CNC milling vs. injection moulding – Which process makes sense when?

When plastic components are to be manufactured, the question often arises: CNC milling or injection moulding? Both manufacturing processes can produce precise plastic parts, but they differ significantly in terms of unit quantities, tooling costs, flexibility and lead time.

Particularly for prototypes, small series or components whose design can still be changed, CNC machining of plastic is an interesting alternative to injection moulding. For high and long-term stable unit quantities, on the other hand, injection moulding plays to its economic advantages.

Which solution is better suited therefore depends not only on the component, but above all on unit quantity, stage of development, material, geometry and the required tolerances.

CNC machining: flexible, fast and precise

CNC machining (Computerized Numerical Control) is a cutting process in which workpieces are formed from a raw material by milling, drilling or turning. Particularly for small unit quantities and complex geometries, CNC milling of plastic plays to its strengths.

In CNC plastic machining, the component is manufactured directly from a plastic semi-finished product. A separate mould as in injection moulding is not required. As a result, new components, variants and design changes can be implemented comparatively quickly.

Advantages of CNC machining

• High precision in the realisation of complex shapes
• Ideal for prototypes, one-offs and small series
• Short lead times, as no injection moulding tool has to be produced
• Large selection of engineering plastics and high-performance plastics
• Design changes can be implemented without a new mould

When is CNC milling worthwhile?

CNC milling is particularly interesting:

• for small to medium unit quantities,
• for prototypes and pre-series,
• when changes to the component are to be expected,
• for highly precise components with tight tolerances,
• when fast availability is important,
• when the investment in an injection moulding tool is to be avoided.

CNC plastic milling can therefore represent an economic injection moulding alternative, particularly in the development phase or for manageable series.

Further machining processes such as turning, laser cutting or waterjet cutting can be found in the plastic machining section.

Injection moulding process: efficient for large unit quantities

The injection moulding process is a primary shaping process in which molten plastic is introduced under pressure into a specially produced tool. After cooling, the finished component is removed from the mould.

The production of the tool initially causes additional effort. However, once the tool is available and the component design is fixed, large unit quantities can be produced with high repeat accuracy and comparatively low unit costs.

Advantages of the injection moulding process

• High economic efficiency for large unit quantities
• Very good repeat accuracy for series components
• Uniform surfaces possible
• Complex component geometries feasible
• Functions such as ribs, clips or fastening elements can be integrated into the design
• Low unit costs at correspondingly high production volumes

When is injection moulding worthwhile?

Injection moulding is particularly suitable:

• for high and recurring production volumes,
• when the component design is largely complete,
• when identical parts are required in consistent quality,
• for complex components with a series character,
• when low unit costs for larger quantities are the priority.

CNC milling or injection moulding? A direct comparison

Criterion	CNC machining	Injection moulding
Economic efficiency	Particularly for small to medium unit quantities	Particularly for high unit quantities
Tooling costs	No injection moulding tool required	Mould required
Lead time	Comparatively short	Toolmaking lengthens the start-up phase
Flexibility	Very high	Lower after tool approval
Design changes	Quickly implementable	Changes to the tool may be required
Unit costs	Comparatively higher as quantity increases	Comparatively low for large series
Material selection	Very large selection of machinable plastics	Large selection of injection-mouldable plastics
Prototypes	Very well suited	Usually only conditionally economic
Series production	Small and medium series possible	Particularly suitable for large series
Precision	Very high	High repeat accuracy in the series

When is CNC milling an alternative to injection moulding?

There is no blanket unit quantity at which a switch should be made between CNC and injection moulding. Economic efficiency depends among other things on component size, geometry, material, machining time, tooling effort and the total quantity required.

CNC milling can be a suitable injection moulding alternative when large series are not yet required or the component design has not yet been fully finalised.

Typical situations are:

• prototypes for functional or installation tests
• spare parts with low demand
• small and pre-series
• frequently changing component variants
• products with a design that is not yet complete
• technically demanding plastic parts in manageable unit quantities

For a successful series product, a switch to injection moulding can then become worthwhile. CNC machining and injection moulding therefore do not necessarily have to be competing processes: in many projects, CNC manufacturing complements the development and start-up phase before larger series are produced by injection moulding.

Which solution suits your project?

The choice between CNC machining and injection moulding depends above all on the planned unit quantity, the stage of development of the component, the technical requirements and the desired flexibility.

For prototypes, small series and precise plastic components, CNC milling offers a direct manufacturing route without an injection moulding tool. Designs can be adapted and variants implemented comparatively flexibly.

For long-term high unit quantities and a stable component design, on the other hand, injection moulding offers advantages through automated series production and low unit costs.

Martan Plastics supports you in selecting the suitable manufacturing process and in the machining of engineering plastic components according to your requirements.

FAQ

Which is cheaper: CNC machining or injection moulding?

That depends on the required unit quantity and the respective component. For small unit quantities, CNC milling can be more economic because there are no costs for an injection moulding tool. As the series size increases, injection moulding gains in economic efficiency through lower unit costs.

Is CNC milling an alternative to injection moulding?

Yes. Particularly for prototypes, small series, spare parts and components with frequent design changes, CNC milling can be a suitable alternative to injection moulding. For very large series, on the other hand, injection moulding often offers economic advantages.

Which plastics can be milled with CNC?

Numerous engineering plastics and high-performance plastics can be CNC-machined. Which material is suitable depends on the mechanical, thermal, chemical and design requirements of the component. An overview of suitable base materials can be found under plastic semi-finished products.

When is CNC milling of plastic worthwhile?

CNC milling of plastic is particularly worthwhile for individual parts, prototypes as well as small and medium series. The process is also interesting when short lead times or subsequent changes to the design are required.

How fast is CNC milling compared with injection moulding?

With CNC milling, the production of an injection moulding tool is eliminated. Therefore, particularly prototypes and small series can often be brought into production more quickly. With injection moulding, the tool must first be designed and produced.

What advantages does injection moulding offer over CNC milling?

Injection moulding offers advantages particularly for high unit quantities. After completion of the tool, identical plastic parts can be produced automatically and with high repeat accuracy. As a result, unit costs fall at corresponding production volumes.

Can metals also be injection moulded?

Special processes exist for metals, such as metal powder injection moulding or Metal Injection Moulding (MIM). This process differs from classic plastic injection moulding.