Mold Venting and Degassing in Injection Molding – Managing Gases in Production

Introduction to Mold Venting

Mold venting is one of the most frequently overlooked yet critically important aspects of proper injection mold design and operation. As the polymer fills the mold cavity, air and moisture must rapidly exit the cavity to allow complete part filling. When gases become trapped in the mold, the result is air bubbles, sink marks, incomplete filling, as well as burn marks and polymer oxidation defects.

Good venting directly impacts: part quality, cycle time, material strength, surface aesthetics, and required injection pressures. This guide discusses the design and practical operation of venting systems, identifying problems, and strategies to reduce defects.

Why Venting is Critical

Impact on Part Quality

Venting directly affects the number of molding defects:

• Air bubbles and voids – air trapped in the material creates structural imperfections
• Short shots (incomplete filling) – gas in the cavity resists material flow, requiring higher pressures or longer injection times
• Burn marks (oxidation defects) – compressed air heated to high temperatures oxidizes and discolors the polymer
• Cracking and fractures – parts with air bubbles are weaker and fail under load
• Warping and internal stress – uneven cooling caused by trapped gases results in dimensional variation and stress

Impact on Production Parameters

Poor venting forces:

• Higher injection pressures – to overcome gas resistance
• Longer hold times – to ensure complete filling
• Higher mold temperatures – to reduce viscosity and overcome resistance
• Longer cycle times – due to extended cooling times and production delays
• Higher energy consumption – more powerful motors, higher pressures, more intensive cooling

Sources of Gases in Injection Molding

1. Air in the Molding Cavity

Before each injection, the mold cavity contains air at atmospheric pressure (1 bar). When polymer enters at 1000+ bar pressure, the air compresses to nearly negligible volume. This compressed mass of air must exit the mold – if it doesn't, defects occur.

2. Moisture and Volatile Compounds from the Material

Polymers absorb moisture from their environment. During molding, this moisture evaporates (temperature exceeds 200°C for most polymers). Volatile molecules from plasticizers, solvents, and additives are also released. If the polymer is not properly dried, the volume of gases increases significantly.

3. Air Entrapment from Flow Patterns

When polymer enters the mold at high velocity, it can shear thin sections, creating micro-bubbles dispersed throughout the part.

4. Chemical Reactions During Processing

Some polymers (especially those with fillers or pigments) release gases during processing, particularly if temperature is too high.

Designing Venting Systems

Vent Geometry – Size and Depth

Vents must be large enough to allow gas escape without causing material leakage:

• Vent width: typically 0.15 – 0.5 mm (depending on material)
• Vent depth: typically 0.025 – 0.1 mm (smaller than width)
• Vent channel length: typically 2 – 6 mm
• Vent spacing: every 10 – 25 mm along the cavity edge

Rule of thumb: a vent should be large enough to allow gas escape but small enough to prevent material flow. Oversized vents cause flash leakage. Undersized vents block gas flow.

Width Versus Depth

Vents with large width and shallow depth are more efficient than narrow and deep vents. Gas escapes more readily from expanded surface area than from a constricted channel.

Number and Placement of Vents

Vent density should be higher:

• Near the gate (where gas is most compressed)
• At flow fronts (where material first arrives)
• In thin sections and ribbed design areas
• Around complex geometry and pockets

High-risk areas include:

• Last fill points – even a small air trap causes defects
• Internal spaces (radii, pockets)
• Flow weld lines – where two material streams converge

Vent Locations in the Mold

Primary Locations

1. Around the Cavity Perimeter

Vents distributed regularly around the cavity edge ensure uniform gas removal. The most common spacing is every 15-20 mm.

2. On Cores

If the part has holes or internal channels, core venting is critical. Vent holes must allow gases to escape.

3. In Variable Thickness Sections

Thicker sections cool more slowly. Gases can become trapped at thickness transitions. Vents should be positioned near these transitions.

4. Near the Gate

The gate is typically where the largest air accumulation occurs. A vent near the gate helps this gas mass escape.

Locations to Avoid

• In sections requiring aesthetic finish (vent marks will be visible)
• Where the flow front may squeeze material through the vent (flash leakage)
• In areas subject to high structural loading

Air Traps and Their Identification

When Air Traps Form

Air traps typically form when:

• Two flow streams converge (weld lines)
• Flow passes around an internal structure (core, metal insert)
• Geometry is complex (many ribs, radii, transitions)
• The flow path is long and narrow

Identifying Defects from Air Traps

• Burn marks (black spots) – indicate compressed air heated to high temperature
• Short shots – the far end of the cavity doesn't fill completely
• Visible bubbles on cross-section – inside the part
• Sink marks – indicate poor consolidation in that area
• Matte spots on the surface – where air contacted the material

Reducing Air Traps

Flow Simulation

Before the mold is manufactured, use CAD/FEA tools to simulate the injection process. Identify areas where air will become trapped while still in the mold design phase.

Geometry Optimization

• Increase fillet radii in high-risk areas
• Reduce the length of narrow sections
• Position gates for more uniform flow

Multi-Level Venting

Don't rely only on surface vents. If a core is internal, it must also have venting leading to the core pin or exit.

Degassing Methods

1. Passive Gravity Venting

Gases naturally escape from the mold through vents, driven by the pressure difference between the cavity and atmosphere. This is the oldest method and works well for many materials.

Advantages: simple, requires no additional equipment

Disadvantages: effective only at low injection pressures; sometimes insufficient for rapid processes

2. Ejector Pin Venting

Ejector pins can serve as vents – allowing gases to escape during part ejection. This venting is sometimes built into the ejection mechanism.

