1. Definition
Mold exhaust system is a channel system that exhausts gases (air, plastic decomposition gas, water vapor, etc.) in cavity through specific structures (such as exhaust grooves, exhaust needles, insert gaps, etc.), and is an indispensable key part of mold design.
2. Function
- Avoid gas retention: prevent gas from being compressed during filling process of molten plastic, resulting in defects such as burning and insufficient filling.
- Improve filling performance: reduce gas resistance, improve melt flow efficiency, and ensure complete mold filling.
- Improve surface quality: eliminate problems such as gas marks and obvious weld lines to ensure appearance requirements.
- Stabilize process parameters: reduce injection pressure fluctuations caused by gas compression and improve molding stability.

 

1. Surface defects
- Burning (gas high temperature compression), gas marks (gas and melt mixing).
2. Structural defects
- Insufficient filling (gas hinders flow), shrinkage (local gas residue affects shrinkage compensation).
3. Performance issues
- Reduced weld line strength and internal bubbles lead to decreased mechanical properties.
4. Process fluctuations
- Need to increase injection pressure or temperature compensation, increase energy consumption and production risks.

1. Design and optimization of exhaust grooves
- Position requirements
- Must be located at the end of melt flow sealing position to ensure that final gathering point of gas can be discharged.
- Exhaust grooves must be opened synchronously at the end of main/divided flow channel (to avoid gas retention in flow channel).
- Quantity and size
- Quantity must be sufficient (calculated according to cavity volume and injection speed) to avoid local exhaust shortage.
- Width should be as large as possible (usually 5-15mm) to increase exhaust area.
- Length of sealing position is controlled at 0.8-1.2mm (too long is easy to overflow, too short reduces sealing).
- Structural design
- Front section is sealing position, and rear section needs to avoid air (deepen or widen to reduce resistance of gas discharge).
- Surface roughness Ra≤1.6μm to avoid obstruction of gas flow.
2. Application of exhaust needles
- Position selection
- It is preferred to be set at sealing position inside plastic part (such as deep cavity, narrow groove and other gas-prone areas).
- Quantity and form
- Quantity must be sufficient (adjusted according to complexity of cavity).
- Adopt standard exhaust needle structure (front cone air guide, rear avoidance groove).
3. Exhaust optimization of special structure
- Rib position/bone position
- Use flat ejector pin for ejection (use gap between its flat structure and core for exhaust).
- Screw column
- Use exhaust sleeve ejection (gap between sleeve and core is designed to be 0.03-0.05mm).
- Mold dead corners
- Add inserts (use insert gap to exhaust, gap is controlled at 0.02-0.04mm).
4. Optimization of process and material compatibility
- Exhaust time control
- Exhaust time must be less than injection time (achieved by adjusting depth or number of exhaust grooves).
- Thin-walled products
- High-speed injection requires increasing density and width of exhaust grooves (to prevent instantaneous gas compression).
- Thermosensitive plastics
- Increase gap of exhaust grooves (such as PA, PC and other materials, gap can be increased to 0.05-0.08mm).

 

Design of mold exhaust system needs to integrate structure, process and material characteristics, and follow principle of "precise position, sufficient quantity and reasonable size". By optimizing gap between exhaust grooves, exhaust needles and inserts, product quality and production efficiency can be significantly improved, especially for thin-walled parts, high-gloss parts and molds of heat-sensitive materials. It is recommended to verify exhaust scheme in combination with mold flow analysis (Moldflow) to ensure that theoretical design is consistent with actual application.