Plastic Mold and Process Solutions for Short Shot Adjustment of Multi-Cavity Molds

Multi-cavity molds often encounter a defect where parts from some cavities show short shot while others are fully filled. Simply raising injection pressure and speed cannot solve this issue fundamentally. Troubleshooting should be conducted from multiple dimensions including gating balance, venting, mold structure, raw material condition and equipment parameters, so as to achieve consistent filling in all cavities.

Gating System Balance Adjustment

For multi-cavity molds, the geometric balance of runners should be verified first. For molds with symmetric cavity layout, check whether the length and cross-sectional size of each branch runner are consistent. Use arc transitions at runner corners to reduce differences in melt flow resistance. For naturally unbalanced runner layout, artificial throttling can be adopted. Add throttling segments on branch runners feeding fast-filling cavities to adjust melt flow rate of each branch and synchronize filling sequence. The gate is the core of balance control. Appropriately reduce gate size for fast-filling cavities to delay melt entering cavities; moderately enlarge gates for short-shot cavities to lower shear resistance. Modify gates in small increments each time to avoid secondary defects such as stringing and flash. Clean carbon deposits and gel residue inside runners regularly, as accumulated carbon will reduce the effective cross-section of runners and change melt flow state, leading to persistent short shot in individual cavities.

Venting Structure Optimization

Short shot in single cavity is frequently caused by trapped gas. Gas accumulated at melt front forms back pressure to block melt filling. Add vent slots at melt end positions of short-shot cavities. The depth of vent slots shall match the flash threshold of raw materials to guarantee gas discharge without flash. Utilize tiny gaps between ejector pins and inserts for auxiliary venting, and regularly clear carbides and gel debris in vent slots. In mass production, carbon buildup will gradually block vent passages. Venting conditions of all cavities in multi-cavity molds should remain consistent. If partial cavities have sufficient venting while others do not, filling difference among cavities will occur directly. For thin-wall products, widen vent slots moderately to improve vent efficiency.

Raw Material Preprocessing Control

Insufficient drying of raw materials generates water vapor, which vaporizes during molding and increases filling resistance to trigger local short shot. For hygroscopic materials such as PA, PBT and PC, strictly control moisture content according to material specification for drying temperature and duration. Limit the proportion of regrind material, since excessive regrind reduces melt fluidity and amplifies flow resistance difference between cavities. Test melt index when switching raw material batches, and fine-tune molding parameters according to melt index variation. Purge carbonized material inside the barrel before production, to prevent carbon particles from blocking nozzles or runners and causing interrupted feeding and short shot in single cavity.

Fine Matching of Injection Molding Parameters

Set injection speed and pressure by stages. Use low speed in the first stage to deliver melt into main runner and branch runners, then switch speed in the second stage according to filling sequence, preventing some cavities from gate freeze prematurely. Confirm filling completion before packing, and never rely on packing pressure to compensate insufficient filling at front stage. Adjust barrel temperature properly to improve melt fluidity within the processing range of raw materials, but avoid excessive temperature which causes material decomposition and carbonization. Modify mold temperature. Low mold temperature of cavities with short shot increases cooling resistance and shortens melt flow length. Raise mold temperature within controllable range to extend melt flow distance. Control injection volume and reserve reasonable margin for screw stroke, as multi-cavity molds have higher requirement on stability of melt total amount.

Special Adjustment for Hot Runner System (Hot Runner Multi-Cavity Molds)

When single cavity short shot occurs in hot runner multi-cavity molds, check temperature control status of each nozzle first. Low temperature of nozzle corresponding to short-shot cavity raises melt viscosity and hinders filling. Verify working status of each heating coil and thermocouple. Temperature drift causes cold material blockage at local gates. For valve gate hot runners, check synchronization of valve pin movement. Delayed opening or insufficient stroke of individual valve pins will directly block melt entering cavities. Clean carbon deposits at nozzle heads. Carbon buildup reduces gate diameter and continuously induces single cavity short shot. Avoid scratching sealing surfaces of nozzles during maintenance, which may cause melt leakage.

Balanced Adjustment of Mold Temperature

Scaling blockage and inconsistent length of cooling channels for cavities in multi-cavity molds result in uneven mold temperature. Melt cools rapidly in low-temperature cavities with shorter flow length and short shot occurs. Flush cooling pipelines regularly to remove scale and ensure smooth circulation of cooling water in each cavity. Optimize layout of cooling channels to make cooling conditions of all molding cavities close and reduce temperature difference between cavities. Inspect mold insulation layer. Failure of heat preservation for hot runner molds leads to local heat loss and affects melt flow.

Segmented Verification in Mold Trial and Mass Production Inspection

Adopt single-cavity blocking method for machine tuning. Temporarily seal fully-filled cavities to test short-shot cavity independently, and judge flow and venting condition of the cavity to locate fault source. Follow single-variable principle for parameter modification, adjust only one parameter each time to judge improvement effect conveniently. Monitor product status of each cavity during mass production and record finished parts under different parameters. Establish regular maintenance plan for long-running molds, clean carbon deposits in runners and vent slots to prevent repeated defects.