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How should I prototype? Machined parts, print 3D parts, or fabricate a prototype mold

Introduction

Once a design engineer completes the first draft of their product design, the next step is to create a plan for how it will be fabricated, tested, and developed. There are three basic methods of prototype fabrication: 1) CNC machining, 2) 3D printing, and 3) Prototype molding. The method to be used depends on the stage of development and the application. There are critical tradeoffs and limitations to consider when deciding which prototyping method to use.

CNC Machining

Before 3D printing became widely available, CNC machining was the primary means of prototype fabrication early in development. CNC machining has the drawback that it is slow and expensive in comparison to 3D printing. While 3D printing strives to provide a wide range of materials that replicate the mechanical properties of various injection molded plastics, the 3D printed materials are only an approximation. CNC machining has the advantage that it allows the engineer to test the actual material to be used in manufacturing without having to compromise by using an approximation. In some cases, the 3D printed material may not be sufficient for testing.  For example, the mechanical wear of gear due to friction is going to be highly dependent on the material used to fabricate the gear. Although there are durable 3D printed materials available it is typically beneficial to test to using CNC machined gear prototypes so test results are not compromised by the use of 3D printed materials. In such a situation, the additional cost and lead time for CNC machined prototypes are typically justified.  The application determines whether the additional cost and lead time of CNC prototypes are necessary. The design engineer must consider the quality of the data to be obtained during testing using the different prototyping methods available. In the gear wear example discussed earlier, if the application requires testing the gear at room temperature, at low speed, and very low stress well within mechanical material limits then it can be justified that the use of 3D printed prototypes is sufficient. It’s only when the mechanical requirements of the application reach a level where test results become questionable using 3D printed prototypes does it become necessary to use CNC machined prototypes.

3D Printing

3D printing is a form of additive manufacturing where parts are fabricated using layers of material. Several different fabrication processes for 3D printing have emerged. 3D printing is a process often used in early product development since prototype parts can be fabricated quickly at a much lower cost than CNC machining. There is such a wide variety of 3D printed materials available that 3D printing has become the primary prototyping method for the early stages of product development. Most of the time, the engineer will simply need to decide which 3D printed material to use and consider using CNC machining only when the application demands it. Let’s review the most common 3D printing processes and the applications where each process is best utilized. Stereolithography (SLA) is a method where a thin layer of UV resin is layered and cured using a laser. The laser beam spot diameter determines the tolerance of SLA features. SLA is one of the oldest 3D printing methods but still the most accurate, allowing features as small as 0.002” (0.05 mm). SLA is available in a variety of materials that are intended to replicate common plastics such as ABS, polypropylene, and polycarbonate. However, it’s important to remember that SLA materials are not injection molded plastics, but UV activated resins that are designed to replicate injection molded plastic material properties.  SLA is best utilized in applications where feature accuracy is most important. Polyjet is another form of 3D printing that also uses UV-activated resins. In polyjet printers, small droplets of the UV activated resin create each layer and then a UV lamp is passed over to harden each material layer. Polyjet offers materials that replicate common plastics, like SLA, but also offers elastomeric material options. Polyjet can fabricate features that are slightly larger than SLA at 0.004” (0.1 mm). Polyjet is best utilized in elastomeric or overmolded applications but can also be used for parts when the high accuracy of SLA is not needed. Digital light processing (DLP) is another form of 3D printing that also uses UV activated resins but utilizes a UV projector to cure each layer. The benefit of DLP is build speed. The projector allows the entire layer to be cured simultaneously. However, the projector has a fixed number of pixels which means that the larger the part that the DLP machine can build, the larger each pixel needs to be. DLP machines are therefore characterized by their feature accuracy and maximum part size. The smaller the feature accuracy, the smaller the overall maximum part size that the machine can fabricate. DLP is best utilized for building prototypes with small, very accurate features, but the maximum part size is limited as well. Similar to polyjet, DLP can be used for larger parts where accuracy is less important. Fused Deposition Modeling (FDM) is a unique form of 3D printing which dispenses melted plastic in layers. Common material options include ABS, polylactic acid (PLA), polycarbonate and nylon. The benefit of FDM is that it allows the engineer to utilize plastics in the prototype rather than UV activated resins that only replicate plastic performance. The drawback of FDM is the much larger tolerances and visible build lines that limit their ability to be used in functional prototypes. FDM is best utilized when testing large parts or when feature accuracy is not required. Selective laser sintering (SLS) is another unique form of 3D printing where the material is in a powder form. A laser is used to fuse each layer. SLS is unique in that metals can be prototyped in addition to plastics. Selective laser sintering is used less often than other 3D printing methods due to its material limitations, higher cost, and larger tolerances.

Prototype Molding

Prototype molding is used later in development after most design issues have been resolved using CNC machined parts, 3D printed parts, or both. The substantial advantage of prototype molding is the use of injection moldable materials that allow for prototypes that closely replicate, and ideally replicate, production molding.  Typically prototype molds have a larger tolerance than production molds since they are fabricated for a fraction of the cost and lead time. However, the engineer wouldn’t want to skip 3D printed or CNC machined prototype parts and start with prototype molds. The reason is that prototype molds are relatively slow and expensive to iterate. It’s better to use 3D printed parts or CNC machined parts for early development when it’s necessary to make iterations quickly. 

When to switch to prototype molding?

When does it make sense to switch from 3D printed or CNC machined prototypes to prototype molding? I use a guideline that at least 80% of development needs to be completed using 3D printed or CNC machined prototypes before switching to prototype molds. In my experience, this guideline ensures that development time to market is optimized using lower cost and rapidly produced 3D printed parts or CNC machined prototypes.  At that point there would be diminishing returns from the continued use of 3D printed parts or CNC machined prototypes. The switch to prototype molding allows for more thorough and reliable testing using actual materials. Risk is reduced as results are more representative of the final product. By that point in development there should be relatively few design changes needed so the higher cost of prototype mold iterations is less likely to be an issue. However, there is a rare situation where it is practical to jump directly to prototype molding and skip 3D printing and CNC machining. An engineer should skip directly to prototype molding when the application requires actual injection molded materials to be used where performance cannot be accurately tested using 3D printed or CNC machined parts. For example, consider an elastomeric part including a seal that must block oxygen transmission through the material. No 3D printed or CNC machined material can block oxygen transmission, so jumping to a prototype mold would be justified since the only means to test the part accurately would be with prototype molded parts.

Summary

For early development, CNC machining and 3D printing are used most often because they can be iterated quickly and inexpensively. The default choice is 3D printing unless application requirements exceed the mechanical properties of the 3D printed materials and CNC machining using actual materials is needed instead. Development continues using 3D printing and CNC machining until approximately 80% of the development is completed, and then prototype molding is used to complete development using actual materials and parts that more closely replicate production.