Injection Molding
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Underfill/short shot of homopolymer polypropylene injection products
Problem Phenomenon
The product fails to completely fill the cavity, with missing material areas, incomplete edges, or local voids.
Short shots directly cause product scrap, usually occurring first at locations with long flow distance, thin walls, or poor venting.
Cause Analysis
(1) Insufficient injection pressure or injection speed too slow; melt solidifies prematurely during mold filling
(2) Barrel temperature too low (below the 200-280°C lower limit); high melt viscosity, poor flowability
(3) Mold temperature too low; melt rapidly cools after contacting the mold wall, flow distance shortened
(4) Gate size too small or runner too long, large melt flow resistance
(5) Poor cavity venting; trapped air hinders melt filling
Solution
(1) Increase injection pressure to 80-150MPa, increase injection speed, shorten mold filling time
(2) Increase barrel temperature to the higher value within the 200-280°C range, reduce melt viscosity
(3) Increase mold temperature to the higher value within the 40-90°C range, delay melt cooling
(4) Enlarge gate and runner sizes, shorten runner length, reduce flow resistance
(5) Check and improve mold venting, add vent grooves or vent pins at the filling end
Key Takeaway
Short shot is the result of insufficient melt flowability or excessive mold filling resistance; it needs to be solved by increasing temperature, pressure, and improving the gating and venting system.
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