7 Types Of Injection Moulding: Materials, Processes & Uses

3. Gas-Assisted Injection Moulding

Gas-assisted injection moulding is a specialised process designed to produce parts with hollow or thick-walled sections while minimising sink marks and internal stresses. By introducing pressurised gas into the melt, this technique creates internal channels that reduce material usage and shorten cooling times. It’s particularly well suited to large, ergonomically shaped components where weight reduction and surface quality are paramount.

3.1 How the Gas-Assisted Process Works

In gas-assisted injection moulding, the cycle begins much like conventional moulding: molten plastic is injected to partially fill the cavity. At a predetermined shot volume—often 50–70% fill—the mould is paused and a precisely timed pulse of nitrogen (N₂) is injected through dedicated gas pins or a runner system. The gas pressure forces the melt outward against the cavity walls, forming hollow sections along the part’s interior.

Once the gas has created the desired channels, the melt cools against the mould surface, solidifying into a dense “skin” while the core remains hollow. A “pinch-off” or seal is formed at the gas entry point to trap the gas inside, and any residual gas is vented back through the runner. The result is a component with a uniform wall appearance, minimal sink marks and significantly reduced mass.

3.2 Material and Design Considerations

Not every polymer can tolerate gas-assist. Commonly used resins include:

• ABS (Acrylonitrile Butadiene Styrene)
• PC (Polycarbonate)
• HIPS (High Impact Polystyrene)
• POM (Polyoxymethylene)
• PA (Nylon)
• PP (Polypropylene)

Designing for gas-assist demands consistent wall thickness—typically 2–8 mm—to guide the gas along straight, uninterrupted runs. Sharp corners, drastic section changes or deep ribs can impede gas flow and cause uneven channeling. Mould tools must incorporate robust venting and dedicated gas pins positioned strategically to ensure the gas front advances smoothly.

3.3 Applications and Examples

This process shines in applications where hollow interiors and smooth exteriors are desirable:

• Hollow Handles: Ergonomic tool grips or power-tool housings that feel sturdy but weigh less.
• Furniture Legs: Lightweight structural members for chairs or tables.
• Large Panels: Wall-mounted enclosures or decorative panels in appliances.
• Automotive Components: Shells for armrests or headrests with embedded gas channels to cut weight without compromising strength.

For instance, an automotive armrest shell moulded with gas assistance can shed up to 25% of its bulk material, reducing both cost and part weight while maintaining a blemish-free finish.

3.4 Advantages and Limitations

Advantages of gas-assisted injection moulding include:

• Material savings of 15–30%, thanks to hollow cores
• Shorter cycle times, as thinner skins cool faster
• Reduced sink marks and warpage in thick sections
• Lower clamp force requirements compared to filling a solid part

However, there are trade-offs:

• Gas channels complicate mould design, making multi-cavity tools challenging
• Clear or transparent plastics may suffer surface fogging where gas interfaces with the melt
• Initial setup and process optimisation require experienced mouldmakers and technicians

By weighing these factors—material compatibility, part geometry and production volume—designers can determine whether gas-assisted injection moulding will deliver the optimal balance of cost, quality and performance.

4. Multi-Material Injection Moulding

Multi-material injection moulding (MMM) elevates conventional processes by integrating two or more polymers—often with very different characteristics—into a single component in one production cycle. Instead of assembling parts post-moulding, MMM builds complex assemblies in situ, combining rigid substrates with elastomeric overmoulds or embedding functional inserts with cosmetic finishes. This approach can reduce part count, streamline assembly and deliver localised variation in material performance exactly where it’s needed.

4.1 Overview of MMM Techniques

Multi-material injection moulding encompasses several distinct methods:

• Multi-component moulding: Sequential injection of different melts via independent injection units.
• Multi-shot moulding: The mould indexes between shots, allowing materials to form distinct layers or sections.
• Overmoulding: A second material—typically a softer elastomer—is injected over a pre-formed plastic core or insert.

