To beat competitors to market, manufacturers must quickly obtain prototype parts to evaluate form (shape, surface finish, texture, color), fit (dimensional accuracy, assembly with other parts), and function (material properties, temperature stability, etc.). The faster, more accurate, and more thorough the evaluation is, the better. Ideally the prototype parts should be made directly, without requiring tools or dies.

There are two primary methods for directly producing prototype parts: additive and subtractive. Additive methods build the part layer by layer, while subtractive methods cut the part out of a solid block of material. The subtractive process has many inherent benefits because of its ability to produce parts in standard, fully dense engineering materials.

Historically the time and cost associated with programming CNC mills for subtractive prototyping drove the invention and growth of the additive, flat-layer based rapid prototyping technologies that are easier to program. Recently, however, that picture has changed. Planning software combined with high-performance parallel processing computers now make it possible for CNC-machined prototypes to be manufactured as easily as those from rapid prototyping methods, minus some of the drawbacks.

In terms of turnaround speed, both subtractive and additive prototyping methods are direct and, therefore, offer the potential of being faster than methods such as cast urethane or injection molding for one to 10 parts, but it is important to understand that this comparison is highly dependent upon the geometry of the desired part.

In the simplest example, subtractive prototyping would be much quicker at producing a solid cube since very little would need to be machined away. By contrast, a hollow, open-top cube (a box) could be created more quickly by an additive process since only the material for the walls would need to be deposited.

Understanding how both methods operate helps to grasp the advantages and disadvantages of each one.

Additive

CNC milling of prototypes permits a broader range of materials to be used.

Some of the more commonly used additive prototyping methods include Stereolitho-graphy (SLA), Selective Laser Sintering (SLS), Fused Deposition Modeling (FDM), Three Dimensional Printing (3DP), and PolyJet (PJET). All these methods vary slightly in their execution, but share the similarity of building a three-dimensional part by stacking up a series of two-dimensional layers. Each of the individual layers is fabricated by the selective deposition of a material or the selective fusing or curing of such materials.

The materials used in additive processes are frequently some type of resin, though some processes can support ceramics or a limited number of metals. Some also allow for a range of available colors.

Due to their direct nature, additive processes tend to be fast and affordable. More importantly, additive methods support highly complex geometries — including geometries that cannot be manufactured in any other way, such as parts with complex internal voids or recesses. For example, an additive process could produce a sphere within a sphere if desired, though one would still have to determine how to manufacture the production version of such a part.

All of the additive methods are limited in the types of materials they support. This can be a disadvantage where functional testing of the parts is desired. In such cases, it is preferable to have the prototype material match the mechanical properties of the material that will be used for the final production of the part. This is difficult to do for a number of reasons, and additive parts generally have inferior strength to injection molded parts or CNC machined parts.

In addition to reduced mechanical properties, additive built prototypes typically possess a non-cosmetic surface finish due to the porosity and stair-stepping inherent in their layering process.

Subtractive

These sample cubes, made by CNC milling, were designed to illustrate the level of part complexity that can be achieved with this process.

As the name implies, subtractive prototyping works by removing material. It begins with a solid block of material clamped into a CNC milling machine, which then selectively cuts away material to produce the finished part. This method produces superior strength and surface finish compared to an additive process because the part has the complete, homogenous properties of the material, not a mixture or layers. This single block can deliver the very high tensile strength of a glass-filled engineering resin, the toughness of polycarbonate, or the heat resistance of PES or PPS plastics. Tight tolerances yield parts suitable for form, fit, and functional testing. Prototypes can be shipped in days, just like additive processes.

The main limitation of the subtractive prototyping process lies in the limited complexity of the parts it can support. At any point in time the computer software can handle parts with a finite complexity, although this is being improved continuously. Currently, Proto Labs’ First Cut service can support 3-axis milling from six sides of a part, which means that most types of external undercuts in the part geometry can be supported, while internal undercuts or those that are at an angle with respect to the surface of the part cannot. A good example of the complex-shaped parts that can be made with this capability is shown in the photo above.

From an overall part size point of view, the only limitation on the size of the part being prototyped is the size of the machine used. Subtractive CNC mills exist that can machine parts up to many feet in length and/or width, and the additive FDM process has produced parts up to about 2 ft. x 2 ft. x 3 ft.

Advances

Up until recently, subtractive prototyping was slowed by the task of converting a 3D CAD model into toolpaths for the CNC mills. If it could be done at all, it was time-consuming and expensive. By contrast, the additive processes allowed for the dramatic simplification of toolpath planning through the slicing of the CAD model into flat layers. So for a long time, the laborious manual toolpath planning for conventional CNC mills has held back the use of subtractive methods.

The key to changing that situation was to automate the toolpath planning process to make it faster and less expensive. Proto Labs has achieved that breakthrough through use of a large-scale compute cluster and proprietary software. As a result, subtractive prototyping is now comparable with additive prototyping in both speed and economy, but without the associated material limitations.

Subtractive prototyping makes parts that are extremely close to real injection-molded parts, both in regards to surface finish and tolerance, improving the accuracy of testing the part design for functionality. Advanced computer technology also makes it easier to obtain automated CNC machining quotes. After uploading the geometry from a 3D CAD file and specifying a material, it is possible to receive an automated quotation in as little as an hour. Subsequent to quote approval, parts can be shipped in as fast as one day.

Suitability

If a plastic part is destined for mid-volume to high-volume production, it is usually designed for injection molding. If a part design is capable of being injected molded, it is typically also capable of being machined, making it a suitable candidate for subtractive prototyping. On the other hand, if the geometry is too complex to be machined, then additive prototyping may be a better choice despite the limitations in materials.

Housings, switches, handles, cases, connectors, structural parts, moving parts, and actuating parts all are representative of the kind of parts one would wish to thoroughly test with functional prototypes before moving to production. Parts that will be subject to usability or customer beta testing are also good candidates for a process that provides good strength and finish.

The key to getting the most value out of prototype parts is in getting fast, accurate, and affordable parts that will closely resemble the look, feel, and function of the final production parts. The more closely the prototype matches the attributes of the final product, the fewer troubles will be encountered when transferring the design to production.

For more information, visit: www.firstcut.com.