The option when producing a prototype in metal – how to decide on process and materials

In most instances metal prototyping is going to cost more than producing the equivalent part in plastic, so your first question should be, do I really need it produced in metal? It might be worth considering some of the interesting RP applications/materials out there, such as metal coated stereolithography for instance – it would be more cost effective.

From a production perspective
Most parts are designed as either CNC, Castings or Die castings in production, although as the capability of DMLS continues to improve we are starting to see some items designed especially to take advantage of layer based manufacture.

Using CNC or DMLS to prototype a part designed to for final production in this way would be the obvious starting point. However, it's worth noting that your required quantities and timescales will have a dramatic effect on price.

Because casting permits the creation of certain geometries that can be very difficult to create any other way, there may be little choice but to cast the part. That said, it may be possible to use a different casting process (plaster, sand or investment for instance) for your prototype. If post machining is required and the geometry is machinable, then machining the entire item can offer cost and time benefits.

Die castings really fall into the same category as sand or investment castings, although typically these parts are smaller - making them more amenable to a potential DMLS solution.

From a process perspective
DMLS is best suited to highly complex, small parts. That said, it is capable of producing some stunning parts up to the limit of the bed size (usually 250mm x 250mm although larger machines are starting to become available). At these kinds of sizes the geometry of the part will drive the decision, as the premium over CNC will be significant. The advantages of DMLS can really be seen where multiple processes are required to tackle the geometry from another angle - for example, a die casting that would need EDM and CNC to produce.

When considering DMLS you must also look at what will happen with the supports, as these have to be removed post build. It is not as simple as it is with stereolithography! With DMLS the supports are usually wire cut from the part or CNC machined, hence the geometry of the parts lower surface can have a big impact on this operation.

CNC machining offers the greatest range of material options and the best potential accuracy, but is limited by cutter reach and geometry. That said, Plunkett Associates has recently worked on two projects where we pushed CNC to its limits with incredible results.

The first involved fabrication and welding to create a geometry that was otherwise impossible to machine. The part was finally dressed and polished such that the join completely vanished.

The second employed five axis machining to progressively chase out material until the desired form was achieved. Whilst this can prove costly in time, the result was a stunning piece of precision machining.

Sand casting or investment casting can be an excellent solution. However, post machining can further increase cost, raising the question as to whether the geometry permits a straight CNC approach.

How does quantity affect my choices?
Your required quantities will influence which process is used. When considering CNC there is the upfront programming time that needs to be amortised into the number of parts. Hence unit prices tend to fall initially, and then stabilise.

Matching supplier to quantity will also make a big difference. Applying a low volume production philosophy as opposed to a prototype philosophy will help to further reduce costs if quantities justify it.

With DMLS, quantity is all about optimisation of platform area and part geometry. Hence just because 100 parts are required, DMLS does not have to be ruled out as a solution - assuming they can all fit in a compact area and that minimal supports are required.

The higher the quantity, the more applicable a cast solution becomes, even if post machining is required. The CNC cost levels out at higher quantities as the material and machine time become fixed.
Casting also has some fixed upfront costs that need to be addressed, but once these are taken care of, the unit price can be attractive alongside a full CNC version.

Is time an issue?
Time is always a factor in assessing which route to take. The important point to make is that this is time from availability of the purchase order to delivery, and not from quote to delivery!

Available capacity will affect your time limits. For example, DMLS may be the quickest solution, but if the job is queued behind an 80-hour build, this can destroy any advantage. That's why meeting delivery dates is frequently more about locating a supplier who can initiate the work immediately, than it is about the process.

CNC machining has the biggest advantage here as it may be possible to drip feed parts, as opposed to alternative processes that require a time to manufacture tooling before part production can commence.

Material choice
When it comes to material, the level of flexibility is linked to the application. Most prototypes are in aluminium or stainless steel, however DMLS offers options such as Cobalt Chrome, Inconel and Titanium. If the purpose is to emulate the mechanical properties of the production part then this will require that the process be matched as well. For example, the grain flow in a casting will not be present in a part CNC machined from wrought billet. (Allowances may still need to be made for different casting techniques, i.e. plaster as opposed to investment.)

Constraining the material to a particular grade or standard is likely to have a significant impact on the process options and hence time to produce the prototypes.

And finally, what affect will the geometry of the part have on my choices?
Features such as thin walls, internal sharp edges, aspect ratio of the features, part size, surface finish, accuracy etc. all have a significant affect on the selection process. In other words, geometry is pretty important! That said, when prototyping, many of these attributes are open for discussion and in comparing quotations it is vital to compare like with like. Internal sharp edges are the classic scenario that can add time and cost when the only reason they are on the data is that no one has thought to add a radius.

What else?
The above are just some of the factors that need to be taken into consideration when deciding how to produce your metal prototype, but the following will also influence your decision:

• Can you apply different options to different shipments? For example, can the first 10 be produced with CNC and the balance cast and post machined?
• How critical is the material call out and how flexible can this be?
• Have you got the correct control drawings in place to control what you purchase?
• Do you have the flexibility to only call up tight tolerances where really required and allow some fluidity of form where it is not so important? (Note a 0.5 mm radius can make a big difference form a machining perspective!)

If youneed help producing prototype and production parts, please get in touch with Plunkett Associates.

Author
Tim Plunkett

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