Analysis of injection moulding problems

Visible by burn marks at the end of the flow path

Cause of deffect:

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

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

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

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

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:

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

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

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

Often visible by matt surface in the gate/gate area

Cause of defect:

If the melt is injected very quickly and diverted by a sharp edge, large orientations and expansions of the molecular chains in the area of the gate area arise. The already solidified edge layer breaks open. This creates a matt area on the surface.

Sources of defect:

Machine:

• Speed regulation inaccurate

Material:

• Material too high viscosity

Method:

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

Mould:

• Changeover from gate to cavity not rounded
• Gate diameter too small
• Gate position unfavourable

Visible by smooth grooves on the surface

Cause of defect:

If the melt is too thick (high viscosity) the plastic core expands in flow direction during injection. At the same time, grooves are generated in the surface layer. This effect can especially be seen at the end of the flow path.

Sources of defect:

Machine:

• Specific injection pressure too low
• Heating power of the temperature control unit too low
• Wrong selection of tempering coolant
• Draught on the machine

Material:

• Material too high viscosity

Method:

• Injection speed too low
• Injection pressure limitation too low
• Changeover point too early
• Mould temperature too low
• Melt temperature too low

Mould:

• Wall thickness too thin
• Flow paths too long, insufficient feed points
• No insulation plates

Visible by sharp edges or plastic film at mould splint lines

Cause of defect:

During the injection process, very high pressures are generated which can deform the mould and cause gaps to form at the split line face or in the areas of moving parts. If the melt gets into such a gap, flash or burrs can be produced.

Sources of defect:

Machine:

• Locking pressure too low
• Locking pressure uneven (bar wear)
• Locating ring hole too big

Material:

• Material too low-viscosity

Method:

• Injection speed too high
• No injection profile, fast – slow
• Volume of melt too large (change over point too late=pressure peak)
• Holding pressure too high
• Melt temperature is too high
• Locking pressure too low

Mould:

• Mould construction not rigid enough
• Mould split line surfaces damaged
• Split line face dirty

Clearly visible flow lines (notch)

Cause of defect:

If two melt fronts meet, they must fuse together. If they do not fuse together properly they leave visible notches. At flow lines colour separation often occurs (see coloured streaks).

Sources of defect:

Material:

• Material too high viscosity
• Colours – glitter particles too big

Method:

• Injection speed too low
• Holding pressure too low
• Melt temperature too low
• Mould temperature too low
• Changeover point too early

Mould:

• Bad venting
• Gate at wrong position
• No cascade gate system

U-shaped, drawn-out streaks / bubbles

Cause of defect:

If there is moisture in the granulate, water vapour is created during melting. These steam bubbles are visible in transparent parts. They are small, multiplied and present at different places of the part. Bubbles, which make it to the surface, can be recognised as u-shaped ‘smudged’ streaks, which are open against the flow direction. Plastic parts with moisture streaks are usually brittle.

Sources of defect:

Machine:

• Hopper is not covered
• No shut-off nozzle
• Dryer defect (in need of maintenance)
• Dryer too small
• Feed route for the material too long

Material:

• Residual moisture in the granulate too high
• Packaging defect of the granulate
• Additives are moisture-sensitive (soot)

Method:

• Decompression path too big
• Decompression speed too fast
• Pre-drying temperature too low /time too short
• Feed line too cold
• Mould temperature too cold

Mould:

• Mould leaking (cooling hole fissure)
• Flow cross-section too small

After some time, visible cracks appear in the part.

Tiger stripes on the visible surface

Cause of defect:

Light and dark stripes appear on flat parts, this effect can be seen especially at the end of the flow path. Tiger stripes particularly appear on PE and PP.

Sources of defect:

Machine:

• Nozzle bore is too small

Material:

• Wax ratio (paraffin) too high
• Wax (sliding and demoulding assistance) too thermally sensitive
• Integration of aggregates insufficient

Method:

• Injection speed too high (too low)
• Melt temperature too low (too high)
• Mould temperature too low (too high)

Mould:

• Flow cross-sections too small
• Flow path too long
• Position of flow lines unfavourable
• Gates too small
• Incorrect design of hot runner

Visible by bubbles, holes in the moulded part at the same place

Cause of defect:

Voids are generated by shrinkage inside the moulded part. They are at the end of the flow path or in thick areas. They are always at the same place in the moulded part. In transparent parts they are visible as bubbles.

Sources of defect:

Machine:

• Nozzle bore is too small
• Non-return valve worn-out.
• Pressure control imprecise

Material:

• Plastic has too high shrinkage

Method:

• Holding pressure too low
• Holding pressure time too short (solidification 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 gate insert wrong
• Too great differences in wall thickness
• Accumulation of melt (ribs too thick)
• Incorrect cooling configuration

Cause of defect:

Sources of defect: