Injection Molding
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Tiger stripes (flow marks) on PP automotive parts
Fenómeno do problema
Light-dark alternating tiger-stripe patterns appear on the surface of large-area PP parts such as bumpers, door panels, and instrument panels, distributed along the flow direction, affecting appearance on high-gloss or grained surfaces.
Análise das causas
Under the filling shear action, the distribution of the rubber (EPR) phase and the matrix in impact copolymer PP fluctuates, forming periodic gloss/brightness differences; injection speed fluctuations and unreasonable gate design causing front instability will 'imprint' this phase fluctuation on the surface.
Solução
①Optimize the injection speed curve to keep the front advancing smoothly (avoid fast-slow alternation); ②appropriately increase the mold temperature and melt temperature to improve phase uniformity; ③enlarge the gate and optimize the gate position to reduce shear fluctuation; ④for occasions with high appearance requirements, select an anti-tiger-stripe special grade or customize with the material supplier; ⑤check the temperature uniformity of the hot runner.
Conclusão principal
Tiger stripes are the 'flow imprint' of the phase distribution of copolymer PP; the core is to stabilize the front and the gate shear, and process-wise the main measures are stable speed and higher temperature; stubborn parts often require a material grade upgrade and cannot be expected to be cured purely by machine adjustment.
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