The Injection Moulding Process: A Complete Guide

What is injection moulding?

Injection moulding is a manufacturing process used to produce accurate, repeatable plastic components. During the plastic injection moulding process, molten plastic is injected into a specially designed mould and allowed to cool into the required shape. Manufacturers can use the process for anything from complex, low-volume parts to large production runs.

The process involves a moulding machine which heats the selected plastic and injects it into a mould cavity under pressure. The material then cools and solidifies before the mould releases the part. The manufacturer can trim, machine, decorate or assemble the moulded component to complete the product.

Service providers such as Gemini incorporate plastic injection moulding into a wider manufacturing process that takes products from initial concept and design through to finished production runs of hundreds or thousands of parts.

What is the injection moulding process?

The injection moulding process follows seven main steps that take a part from initial idea to finished product. These stages are product design, mould creation, clamping, injection, cooling, ejection, and finishing and assembly. Below, we explain the injection moulding process step by step.

1. Product Design

The engineer converts the client’s drawings into a 3D model and assesses important factors such as wall thickness, draft angles, material flow and shrinkage.

2. Mould Creation

The manufacturer then produces the mould, typically from hardened steel or aluminium. The mould contains a cavity which is shaped like the desired product. This is typically the most expensive part of the entire injection moulding process. While you manufacture with Gemini, we cover the cost of maintaining and replacing your mould tool.

3. Clamping

The operator mounts the mould in the machine, and the clamping unit closes its two halves under high pressure.

4. Injection

Molten plastic is injected into the mould cavity, filling the mould.

5. Cooling

The plastic cools and solidifies inside the mould.

6. Ejection

The mould opens and ejector pins push the finished part out of the mould. The mould then closes and the cycle repeats. The full cycle can take seconds or minutes, depending on the component size, wall thickness and selected material.

7. Finishing and Assembly

Manufacturers with in-house finishing and assembly capabilities, such as Gemini, can trim, machine, decorate and assemble the moulded component to meet the final product specification.

What types of injection moulding are there?

The main types of injection moulding include thermoplastic moulding, insert moulding, overmoulding, two-shot moulding, liquid silicone rubber moulding and gas-assisted injection moulding. Each method works in a slightly different way and suits different materials, part designs and performance requirements.

Thermoplastic Moulding

Thermoplastic injection moulding is the most common form of injection moulding, where a thermoplastic material is heated until molten, then directly injected into a mould cavity to create a part.

Insert Moulding

Insert moulding places a separate component, such as a threaded metal insert, electrical contact or magnet, inside the mould before the plastic is injected. The plastic forms around the insert, creating a single integrated component.

Overmoulding

Overmoulding involves moulding one material over another. For example, a softer, rubber-like material may be moulded over a rigid plastic handle to improve grip, comfort and impact resistance.

Two-shot Moulding

Two-shot moulding uses two different materials or colours within the same production cycle. This can create components with contrasting colours, flexible sections, seals or integrated buttons without requiring separate assembly.

Liquid Silicone Rubber Moulding

Liquid Silicone Rubber Moulding (LSR Moulding) is used to manufacture flexible, heat-resistant and chemically resistant silicone components. Typical products include seals, medical parts, electrical connectors and food-grade items. Gemini specialises in LSR moulding for the medical industry.

Gas-assisted injection moulding

Gas-assisted injection moulding introduces pressurised gas into the molten plastic. The process creates hollow sections, reduces component weight and minimises visible sink marks in thicker parts.

Which injection moulding method is best for my product?

Get in touch with us and tell us about your part; we run many different types of injection moulding methods in-house and can tell you which one is best for you.

What materials are used for injection moulding?

Manufacturers select injection moulding materials based on the part’s required strength, heat resistance, chemical resistance, appearance and budget. Common materials include polypropylene, polyethylene, ABS, nylon, polycarbonate, acetal and acrylic. Specialist and high-performance materials can also be used for more complex or demanding applications.

The table compares the mechanical properties, heat performance, chemical resistance, appearance, density, applications and relative cost of common injection moulding materials.

