The volume curve

If you are looking for production plastic parts, then injection moulding is a very cost effective solution for quantities down to and even below 100.

However, if your quantities are uncertain or the design not frozen, then CNC machining (computer numerical control) could offer a very economic alternative and comes with the added advantage that quality of finish and accuracy are far superior to alternative prototyping options.

Rapid Prototyping (RP) gets a lot of press for its capabilities as an Additive Manufacture (AM) process, and the 'freedom' it affords to design and manufacture anything. However, when it comes to production the reality is less about the geometry and more about the issues of poor surface quality and finish. If your part needs to be smooth, then currently the only way of achieving this is by vibro-finishing, but this is usually to the detriment of all sharp edges and not just those layers.

CNC offers a good alternative, but the set up and cost per part is usually deemed prohibitively high. The good news is, there is a way round this so that you can benefit from the wide range of materials, colours and the quality of finish that CNC delivers. How? By processing a 'sheet' of parts at a time. Depending on part size and geometry, over 100 parts can be cut from a single sheet - and very quickly.

If we then allow a level of fabrication, even a 'box' can be made by machining, folding and bonding. Using this process as opposed to creating a box from machining a solid block saves both time and money. It's almost like sheet metal fabrication but in plastic! Process time and waste is significantly reduced; structurally the 'box' can be very strong and aesthetics can be maintained up to and including some texture on the outside.

What's more, it offers great design flexibility – with a little lateral thinking even rounded corners can be achieved. Mounting features can then be assembled either as more machined parts, or if they're internal, created as RP parts that dowel into the cosmetic housing.

Design freedom may not be as great as with injection moulding, but on the plus side you don't have to wait weeks to get the first part or have an upfront cost to contend with. Components can be drip fed from early in the process and design changes can be easily incorporated without the need to alter any tooling.

The same principle of manufacturing a 'sheet' of materials can be applied to millturn parts when the technology is used with a bar feeder. Material up to 60mm can be readily used with millturn machines and once programmed the process will run unattended.

We still tend to associate turning with round parts, however modern technology allows for multiple tools such that turning, milling and drilling to be carried out in a single operation. Features can be created off axis, drilled and threaded such that the part is fully finished by the time it's parted off.

All of a sudden we have highly complex parts produced in a single operation with dramatically less labour involved…and yes this applies to metals as well.

With the finish and accuracy of CNC being brought to batch production of plastic parts and pushing quantities higher than RP would economically allow, CNC is a viable contender when quantities are too low or uncertain for moulding.

If you have a project you wish to discuss call Tim Plunkett on 01452 386608 or email tim@plunkettassociates.co.uk.

Author
Tim Plunkett, Plunkett Associates

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