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Mould problems solution

Plastic mould defects-moisture streaks

Nov. 09, 2016

Plastic Mould Defects-Moisture Streaks

During storage or processing, moisture is absorbed by the granules, forming water vapor in the melt. Due to the velocity profile at the flow front, gas blisters are pushed to the surface of the melt. Since they want to compensate the pressure, the blisters burst, are deformed by the moving flow front and freeze at the mold wall.
Possible causes for moisture streaks:
leaky mold temperature control system (moisture on the mold surface)
condensation water on the mold walls (moisture on the mold surface)
insufficient pre-drying of the material (moisture in / on the granules)
wrong storage of the material (moisture in / on the granules)
Flow of the water vapor blisters near the flow front

Moisture on the mold surface? If yes, then
(1) check mold cooling system for leaks
(2) increase mold wall temperature
(3) use dry-air-curtain system*1)
application appropriate if:
shorter cooling and cycle times can be achieved by lower temperature control medium
optimum temperature of the temperature control medium cannot be maintained due to condensed moisture
If no, then reduce moisture content:
(1) sufficiently pre-dry material*2) (pay attention to drying instructions by the raw material producer)
(2) check material packaging
(3) check storage of material
(4) reduce residence time in material hopper (if possible, feed directly from the dryer in a closed system)

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