Solutions for Poor Venting in Plastic Injection Molds

Poor venting is a primary cause of defects in plastic injection molding, directly leading to issues such as air traps, silver streaks, burn marks, and short shots, which severely compromise both the appearance and mechanical properties of the final product. The root cause is the inability of air and volatile gases to escape the mold cavity and runner system efficiently as the melt advances. This results in gas compression, combustion, or entrapment. Effective solutions require a systematic approach encompassing mold design, processing parameters, and auxiliary technologies, balancing cost-efficiency with production stability.

I. Identification and Core Causes

1. Identification Methods

Defects are often visually apparent: irregular bubbles, surface silvering, or localized yellow/brown burn marks typically appear at the last point of fill. During mold trials, symptoms such as "jetting" or hesitation of the melt front as it reaches the cavity end strongly indicate insufficient venting.

2. Core Causes

The primary cause is flawed mold design, specifically the absence of vents at the last point of fill or improperly sized vent channels. Secondary causes include mismatched processing parameters, such as an injection speed that is too fast, compressing air before it can escape. Additionally, excessive moisture content or high levels of low-molecular volatiles in the raw material increase the total gas volume, exacerbating the problem indirectly.

II. Targeted Improvement Measures

1. Mold Design Optimization

This is the fundamental solution.

Adding Venting Channels: Vents must be strategically placed at the last point of fill, runner ends, and weld line locations. Standard industry practice recommends a width of 3–5mm. The depth is material-dependent: 0.04–0.05mm for low-viscosity materials (e.g., PE, PP), 0.02–0.03mm for medium-viscosity materials (e.g., ABS, PS), and 0.01–0.02mm for high-viscosity materials (e.g., PC, PMMA). Channels should extend 5–10mm outside the mold to ensure direct gas evacuation.

Utilizing Clearances: Leverage the mating clearances between ejector pins and their holes, or between inserts and the cavity, typically controlled within 0.01–0.03mm, to facilitate venting while preventing flash.

Specialized Components: For complex or deep cavities, design venting inserts or pins to channel trapped gases out effectively.

2. Adjustment of Processing Parameters

Injection Speed Profiling: Reduce the overall speed and implement a multi-stage profile. Use a slow speed (30%–40% of rated) initially to avoid rapid gas compression, a medium speed (60%–70%) for bulk filling, and a slow speed (20%–30%) at the end to allow sufficient time for gas evacuation.

Temperature Control: Increase the mold temperature by 5–10℃ to reduce flow resistance caused by premature cooling, aiding gas displacement.

Material Drying: Strictly control the drying process (e.g., ABS at 80–90℃ for 2–3 hours, PA6 at 90–100℃ for 3–4 hours) to minimize moisture and volatile content.

3. Auxiliary Venting Technologies

Vacuum Venting: For precision or complex parts, connect a vacuum pump to the mold vents to actively extract air and volatiles, increasing efficiency by over 50% compared to passive vents.

Defoaming Masterbatches: Add 0.5%–1% defoaming masterbatch to the raw material to absorb moisture and volatiles at the source.

III. Validation and Technical Trends

1. Validation Methods

Post-modification validation involves mold trials to check for the elimination of burn marks and bubbles. The "smoke test" method—injecting smoke into the cavity to simulate melt flow—provides a visual representation of trapped gas pockets for further refinement.

2. Technical Trends

The industry is increasingly adopting CAE flow simulation during the design phase to predict gas trap locations accurately, optimizing vent placement upfront and reducing trial-and-error. High-end molds are also integrating intelligent monitoring to provide real-time feedback on cavity pressure, enabling dynamic process adjustments for enhanced stability.

In summary, resolving poor venting requires mold design optimization as the core, supported by parameter tuning and auxiliary technologies. Leveraging CAE simulation ensures precise implementation, fundamentally solving gas entrapment issues and guaranteeing consistent part quality.