Introduction
Gas injection moulding is a specialized manufacturing process that solves problems traditional injection moulding struggles with. Sink marks, warpage, and excessive weight in thick sections—these common defects have a proven solution.
The process uses nitrogen gas to create hollow channels inside plastic parts. This reduces material use, cuts cycle times, and improves surface quality. For manufacturers making large or thick-walled components, gas injection moulding offers significant advantages.
This guide explains how the process works, when to use it, and what results you can expect. You will learn the key stages, material considerations, and real-world applications. By the end, you will understand whether gas injection moulding fits your next project.
What Is Gas Injection Moulding?
Gas injection moulding is a variation of traditional injection moulding. It combines plastic injection with high-pressure gas to create parts with hollow internal structures.
How Does It Differ from Traditional Moulding?
| Feature | Traditional Injection Moulding | Gas Injection Moulding |
|---|---|---|
| Part structure | Solid throughout | Hollow core in thick sections |
| Material usage | Full shot volume | 20–40% less material |
| Sink marks | Common in thick areas | Eliminated |
| Warpage | Moderate to high risk | Reduced |
| Cycle time | Longer due to thick cooling | Shorter; hollow sections cool faster |
| Surface finish | May show sink marks | Excellent; no sink marks |
What Role Does Gas Play?
The gas—typically nitrogen—does not mix with the plastic. Instead, it pushes into the molten material, displacing it toward the mold walls. The result is a part with a solid skin and a hollow core.
Nitrogen is used because it is:
- Inert – Does not react with molten plastic
- Dry – Contains no moisture that could cause defects
- Readily available – Easy to store and handle
How Does the Process Work?
Gas injection moulding follows a sequence of stages. Each stage must be precisely controlled for consistent results.
Stage 1: Plastic Melting and Injection
The process begins like traditional injection moulding. Plastic pellets feed into a heated barrel. A screw melts the material and pushes it forward.
Melt temperatures depend on the material:
- Polypropylene (PP): 180–220°C
- Polyethylene (PE): 160–210°C
- Nylon (PA): 240–280°C
- Polycarbonate (PC): 260–300°C
The plastic is injected into the mold, but only partially. Typically, the mold fills to 60–90% of its volume before gas is introduced.
Stage 2: Gas Introduction
At the precise moment, nitrogen gas enters the melt through a specially designed gas nozzle. The timing is critical.
- Too early – Gas disrupts the initial fill; incomplete parts
- Too late – Plastic solidifies; gas cannot penetrate
Gas pressure typically ranges from 5 to 30 MPa (about 700–4,300 psi). The gas pushes the molten plastic forward, completing the fill while creating a hollow core.
Stage 3: Gas Penetration and Packing
As gas flows into the plastic, it follows the path of least resistance. It pushes toward the thickest sections and the ends of the flow path.
The gas serves two functions:
- Completes filling – Replaces the remaining plastic volume
- Provides packing pressure – Maintains uniform pressure as the part cools
Because gas applies pressure evenly from inside, the plastic compresses against the mold walls. This eliminates sink marks and reduces internal stresses.
Stage 4: Cooling and Solidification
Cooling begins immediately after gas injection. The hollow core allows heat to dissipate faster than a solid section.
Cooling time reduction can reach 20–40% compared to solid parts of equivalent thickness. The gas pressure remains during cooling to compensate for shrinkage.
Stage 5: Gas Exhaust and Ejection
Before the mold opens, gas pressure is released. The gas vents to the atmosphere or is recovered. The mold opens, and the part is ejected—now with a hollow internal structure.
What Are the Key Benefits?
Gas injection moulding offers several advantages over conventional processing.
Material Savings
By creating hollow sections, the process reduces material usage by 20–40%. For large parts or high volumes, this adds up to significant cost savings.
A real-world example: A manufacturer producing plastic handles for power tools switched from solid moulding to gas injection. Part weight dropped from 480 grams to 320 grams. Annual material savings exceeded $45,000.
Elimination of Sink Marks
Sink marks occur when thick sections cool and shrink unevenly. With gas injection, the hollow core eliminates the thick mass that causes sinks. Surface finish remains flawless.
