Optically clear parts are notoriously difficult to prototype. CNC machining, 3D printing, vacuum casting and injection molding are compatible with this prototyping application, but each technique has drawbacks in addition to benefits. They are outlined here.
CNC machining from billet is one of the quickest options for prototyping a clear plastic part. Depending on your part and the geometry, CNC machining can produce very good results.
The two main options in terms of materials are poly(methyl methacrylate) (PMMA) and polycarbonate (PC). Even if PC is the production material, it might not be the best choice for the prototype. PMMA, or acrylic, is much easier to machine and polishes to a glass-like, transparent finish. PC, on the other hand, is much more difficult to polish; achieving a high optical finish may require a vapor polish after several rounds of hand work with finer and finer grit sandpaper. Even after all this, the machined PC may not have an even finish.
If you are looking for transparent parts for a prototype or model for aesthetic or display purposes, PMMA is a better option. You may still want to use PC if you need to test the parts functionally, but bear in mind the limitations of the surface finish.
The CNC machining process may require spliting the part into multiple sections to machine and then joining them together with glue. Although this can be done to a very high level by professional model makers, there is still going to be a visible join and an inherent weakness.
While additive manufacturing has evolved significantly, it is still fairly limited when it comes to printing transparent parts. The best results can be achieved via stereolithography (SLA).
SLA machines and resins are increasingly diverse, with a number of different options available for clear parts. Quite extensive manual processing is required after printing to get a clear finish, but good results can be achieved, especially for appearance prototypes and display models. The finish and material characteristics won’t match those of a machined part, but the capability of 3D printing to produce difficult and complex geometries might give it the edge, depending on the project. Another drawback is the cost, which will be quite high compared with a non-transparent 3D printed part.
Vacuum casting has a number of unique advantages over the other processes on this list. Silicone molds are used to cast polyurethane resins to form the final parts. A number of transparent PU resins are available, designed to replicate other production plastics such as PC or PMMA.
Vacuum casting clear parts will be cheaper than some of the other methods, if you are looking at a low-volume run rather than just a one off. Achieving good optical transparency does not require as much manual labour as CNC machining or additive manufacturing do.
Other advantages include the capability to cast large or complex parts in a single piece and adding a tint or color to the clear part. The disadvantage is that this can be quite costly
Here the main advantages are the lower cost of parts in low-volume production runs, the range of material choices available, and the quality and tolerances achievable. Rapid injection molding will give the best results, but will cost the most.
Rapid injection molding for prototypes allows use of the intended production material and the eventual production technology, so the prototypes will be as close to the real thing as possible. Tools to manufacture parts in low to mid volumes can be used while the production tools are being built or for initial pre-production market testing.
The only real disadvantages of injection molding are the initial tooling costs and the increased time needed to make them. Clear parts need to be molded from hard steel tools, polished to a very high surface finish. The cost and time required are commensurate.
Transparent parts are essential for many applications or they may just need to look great. To achieve the best results, it is essential to consider the appropriate technology.
Image courtesy Fernando/Adobe Stock.
About the author
James Murphy is the co-founder and Vice General Manager of HLH Prototypes Co. Ltd., one of China’s leading providers of rapid prototyping and low-volume injection molding services. Originally from the UK, Murphy has lived and worked in China for more than 12 years. He is passionate about hardware and enabling companies large and small to bring world-changing innovations to market faster.