Each technique demands specific tooling and cycle strategies, yet all share the objective of producing fully integrated parts without secondary assembly. For a deeper explanation, see Multi-material injection moulding on Wikipedia.

4.2 Common Material Combinations

Designers choose polymer pairings to exploit contrasting properties:

• Rigid + soft: ABS or PC cores with TPE or TPU overmoulds for comfortable grips and seals.
• Blends: PC/ABS alloys for balanced impact resistance and thermal performance.
• Hard plastic + rubber: Functional zones—such as anti-slip handles or soft-touch buttons—moulded directly onto a sturdy base.

Successful bonding often relies on chemical compatibility or dedicated adhesion promoters. It’s also crucial to accommodate differences in shrinkage and thermal expansion to prevent delamination.

4.3 Applications and Use Cases

Multi-material parts are prevalent across industries:

• Toothbrush handles combining rigid polypropylene with a soft TPE grip in one cycle.
• Ergonomic power-tool handles featuring a nylon core and rubberised overmould for vibration damping.
• Medical instruments with sterile, soft-touch finger pads over a rigid housing.
• Decorative or functional inserts, such as two-colour logos or transparent windows, moulded without post-assembly.

This seamless integration of form and function enhances both product aesthetics and user experience.

4.4 Benefits and Constraints

Benefits of multi-material injection moulding include:

• Assembly elimination: No adhesives, welds or manual fitting steps.
• Localised material properties: Stiffness, flexibility or grip exactly where required.
• Design freedom: Overmoulded seals, multi-colour elements and payload inserts in a single tool.

Constraints to consider:

• Complex mould design: Precise alignment, multi-stage gating and shot sequencing are essential.
• Higher tooling cost: Additional injection units, rotating platens or slides increase investment.
• Extended cycle times: Each extra shot lengthens the overall moulding cycle, affecting throughput.

By balancing material ambitions, projected volumes and budget, you can determine whether multi-material injection moulding is the ideal solution for your next project.

5. Micro Injection Moulding

When component dimensions shrink to the millimetre and sub-millimetre scale, standard injection moulding gives way to micro injection moulding. This highly specialised process produces parts weighing as little as 0.1 g, with feature sizes down to a few tens of micrometres. It’s critical in applications where precision and miniaturisation are non-negotiable, from medical implants to watchmaking and micro-fluidic devices.

5.1 Defining Micro Moulding

Micro moulding refers to any injection process that handles shot weights typically between 0.1 g and 1 g and cavity features measured in micrometres. Components can be smaller than a pea yet require tight dimensional tolerances—often ±0.01 mm or better. For a thorough overview, see Micro injection moulding on Wikipedia.

5.2 Equipment and Process Adaptations

Producing micro parts demands tailored machinery and tooling. Micro-injection presses employ tiny screws—commonly 8–15 mm in diameter—to meter and inject precise melt volumes. Moulds are manufactured to sub-micron surface finishes, with mould cavities only a few millimetres across. Process controls must regulate injection speed, melt temperature and back-pressure with exceptional accuracy. Even small fluctuations in barrel temperature or screw position can lead to flash, short shots or dimensional drift.

5.3 Materials for Micro Parts

Not all polymers adapt well to ultra-small geometries. Low-viscosity grades of polyoxymethylene (POM), polycarbonate (PC) and polypropylene (PP) are favourites for their good flow and minimal shrinkage. In certain micro-seal or micro-valve applications, liquid silicones offer the flexibility and biocompatibility required at tiny scales. Material selection often hinges on balancing flowability, mechanical performance and the end-use environment.

