Injection Molding
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Underfill/short shot appearing in random copolymer polypropylene injection molded products (variant: strengthened measures)
Problem Phenomenon
The product fails to completely fill the cavity, with material-deficient areas, incomplete edges or local voids.
Short shot directly leads to product scrapping and usually occurs first at positions with long flow distance, thin walls or poor venting.
Especially, additional attention is needed on top of the original basis; this problem is more prominent and requires focused attention.
Cause Analysis
(1) Injection pressure is insufficient or injection speed is too slow; the melt solidifies prematurely during filling
(2) Barrel temperature is too low (below the 190-260°C lower limit); melt viscosity is high and fluidity is poor
(3) Mold temperature is too low; the melt cools rapidly after contacting the mold wall, shortening the flow distance
(4) Gate size is too small or the runner is too long, resulting in large melt flow resistance
(5) Poor cavity venting; trapped air hinders melt filling
(6) On top of the original basis, additional attention is needed as process parameter fluctuations or equipment condition changes aggravate this problem
Solution
(1) Increase injection pressure to 80-150MPa and increase injection speed to shorten filling time
(2) Increase barrel temperature to the higher value within the 190-260°C range to reduce melt viscosity
(3) Increase mold temperature to the higher value within the 40-80°C range to delay melt cooling
(4) Increase gate and runner sizes, shorten runner length and reduce flow resistance
(5) Check and improve mold venting, adding vent grooves or vent pins at the filling end
(6) And strengthen process monitoring; on top of the original basis, additional attention is needed to pay special attention to parameter stability
Key Takeaway
Short shot is the result of insufficient melt fluidity or excessive filling resistance and needs to be solved by increasing temperature and pressure and improving the gating and venting system.
In different production scenarios, targeted preventive measures need to be taken.
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