Metal Prototyping: How to Choose a Material and Technology
How to leverage metal materials for your prototyping needs
What is Metal Prototyping?
As with any part you design, the final product is the end-result of numerous steps, or iterations, that get you to a functional part for your application. Metal prototyping is the process of taking your CAD model and manufacturing a physical metal version that you can assess and adjust, as needed.
By having an actual part in your hand, you can determine if there are any design flaws. Beyond that, you can test the part for functionality, durability, and performance, offering real data you can evaluate.
Prototyping in plastic is always available, but if your final product will use a metal part for a particular setting, you have multiple choices on how to run with metals right from the get-go.
Choosing the Right Metal
Metals, by and large, are tough, stiff, and dimensionally stable. The decision on which material you choose comes down to the requirements of your part. If you’re looking for a biocompatible material, cobalt chrome and titanium are excellent choices. If electrical conductivity is a necessity, look to aluminum or copper. Inconel (3D printing only) and titanium are known for their strength and durability, while brass and copper are aesthetic winners.
Here are some very basic characteristics of the metals we offer via 3D printing, CNC machining, and sheet metal fabrication. Note that these are overarching descriptions of these materials, and that some of these are available in multiple variants of their base metal.
Metals and Their Primary Properties
| Manufacturing Technique | Characteristics | ||||||
|---|---|---|---|---|---|---|---|
| 3D Printing | CNC | Sheet Metal | Biocompatibility | Corrosion Resistance | Electrical Conductivity | Heat Resistance | |
| Aluminum | • | • | • | • | • | • | |
| Brass | • | • | • | ||||
| Cobalt Chrome | • | • | • | • | |||
| Copper | • | • | • | • | |||
| Inconel | • | • | • | ||||
| Pre-Plated Steel | • | • | |||||
| Stainless Steel | • | • | • | • | |||
| Steel Alloy | • | • | • | ||||
| Titanium | • | • | • | • | • | ||
Strength-to-weight Ratio
Strength-to-weight ratio tends to be one of the biggest starting points. Not surprisingly, this is more critical in aerospace but certainly affects just about every industry. Heavier/denser metals typically come at a higher cost, going beyond just the mechanical properties. That’s probably the biggest reason we see so much demand for aluminum—specifically 6061—in CNC machining and sheet metal fabrication. It’s a workhorse metal, doesn’t weigh as much as a steel alternative, and tends to cost less.
Overall weight can really matter if you’re evaluating the cost to put a product in the air or into space, considering how physically demanding a user experience might be with heavier parts. Overall wear and strain on a mechanical assembly over years of operation is also relevant. All that said, if you’re looking for absolute strength and durability—as is the case with production-quantity injection molding tools—making the jump from lightweight aluminum to steel makes good sense.
Corrosion Resistance
Corrosion resistance is a factor, too, but how much is dependent on your application. A marine environment would be far more demanding compared to a cool, dry workshop. Still, there is a high likelihood that parts will be exposed to water or chemicals at some point. It could be sea water, rocket fuel, or simple cleaning solutions, so corrosion resistance should always be considered. Note that metals can be plated or coated either before manufacturing or as a post-processing option, which can enhance corrosion resistance.
Biocompatibility
Biocompatibility is often more of a forward-looking concern. For example, a medical company might be considering future production of their part and looking at materials with needed biocompatible properties. Sometimes during the prototyping stage, parts are manufactured that don’t carry requirements as stringent as ISO 13485 or extensive cleaning options. That said, getting that feedback via prototypes helps you gain confidence moving into production down the road.
Conductivity
Conductivity is a concern in two forms: thermal and electrical. One or both might be an issue, depending on your application. Thermal conductivity—the ability to wick away heat to cool a part—is often a concern for general industrial applications, aerospace scenarios, and a wide variety of more niche projects, too. In some cases, certain elements of an assembly or product need to be protected from heat, like sensitive electronics or instruments. Electrical conductivity is similar. Sometimes you want electricity to flow through a part, and other times you don’t. You can enhance electrical conductivity by choosing options such as nickel plating, which provides a double advantage in that it increases conductivity while enhancing corrosion resistance.
Vibration Testing
Vibration testing plays a role in very specific applications. For example, aerospace parts are subject to intense shaking that can destroy a part. In these situations, vibration testing during prototyping can be critical. Beyond aerospace, vibration properties can be important even for something like industrial machines that run every day. While you have the NASAs and SpaceXs of the world thinking about that, it’s also a concern for more earthbound companies, such as Caterpillar and John Deere.