5.4 Example Applications

Micro injection moulding underpins several high-precision industries:

• Medical Devices: Micro-valves, micro-tubing connectors and implantable components that must meet sterilisation standards.
• Timepieces: Micro-gears and escapement parts in luxury watches, where surface finish and tolerance are paramount.
• Micro-Optics: Lens arrays and micro-prisms for sensors or lab-on-a-chip devices, requiring optical clarity and precise geometry.
• Micro-Electronics: Housings and connectors for MEMS sensors and miniature actuators.

5.5 Advantages and Challenges

Micro injection moulding offers unrivalled repeatability and precision, enabling features that no other mass-production process can match. However, these benefits come with trade-offs:

• High Capital Cost: Equipment and mould tooling command a premium, often several times the cost of conventional systems.
• Stringent Environment: Temperature, humidity and vibration must be tightly controlled to avoid defects.
• Complex Maintenance: Tiny feed ports and fine tolerances require meticulous upkeep and specialised technician skill.

Despite these hurdles, micro moulding remains the go-to for any project where miniaturisation and high throughput converge—delivering consistent quality part after part.

6. Liquid Silicone Injection Moulding

Liquid Silicone Injection Moulding (LIM) stands apart from thermoplastic processes by using liquid silicone rubber (LSR), a two-component elastomer that cures inside the mould. Rather than melting a polymer granule, LIM mixes cold silicone components, injects them into a heated cavity and triggers vulcanisation in situ. This specialised approach delivers high-precision rubber parts with excellent consistency—ideal for applications demanding tight tolerances, biocompatibility or extreme resistance to heat and chemicals.

6.1 What Is Liquid Silicone Injection Moulding (LIM)?

In LIM, two liquid silicone cartridges—often denoted A and B—are metered and mixed under pressure before entering the mould. The tool is held at temperatures between 150 °C and 200 °C, where the silicone undergoes peroxidic or platinum-catalysed vulcanisation. Unlike thermoplastics, LSR never truly “melts” and there is no cooling phase in the classical sense: the part solidifies as the silicone cross-links within the hot mould cavity. Once cured, the elastomer retains its shape and properties without further annealing.

6.2 Properties of Silicone Elastomers

Silicone elastomers offer a combination of attributes unmatched by most plastics:

• Biocompatibility and food-grade approval for medical or kitchenware applications
• Broad temperature range (−60 °C to +230 °C) with minimal change in mechanical performance
• Excellent chemical resistance to oils, solvents and UV exposure
• Low compression set and high elasticity for durable seals and gaskets
• Electrical insulation properties for electronic over-moulds

These intrinsic properties explain why engineers often specify LSR where standard thermoplastics fail to meet demanding service conditions.

6.3 Typical Applications

LIM parts feature across diverse industries:

• Medical Devices: Catheter tips, implantable seals and ventilator masks where sterilisable, biocompatible components are essential.
• Consumer Goods: Kitchen utensils, child-safe bottle nipples and wearable products that require food-safe, flexible materials.
• Electronics: Keypads, gaskets and cable-overmoulds that benefit from silicone’s dielectric strength and tactile softness.
• Automotive Seals: Door gaskets, engine bay connectors and vibration dampers exposed to heat, oils and ozone.

By producing net-shape parts with integrated features, LIM reduces assembly steps and enhances product reliability.

6.4 Equipment and Process Requirements

Executing a reliable LIM cycle depends on precision equipment and tooling:

• Metering/Mixing Heads: Twin-plunger or static-mixer systems ensure exact A:B ratios and homogeneous mixing.
• Heated Moulds: Thermally controlled cavities maintain uniform temperatures to guarantee consistent cure times across all cavities.
• High-Pressure Injection Units: Pumps or syringes designed for low-viscosity fluids deliver shot weights from a few grams up to several hundred grams without shear degradation.
• Sealed Tooling and Venting: Tight parting lines prevent flash, while micro-vents allow trapped air to escape without LSR leakage.

Strict process controls on temperature, pressure and shot timing are crucial—any deviation can lead to under-cured spots or excessive flash.

