Troubleshooting Guide for Injection Molding II

Troubleshooting Guide for Injection Molding II

Moisture Streaks

Physical cause
During storage or processing, moisture is absorbed by the granules, forming water vapour in the melt. Due to the velocity profile at the flow front, gas blisters are pushed to the surface of the melt as shown in Figure. Since they want to compensate the pressure, the blisters burst, are deformed by the moving flow front and freeze at the mould wall.

Flow of water vapour blisters near the flow front
Flow of water vapour blisters near the flow front

Possible causes for moisture streaks:
(1) Moisture on the mould surface
     • leaky mould temperature control system
     • condensation water on the mould walls
(2) Moisture in/on the granules
    • insufficient pre-drying of the material
    • wrong storage of the material
Correcting moisture streaks
Check and/or change machine settings, change mould or moulding compound, start new cycle.

Colour Streaks

Physical cause
During pigmentation, pigment agglomerations can lead to differences in the concentration. To some extent this can be mitigated by an increase in shearing as shown in Figure and increases in back pressure can be applied during the plastication stage to increase mixing. This kind of poor distribution can be caused by the plastic, the processing parameters, adhesives and other additives. With in-plant colouring using dyes, the defect can occur due to uncompleted solution of the dye particles in the melt.

Smaller differences in pigment concentration due to higher shearing
Smaller differences in pigment concentration due to higher shearing

Similar to thermoplastics, pigments and dyes are sensitive to excessive processing temperature and residence times. If thermal damage is the reason for colour streaks, they should be considered as burnt
streaks.
Extensive stress or warpage can also cause colour differences. The deformed areas break the light in a different manner than other areas.
Note: If using master batches for colouring, make sure the substrate is compatible with the plastic to be coloured. The effect of the use of an incompatible masterbatch is shown in Figure

Colour streaks due to an incompatible masterbatch
Colour streaks due to an incompatible masterbatch
Orientation of metal-effect pigments caused by the flow
Orientation of metal-effect pigments caused by the flow

Correcting colour streaks
Check and/or change machine settings, change mould or moulding compound, start new cycle

Air Streaks/Air Hooks

Physical cause
Air which cannot escape in time during the mould filling, is drawn to the surface and stretched in the direction of the flow. Especially near writings, ribs, domes and depressions, the air can be rolled over and thus entrapped by the melt as shown in Figure 9.10. The result is the formation of air streaks or air hooks.

Formation of an air streak behind an engraving
Formation of an air streak behind an engraving

If air is sucked into the area in front of the screw during decompression, air streaks will appear near the gate. Here, air is transported into the cavity during the injection, and is then pushed towards the mould
wall where it freezes.
Correcting air streaks/air hooks
Check and/or change machine settings, change mould or moulding compound, start new cycle.

Glass Fibre Streaks

Physical cause
Due to their length, glass fibres orientate themselves in the direction of flow during injection. If the melt suddenly freezes when touching the mould wall, the glass fibres may not yet be sufficiently surrounded
with melt.
In addition to that, the surface can turn rough because of the big differences in shrinkage (glass fibre: plastic = 1: 200). The glass fibres impede shrinkage of the cooling plastic, especially in the longitudinal
direction of the fibre, thus producing an uneven surface as shown in Figure.

Glass fibre streaks clearly visible weld line
Glass fibre streaks clearly visible weld line
Moulded part with rough silvery surface
Moulded part with rough silvery surface
Glass fibre streak due to orientation near sprue
Glass fibre streak due to orientation near sprue

Gloss/Gloss Differences

Physical cause
The gloss of a moulded part is the appearance of its surface when exposed to light.
If a ray of light hits the surface, its direction will change (refraction of light). While one part of the light will be reflected on the surface, another part will reflect inside the part or penetrate it with different intensities. The impression of gloss is at an optimum, the lower the surface roughness. To achieve this, a polished mould wall should be as good as possible, a textured mould wall would not. This is illustrated in Figure. Gloss differences are caused by different projection behaviours of the plastic at the mould wall, due to different cooling conditions and shrinkage differences.

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