Reduced Warpage
Internal stresses cause warpage in solid parts. Gas pressure applies uniform force from inside, reducing stress concentration. Parts come out straighter and more dimensionally stable.
Shorter Cycle Times
Thick sections take longer to cool. Hollow cores cool faster. Cycle time reductions of 15–30% are common.
Lower Clamping Force
Gas pressure helps fill the mold, reducing the injection pressure needed from the machine. This allows the use of smaller clamping tonnage—sometimes 30–50% less than a solid part would require.
Improved Strength-to-Weight Ratio
The hollow structure behaves like an I-beam or tube. Stiffness remains high while weight drops. This is especially valuable for structural components.
What Are the Common Applications?
Gas injection moulding is used across industries where large, thick, or structural parts are needed.
Automotive Industry
- Door handles and grab handles
- Armrests and interior trim
- Spoilers and body panels
- Engine cover components
A case example: An automotive supplier switched to gas injection for door handles. The solid handles weighed 220 grams and showed sink marks at the thick grip section. Gas-injected handles weighed 150 grams, had perfect surface finish, and passed all strength tests.
Consumer Products
- Power tool housings
- Furniture components
- Appliance handles
- Large enclosures
Industrial Equipment
- Structural frames
- Heavy-duty handles
- Pump housings
- Roller cores
What Materials Work Best?
Most thermoplastics used in traditional moulding work with gas injection. Some materials perform better than others.
Preferred Materials
| Material | Why It Works Well | Typical Applications |
|---|---|---|
| Polypropylene (PP) | Good flow; gas penetrates easily | Handles, automotive trim, containers |
| Polyethylene (PE) | Excellent flow; low viscosity | Large structural parts, tanks |
| Nylon (PA) | Good strength; semi-crystalline | Engine components, structural parts |
| Polycarbonate (PC) | High strength; thick sections benefit most | Housings, protective covers |
| ABS | Good flow; good surface finish | Consumer products, electronics |
Filled Materials
Glass-filled and mineral-filled materials also work, but with considerations:
- Glass fibers can affect gas penetration depth
- Higher injection pressures may be needed
- Mold design must account for flow changes
How Is the Mold Designed Differently?
Gas injection molds require specific design features.
Gas Nozzle Placement
The gas nozzle is positioned where gas should first enter the melt. This is typically at the thickest section or along the main flow path. Multiple gas nozzles may be used for large parts.
Gas Channel Design
The mold may include gas channels—dedicated paths where gas is intended to flow. These channels are often in thick sections where material reduction is desired.
Sealing Requirements
The mold must seal tightly to prevent gas from escaping at the parting line. Higher clamping forces may be needed compared to solid molding.
Gate Design
Gates must allow both plastic and gas to flow. Larger gates or special gas-injection gates are common. Some designs use the gas nozzle as the primary entry point, with plastic injected through a separate location.
What Process Parameters Matter Most?
Success in gas injection moulding depends on controlling several key variables.
Gas Injection Timing
The percentage of mold filled before gas injection is critical. Typical range: 60–90% filled.
| Fill Percentage | Effect |
|---|---|
| Low (60–70%) | More gas penetration; larger hollow core |
| Medium (70–80%) | Balanced; most common starting point |
| High (80–90%) | Less gas penetration; solid skin thicker |
Gas Pressure and Hold Time
Gas pressure must be higher than plastic injection pressure to penetrate. Hold time should continue until the part solidifies sufficiently to hold its shape.
Melt Temperature
Higher melt temperatures allow gas to penetrate more easily but increase cycle time. The optimal temperature is usually at the higher end of the material's recommended range.
Mold Temperature
Lower mold temperatures shorten cycle time but may affect surface finish. The ideal temperature balances cooling speed with part quality.
What Defects Can Occur?
