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Prototype Designs: Comparing Additive and Accelerated Conventional Methods

Prototype Designs: Comparing Additive and Accelerated Conventional Methods

Twenty years ago, designers identified a problem shared by the traditional production methodologies used in the production of plastic parts. Injection molding and CNC machining both entailed lengthy and expensive setup. These non-recurring costs - converting 3-D CAD models to toolpaths and, in the case of injection molding, machining of steel molds - could be easily justified for high-volume production, but were prohibitive for short-run production or prototyping.

To fix the problem of costly setup associated with injection molding and CNC machining, some engineers set out to invent new ways of producing parts. The result was a variety of additive processes for creating low volumes of parts. Unlike traditional injection molding and machining, these additive processes - stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling (FDM), three dimensional printing (3DP), and polyjet (PJET) - used sophisticated software to "slice" a 3-D CAD model into thin virtual layers. The systems then physically replicate those slices, adding successive layers of liquid or powdered material and solidifying that material to build up a finished part.

Slicing algorithms were relatively easy to write, which led to the development of a number of competing technologies. The dream these technologies shared was to advance their methods to a point at which they could compete with, and possibly even replace, CNC machining and injection molding. In that scenario, additive processes would provide a massively scalable production capability in which fully functional parts could be produced singly on a desktop machine or in the thousands by factories equipped with large numbers of additive machines running in parallel.

Additive Production

The mere introduction of additive production was a huge breakthrough, and over the following decades, associated technologies have continued to develop. Desktop part production is now a reality and, according to the Wohlers Report, related equipment sales are skyrocketing. Designers can create a model using CAD software and hold the actual part a short time later. It is predicted that this sort of equipment will eventually be as ubiquitous as laser printers are today, producing solid realizations of computer aided designs, literally, in minutes.

Though parts produced by additive methods are shaped like real parts and are produced by standard manufacturing methods, they are not equivalent. The main reason for this is that, while thousands of different resins are available for traditional manufacturing, only a handful of them can be used in additive processes. In addition, even if the resin were identical to that used in the manufacturing process, the bonded layers produced by additive methods cannot attain the material properties of an injection molded part or the solid stock used for CNC machining. The resulting parts are, therefore, often unsuitable for functional testing of prototypes. Another issue is that the layering process leaves a "stepped" surface that cannot match the finish of an injection molded or CNC machined part. As such, their benefits remain limited to narrow niches in the product development marketplace.

Because of these issues associated with additive methods, injection molding and CNC machining remain the leaders in high-quality part production. The good news is that, while the attempt to replace machining and molding with additive processes has encountered obstacles, engineers have devised a different set of technologies to address the problem of high setup costs.

Rapid Injection Molding and Automated CNC Machining

The non-recurring costs of injection molding and CNC machining are primarily due to the complex process of converting 3-D CAD models to toolpaths for mold making or direct machining of parts. Using traditional methods, that conversion process can take weeks and thousands of dollars in manpower. For years, the complexity of the conversion process kept anyone from writing software to effectively automate it. In 1998, however, the first example of this software, consisting of over 1 million lines of code, was created and used to convert customer models directly to toolpaths for mold milling equipment. By making molds of aluminum instead of steel, the time needed to produce molds was similarly shortened.

Rapid injection molding - the term used to refer to this software-driven method of toolpath development - cut production time for molds and parts from months to as little as a day, while simultaneously slashing costs. Development of that software continued to the point where it is now used to produce toolpaths for direct CNC machining of parts in a wide variety of plastic resins and, more recently, metal. As with molding, automating the machining setup process eliminated the high cost and long delays of traditional methods.

By drastically reducing setup time and costs, rapid injection molding and automated CNC machining have made molding and machining cost-effective and practical for both short-run production and prototyping. Both processes can use a vast array of resins, produce parts with the material properties of parts in full-scale manufacturing, and offer a variety of surface finishes. Because molded and machined prototypes share the characteristics of high-volume manufactured parts, they can be used for the sort of functional testing that cannot be done with parts produced by additive methods. They perform like real parts because they are real parts.

Weighing the Options

This, of course, does not detract from the virtues of the additive methods. While rapid injection molding and automated machining can now produce parts from 3-D CAD models in a day, additive desktop equipment can produce facsimiles in minutes. That is a highly valuable capability in the early phases of product development. In fact, it can be a critical step in the development process.

Today, an increasing number of developers are using desktop additive part production as an intermediate step in their design process. They begin by evaluating 3-D CAD models and then creating solid models using additive processes on desktop equipment to get a "feel" for the product. When ready for functional testing, they turn to automated CNC machining for parts cut from solid blocks or their specified resin, typically in quantities of one to 10. Then, for the larger quantities needed for expanded functional testing, market trials or bridge tooling, they use rapid injection molding. At that point traditional injection molding is typically the choice for high production quantities.

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TAGS: Materials
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