Exhaust Methods for Thin-Wall Injection Molding

Thin-wall injection molding is a technically challenging process where the cavity wall thickness is typically ≤1mm. The high-speed filling and long flow paths cause air to become trapped and compressed, leading to defects such as short shots, burns, weld lines, and bubbles. Scientific exhaust system design and optimization are crucial to ensuring quality. This document outlines comprehensive exhaust methods from four dimensions: mold structure design, process parameter optimization, material pre-treatment, and system maintenance.

I. Exhaust Methods via Mold Structure Design

Mold design is the primary means of exhausting air. By creating dedicated channels, air is guided out of the cavity directly, addressing the root cause of trapping.

Vent Groove Exhaust: This is the most common method. Vents must be precisely placed at the end of flow paths, at weld lines, and in dead corners. Dimensions depend on material viscosity: for low-viscosity materials like PE and PP, the depth is 0.02-0.05mm and width 8-15mm; for high-viscosity materials like ABS and PC, depth is reduced to 0.015-0.03mm with a width of 10-20mm. A straight-through design is recommended with a length ≤10mm, connected to a scrap trap to prevent clogging.

Insert Joint Exhaust: For complex cavities, insert structures are used where the mating gap between inserts acts as a natural vent. The gap must be tightly controlled at 0.01-0.02mm to prevent flashing while allowing air escape. This is ideal for side walls and deep cavities where traditional vents are impractical, especially for parts ≤0.5mm thick.

Ejector Pin Clearance Exhaust: Utilizes the clearance between ejector pins and their guide holes. The clearance is controlled at 0.01-0.02mm. To enhance efficiency, longitudinal grooves are often machined at the rear of the pins to channel air out. This method is suitable for internal surfaces or cosmetic areas where surface vents are undesirable.

Vacuum Exhaust: For high-precision, transparent parts (e.g., optical lenses), a vacuum system actively removes air. A vacuum port is placed at the cavity end or parting line, connected to a pump maintaining 0.06-0.09MPa. This removes over 90% of air, eliminating burns and bubbles. A robust mold seal is required to prevent air backflow, improving efficiency by 15%-20%.

II. Exhaust Methods via Process Parameter Optimization

Optimizing process parameters creates favorable conditions for air evacuation, working synergistically with mold design.

Segmented Injection Speed: A "Slow-Fast-Slow" profile is adopted. The initial phase uses low speed (50%-60% of normal) to allow air to escape ahead of the melt front. The middle phase uses high speed to ensure filling before solidification. The final phase slows again to prevent excessive air compression. When combined with vents, this strategy improves exhaust efficiency by over 30%.

Temperature & Pressure Adjustment: Moderately reduce melt temperature by 5-10℃ and mold temperature by 3-5℃ to minimize gasification and reduce air thermal expansion. Additionally, increase back pressure to 0.3-0.8MPa to purge air and volatiles from the barrel, preventing them from entering the cavity.

Packing Optimization: Shorten packing time or reduce pressure to avoid squeezing trapped air into the part, which causes bubbles. For thin walls, packing time is typically 1-3 seconds at 60%-70% of the injection pressure, adjusted based on specific geometry and material.

III. Exhaust Aids via Material Pre-Treatment

Moisture and volatiles in raw materials are major gas sources. Proper pre-treatment reduces gas generation, easing the load on the exhaust system.

Hygroscopic materials (ABS, PC, PA) require strict drying: ABS at 80-90℃ for 2-3 hours; PC at 110-120℃ for 3-4 hours; PA at 90-100℃ for 4-5 hours. Moisture content must be ≤0.02% to prevent vaporization. For non-hygroscopic materials (PE, PP), screening removes impurities and clumps that can block vents.

IV. Maintenance & Precautions

Regular maintenance ensures the exhaust system remains effective.

Regular Cleaning: Vents must be cleaned to prevent clogging by debris. It is recommended to clear vents with a 0.1mm wire and clean scrap traps every 1000-2000 cycles.

Material Specifics: Transparent materials (PMMA, PC) demand the highest exhaust standards (often vacuum + vents). Glass-filled materials generate more debris, requiring wider vents and more frequent cleaning.

Cosmetic Considerations: Avoid placing vents on visible surfaces to prevent vent marks. Use ejector pin or insert clearance methods instead.

Troubleshooting: If burns or bubbles appear, first check for blocked vents before adjusting process parameters.

In summary, successful thin-wall exhaust requires a "Mold First, Process Second" approach, supported by proper material handling and maintenance, to ensure high-quality production.