Injection moulding process - structure, workflow, and benefits of modern plastic manufacturing

Thermoplastics are by far the most common materials in injection moulding. They are fed as granules, melted, injected into the tool, and solidify again during cooling. This process is reversible, so thermoplastics can generally be remelted, which supports recycling and the use of regranulate.

Thermosets behave differently in injection moulding because they crosslink in a chemical curing process. After curing, they can no longer be melted, only mechanically processed.

In practice, thermosets are used when thermoplastics reach thermal or dimensional limits, for example permanently hot or mechanically loaded electrical housings and components in engine compartments or other high-temperature areas. The trade-off is longer cycle time and stricter tool-temperature and process control.

Elastomers are rubber-like plastics that stretch under load and return to their original shape. In injection moulding they are processed similarly to thermosets: crosslinking takes place through heat in the hot tool, often by vulcanisation.

They are useful when parts must stay flexible, absorb movement, or isolate vibration, for example in drive technology, mechanical engineering, and sealing systems.

Many injection moulding materials are upgraded with additives and fillers so mechanical, thermal, electrical, and optical properties can be adjusted precisely.

Glass- or carbon-fibre-reinforced grades behave differently during filling, shrinkage, and surface formation than unfilled materials. Material choice should therefore always be considered together with part geometry and tool concept.

At assemblean, we support material selection already during the design phase based on temperature requirements, mechanical load, and desired surface quality. If the material you need is not listed, contact us; we can coordinate production with your desired material.

Tools with one cavity produce exactly one part per cycle. They are useful for prototypes, smaller series, complex geometries, or dimension-critical parts. Design effort is lower and changes to parting lines, gates, or venting can be implemented comparatively quickly.

Unit costs are higher because only one component is produced per shot.

Multi-cavity tools contain several identical cavities and produce several parts per cycle. This significantly increases output and reduces part cost at medium to high quantities. Runner and cooling design is more complex because all cavities must fill and cool evenly to avoid dimensional deviations and weld lines.

Family tools combine several different but related components in one mould, for example the upper and lower halves of a housing. All parts are moulded in one cycle, saving tool cost because each component does not need a separate tool.

The parts should have similar volumes and wall thicknesses so filling and shrinkage remain balanced.

Single-cavity, multi-cavity, and family tools describe how many and which parts a mould produces per cycle. Independently of that, each tool can use a cold runner or hot runner system, meaning the way the plastic melt reaches the cavities.

In a cold runner tool, the plastic in the runner solidifies together with the component. The runner is separated afterwards and, depending on the material, recycled or discarded.

Cold runner systems are robust, relatively simple, and suitable for nearly all plastics. They fit small to medium quantities, but create extra material use and more visible gate marks.

Hot runner tools keep material in the distribution channel molten through heaters, so only the part solidifies when the mould opens. This reduces material waste, often shortens cycle time, and allows precise control of pressure and temperature.

It is especially useful for expensive or technical plastics and high-volume production, but investment, maintenance, and design complexity are higher.

Uniform wall thickness helps avoid sink marks and warpage. Depending on material, a typical range is 1-4 mm.

At least 1° to 2° per side makes demoulding easier.

Ribs increase stiffness without increasing the main wall thickness.

Internal radii should be 0.5-1 x wall thickness to avoid stress peaks.

If undercuts cannot be avoided, they must be feasible with slides, lifters, or ejector concepts.

Pin, film, tunnel, or hot runner gates are selected according to part geometry.

Already during design, assemblean supports customers in applying these guidelines. If needed, a manufacturability review identifies and optimises risks such as undercuts or uneven wall thickness.

Even with careful design and precise tooling, typical defects can occur. The important point is to know their causes and prevent them in the process.

Flow lines occur when the melt front cools unevenly or overlaps in areas with different flow speeds. They appear as visible lines or matte zones. Uniform wall thickness, higher injection speed, optimised tool temperature, and adjusted gate position help distribute material more evenly.

Weld lines form where two flow fronts meet and do not fully bond. They are often caused by low temperature or low injection speed. Higher tool and melt temperature, adjusted pressure, and better gate design help the fronts merge more reliably.

Air traps and bubbles are usually caused by insufficient venting or excessive injection speed. Precise vent grooves, adjusted filling speed, careful drying of hygroscopic plastics, and a stable holding-pressure phase help prevent voids.

Short shots are incompletely filled parts. They occur when material pressure or temperature is not sufficient to fill the mould. Higher injection pressure, improved venting, optimised runners, or multi-point injection can stabilise flow.

