How to Improve Poor Venting of Plastic Molds? Optimization Skills of Venting System

During injection molding, poor venting is a frequent inducement of defects. When molten plastic fills the cavity at high speed, if air inside the cavity cannot be discharged smoothly, burning marks, bubbles, silver streaks, insufficient weld line strength and short shot will occur, directly affecting the appearance and service performance of plastic parts. Unreasonable design of mold venting system and blocked vent grooves during production are main sources of this problem. Combined with mold design and on-site production practice, improvement schemes for poor venting and optimization skills of venting system are sorted from early design, mold machining, on-site mold modification and daily maintenance dimensions.

1. Identify Typical Positions of Poor Venting

Melt converging weld line zones, terminal positions of final product filling, deep ribs and thick wall corners, matching gaps of sliders and inserts are positions where gas is easy to accumulate. Deep ribs and closed areas in product structure trap gas inside cavities, which is hard to cover by conventional venting. At the mold trial stage, confirm defect positions preferentially, judge whether the problem comes from missing venting structure in design stage or vent grooves blocked by carbon deposits and glue scraps. Distinguish root causes before optimization to avoid blindly machining vent grooves and introducing new flash defects.

2. Optimization of Basic Vent Groove Parameters

Matching vent groove depth with molding plastic type is the core point of venting design. For general low-viscosity plastics such as PP and ABS, vent groove depth is controlled at 0.02mm to 0.03mm. For PC, PMMA and flame-retardant plastics, the depth ranges from 0.01mm to 0.02mm. Excessively large depth is very easy to cause product flash. The width of vent grooves should be increased as much as possible. The width of single vent groove is recommended from 8mm to 15mm, and multiple vent grooves are arranged at intervals. Vent grooves adopt stepped structure, shallow near the cavity and gradually deepened outward. The shallow section prevents material overflow, and the rear section expands the channel for fast gas export. The trend of vent grooves keeps straight with fewer corners to prevent plastic residue accumulation and carbon deposit generation.

3. Mold Structure Schemes for Additional Venting

For closed cavities and deep rib positions where planar vent grooves are difficult to set, inserts, sliders and ejector pin gaps can be used for auxiliary venting. Split mold cores at melt converging positions and use splicing gaps of inserts for venting. Matching gaps of ejector pins and flat ejectors can serve as auxiliary vent channels, but the gap size must be set strictly according to raw materials and cannot exceed overflow threshold. Add vent inserts at melt flow terminals of large plastic parts, convenient for carbon deposit cleaning later without integral mold core polishing. For multi-cavity molds, each cavity needs independent venting instead of shared vent channels, preventing mutual venting interference among cavities and single-cavity nonconformity.

4. Auxiliary Optimization of Gating and Molding Process

Reasonably adjust gate positions to avoid melt flow wrapping air and reduce gas trapped inside cavities. Match gate size with injection speed. High-speed filling will seal the cavity quickly before gas is discharged. During mold trial, reduce injection speed when melt flows to weld line zones and reserve time for gas exhaust. Appropriately raise barrel temperature and mold temperature to reduce melt viscosity and reduce gas wrapped inside melt. Adjust holding parameters. Excessively high holding pressure will squeeze melt into vent grooves and cause blockage. Shortening holding time can reduce glue accumulation in vent grooves and extend the stable production cycle of venting system.

5. Production On-site Maintenance and Mold Modification Control

During mass production, vent grooves continuously accumulate carbides and plastic scraps, gradually reducing and invalidating vent channels. Regular cleaning mechanisms should be established. At the end of each shift, use fine oil stones and copper blades to clean carbon deposits inside vent grooves. Hard steel blades are forbidden to scrape, to avoid scratching the working surface of vent grooves and changing groove depth to produce flash. Clean vent grooves and spray rust inhibitor before mold storage to prevent rust from shrinking vent channels. When adding vent grooves due to insufficient venting found in mold trial, only carry out local machining at gas accumulation areas instead of large-area vent groove processing, balancing venting effect and mold strength.

Control methods, conduct mold flow analysis before new product machining to predict trapped gas positions and reserve venting structure before mold processing, reducing later mold modification cost. After vent groove machining, inspect depth with feeler gauges to ensure uniform depth of all vent grooves. Record venting effect corresponding to different raw materials during mold trial, establish venting parameter ledgers, which can be directly referred for subsequent products of the same material and shorten mold trial cycles.

Summary 

Improvement of poor plastic mold venting needs to balance early mold design, machining precision, molding process and daily maintenance. Reasonably select vent groove depth, find trapped gas positions, cooperate with auxiliary venting of inserts and ejector pins, and standardize vent groove cleaning in production. These measures can effectively eliminate burning marks, bubbles and weld line defects. Perfect venting system can improve plastic part molding quality, reduce mold trial and modification times and stabilize yield of long-term mass production.