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LSR Silicone Mold Venting Design and Entrapped Air Defect Prevention

During LSR injection molding, if air inside the mold cavity cannot be effectively expelled, entrapped air defects will occur. Entrapped air causes short shots, burn marks, bubbles, and surface blemishes, making it the most common and impactful defect type in LSR molding.

I. Causes and Consequences of Entrapped Air

1. Formation Mechanism

• LSR has low viscosity (3000-30000 mPa·s), fast injection speed, air cannot escape in time
• Silicone vulcanization reaction produces no gas; all gas in cavity is original air
• Parting line, insert gaps, and ejector pin holes restrict airflow
• Complex product structures (deep cavities, thin walls, undercuts) impede venting

2. Manifestations

• Short shot: air occupies space, silicone cannot fill
• Burn mark (Diesel effect): trapped air adiabatically compressed under pressure, temperature rises above 200°C, causing local burning
• Surface bubbles: bubbles on or near product surface
• Silver streaks: traces of air mixing at flow front
• Air traps at weld lines: air entrapment where multiple gates converge

3. Impact Scope

• Appearance parts: surface defects cause rejection
• Seals: bubbles cause seal failure
• Medical parts: bubbles become bacterial colonization points
• Electrical parts: bubbles affect insulation performance

II. Venting System Design Principles

1. Vent Location Determination

• Last-fill area: determined through mold flow analysis
• Parting line surface: primary venting channel
• Insert mating surfaces: gap venting between insert and mold plate
• Ejector pin mating surfaces: utilizing pin-to-hole gap for venting
• Overflow wells (cold slug wells): accommodating small amounts of silicone carried out by venting

2. Vent Slot Dimension Design

• Vent slot depth: 0.005-0.02mm (depending on silicone viscosity and flowability)
• Vent slot width: 3-10mm
• Vent slot length: < 5mm (short vent path for high efficiency)
• Vent slot spacing: 8-15mm (uniform distribution)

3. Vent Volume Calculation

• Cavity volume V_cavity
• Required vent area A_exhaust ≥ V_cavity / (v_injection × t_fill)
• Actual vent area should include 2-3× safety factor

III. Venting Methods Classification

1. Parting Line Venting

• Most common and effective venting method
• Vent slots along parting line, connecting cavity to mold exterior
• Vent slot ends connect to collection grooves, preventing silicone overflow contamination

2. Insert Venting

• Insert-to-mold plate fit gap 0.01-0.03mm for venting
• Micro-vent slots on insert sides (0.005-0.01mm depth)
• Suitable for deep cavity structures and localized air trap areas

3. Ejector Pin Venting

• Utilizing ejector pin-to-hole fit gap for venting
• Gap 0.01-0.025mm
• Limited venting area, serves as auxiliary venting method

4. Vent Pins/Vent Plugs

• Specialized venting elements with micro-vent channels on surface
• Installable in bored holes at any location
• High venting efficiency, but requires periodic cleaning

5. Vacuum Venting

• Before mold closure, cavity evacuated through dedicated vacuum port
• Vacuum level: -0.08 to -0.095 MPa
• Thoroughly eliminates entrapped air, suitable for high-precision products
• Requires vacuum system and sealed parting line

IV. Entrapped Air Defect Prevention Measures

1. Gate System Optimization

• Gate position should avoid air being trapped in blind areas
• Multi-gate design ensures simultaneous filling from each flow path (balanced flow)
• Tab gates and fan gates help push air toward vent slots

2. Injection Parameter Adjustment

• Reduce injection speed: give air time to escape, especially at final fill stage
• Stepped injection: slow-fast-slow three-stage injection speed
• Reduce injection pressure: reduce air adiabatic compression temperature rise
• Delay before holding pressure: allow residual air to escape before packing

3. Mold Temperature Control

• Moderately lower mold temperature: slow vulcanization speed, extend flow time
• Mold temperature uniformity: avoid localized overheating accelerating vulcanization and blocking flow

4. Vacuum-Assisted Molding

• Vacuum system configuration: vacuum pump + reservoir + control valve
• Vacuum time: 3-8 seconds
• 0.5-1 second delay after mold closure before injection, ensuring vacuum establishment

V. Venting System Maintenance

1. Regular Cleaning

• Clean vent slot overflow every shift
• Check vent pins/plugs for blockage weekly
• Deep clean mold venting system monthly

2. Vent Efficiency Monitoring

• First article inspection: appearance and dimensional confirmation
• Periodic sampling: monitoring bubble and short shot trends
• Mold flow analysis update: re-verify venting design after product changes

3. Vent System Repair

• Repair worn and enlarged vent slots
• Replace inserts when vent gap increases
• Clean or replace blocked vent pins

LSR mold venting design is the foundation of molding quality; a properly designed venting system combined with fine-tuned parameter adjustment can fundamentally eliminate entrapped air defects and improve product yield.