In the injection molding, the short shot injection, also called underfill , refers to the injection plastic flow end of the phenomenon of partial incompleteness or a part of a mold cavity is not filled, especially the thin-walled area or the end of the flow path area. The performance of the melt in the cavity is not filled with condensation, the melt into the cavity is not filled completely, resulting in the product lack of material.
What is the reason to cause the short shot injection?
The main reason for short injection is excessive flow resistance, causing the melt to be unable to continue to flow. Factors affecting melt flow length include: wall thickness of the part, mold temperature, injection pressure, melt temperature and material composition. These factors can cause short injection if not handled properly.
Hysteresis effect: also called stagnant flow, if there is a relatively thin structure, usually reinforcement bars, etc., at a location closer to the gate or in a location perpendicular to the flow direction, then during the injection process, the melt will encounter a relatively large forward resistance when passing through the location, and in the flow direction of its main body, due to the smooth flow, no flow pressure can be formed, and only when the melt is filled in the main body direction, or enters the The holding pressure will only form enough pressure to fill the stagnant part, and at this time, because the location is very thin and the melt does not flow without heat replenishment, it has been cured, thus causing short shot injection.
How to solve it?
1.Material:
—Increase the fluidity of the melt.
—Reduce the addition of recycled materials.
—Reduction of gas decomposition in raw materials.
2.Tool:
—The location of the gate is designed to ensure that it fills the thick wall first to avoid stagnation, which can lead to premature hardening of the polymer melt.
—Increase the number of gates to reduce the flow ratio.
—Increase the runner size to reduce the flow resistance.
—Proper location of the venting port to avoid poor venting (see if the under-injection area is burnt).
—Increase the number and size of exhaust port.
—Increase the design of cold material well to discharge cold material.
—The distribution of the cooling water channel should be reasonable to avoid causing the local temperature of the mold to be low.
3.Injection machine:
—Check if the check valve and the inner wall of the barrel are badly worn, which will lead to serious loss of injection pressure and injection volume.
—Check if there is material at the filling port or if it is bridged.
—Check whether the capacity of the injection molding machine can reach the required molding capacity.
4.Injection process:
—Increase injection pressure.
—Increase injection speed to enhance shear heat.
—Increase the injection volume.
—Increase the barrel temperature and mold temperature.
—Increase the melt length of the injection molding machine.
—Decrease the buffer volume of the injection molding machine.
—Extend the injection time.
—Reasonably adjust the position, speed and pressure of each injection section.
5.Product structure:
—Remove the thin area
—Remove the ribs that caused the bad flowability.
—Have uniform wall thickness.
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Post time: Jan-03-2023