosity
- Viscosity fluctuates
- Percentage of recycled material fluctuates
Method:
- Decompression too little, too slow.
- Injection pressure, holding pressure too low
- Residual melt cushion too small
- Injection volume too small
- Back pressure too low
- Screw speed too high
Mould:
- Wall thickness too thin, no flow aids.
- Gating system not balanced, too small
- Temperature control at hot runner inaccurate
- Venting within the tool not sufficient, blocked
- Hot runner nozzles (wearing of the tips/unequal nozzle lengths)
Defective surface gloss
Defective surface gloss
Too little surface gloss or gloss spots
Cause of defect:
The more precisely the surface is reproduced by the plastic, the glossier the part in highly polished moulds and the more equally structured is the part when using etched moulds (no gloss/speck spots).
Sources of defect:
Machine:
- Specific injection pressure too low
- Temperature control unit issue, possible problem with tempering medium
Material:
- Material too high viscosity
Method:
- Injection speed too low
- Mould temperature too low
- Melt temperature too low
Mould:
- 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 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
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