Machinable Metals vs 3D Printed Polymers

As an Application Engineer with TriMech, my role often takes me into many manufacturing environments ranging from mom-and-pop machine shops to aerospace manufactures. As different as you may think these facilities would be, we often find a common trend – most custom fixtures, jigs, equipment, and assembly tools are made from aluminum. The biggest reason when asked about this is because “it’s what we have always used,” and “it’s what the toolroom prefers to work with.” When probed further we usually hear “aluminum is more than strong enough,” so it is chosen for a task that nowhere near approaches aluminum’s tensile strength benefits. There is a wide variety of materials out there other than aluminum that may fit some of these needs better. In this article we’re going to take a look at how printed polymers stack up against it.

Applications First – Material Selection Second

Don’t get me wrong, I am a fan of aluminum and easy to machine metals. I have a small machining setup in my basement and will crank out aluminum doodads all day long (…or maybe I am just hiding from my other household chores). I am not riffing on aluminum and other common metals, but rather nudging you to ask yourself what your application is and what the key properties are that should drive your materials selection. You may be surprised to find that tensile strength isn’t always the driving factor.

The biggest question to ask ourselves when looking to manufacture any part is “What is the application?” What I mean by this is how will this part be used, what stresses will it be subjected to and what are key properties needed for the part to be successful? Often, there are multiple performance criteria that must be taken into consideration. This can lead to overbuilding or adding too many specifications to materials. This can lead to a list of problems including increased material cost and manufacturing time.

Material Properties for Your Consideration

Countless books, engineering classes, and whole degrees are focused on material science and properties. However, I want to highlight a few important points to challenge the mindset that metal (typically aluminum) should be the go-to material for a project.

Tensile Strength

It’s true, aluminum can outperform the strongest polymers and composites in some applications, but do you need that level of strength? For example, a carbon fiber filled nylon like Stratasys Nylon12CF has a tensile strength of roughly 11,000 PSI (76MPa) but is half the density of aluminum. If strength is key, metal is the material of choice. However, if pairing strength with weight reduction or ease of manufacturing then the needle may start to move towards the polymer side of things – even with a strength requirement leading the charge.

Anti-Static/Static Dissipative

Common practice in the electronics and aerospace industries is to make metal tooling and goods static dissipative. This usually involves costly coatings and plating operations, which makes it cost-prohibitive and time-consuming to produce uncommonly machined metals.  Several polymers are custom designed for static dissipative properties by using fillers like carbon-nanotubes mixed into their resins, such as PEKK Antero 840CN03. Materials like this are also formulated to have extremely high heat/chemical resistance and exceptional strength characteristics, providing a high strength solution that does not require additional coating or plating processes.

Electrical Insulated

Without additional coatings, metal is problematic when there are concerns of electrical conductivity. Perhaps there is an electrolysis worry or an arcing hazard. Polymers, when not carbon loaded, are natural insulators. This allows for polymer tooling to excel in applications where there are concerns of equipment and worker safety in electrical operations.

Light Weight

Let’s face it, even compared to aluminum, polymers are lightweight solutions. If weight reduction is a project requirement, then looking at high performance plastics like Ultem, Antero (PEKK), and carbon fiber reinforced nylons (Nylon12CF) are perfect places to start. Perhaps you run the numbers and find that these 3D printed materials don’t offer the stiffness to weight ratio required.  From this you determine composites are the material of choice. 3D printed polymers can still provide a time and cost savings to your project. Materials like Ultem1010, favored for its high thermal resistance, low coefficient of thermal expansion, and high rigidity are the perfect solution for producing composite layup tooling. These high-performance polymer tools offer greatly reduced tool weight (easier to handle), allow for faster cooldown between lay-up cycles (faster thruput on serial production of composite parts), and are generally more economical and faster to produce than traditional CNC aluminum tooling.

High Heat Tolerance

I will not make the claim that any polymer can withstand the heat resistance of even common steel and aluminum alloys. If extreme heat tolerance is your thing, then metal is the way to go. However, I ask you to really look at the numbers. If you are producing a part that is continually subjected to temperatures at or below 300 degrees F, there is a good portfolio of polymers that can thrive in those environments (such as Ultem, PEKK Antero, and PPSF).

Low Thermal Conductivity

While talking about heat, polymers are also poor conductors of heat. This is bad if you are trying to make a heatsink, but a great advantage if you are making tooling that needs to cool down rapidly or reduce the risk of a burn hazard to workers. When making things like composite tooling or molds/patterns that require cooling before being ready to re-use, polymer tools might be a great solution to reduce your cycle times.

Chemical/Corrosion Resistance

Many stainless-steel alloys and nickel super alloys have great corrosion and chemical resistance. Unfortunately, the most common machining metals often have poor corrosion resistance. Even the humblest of plastics, like ABS, have extremely high resistance to strong bases like sodium hydroxide, which will heavily corrode aluminum. This isn’t to say that one plastic is a silver bullet against all chemicals, but different families of polymers have unique chemical makeups that promote resistance to their own unique group of chemicals. Be it strong acids, bases, keytones, or petroleum, there most likely is a 3D printable polymer that can hold up to your chemical needs.

>> Related article: FDM Chemical Resistance

Flexible/Impact Absorbing

Compared to metals, polymers have an upper hand when projects require impact absorbing properties. Various plastics like PC-ABS, Nylon, and TPU (thermoplastic urethane rubbers) excel at impact resistance, shock absorption, and acoustical/harmonic vibrations over metal. Some 3D printing technology, like PolyJet, allow for multiple materials of different durometers to be produced simultaneously within one single part, opening the doors for new fixturing and inspection tooling applications.

Non-Marring/Low Surface Friction

Similar to impact and shock damping, many plastics like Nylon and Diran 410MF07 (Mineral filled Nylon 6,6) have extremely low surface friction and offer lubricating properties unlike their metal counterparts. These polymers are great in fixturing and work holding applications where tooling contacts finished critical surfaces while still offering the strength and work-holding properties desired from metal fixturing.

Transparency/Color

Transparent, solid color, or full-color parts is not easily achieved without painting or powder coating. However, most 3D printable polymers on the market are available in a wide range of colors. Some polymer 3D printing processes, like PolyJet, allow for full-color with text and images produced directly on final parts with no additional post processing. This can make marking and identifying fast and easy.

>> Related article: PolyJet Materials – A Range of Possibilities

Ease of Manufacturing/Unattended Manufacturing

While manufacturability is not necessarily a material property, it is certainly a major aspect to part production. Machining of traditional metals like aluminum, or machined plastics like acetal, require the presence of a machine operator to setup and oversee the machining operation. This marks a key benefit of additive manufactured polymer parts. They require no user interaction or oversight during the building process and only a few minutes to set up the job. Polymer 3D Printing is viewed as a lights-out operation, meaning that once the print is started, the user can simply leave the machine unattended for the duration of the process. The secondary benefit of this style of manufacturing is the ability to produce extremely complex parts that exceed the capabilities of even 5-axis machining operations with little to no additional setup or manufacturing time.

The Bottom Line

Whether you decide to go with a machinable metal or 3D printed polymers, it’s important to take into consideration the purpose and expectations of your part. A deeper understanding can steer you in the right direction for the best material to use. There are benefits of both technologies but choosing the right one for the job will help you maintain quality while demonstrating time and cost-savings.

Need help determination which material and process is best for your specific application? TriMech’s Project Engineering Group can help!