Advantages: functions as the part is ejected

Disadvantages: too late – most air must already be removed

3. Vacuum Venting

Special vacuum channels can be connected to areas particularly prone to air entrapment. Vacuum actively removes air from the mold during injection.

Advantages: very effective for complex geometry; allows higher injection speeds and pressures

Disadvantages: additional complexity, requires auxiliary equipment (vacuum pump), higher mold cost

4. Material Drying

Many gas-related defects come from moisture in the material. Proper drying of the resin before injection reduces volatile compounds.

Drying Parameters:

• Temperature: 60-90°C (depending on material)
• Time: 2-8 hours
• Relative humidity: below 0.1% (for hygroscopic materials)

5. Material Temperature Control

The polymer temperature during injection must be optimized:

• Too low – high viscosity, gas cannot escape
• Too high – material degradation, gas release, oxidation

Proper temperature reduces both viscosity and volatile compound release.

Vacuum Assistance

How Vacuum Assistance Works

Vacuum creates negative pressure in selected mold channels. As polymer enters, air is actively drawn out rather than becoming trapped. This allows:

• Faster material flow
• Lower injection pressures
• Elimination of bubbles even in the most difficult geometries

Vacuum Implementation

Vacuum Channels: small channels leading to selected vents and connected to a vacuum pump.

Vacuum Pump: a special pump connected to the molding machine or mold. Typically achieves 0.1-0.5 bar negative pressure.

Activation Timing: vacuum is usually started before or at the beginning of injection and stopped shortly after.

Vacuum Conditions

Parameters to control:

• Vacuum depth: -0.1 to -0.9 bar (relative to atmosphere)
• Duration: typically equal to injection time or slightly longer
• Channel specification: similar to venting, but with channels leading to the pump

Defects Caused by Poor Venting

1. Burn Marks (Oxidation Defects)

Cause: air compressed to 1000+ bar heats to 200-300°C, oxidizing the surface layer of the polymer.

Appearance: black or brown spots on the part surface, usually at the last fill points.

Solution: add vents near the affected area, increase vent size or number, or implement vacuum assistance.

2. Air Bubbles

Cause: air becomes trapped in the material during injection.

Appearance: visible voids inside the part (subsurface) or on the cross-section, sometimes macroscopic.

Solution: simulate flow, identify air trap locations, add vents there.

3. Short Shots (Incomplete Filling)

Cause: air in the cavity resists material flow, requiring higher pressures or longer times.

Appearance: parts are not completely filled, material doesn't reach the end of the cavity.

Solution: increase vent number and size, increase injection pressure, increase material temperature.

4. Sink Marks

Cause: air trapped below the surface causes poor consolidation during cooling.

Appearance: depression on the part surface, usually in thicker sections or near trapped air.

Solution: increase venting in that area, increase cooling time, reduce section thickness there.

5. Matte Spots and Discoloration

Cause: air in contact with hot material causes surface oxidation.

Appearance: variable matte spots, discoloration, degraded surface.

Solution: improve venting, reduce mold temperature if possible, increase material flow rate.

Best Practices for Venting

1. Plan Venting in the Mold Design Phase

Don't add vents ad hoc after the mold is manufactured. Plan them in 3D CAD, check for interference, ensure they won't cause leakage.

2. Use Flow Simulation

Software such as Moldex3D, Autodesk Simulation, or Solidworks Plastics allows you to simulate the injection process and identify venting problems before the mold is complete.

3. Distribute Vents Evenly

If one area has few vents, gases will accumulate there. Space vents every 15-25 mm around the perimeter.

4. Test Venting on Prototype

If possible, create a quick, inexpensive prototype mold (such as with metal 3D printing or epoxy) and test venting before moving to production.

5. Monitor Defects

Collect production data – which areas of the part most frequently have burn marks or bubbles. This data guides venting improvements in future iterations.

6. Consider Material Drying

Especially for hygroscopic materials (PA, ABS, PMMA, polycarbonate). Drying reduces the volume of gases to evacuate.

7. For Complex Geometry: Vacuum

If the mold is complex and traditional venting is insufficient, vacuum assistance is a worthwhile investment.

Troubleshooting Guide

Problem	Venting-Related Cause	Solution
Black spots (burn marks)	Compressed air in that area	Add vents near the spot, increase their size or number
Internal air bubbles	Air couldn't escape	Simulate flow, identify traps, add vents there
Incomplete filling	Air resistance or too low temperature	Increase venting, increase temperature or pressure
Sink marks	Air under the surface	Increase venting in that area, increase cooling time
Flash/leakage through vents	Vent too large	Reduce vent size or depth, reduce pressure
Matte spots	Air-induced surface oxidation	Improve venting, reduce temperature

Summary

Mold venting is a fundamental aspect of producing high-quality injection-molded parts. Good venting eliminates bubbles, burn marks, and reduces required pressures and process temperatures. Key points:

• Plan venting during mold design – not ad hoc
• Distribute vents evenly and at proper sizes – 0.15-0.5 mm width, 0.025-0.1 mm depth
• Identify air traps – especially weld lines and internal areas
• Dry the resin – reduces gas volume to evacuate
• For complex geometry consider vacuum – very effective for difficult parts
• Monitor defects and iterate – production data guides improvements

Investment in good venting pays off through higher part quality, lower pressures and cycle times, and long-term reduction in scrap and improved production efficiency.