Material	Mechanical properties	Heat performance	Chemical resistance	Appearance	Density	Typical applications	Relative cost
Nylon (PA6)	Tensile strength: ~85 MPa Modulus: ~3,200 MPa Strong, stiff and wear resistant	Melting point: ~220°C HDT: ~76°C at 1.8 MPa	Good against oils, greases and hydrocarbons; absorbs moisture	Opaque; usually cream or natural	~1.13 g/cm³	Gears, bearings, clips and under-bonnet parts	High
Polycarbonate (PC)	Modulus: ~2,300 MPa Exceptional impact resistance	High-heat grades: HDT up to ~183°C at 0.45 MPa	Moderate; some solvents and alkalis cause stress cracking	Transparent, tinted or opaque	~1.20 g/cm³	Guards, lighting, medical housings and safety covers	High
Acetal (POM)	Rigid, dimensionally stable, low friction and wear resistant	HDT: up to ~100°C, depending on grade	Excellent against fuels, solvents and strong alkalis	Smooth, opaque and slightly glossy	~1.39–1.42 g/cm³	Gears, valves, bearings and precision parts	Mid-range
ABS	Tensile strength: ~45 MPa Modulus: ~2,300 MPa Impact strength: ~299 J/m	HDT: ~78°C at 1.8 MPa	Limited against fuels, ketones and organic solvents	High gloss; easy to colour, paint or plate	~1.04 g/cm³	Housings, automotive trim, panels and appliances	Mid-range
Polyethylene (PE)	Flexible, lightweight and impact resistant; properties vary by grade	Generally low; varies between LDPE, LLDPE and HDPE	Good against many acids, alkalis and chemicals	Waxy; translucent or opaque	~0.91–0.96 g/cm³	Containers, caps, closures and flexible parts	Low
Polypropylene (PP)	Modulus: ~1,100–1,900 MPa Flexible with excellent fatigue resistance	HDT: ~70–105°C at 0.45 MPa Vicat: ~127–155°C	Very good against acids, alkalis and chemical solutions	Translucent or opaque; matte to glossy	~0.89–0.91 g/cm³	Living hinges, packaging, caps and automotive trim	Low
HDPE	Tensile strength: ~24 MPa Modulus: ~850 MPa Tough and stiffer than LDPE	Melting point: ~130°C HDT: ~70°C at 0.45 MPa	Very good; strong stress-crack resistance	Opaque with a waxy or matte finish	~0.94–0.97 g/cm³	Chemical containers, valves and fluid-handling parts	Low
Acrylic (PMMA)	Modulus: ~1.5–3.2 GPa Rigid but less impact resistant than PC	Vicat: ~90–105°C	Moderate; limited against ketones, aromatic solvents and some alcohols	Clear, high gloss; light transmission often above 90%	~1.08–1.19 g/cm³	Lenses, displays, lighting and transparent covers	Mid-range to high

What are the applications of injection moulding?

Injection moulding can be used to manufacture both simple and highly technical products. Common applications include caps, containers, handles, clips, fasteners, casings, switches, plugs, brackets, gears and protective covers. Specialist manufacturers like Gemini Manufacturing can also use the process for lower-volume, higher-complexity parts that require consistent dimensions or repeatable performance.

Manufacturers can incorporate decorative textures, branding and identification markings into the mould tool, which transfers them to each finished component.

What industries can injection moulding be used for?

Injection moulding is used across industries including automotive, medical and healthcare, electronics, packaging, construction, aerospace, agriculture, telecommunications, consumer goods, food production and industrial machinery. Applications range from dashboards and medical-device housings to electrical connectors, packaging closures, irrigation fittings and precision machinery components.

Industry	Example applications
Automotive	Dashboards, clips, connectors, vents, lighting components, fluid reservoirs and interior trim
Medical and healthcare	Diagnostic equipment, syringe components, device housings, laboratory products and disposable medical items
Electronics	Plugs, sockets, switches, cable-management components, connectors and protective enclosures
Packaging	Caps, closures, tubs, containers, dispensers and tamper-evident components
Construction	Pipe fittings, electrical boxes, fixings, handles, protective covers and insulation components
Aerospace	Lightweight interior fittings, clips, housings, connectors and precision components
Agriculture	Irrigation fittings, equipment housings, trays, clips, handles and machinery covers
Telecommunications	Cable connectors, network housings, sockets, terminal components and equipment enclosures
Consumer goods	Toys, kitchenware, storage products, appliance housings and personal-care packaging
Food production	Food-grade containers, utensils, caps, dispensers and processing-equipment components
Industrial machinery	Gears, guards, control knobs, handles, seals, housings and other engineered components

When should I use injection moulding vs. other plastic moulding methods?

Injection moulding suits projects that require consistent dimensions, production-grade materials, complex parts and repeatable performance. Although commonly associated with medium- and high-volume production, it can also work well for complex, low-volume projects. Other moulding methods may suit hollow products, shallow parts, prototypes or specialist materials better.

Low-volume injection moulding is particularly useful for bridge production, product launches, specialist components, replacement parts and applications where 3D printing cannot provide the required strength, finish, accuracy or material performance. Using aluminium tooling, simplified mould designs or single-cavity tools can also reduce the initial investment for smaller production runs.

Blow moulding is generally more suitable for hollow products such as bottles, tanks and containers. Rotational moulding often suits large hollow components produced in lower volumes, including storage tanks and outdoor equipment.

Thermoforming can be cost-effective for large, relatively shallow components made from plastic sheet. Compression moulding may suit thermoset plastics, rubber components and fibre-reinforced materials.

The best method depends on tooling cost, production volume, part complexity, material requirements, tolerances, surface finish and expected product lifespan. For complex parts that require production-quality performance, low-volume injection moulding can offer a practical middle ground between prototyping and full-scale manufacture.

If you have a part you want to prototype or manufacture but aren’t sure of the best moulding method, contact us and we can let you know.

How to get started with injection moulding

To begin the injection moulding procedure, send us your drawings or 3D model, expected quantities, material requirements and intended application. We will review the information, contact you within 24 hours if we need any clarification and recommend the next step with clear pricing.

Injection moulding FAQs