Machinability
Machinability of a material can play a role—not surprisingly—with CNC-machined parts. Smart designers and engineers think about this criterion, along with ease of fabrication. That’s part of the reason so many parts are made with aluminum 6061 and 300 series steels (available via CNC machining and sheet metal fabrication).
Metal Prototyping Techniques
How you build your part out before and during prototyping is entirely dependent on the manufacturing method you choose, which means you might have to think differently for each part. Here are some tips that cover CNC machining, 3D printing, and sheet metal fabrication.
CNC Machining
The biggest advantage for prototyping metal parts through CNC machining is being able to exactly replicate an end-use production part without the cost and investment. By comparison, when you’re looking at plastic parts, engineers and designers commonly use machining or 3D printing during their prototype phase with injection molding being the end production solution.
While a material’s mechanical properties can be similar across different fabrication methods, there are still inherent nuances that vary with each manufacturing option. That means that the performance of a part can vary depending on whether it is a machined prototype or a molded production counterpart.
When you’re talking about metals, prototyping and production methods are much more closely aligned. Typically, a machined prototype part will also be machined when that part is ready for production. Even in cases when casting will be the end production solution, you’re still seeing less part performance variation from machined prototype to cast production part as compared to a 3D-printed plastic prototype moving to an injection-molded production part.
Engineers and designers will have a higher level of confidence in their analysis as they move through design validation and performance testing when prototypes accurately represent what the production environment will yield.
You can test a handful of machined aluminum parts with an anodized finish, for example, and know that those parts will function properly even when you bump up to production quantities. With digital manufacturing techniques, you’re able to expedite iteration testing without sacrificing development timelines. We see customers send revision after revision through our digital quoting platform knowing that they can design a part, have that part in hand within a matter of days, conduct testing as needed, redesign and run it through the same analysis process all within the course of a week or two. That means getting insights faster, a shorter path to the right design, and ultimately, a quicker final design state.
CNC machining’s biggest limitation comes in the form of the process itself. Machining is a subtractive manufacturing process with physical limitations to what even the most sophisticated mills and lathes can achieve. An angular, blocky metal design can be perfect for machining from prototype to production, while a more organic, curvy design might be better suited for metal 3D printing using direct metal laser sintering (DMLS).
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The applications for machined metal parts are as expansive as they are creative. Companies use machining for surgical tools, cutting-edge imaging solutions, prosthetics, and countless other developing technology needs. Aerospace companies request metal prototypes in the development of airborne drones, microsatellites, planetary rovers, complex rocket engines, and a galaxy of other innovations that continue to push engineering boundaries.
While those industries stand out in their roles as modern marvel machines, they’re only the tip of the iceberg. CNC machining supports advancements in robotics, industrial machinery, quantum computing, and a wide variety of other creative endeavors that will shape our future, including design and testing facilitation. And of course, where would injection molding be without machined tooling?
As for materials, some frequently chosen metals include varieties of aluminum and steel, titanium, and softer metals like copper. Our network expands our materials list substantially to include options such as: MIC 6 aluminum, A2 tool steel, Inconel, and more.
3D Printing
The key advantage to using additive manufacturing—specifically the 3D printing process called direct metal laser sintering (DMLS)—for prototyping is that it doesn’t require tooling. Metals are commonly used to prototype parts that will eventually be cast or metal injection-molded. It can also be used for complex prototypes of parts that will be machined, specifically in harder materials such as titanium. In addition to saving the cost of tooling, metal printing also saves the time it takes to create tooling. In most cases, you can get a few printed metal parts far sooner than you could if they were cast.
Metal printing has similar surface finish, design considerations, materials, and tolerances compared to casting. Moreover, the prototypes made using DMLS are viable stand-ins for production parts.
The most notable limitation of printed metal prototyping is that material selection for DMLS is much smaller than that of other metal manufacturing processes. Materials are designed with the manufacturing process in mind, and additive manufacturing is not only relatively new, but also has unique material requirements compared to casting or machining. Therefore, the current material selection isn’t as broad.
We commonly see DMLS used for functional prototypes when it’s important to use a material as close to the production material as possible. This allows for testing attributes such as part strength or flow profile.
Since the surface roughness and tolerances of printed metals are comparable to cast parts, prototypes can be used to validate additional processing steps such as post-machining, vibratory finishing, or coatings. DMLS can also be used to prototype complex geometries that will eventually be machined from mechanically hard materials such as titanium or Inconel.
