Complete Troubleshooting Analysis of Short Shots in Injection Molding

Short shot, also known as insufficient filling, is one of the most common defects in injection molding production, which is manifested as incomplete filling of product edges, unfilled thin walls, missing ends of reinforcing ribs and local cavity vacancy. Short shot is not caused by a single factor, but a comprehensive defect induced by abnormal raw materials, unreasonable mold structure, improper process parameters and equipment failure. Blindly increasing injection pressure and speed on site cannot solve the problem fundamentally, but will easily cause secondary defects such as flash, burning and internal stress. Systematic hierarchical troubleshooting is required to locate the root cause and completely eliminate the defect.

1. Raw material failure troubleshooting

Raw material performance is the foundation of melt filling. Most intermittent and recurrent short shots are derived from raw material problems. High-viscosity materials such as PC, flame-retardant ABS and glass fiber modified plastics have large flow resistance, which is easy to cause insufficient filling of long-flow and thin-wall products. Mixing different grades of raw materials or excessive recycled materials will lead to broken molecular chains and reduced fluidity, resulting in unstable filling and intermittent short shots.

Excessive moisture in raw materials is a hidden high-frequency cause. Hygroscopic materials such as PC, PA and PET will vaporize water in the high-temperature barrel if not fully dried, forming air resistance in the cavity, squeezing the filling space, and causing incomplete filling at product corners and ends, often accompanied by silver lines and air patterns.

In addition, hopper bridging, material blockage and unstable feeding will lead to insufficient screw metering, fluctuating injection volume per mold, and irregular short shots. Dust and impurities accumulated at the nozzle will also block the melt flow and cause stubborn filling defects. Standard drying, material inspection and feeding inspection are effective means to solve raw material-induced short shots.

2. Mold structure and failure troubleshooting

Fixed-position short shots are mostly caused by mold problems. Insufficient exhaust or blocked exhaust grooves at deep ribs, dead corners and flow ends will cause air trapping in the cavity. The compressed air forms reverse pressure to block the melt flow, resulting in unfilled local positions, which is the most common cause of short shots.

Unreasonable gate and runner design is also a key factor. Too small gate size, insufficient gate quantity and too long flow path will cause serious pressure and temperature loss of melt, insufficient filling power at the end, and continuous short shots. Unbalanced runner of multi-cavity molds will lead to full filling of individual cavities and insufficient filling of part cavities.

Low mold temperature and uneven cooling will cause the melt to solidify rapidly after entering the cavity, resulting in premature sealing and incomplete filling. Wear of mold parting surface, inserts and top pin gaps will lead to pressure leakage during high-pressure injection, loss of effective filling pressure, and difficulty in filling the cavity even by adjusting process parameters. Regular cleaning of exhaust grooves, optimizing gate layout and repairing mold gaps can effectively solve mold-type short shots.

3. Injection molding process parameter troubleshooting

Unreasonable process parameters are the most direct inducement of short shots. Insufficient injection pressure and speed cannot overcome the melt flow resistance; too slow injection speed will cause the melt to cool down and solidify in advance during the flow process, resulting in end filling defects.

Too low barrel and nozzle temperature will lead to incomplete plasticization of raw materials, high melt viscosity and poor fluidity; excessive temperature will cause thermal degradation of materials, generate pyrolysis gas, form air resistance and aggravate short shots. Insufficient metering volume and buffer volume belong to physical filling shortage, which cannot be solved by parameter adjustment.

Too low back pressure and unreasonable screw speed lead to uneven plasticization and porous melt, resulting in unstable filling quality. Too early switching from injection to pressure holding will cut off the filling power in advance and cause short shots; too late switching will cause flash defects. Accurate process parameter matching is the core of solving process-type short shots.

4. Equipment mechanical failure troubleshooting

Many recurrent short shots cannot be solved by adjusting parameters, which are caused by equipment wear. Serious wear of screw, barrel and check ring leads to excessive matching gaps, melt backflow during injection, sharp attenuation of actual pressure and injection volume, and intermittent short shots.

Excessive clamping force will completely compress the mold parting surface, block the exhaust gap and cause severe air trapping and short shots; too small clamping force will lead to material overflow and pressure leakage. When the product weight is close to the maximum injection volume of the equipment, the machine output is insufficient, resulting in permanent filling defects. Equipment failure needs professional maintenance and calibration to solve fundamentally.

5. Comprehensive summary

Injection short shot is a comprehensive molding defect involving raw materials, molds, processes and equipment. On-site debugging errors are mostly blind pressure and speed increase, which leads to more secondary defects. Fixed-position short shots focus on mold exhaust and gate structure; intermittent short shots focus on raw material drying and equipment wear. The standardized solution logic is to stabilize raw material quality first, eliminate mold and equipment hidden dangers, and finally fine-tune process parameters. Layered troubleshooting can effectively solve short shot defects, stabilize product quality and improve mass production yield.