Even with gas injection, defects can happen. Understanding them helps with troubleshooting.
| Defect | Cause | Solution |
|---|---|---|
| Gas breakthrough | Gas escapes through surface | Reduce gas pressure; check nozzle position |
| Incomplete hollow core | Gas penetration too shallow | Increase gas pressure; inject earlier |
| Blow-through | Gas pushes through thin wall | Relocate gas nozzle; increase wall thickness |
| Finger effect | Uneven gas flow; multiple channels | Improve gate design; adjust timing |
| Surface marks | Gas disturbance at injection point | Optimize nozzle position; adjust timing |
A case example: A manufacturer making hollow tubular parts experienced "finger effect"—gas created multiple small channels instead of one clean hollow core. After adjusting gas injection timing from 75% fill to 70% fill, the gas flow consolidated into a single channel, eliminating the defect.
How Does It Compare to Structural Foam?
Gas injection moulding is sometimes confused with structural foam moulding. They are different processes.
| Feature | Gas Injection | Structural Foam |
|---|---|---|
| Gas type | Nitrogen (inert) | Chemical blowing agent or nitrogen |
| Internal structure | Clean hollow core | Cellular foam structure |
| Surface finish | Excellent (solid skin) | Swirl marks typical; requires painting |
| Weight reduction | 20–40% | 10–30% |
| Part thickness | Any; thick sections best | Typically thicker walls |
| Cycle time | Shorter | Longer due to foam formation |
Conclusion
Gas injection moulding is a powerful technique for producing large, thick, or structural plastic parts. It eliminates sink marks, reduces warpage, cuts material usage by 20–40%, and shortens cycle times by 15–30%.
The process works by injecting nitrogen gas into the molten plastic, creating hollow cores while maintaining a solid outer skin. It is well-suited for materials like PP, PE, nylon, and polycarbonate. Automotive components, consumer products, and industrial parts all benefit from this technology.
Success requires careful control of gas timing, pressure, and mold design. When done correctly, gas injection moulding delivers parts that are lighter, stronger, and more dimensionally stable than their solid counterparts.
Frequently Asked Questions (FAQ)
What types of plastics are most suitable for gas injection moulding?
Most thermoplastics work, but polypropylene (PP), polyethylene (PE), nylon (PA), and polycarbonate (PC) are most common. PP and PE flow easily, allowing gas to penetrate smoothly. Nylon offers excellent strength for structural parts. PC is ideal for thick sections requiring high impact resistance. Glass-filled materials can also be used but may require higher gas pressure and careful mold design.
How much material can gas injection moulding save?
Typical material savings range from 20% to 40% compared to solid parts. The exact amount depends on part geometry and gas channel design. Parts with thick sections see the highest savings. For high-volume production, material savings alone often justify the investment in gas injection equipment.
What is the difference between gas injection moulding and structural foam?
Gas injection creates a clean, smooth hollow core with a solid skin. Surface finish is excellent and often requires no post-processing. Structural foam creates a cellular foam structure throughout the part. It produces a swirl pattern on the surface that usually requires painting. Gas injection is better for parts needing good surface appearance; structural foam is often chosen for very large parts where surface finish is less critical.
Can gas injection moulding be used with existing molds?
Sometimes, but modifications are usually needed. The mold requires a gas injection nozzle, which may mean adding a new entry point. Gas channels may need to be added or existing geometry adjusted. Sealing requirements are more demanding. A mold designed specifically for gas injection performs better than a converted solid mold.
What is the typical cycle time reduction with gas injection?
Cycle time reductions of 15–30% are common. The hollow core cools faster than a solid section of the same thickness. For parts with thick walls, the reduction can be even greater—sometimes 40% or more. Faster cooling translates directly to higher output and lower per-part costs.
Contact Yigu Technology for Custom Manufacturing
At Yigu Technology, we have extensive experience with gas injection moulding for complex plastic and plastic-metal composite parts. Our engineering team understands how to optimize the process for your specific material and geometry.
We offer:
- Gas injection mould design with proper nozzle placement and gas channels
- Process development to dial in timing, pressure, and temperature
- Material selection guidance for optimal gas penetration and part strength
- Quality control with inspection of hollow core consistency and surface finish
Whether you need large structural components, automotive parts, or consumer product housings, gas injection moulding can deliver lighter, stronger parts with better surface quality.
Contact us today to discuss how gas injection moulding can improve your next project.