We offer the following materials for DMLS:
- Aluminum (AlSi10Mg)
- Cobalt Chrome (CoCrMo)
- Inconel 718
- Stainless Steel 17-4PH
- Stainless Steel 316L
- Titanium (Ti6Al4V)
All materials expect cobalt chrome undergo a stress relief cycle after building. All stainless steel 17-4PH parts undergo an H900 cycle as part of our standard finishing. Additional heat treatments are available, and out offering is open ended—just let us know the required specification or cycle. We also offer hot isostatic pressing (HIP).
Sheet Metal Fabrication
Sheet metal stands alone in the pantheon of parts and designs as a technique which has stood the test of time and countless innovations. To this day, it has retained the artistry and craftsmanship of its foundational years. Two approaches to sheet metal design have dominated over the years. The first is a tangible approach that ultimately leads to a digital model. The second is a fully computer-aided design (CAD) that starts from, and lives in, the digital space.
CAD isn’t available in every circumstance. Many engineers work with legacy equipment that predates 3D models and rendering. Sometimes, they don’t have the time to create the digital models necessary to design their parts. Situations like this call for the unchallenged master of sheet metal design: You can call it cardboard-aided design.
Cardboard can be an analog to sheet metal for the design and development process. A resourceful engineer can find different kinds of cardboard with different stiffness characteristics and shapes, plus you can make your cardboard model as simple or as detailed as you want. This is a viable path to modeling and prototyping. There are distinct benefits to these physical models:
- Having a part to hold helps you understand things like installation and tooling access.
- It allows you to iterate to fix issues, such as clearance, as other parts of the assembly are developed.
- Most businesses have waste cardboard, so materials are free.
A designer can take everything learned from the cardboard analog and transfer dimensions and characteristics to CAD. From there, the designer is faced with making discrete decisions about things like material thickness, bend radius, and bend reliefs. There are questions of fabrication, such as whether your design requires welding. Plus, not everything you can make from cardboard—certainly not everything you make in CAD—can be produced in sheet metal with conventional tooling and equipment.
On the other side, starting in the CAD environment, for all its benefits, has some distinct drawbacks. It is hard to get a sense of scale when a 1 in. x 1 in. (25.4mm x 25.4mm) part takes up the same space on your screen as a huge part. Furthermore, the real world, even cardboard, has many more meaningful limitations than exist in a 3D-modeling environment.
One example is that a CAD package will allow you to drop a screw anywhere in a model. If you don’t take real world limitations seriously, you will design and produce an assembly that cannot be assembled because there is no way for a tool to access the fastener. You can model anything in CAD, but when working in cardboard you will quickly learn about the need for bend reliefs—cuts next to a bend that allow the material to form separately from the base flange. You will also learn that features can’t overlap in the flat; profile changes or welding will always be required but extruding four adjacent walls from a base is simple in a CAD platform.
Whether you start with a tangible analog like cardboard or you’re a digital designer working exclusively in CAD spaces, there are a lot of fine details that can help you prototype sheet metal parts. Some of these you learn from working material or through iterations of design and prototyping, while others you learn from valuable, often automated design for manufacturability feedback, if your manufacturer offers it. These tips will take you far while you prototype your sheet metal parts:
- Choose the tightest bend radius that suits your design. Tools do not necessarily align with material thickness. To best control cost and lead time choose either 0.030 in. (0.762mm), 0.060 in. (1.524mm), 0.090 in. (2.286mm), or 0.120 in. (3.048mm). Other standard sizes include 0.188 in. (4.7752mm), 0.250 in. (6.35mm), 0.375 in. (9.525mm), and 0.500 in. (12.7mm)
- Be ready for alternative fabrication methods. Sometimes we must weld flanges or split your part into a multi-piece assembly to achieve your design.
- Remember that press brake forming has three points of contact; two spots on the bottom die and one on the top die. Because of this, it’s important to remember that a bend needs a flat region before and after the bend. Offsets are a special situation because, when space allows, they can be hit at once and not bent. If you need organic curves and complex forms, you will have to consider a stamping operation. Stamping is significantly different from conventional press brake forming.
There is a handy rule offering a guideline you can use in multiple situations: If you model your flanges or hole features at 4x material thickness away from the outside bend line, you will ensure formability and minimize or eliminate deformation due to hole proximity to the bend. Note this rule does not seamlessly apply to larger bend radii.
Following these rules in the prototyping phase can speed up your design and iteration time during prototyping for sheet metal parts.
Our objective is to make it easy to make hard parts, like metal prototypes. If you have any questions about best strategies for your application, reach out to us at 877-479-3680 or [email protected].
Thank you to Chris Gottlieb, Dan Snetselaar, Chloe Vollaro, and James Annis for their contributions to this design tip.