6.5 Pros and Cons

Pros

• Near-net-shape accuracy, eliminating secondary machining or finishing
• Permanent elastomeric properties with excellent physical and chemical resistance
• Fully automated mixing and injection for high repeatability
• Biocompatibility and regulatory approvals for medical and food applications

Cons

• Irreversible cure prevents regrinding or recycling of scrap silicone
• Higher tooling and start-up costs compared to thermoplastic moulds
• Need for specialised injection units and precise viscosity management
• Limited raw-material shelf life once cartridges are opened

By weighing these factors, designers can determine whether the performance advantages of LIM justify its upfront investment and process complexity.

7. Structural Foam Moulding

Structural foam moulding—or moulded structural foam—is a specialised injection process that introduces a blowing agent or inert gas into the polymer melt, producing a part with a low-density, foamed core and a solid outer skin. This skin–core structure grants the part exceptional stiffness-to-weight ratios, making it ideal for large components where weight reduction, material savings and structural rigidity are paramount. By combining form, function and economy, structural foam moulding enables the production of sizeable, lightweight parts that would be costly or difficult to achieve through conventional solid moulding techniques.

7.1 Process Fundamentals

In a typical structural foam cycle, the polymer resin and a chemical blowing agent—or an inert gas such as nitrogen—are either mixed directly in the injection barrel or introduced into the mould cavity after shot injection. As the melt enters the cooled cavity, the temperature drop triggers the blowing agent to decompose or the gas to expand, creating microscopic bubbles throughout the core. The outer surface against the steel tool collapses under pressure, forming a dense “skin,” while the interior remains foamed. This dual-phase structure cools faster than a fully solid part, and the reduced material density helps to minimise sink marks and internal stresses.

7.2 Materials and Blowing Agents

Common resins for structural foam moulding include polyurethane, acrylonitrile butadiene styrene (ABS), polycarbonate, polyphenylene oxide (Noryl) and polybutylene terephthalate. Chemical blowing agents—often azodicarbonamide or sodium bicarbonate compounds—decompose at predefined temperatures to release gases that expand the melt. Alternatively, nitrogen gas can be injected under controlled pressure. Selection of resin and blowing agent must account for melt viscosity, compatibility with foaming chemistry and desired mechanical properties of the finished part.

7.3 Applications

Structural foam moulding excels in producing large, hollow or semi-hollow parts that require good strength without excessive weight. Typical applications include:

• Automotive roof and door panels, offering rigidity with reduced mass
• Housings and enclosures for medical devices and laboratory equipment
• Cabinet doors, large consumer electronics casings and furniture components
• Sporting goods such as kayak hulls or sled frames

Across these applications, structural foam parts deliver high stiffness while conserving material and energy.

7.4 Advantages and Disadvantages

Advantages:

• Significant weight savings and material reduction, lowering part cost
• High stiffness-to-weight ratio—up to eight times stiffer than comparable solid polymers
• Capability to mould large parts in single pieces, reducing assembly steps
• Resistance to warpage and lower incidence of sink marks due to uniform foaming

Disadvantages:

• Coarser surface finish that may require additional post-processing for aesthetic applications
• Minimum wall thickness typically above 0.25 in (≈6.35 mm), limiting very thin part designs
• Longer cycle times compared to conventional moulding, as foaming and cooling stages require careful control
• Potential for slight dimensional variability inherent in foamed cores

7.5 Design Guidelines

Effective structural foam designs balance flow symmetry and wall thickness to ensure even foaming and cooling. Recommended wall thicknesses start at 0.25 in (≈6.35 mm) or greater, with gradual transitions between sections to avoid stress concentrations. Moulds should incorporate generous radii, venting channels to release excess gas and robust draft angles for reliable part ejection. Balanced runner systems and gated entry points help maintain consistent skin thickness around the cavity. By adhering to these guidelines, designers can harness structural foam moulding to deliver large, lightweight components with outstanding structural performance.