Injection Molding
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Underfilling/Short Shot of High-Density Polyethylene Injection-Molded Products
Problem Phenomenon
The product fails to completely fill the cavity, with material-short areas appearing, incomplete edges, or local voids.
Short shot directly leads to product scrapping and usually occurs first at parts with long flow distances, thin walls, or poor venting.
Cause Analysis
(1) The injection pressure is insufficient or the injection speed is too slow, and the melt solidifies prematurely during mold filling
(2) The barrel temperature is too low (below the lower limit of 180-260°C), the melt viscosity is high, and the fluidity is poor
(3) The mold temperature is too low, the melt cools rapidly after contacting the mold wall, and the flow distance is shortened
(4) The gate size is too small or the runner is too long, and the melt flow resistance is large
(5) The cavity venting is poor, and trapped gas hinders melt filling
Solution
(1) Increase the injection pressure to 80-150MPa, increase the injection speed, and shorten the filling time
(2) Increase the barrel temperature to the higher value within the range of 180-260°C to reduce the melt viscosity
(3) Increase the mold temperature to the higher value within the range of 30-80°C to delay melt cooling
(4) Increase the gate and runner sizes, shorten the runner length, and reduce flow resistance
(5) Check and improve mold venting, and add vent grooves or vent pins at the filling end
Key Takeaway
Short shot is the result of insufficient melt fluidity or excessive filling resistance, and needs to be solved by increasing the temperature and pressure and improving the gating and venting system.
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