What to do when the quantities rise?

There are times when the quantities of prototypes required rise well beyond the conventional 1 – 10 type range. This may be due to the type of evaluation that is required, the number of people involved, the fact that the design is still uncertain or that they are really production items! 

Methods can vary but typically the greater the numbers, the closer the prototyping method gets to the production method. 

As quantities rise, the options become more limited and factors such as the geometry and size play a greater role. For example, a small complex part can be laser sintered in significant volumes (100's) at a competitive price. As complexity reduces and size increases, both polyurethane (RIM and vacuum casting) and injection moulding/tooling have a greater role to play. The greater the quantity, the more competitive the tooling options will be over 3D printing or machining.

So, assuming the part is small and an SLS finish is acceptable (i.e. we are more interested in function than aesthetics or the part can be rumbled to improve the finish), SLS is often the place to start.  Similarly high-resolution SLA can be used for small batches of very detailed components requiring specific attributes, such as stiffness, or DMLS used if the part is metallic.

Most often if parts are being made with additive manufacture in production, then they contain features that prevent alternative manufacturing options. For example lattice structures in the core to reduce weight, or they are the combination of several conventional parts fused together. In these cases there is little chance of employing anything apart from the intended production process.

CNC machining (computer numerical control) also has a role to play, offering high accuracy and a large range of plastic and metallic materials; small batches can be economically produced. Components can be drip fed from early in the process and design changes can be easily incorporated without the need to alter any tooling.

As quantities rise there are two PU options; the first is Vacuum Casting into silicone tools, the second is Reaction Injection Moulding (RIM) into composite (hard) tools. Whilst requiring tooling, they both have specific attributes that make them attractive to producing quantities of large or complex parts. 

As a general guide, RIM is more applicable to larger, heavier sectioned parts, whilst Vacuum Casting is more applicable to complex, thin walled items. 

To master the complexity, Vacuum Casting utilises a flexible silicone for the tool. This allows the tool to be deformed and the part released, thus avoiding compressible cores and side actions, which would be used in a 'hard' tool.

The limitations are that the silicone progressively hardens, leading to a tool 'life' of around 20 – 30 shots. So 200 parts could therefore require 8 – 10 tools (or cavities) depending on complexity. However these can run in parallel and the parts can be part shipped from about 2 weeks onwards.

Vacuum casting can emulate colour and texture in a variety of grades of polyurethane (PU) resin, both hard and flexible.

Similarly RIM requires tooling, but due to the low pressures employed, facilitates the production of larger parts, without the dramatically increasing price of a similar injection moulding.

We regularly endorse injection moulding as a cost effective solution for quantities down to and even below 100. Offering a range of tooling options; leadtimes can be competitive with vacuum casting and depending on quantities, price can be competitive as well. All with the significant upside of having prototypes in production intent materials.

Development tooling can be economically viable from as low as 50 parts depending on the part in question, and is applicable into the 1000's. Part cost is very low compared to the alternatives, although the tooling is more expensive. Unlike Vacuum Casting, parts cannot be shipped progressively and only start to become available after T1 (first trial). This remains the only way to produce production intent material parts where fillers and material grades are important.

If the requirement is for metal components, then the options tend to revolve around CNC machining, casting, diecasting, or metal injection moulding (MIM). Frequently the solution is to machine and post process, although casting plays a significant role as well. Die casting and MIM are often excluded due to tooling costs and leadtimes, not always correctly! And now we have metal 3D printing to add to the mix. These processes are applicable across the quantity range depending on material, surface finish, size and geometry. 

In summary, the route chosen needs to be decided upon based on size, complexity, aesthetics, material requirements and time, and the economics become a consequence!

With no vested interest in any one process, we can offer truly impartial guidance on the options, and support you all the way from initial prototypes through design approval/launch to subsequent tooling and production.