Adjustment methods for addressing short shots in a single cavity of a multi-cavity mold
In multi-cavity molding, if only specific cavities exhibit underfilling while others are fully formed, the issue is typically caused by uneven melt distribution, blocked venting in individual cavities, or excessive localized flow resistance. Remedial measures can be implemented by systematically adjusting the mold cooling channels, venting systems, runner geometry, and molding process parameters.
Check the branch runner leading from the distribution runner to the cavity experiencing short shots.
If branch runners have undersized cross-sections, excessive turns, or flow-impeding steps, the melt distribution becomes unbalanced, potentially leading to under-filling in specific cavities. To address this, the corresponding branch runners can be moderately widened and deepened, the number of turns reduced, and the runner walls polished to lower flow resistance, thereby balancing the melt intake across all cavities.
Checklist for pin-point gate specifications
Short-shot issues arise when the cavity gate is too thin or narrow, restricting the melt flow rate. Moderately increase the gate dimensions-without generating excessive gate scrap-to enhance the material feed rate.
Clean the single-cavity venting slot
A blocked single-cavity vent or insufficient vent depth prevents air from escaping the cavity during filling; the resulting back pressure impedes the melt flow, leading to short shots. Disassemble the mold to remove carbon deposits and plastic residue, and deepen or extend the corresponding cavity vents to ensure smooth air evacuation.
Check the temperature difference in the cavity cooling system
Short shots are caused by clogged cooling channels in the mold cavity or excessive heat dissipation, leading to premature cooling and solidification of the melt and reduced flowability. Clear the affected cooling channels, adjust the coolant flow rate, and minimize temperature differences across the cavities.
Check the cavity for undercuts or thin-rib structures that create flow resistance
If areas with short shots feature large thin-walled sections, deep ribs, or undercuts-resulting in high melt flow resistance-consider locally optimizing the wall thickness or adding auxiliary runners.
Adjust multi-stage injection speed and pressure
Employ a multi-stage injection profile: moderately increase the injection speed during the intermediate stage to enhance melt flow, and slightly raise the injection pressure to overcome mold-filling resistance caused by venting constraints in individual cavities; avoid a sudden, significant pressure spike to prevent flash formation in the other cavities.
Adjust the injection-to-holding switchover position.
Advance the switchover point to enter the holding phase earlier, ensuring continued material feed into underfilled cavities to alleviate sink marks caused by material shortage.
Moderately increase barrel and mold temperatures
Slightly raise the temperature of the middle section of the barrel and the mold temperature corresponding to the underfilled cavity; this reduces melt viscosity, enhances flowability, and improves material filling in the distant cavities.
Appropriately increase the melt cushion
Insufficient filling in a single cavity can result from an inadequate melt cushion; slightly increase the melt injection stroke to ensure a sufficient supply of material to all cavities in the mold assembly.
Optimize melt back pressure
Moderately increase back pressure to improve melt uniformity and reduce uneven cavity filling caused by variations in melt density.
Check the injection stroke transducer and pressure sensor
Sensor signal drift can result in insufficient actual injection volume; calibrate the sensing components to ensure stable injection output.
Control the dryness of raw materials
Raw materials containing moisture or dust are prone to generating gas bubbles that hinder mold filling; therefore, dry the materials according to material specifications to minimize the inclusion of impurities.
Lowering the feed throat temperature
Overheating at the feed throat causes raw material agglomeration and unstable feeding, leading to fluctuations in melt supply, which indirectly increases the frequency of short shots in individual cavities.
During mold changes and the initial stages of mass production, monitor the filling status of each cavity; if short shots occur in a specific cavity, prioritize clearing the venting before making fine adjustments to the process parameters. For multi-cavity molds in long-term production, regularly polish the runners and clean the vent channels to mitigate persistent filling defects caused by carbon buildup.
Provided that runner feed is balanced, venting is unobstructed, and temperature parameters are matched to the material's characteristics, the issue of short shots in individual cavities can be alleviated, and the filling consistency across cavities can be improved.
