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 and go through Flow Chart 9.2 reducing melt temperature.
9.2.4.2 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 9.7. Since they want to compensate the pressure, the blisters burst, are deformed by the moving flow front and freeze at the mould wall.
