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Plastic mould defects-burnt streaks (brown or silver)

Nov. 03, 2016

Plastic Mould Defects-Burnt Streaks (brown or silver)

Signs for burnt streaks:
. the streak appears periodically
. the streak appears behind narrow cross-sections (shear points) or shape edges in the mold
. the melt temperature is near the upper processing limit
. lowering the injection speed has a positive impact on the defect
. lowering the melt temperature has a positive impact on the defect
. long residence time in the plasticizing unit or the space in front of the screw (due to e.g. cycle breaks or low shot volumes)
. high reclaim content, or a part of the material has already been melted several times before
. the mold is equipped with a hot-runner
. the mold is equipped with a shut-off nozzle
Signs for moisture streaks:
. the material tends to absorb moisture (e.g. PA, ABS, CA, PBTB, PC, PMMA, SAN)
. when purging the melt shows blisters and/or is steaming
. the solidified flow front of a partial filling shows crater-like structures
. the moisture content of the material before the processing is very high
. the moisture content in the environment is very high (especially in combination with cold molds and cold granules)
Signs for air streaks:
. the defect becomes smaller with lower decompression
. the defect becomes smaller with lower injection speed
. blisters in the injected material are visible
. the solidified front flow of a partial filling shows crater-like structures

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 macro molecules (brownish discoloration).
Possible causes for thermal damage:
. temperature too high or residence time too long during pre-drying
. melt temperature too high
. shearing in the plasticizing unit too high (e.g. screw speed too high)
. residence time in the plasticizing unit too long
. shearing in the mold too high (e.g. injection rate too high)
Purge material in order to check the melt temperature. Measure temperature with needle thermometer.
Thermal degradation of the plastic has a negative impact on its mechanical properties, even if no damage is visible on the surface.
Melt temperature above the processing range? If yes, then
Reduce melt temperature:
(1) vary cylinder temperature (-)         
(2) vary screw speed (-)
(3) reduce back pressure
Is the melt residence time within the critical range? If yes, then
(1) reduce cycle time
(2) increase plasticizing time delay 
(3) use smaller plasticizing unit
(4) reduce reclaim part
Burnt streaks appearing periodically or visible after purging? If yes, then
(1) avoid dead spots and flow impeding areas in the gate system and in the plasticizing unit
(2) check plasticizing unit for wear
(3) check granules condition
Burnt streaks near the gate? If yes, then
(1) lower injection rate (injection profile slow-fast)
(2) check hot-runner system
(3) avoid sharp edges in the gate system
If no, then
(1) lower injection rate
(2) avoid sharp edges
(3) avoid small runners *1 (use rheological mold design)
(4) check sprue and gate system (use rheological mold design)
(5) check nozzle cross-section
(6) check functioning of shut-off nozzle
(7) check pre-drying of material (thermal damage possible due to excessively long or hot drying process)
(8) reduce reclaim part
(9) use molding compound or coloring agents with higher thermal stability

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