By Gordon Styles, CTO and founder, Star Rapid

The use of 3D printing in plastic is now well established for making rapid prototypes and, increasingly, as a means of low-volume manufacturing for relatively simple parts. Compare this to metal 3D printing, which has been in development for a far shorter period of time but ultimately represents a much higher potential value for industry, reaching $10 billion in the next 15 years according to a report from McKinsey. If you’re a product developer considering using either of these additive manufacturing processes for your project, you should be aware that they differ not only in the behavior of the raw material but also in the fundamental physical performance of the hardware. Therefore, they are not interchangeable as manufacturing solutions and this knowledge needs to be applied early in the design stage for the best results.

Since 3D CAD design software programs such as AUTOCAD, SolidWorks, NX and CATIA don’t yet distinguish between metal or plastic as the substrate, it’s up to the engineer to exercise the proper judgment when inputting CAD data. Poor design practices at this stage will lead to failed parts when it comes time for production. This is especially important if you’re transferring a design to metal printing that was previously made in plastic. The first steps to converting a design are to clearly understand the role of heat, location of supports and the importance of post-finishing.

Heat

Generally speaking, heat is less of a critical issue for plastic parts than it is for metal. The most common form of plastic 3D printing is fused deposition modeling. Here the relatively low melt temperatures are safe. If a part is melted or warped by excessive heat, it’s inexpensive to try again until the process is dialed in. There are also few, if any, dire consequences to the processing equipment itself if the part fails.

The same is not true with metals. Sintering or melting powder with a high-powered laser can produce temperatures of 2500° F or more. Some of this heat may be absorbed by the build plate as well as the surrounding unused powder, but certainly not all. Without proper design engineering there will be warping and curling of the part, which can potentially damage the wiper of a powder-bed 3D printer. In addition, it’s a much more expensive proposition to remake a failed metal part.

For these reasons, product developers need to work closely with process technicians to understand the exact power characteristics of the machine being used and the ideal parameters to achieve the best results. One of the most important pieces of advice is the correct use and placement of supports.

Where’s the support?

Powder bed printing in plastic or metal requires the use of supports. Plastic parts tend to have more of them, for a few simple reasons.

One, plastic is less intrinsically rigid and so may need more additional stability to withstand the mechanical stress of printing. Two, plastic supports are relatively easy to remove, and the resulting marks cleaned up on the finished part. And three, the raw material is less expensive so there’s relatively little economic incentive to conserve on their use.

However, for metal printing there are different dynamics at work. Supports are necessary but it’s usually best practice to keep them to a minimum. To do this, part orientation is more critical to achieve an alignment that preserves the part’s geometry with the fewest supports.

Why is this? One of the reasons is again due to heat. The high temperatures formed at the focus of the laser induce shear forces in the surrounding metal. To a certain extent this shearing action is held in check by the presence of the metal support. When it’s later removed, this force is released like a spring and the part may deform badly. Smart product developers, working with their process control technicians, will anticipate this and will allow for such movement in the design of the part – something that is not a critical factor in a corresponding plastic piece. Also, powdered metal can be an expensive raw material so it’s costly to use it unnecessarily – another good reason to be conservative.

But the main difference is that removing metal supports is a difficult, time-consuming operation that can require a lot of post-machining. One does not simply snap off titanium struts by hand. This adds considerably to the project’s cost and also makes it harder to achieve a nice finished surface.

Finish with perfection

Both plastic and metal parts will require some form of finishing after being printed. Some of these steps may be common to both, like support removal, sanding or drilling holes. Other processes for metal are much more extensive and demanding of manufacturing resources, like grinding, CNC milling, stress relief and heat treatment.

One of the advantages of plastic printing is that post-processing can often be done by hand or with simple tools, which makes this an attractive operation for DIY product developers or small prototype shops. For metal printing, this post-processing work is not a trivial concern and it must be done professionally. That’s another reason why a plastic printed CAD design cannot simply be switched over to a metal printer to achieve a similar result.

Also, each type of printer creates an intrinsic surface finish on the part. Some processes show a more pronounced layering effect that may need to be sanded or polished off before a part can be painted, plated or anodized.

Putting it all together

New additive manufacturing technologies for both plastic and metal are being introduced to the marketplace all the time. Most of these are focused on trying to scale up from one-off prototyping to true volume production. HP’s Jet Fusion printer for plastic and Desktop Metal’s Production System for metal are two such platforms, although they still remain as niche products and have not been widely adopted. There are some product development difficulties that are still being ironed out in real-world testing, which is further evidence that, so far, the great advantage of 3D printing does not lie in speed or volume, where conventional manufacturing still dominates.

The true value of 3D printing is in the creation of complex geometries that combine light weight with high strength, as well as the ability to make finished parts without the need for expensive tooling. Redesigns can be done quickly, again saving on tooling costs and development times. But as we’ve seen, the product developer needs to clearly understand the differences between metal and plastic printing technologies and to design for the specific process and equipment that’s going to be used. Working in partnership with the machine operator is crucial to achieving reliable results.

Star Rapid
www.starrapid.com