Injection Molding
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Tiger stripes (flow marks) on PP automotive parts
Fenómeno del 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.
Análisis de la causa
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.
Solución
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.
Conclusión clave
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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