CNC machining and injection moulding have both been around for a long time, but until relatively recently neither was a viable option for prototyping of plastic parts. An injection mould could crank out parts by the thousand, but setting up to produce that first part could take weeks or months and cost tens of thousands of dollars. Similarly, CNC machining could produce the same part over and over, but not until the toolpaths had been created, and those too took substantial time and manpower. The setup costs for either could be amortized if the number of parts was large enough, but prototyping is all about small numbers of parts produced quickly and inexpensively, to be examined briefly and then set aside. Until relatively recently the only way to make prototypes was working by hand from prints. It was a laborious and occasionally error-prone, but necessary, process.
The first practical technology for automated prototyping of plastic parts was 3D printing, an additive method that was invented in the 1980s and commercialized in the '90s. 3D printing was the child of Autodesk CAD software and the computer printer. Autodesk and other CAD packages, first in two dimensions and then in three, allowed designers to create, in the "mind" of a computer, a virtual model fully defining a solid object. Meanwhile, the printer could lay down a two-dimensional image that came from that same electronic brain. The replacement of ink with either a liquid that could be solidified or a fusible powder, and the stacking of "two dimensional" layers one upon another, was a logical, if technologically challenging, next step. Suddenly designers could create a 3D CAD model at the desktop and have a 3D part in hand in hours or days (see Figure 1). The part, at least in overall form, duplicated the CAD model, human error in translation from plan to part was eliminated, and designers no longer had to try to imagine how a paper design would look and feel in physical form. 3D printing quickly became the method of choice for plastic prototyping.
Figure 1: FDM or Fused Deposition Modelling part sample. FDM process builds parts from the bottom up through the use of a computer-controlled print head.
3D printing technologies have continued to grow in scope and capabilities, but they have remained limited in both the range of materials they can use and the structural strength of their products. Machining, on the other hand, being a subtractive process, has long been able to produce solid objects in any of hundreds of materials (see Figure 2). Realising machining's potential as a practical prototyping method, however, was challenging.
Figure 2: CNC Machined (Computer Numerically Controlled) part sample. A solid block of plastic or metal is clamped into a CNC mill and cut into a finished part.
Machining did have one advantage. Unlike 3D printing, which required new production technologies, CNC machining already had the equipment in place. The challenge was developing software for converting CAD models to toolpaths. 3D printing's process of slicing a solid into layers was relatively straightforward. Completely automated conversion of CAD models to machine-tool motions in three axes, along with automatic fixture generation, was more complex, and the goal was not reached until 2007. Now that the software exists, however, CNC machining of individual parts is a viable, affordable prototyping option. In some cases it can cost a bit more than 3D printing and there is no desktop option, but the ability to prototype in actual production-equivalent materials allows functional testing of a part's mechanical, electrical, chemical, thermal, and optical properties. This becomes increasingly important as the number of special purpose plastics continues to grow. Machining also can be used to produce prototypes in metals as well as resins.
Figure 3: Injection moulded part sample. Rapid injection moulding is done by injecting thermoplastic resins into a mould. What makes the process "rapid" is the technology used to produce the aluminium mould instead of traditional steel used in production moulds.
Like 3D printing, rapid CNC machining offers no significant economies of scale as production volume grows. This is where rapid injection moulding excels. Like machining, it uses software to quickly turn 3D CAD models into toolpaths for milling aluminium moulds. Once the mould has been made, the cost per moulded part drops quickly, making the process ideal for turning out dozens or hundreds of prototypes for functional or market testing, or thousands to take to market. As a prototyping method it is ideal because it can produce parts in virtually any of hundreds of injection mouldable resins (see Figure 3). And, in addition to functionality, it tests mouldability. Both 3D printing and machining can make parts with features that would be difficult or impossible to produce in a mould.
Clearly, each of these methods can serve a purpose in product development. For fast, early, individual prototypes, perhaps even made at your desktop, 3D printing is hard to beat. For low-volume functional prototypes in production-equivalent materials, rapid CNC machining is ideal. And for larger numbers of prototypes in actual production materials for mouldablity testing or for low volume production, rapid injection moulding is the perfect choice.