Now more than ever, technological advancements drive the product design process.Increasingly powerful CAD programs allow more complex product designs, which in turn drive the demand for more complex prototypes. At the same time,fast-moving competitive markets require frequent design changes, shorter lead times, and tighter budgets. In short,prototyping must be faster, better, and less expensive.

While traditional rapid prototyping is still widely used, a growing number of design engineers are turning to rapid injection molding for prototype development. Rapid injection molding offers superior prototype quality while dramatically reducing the time and cost normally associated with conventional injection molding. As acceptance grows, rapid injection molding is changing the way designers think about prototyping in industries like aerospace, appliances, automobiles, electronics, and medical devices.

ImageWhile rapid prototyping and rapid injection molding both start with a 3D CAD part model, the actual processes and end results are very different. Rapid prototyping — which includes technologies like stereolithography, selective laser sintering,fused deposition modeling, laminated object manufacturing, and three-dimensional printing — creates a prototype layer-by-layer to form the end product.

Rapid injection molding, on the other hand, uses the familiar process of injecting heated thermoplastics into a metal mold, where the material cools into the desired shape. Unlike rapid prototyping, rapid injection molding produces a fully functional, injection-molded part. The resulting quality difference is so significant that many design engineers who test form and fit using rapid prototyping will still check the functionality of their prototypes using rapid injection molding. Also,while conventional mold-making is very labor-intensive, rapid injection molding fully automates this step, typically reducing tooling cost and lead time by two-thirds. Rapid prototyping may be an acceptable choice for creating small numbers of prototypes in very short lead times — typically fewer than 10 parts in one to five days. But rapid injection molding can economically deliver 25 to 1,000 production-quality prototypes in three to 15 days. When production-quality surface finishes are not required, rapid prototyping may be an acceptable choice, but the "stairstep" surface it leaves on parts is a significant disadvantage, as it keeps parts from fully reproducing the intended design. Rapid injection molding, on the other hand, uses a CNC-machined metal mold to create the part shape, so it can replicate the intended shape much more accurately, just as with conventional injection molded parts. This can greatly increase the value of the prototype to the design engineer.

Furthermore, rapid prototyping is very limited with respect to materials that can be used and often creates prototypes too fragile for rigorous physical testing. Rapid injection molding can utilize a wide range of production-grade resins to manufacture fully functional parts, allowing the design engineer to consider both mechanical properties (e.g.strength, temperature resistance, etc.) and cost when ordering the prototypes.

Rapid injection molding can be utilized from the earliest stages of product development through bridge tooling and ongoing production. After rapid injection molding is used to create prototypes, it can support the manufacture of 1,000, or even 10,000 parts for pilot production or market testing at no additional tooling cost. If high production volumes do justify production steel tooling, rapid injection molding can be used as bridge tooling, to produce fully functional production parts until the steel tool is available. And when production volumes are moderate, rapid injection molding can be a complete, cost-effective production solution.

Rapid injection molding is a unique, highly automated method of producing injection molded parts from a 3D CAD part model. The core technology is proprietary software that automatically converts the part model into toolpaths for CNC milling machines. These, in turn, produce the mold components that, when assembled and mounted on an injection molding press, produce the desired part. Although a vast array of geometries can be produced, some limitations in part size and complexity exist due to the highly automated nature of the mold making process. For example, CNC milling results in rounded external part corners. Some high-aspect ratio features, e.g.thin ribs, may not be machinable. And the ability to produce "under- cut" part geometry features is limited.

The software provides a rapid injection molding compatibility review, often suggesting changes that will improve moldability or reduce tooling cost. This automatic analysis highlights undercuts, wall thicknesses that could cause fill or sink problems,and areas where draft is required. The system also provides color-coded indications of radii resulting from the mold-milling process and areas where a minimum thickness might be required. The software automatically generates the mold design,including core and cavity geometries, shutoff surface generation, gate-design layout, and ejector-pin placement,and outputs toolpaths to CNC machine cells to manufacture the required mold components.

Before rapid injection molding,design engineers faced a large gap between the capabilities of rapid prototyping and conventional injection molding. Rapid prototyping parts were fast and relatively cheap, but weren 't "real" Injection molded prototypes were real parts, but neither cheap nor fast. With rapid injection molding it is now possible to get real parts, real fast, and at a real savings.

Rapid injection molding enables companies to experiment and perform functional tests with real parts without worrying about prohibitive cost overruns or skyrocketing design costs. Unshackled from the restraints of long lead times and high production costs, design engineers can instead focus their valuable time, efforts, and expertise on creating innovative, high- quality products.

This article was written by Brad Cleveland,president and CEO of The Protomold Company, Maple Plain, MN. Contact The Protomold Company at Tel: 763-479-3680, or visit www.protomold.com .


NASA Tech Briefs Magazine

This article first appeared in the November, 2004 issue of NASA Tech Briefs Magazine.

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