Sink marks appear over ribs or thicker wall sections when the material shrinks unevenly during cooling. Uniform wall thickness, optimised holding pressure, and homogeneous tool temperature reduce the risk.

Flash is caused by excessive injection pressure, worn parting surfaces, or insufficient clamping force. Regular tool maintenance and correct machine sizing prevent material from escaping at the parting line.

Warpage results from uneven cooling or internal stresses. Symmetrical wall thickness, even temperature control, and sufficient cooling time reduce deformation after demoulding.

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1. Economical series production

After the tool has been built, injection moulding offers very low unit costs. The process is highly automated, delivers consistent quality, and is ideal for medium to large quantities.

2. Precision and repeatability

Every part follows the mould geometry. Tolerances in the range of a few tenths of a millimetre are common, making the process suitable for components that must fit precisely or be processed automatically.

3. Broad material range

From ABS and PP to engineering plastics such as PA and POM and high-performance materials such as PEEK or PEI, injection moulding covers many requirement profiles. Additives and fillers allow targeted changes in strength, surface hardness, or chemical resistance.

4. Design freedom and functional integration

Complex geometries, snap fits, ribs, and living hinges can be formed directly in the tool. Several functions can be combined in one part, reducing assembly steps.

5. High surface quality

Precise tool machining creates surfaces from high gloss to fine texture, often without post-processing.

6. Short cycle times and scalability

A complete production cycle often takes only seconds. Multi-cavity tools produce several parts at once, lowering unit cost and enabling high output.

1. High initial tooling cost

The largest upfront cost is toolmaking. Depending on complexity, the investment can reach several thousand euros, so the process usually pays off from medium quantities upward.

2. Longer lead time

Tool design and manufacturing take time. Design changes after tooling usually require tool modifications or a new tool.

3. Design requirements

Injection moulding needs manufacturing-friendly design: uniform wall thickness, draft angles, and as few undercuts as possible.

4. Tool maintenance and wear

Glass-fibre-reinforced plastics in particular increase tool wear. Regular cleaning and maintenance secure part quality over the tool lifetime.

5. Limited economics for small batches

Below roughly 500 to 1,000 parts, additive manufacturing or vacuum casting is often more economical. assemblean helps choose the right path from 3D printing and rapid tooling to full injection moulding series.

The economic advantage is scalability: the larger the quantity, the lower the price per part. From a few hundred parts onward, injection moulding can become cheaper than additive manufacturing or milling.

If a tool costs EUR 8,000 and 10,000 parts are produced, tooling cost per part is only EUR 0.80. Once material and machine costs are added, injection moulding is usually clearly ahead at this volume.

The key decision is the relationship between fixed costs, such as tooling and setup, and variable unit costs. Higher production volume amortises fixed costs. Alternative processes such as 3D printing or CNC machining remain better for small quantities or frequent design changes.

In early phases, short iteration cycles and low start-up cost matter more than minimum unit cost. Additive manufacturing is often the first choice. SLS and MJF deliver robust functional prototypes and small series without tooling, while SLA offers very fine surfaces and accuracy.

Vacuum casting is another proven small-series route using silicone moulds made from a master pattern.

Rapid tooling becomes relevant here, for example 3D-printed or softer metal mould inserts. It allows real injection moulding materials and more realistic cycle parameters at lower tool cost and limited tool life. Vacuum casting can also bridge the gap when surface appearance and material choice fit.

This is where injection moulding is strongest. Hot-runner multi-cavity tools reduce unit cost through short cycles and high output while dimensional accuracy remains stable. A sensible path can be rapid tooling, then pilot series, then multi-cavity tooling.

For thin-walled covers, blisters, or housing shells, thermoforming with 3D-printed moulds can be cost-effective in low quantities. For soft or medical parts, silicone parts can be cast economically with 3D-printed mould inserts.

These bridge methods close the gap between prototype and series without immediately investing in durable steel tools.

In the automotive industry, injection moulding is used for technical plastic components in interiors, such as ventilation elements, trim parts, and brackets. These components must look good and withstand high temperature and load requirements.

Glass-fibre-reinforced plastics make it possible to produce lightweight, dimensionally stable, vibration-resistant parts. Injection moulding also allows several functions to be integrated into one component, for example clips, sealing lips, or guide features.

The high repeatability ensures that components from different production batches fit together reliably in modern vehicle assembly.

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Injection moulding is the key technology for economical, precise, and repeatable plastic parts. Its modular process, from material selection and tool technology to quality assurance, offers flexibility for many applications.

assemblean combines this proven technology with a modern, digitally managed production structure. Customers benefit from:

This makes injection moulding efficient, plannable, traceable, and future-ready.