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Injection moulding is the best choice when the design is fixed and you need multiple identical parts. CNC plastic machining is the best choice when you need very few parts or the design is not yet finalised. Below, we compare how the two methods affect part price, lead time, material choice and design.
The choice mainly depends on how many parts you need, how fixed the design is, and your requirements for surface finish and material.
| Injection moulding | Plastic machining (CNC milling) | |
|---|---|---|
| Start-up cost | Tool (one-off investment) | Set-up, fixturing and programming |
| Part price at higher volumes | Falls as volume increases | Roughly the same per part |
| First parts | Once the tool is made. Prototypes at Idé-Pro: 3–6 weeks, with Fast Track 1–3 weeks | Typically from a few days to a couple of weeks |
| Typical tolerances | ±0.1–0.3 mm on dimensions between 10 and 100 mm | ±0.1 mm, down to around ±0.05 mm on dimensionally stable materials |
| Materials | Over 2,000 plastic materials at Idé-Pro, including glass-fibre and talc-filled grades | Plastic types and grades available as sheet, rod or tube |
| Surface finish | Defined in the design and produced in the tool | Machining marks, often requiring polishing or sanding |
| Design | Ribs, undercuts and small wall thicknesses are moulded into the part | The cutting tool must be able to reach all surfaces |
| Material utilisation | Almost no waste; surplus can often be reused | Material removed during machining becomes chips |
Injection moulding forms the part in a tool, while CNC machining cuts the part from a plastic block. As a result, the two methods also use material differently.
In plastic injection moulding, pellets are melted and injected into the tool cavity under high pressure. The material cools in the cavity, solidifies and is ejected as a finished part. Surplus plastic from the moulding process, for example from the sprue and runners, can often be reground and reused.
CNC milling is a subtractive form of plastic machining in which a cutting tool removes material from a sheet, rod or block until the part has the required shape. The material milled away becomes chips, which need to be handled in production.
Injection moulding has a higher start-up cost, but the part price falls the more parts you need. Plastic machining has a lower start-up cost, but the part price stays roughly the same regardless of quantity.
Before a part can be injection moulded, the tool has to be made, and this is where the main start-up cost lies. For prototypes and low-volume production, Idé-Pro uses aluminium tools, which are quicker and cheaper to make than steel tools. After that, the part price is mainly determined by cycle time and material.
With plastic machining, the start-up cost consists of set-up and programming. This cost is spread over more parts as the quantity increases, but the machine time and material per part stay the same. As a result, the part price only drops slightly.
Exactly when injection moulding becomes cheaper depends on the part’s size, design, material and volume, so the break-even point varies from part to part. Idé-Pro can help you assess which method is most cost-effective for your part.
X Vent switched two parts for a degassing system, a wing and a set of spokes, from CNC milling to injection moulding. Before the switch, the milling machines had to run for 1,000 hours to produce 4,000 wings, and the spokes were milled from a 20 mm solid plate, with 85% of it ending up as waste.
With injection moulding, X Vent achieved a lower part price and a shorter production time. The parts no longer need post processing, and logo and text are moulded in directly. Read the full X Vent case.
CNC plastic machining is usually quicker than injection moulding, as set-up and programming can start straight away based on the 3D file. The lead time for machined plastic parts typically ranges from a few days to a couple of weeks, depending on the part’s complexity, material and quantity. With injection moulding, the tool has to be made first, and the part designed for the process.
At Idé-Pro, injection-moulded prototypes are normally delivered in 3–6 weeks, or 10–12 weeks for large and complex parts. With Fast Track, the lead time can be reduced to 1–3 weeks. Our tool shop and production are under one roof in Skive, and we manufacture up to 20 tools per week.
A wide range of thermoplastics can be injection moulded. CNC milling uses materials available as semi-finished products in the form of sheet, rod or tube.
Plastic pellets for injection moulding come in many grades: with added glass fibre, carbon fibre or talc, in a range of colours, and with additives such as UV stabilisers and flame retardants. Idé-Pro works with over 2,000 plastic materials for injection moulding, from commodity plastics such as PP and PE to engineering plastics such as PA, PC, ABS, PPS, PSU and PEEK.
The range available for plastic machining is smaller. Plastics such as POM, PA, PE and PEEK are widely stocked as semi-finished products, and some glass-fibre-reinforced grades, such as PA66 GF30 and PEEK GF30, are also available.
The manufacturing process determines the structure of the plastic, including fibre orientation, crystallinity and internal stresses. As a result, a machined part has different properties from an injection-moulded part, even when the material is the same.
Sheet and rod cool slowly during manufacture, which can give them a different structure and higher internal stresses than a moulded part. In an injection-moulded part, the fibres align with the flow direction in the tool. Test results from a machined part may therefore differ from those from the injection-moulded part. This is why an injection-moulded prototype provides the most representative basis for testing.
For injection moulding, the part must be designed so that it can be ejected from the tool. In return, small wall thicknesses, ribs and undercuts can all be moulded into the part.
For CNC milling, the cutting tool must be able to reach every surface. Internal corners therefore always have a radius, and deep, narrow slots are difficult to machine. Complex parts are often split into several pieces and assembled afterwards. Wall thickness, however, can vary freely.
In injection moulding, the part takes on the surface finish of the tool cavity. It comes out with its final surface, and logo and text can be moulded in. In CNC machining, the cutting tool leaves visible marks, so a completely smooth or textured surface often requires manual post processing such as polishing or sanding, which increases the part price.
The two methods also differ in the tolerances they can achieve. An injection-moulded part is typically within ±0.1–0.3 mm on dimensions between 10 and 100 mm, and because all parts are moulded in the same tool cavity, repeatability is high. For plastic machining, ±0.1 mm is common, and on dimensionally stable materials such as POM and PEEK, individual dimensions can be held to around ±0.05 mm. However, temperature, moisture and stresses released during machining can cause the part’s dimensions to change.
For parts that are going to be injection moulded, we can help you optimise the design for moulding, whether you are starting from a drawing or already have machined prototypes.
If you are unsure which method suits your part best, contact Idé-Pro. We are happy to go through it with you.
Yes. Idé-Pro injection moulds functional prototypes in aluminium tools, with a normal lead time of 3–6 weeks, or 1–3 weeks with Fast Track. The prototypes are moulded using the same process and material as the final product, so you get realistic test results.
Yes, if the part has vertical walls, sharp external edges or varying wall thickness. For injection moulding, the part typically needs draft angles, a more uniform wall thickness and radii so that it can be ejected from the tool. Idé-Pro can help adapt the design.
Yes. Logos and text can be built into the tool and moulded in, so the part is complete as soon as it is moulded.
Yes. Machined prototypes can be used in the early design stages, and once the design is fixed, production can move to injection moulding. An injection-moulded part can also be CNC machined afterwards if individual features need tighter tolerances than moulding can achieve.
At Idé-Pro, we injection mould plastic parts from 0.01 g up to 41.5 kg on more than 50 machines with a clamping force from 6 to 4,000 tonnes.