Burning and Short Shot Troubleshooting Caused by Poor Mold Venting

When air and decomposition gas inside the mold cavity cannot escape smoothly during injection molding, burning marks and short shots will occur. Many on-site operators simply increase injection pressure and speed to fix filling issues, which often only bring temporary relief and may worsen burning and carbon buildup. Burning and short shot frequently appear together. Trapped gas is compressed and heated adiabatically, burning the plastic material, while the gas barrier blocks melt flow and causes incomplete filling. Effective troubleshooting needs to distinguish defect characteristics, locate blocked vent areas and implement targeted mold and process modifications.

Characteristics and Venting-Related Causes of Burning Defects

Burning caused by poor venting usually appears at melt ends, weld lines and deep thin ribs, showing fixed black or yellow carbonized traces. Unlike burning from overheated barrels, this defect changes little when adjusting melt temperature. Fast filling compresses enclosed air, raising temperature above the thermal decomposition threshold of plastic. Flame-retardant PC, PBT and other high-temperature engineering plastics are most sensitive to this issue. Other root causes include blocked vents by carbon deposits, overly shallow vent depth, insufficient auxiliary gaps between inserts and slides, and poor gate layout that traps gas in closed pockets.

Characteristics and Venting-Related Causes of Short Shot Defects

Venting-induced short shot differs from short shot caused by insufficient material or low injection pressure. Even after raising pressure and extending packing time, the cavity still cannot be fully filled, and burning marks may exist nearby. The trapped gas creates a barrier that melt cannot penetrate. Thin-wall features, micro holes, narrow long runners and multi-rib structures are high-risk zones. Moldflow simulation can identify gas trapping regions. Many molds only add vents on the parting line, leaving deep cavities and independent inserts without proper gas escape channels.

On-site Rapid Screening Method

First check whether defects are located at melt convergence ends or dead pockets. If higher injection pressure and speed aggravate burning without improving filling, poor venting is highly suspected. Eliminate other factors such as overheated barrel, insufficient drying and excessive shear before inspecting vents for carbon buildup and plastic debris. Reduce filling speed for testing: burning and gas-locked short shot will be relieved if venting is the root cause. If defects remain unchanged after slowing down, focus checks on material, gate and cooling systems.

Vent Parameter Verification and Modification Principles

Vent depth must match the plastic type. For ABS and PC/ABS, the standard depth is 0.02–0.03 mm. Soft PP and PE can use slightly deeper vents, while glass-filled PA66 and PBT are limited to 0.015–0.02 mm. Too deep vents will generate flash, while too shallow vents cannot exhaust gas efficiently. Vent width should be maximized to increase cross-sectional area, followed by expansion grooves to release gas out of the mold. Vents are placed at melt ends, weld lines and rib bottoms. For narrow thin features, vent pins and inserts are used to create extra gas escape paths.

Auxiliary Venting Optimization for Mold Structure

Vents only on parting lines cannot solve deep cavity gas trapping. Clearance of slides, lifters and inserts can be used as auxiliary vents with strict flash control. Flash grooves are an option for non-cosmetic structural parts to guide trapped gas and small melt overflow. For appearance parts, multi-point gating and adjusted gate directions are adopted to avoid early melt closure and gas encapsulation. If weld lines keep trapping gas, add vent inserts or relocate gates to move weld lines to ventable areas.

Vent Maintenance During Mass Production

Vents are wearing parts. Carbon deposits formed by plastic decomposition and flame retardant precipitation gradually block channels, and defects grow worse as shot count increases. Establish regular cleaning schedules, using soft copper brushes and dedicated cleaning agents. Hard tools are forbidden to prevent vent depth damage. Polishing and parting line fitting must not seal vents. Flame-retardant and high-temperature plastics build carbon faster, requiring shorter cleaning cycles.

Supporting Process Optimization

After mold vent modification, adjust molding parameters accordingly. Use multi-stage injection, slow down speed when melt reaches gas trapping zones to reduce adiabatic heating. Stabilize mold temperature to prevent premature melt freezing that seals vents. Strict drying standards reduce water vapor and extra gas load, lowering the risk of burning and gas-locked short shot. Troubleshooting for venting-related burning and short shot follows a clear logic: locate trapped gas zones, check vent accessibility and depth, prioritize mold structural changes and use process adjustment as auxiliary control. Regular maintenance keeps venting systems effective for stable molding.