3D printing has been widely adopted for rapid prototyping as well as for low-volume, end-use parts. The term “3D printing” is synonymous with additive manufacturing, and reveals how parts are made: by growing them out of base material in the shape of a 3D model. In other words, a 3D printer will use a small material like a filament, resin, or powder, and bond cross-sections together to build a 3-dimensional part.
Lately, metal 3D printing has been getting all the buzz in the media. These metal parts are increasingly being used for critical items in aircraft engine components, medical implants, and other impressive applications. However, the reality is that most 3D prints are actually plastic parts versus metal. So why the discrepancy? This article will help explain the differences between metal and plastic 3D printing and how those differences lead to the use of metal printing for highly specialized applications and the use of plastic 3D printing for everything else.
Material Considerations and Costs
Metal printing, like direct metal laser sintering (DMLS), uses a powdered metal alloy as its raw material. As previously mentioned, since DMLS prints are used in highly specialized, end-use applications where precision and durability are critical to, for example, an aircraft or medical device’s function, the materials must meet high-quality standards. Metal 3D printing materials must be auditable by lot and alloy in order to achieve rigorous specifications for production printing. This contrasts with plastic 3D printing materials, which are typically subject to lower traceability and quality standards since these prints are used more often in prototyping rather than production.
The costs of the plastics and metal 3D printing materials differ greatly as well. By weight, the powdered metal used in metal 3D printing is more expensive than its billet counterparts used in machining, and is certainly more expensive than plastics. Powdered metal is costly because of how it is made, via a process called gas atomization. In this process, liquified metal is atomized by supersonic gas spray where it cools into microspheres.
The costs of 3D printed plastic materials range significantly based on the machines and feedstock, but most standard materials like ABS, Nylon, and many resin photopolymers can be much cheaper than metals. With a lower price, plastics are often much easier on the wallet and has resulted in its widespread availability from various competing sources. In contrast, metal 3D printing material manufacturers are fewer and far between with a small handful of companies doing the majority of metal powder production.
Production Environment: Controlled vs. Open
3D printers are often thought of as smaller desktop machines that are about the size of a microwave and that make parts out in the open air. This is indeed the case for desktop 3D printers, where the materials will not significantly alter with exposure to oxygen. However, metals corrode or oxidize pretty regularly. And since the microspheres used in 3D metal printing have a lot of surface area and not a lot of material, premature oxidation can affect the material’s ability to fuse and cause the printed part to become brittle.
This means metal 3D printers are, by necessity, more complex than their plastic counterparts, requiring a gas-tight enclosed area as well as specialized means of storing, sifting, and retrieving unfused powder without exposing it to air. To prevent quality issues, metal 3D printers usually feed their stock into an inert environment. For example, nitrogen is typically used for steels and argon for aluminum and titanium alloys. This gas is usually fed by canisters that get replaced when empty. Nitrogen can also be separated from the air using a nitrogen generator. Stock metal powder is usually contained in metal bottle-like chambers with airlock-style shutoffs for removal and replacement. Imagine needing that for your desktop!
Heat Requirements and Support Structures
Metal 3D printing uses a laser to generate enough heat to selectively fuse the metal powder together, creating a fully dense part. With DMLS, this requires a high wattage laser to create small micro-welds on each layer. Metal melting requires a lot of heat. Softer metals melt at 600°C and harder metals ranges to over 1000°C. Plastic 3D printing requires melting its base material, but most melting points are between 100°C-200°C. With a lower melting point, plastic printers are more consumer-friendly and business-friendly, allowing hobbyists and companies to create rapid prototypes. Fist-sized extruder heads are an example of what plastic 3D printers require to melt filament fed from plastic reels to create a shape. DMLS platforms require lasers that individually cost tens of thousands of dollars and require peripheral equipment like liquid-cooling chillers to prevent overheating. All the infrastructure required for DMLS printers creates a higher barrier to entry versus plastic printer options.
Heat also builds stress inside parts as they are growing in a 3D printer. For most processes, a support structure, which is removed post-printing, is required to hold the part still as a material is being deposited to form the part. The printed support structure is vital in creating features like overhangs, where the material needs to be “floating” from the part’s base. Metal 3D printing support structures are made of the same metal as the part and need to be strong enough to hold the part while it faces extreme heat stress and tension during manufacturing. The stress that forms in metal 3D printed parts can be strong enough to tear through metal supporting structures on thick features and may require additional material just to hold the workpiece still. In fact, many metal parts are not removed from their supporting base until after post-thermal annealing to prevent warping.
Beyond using a thermal treatment oven for removing stress from metal prints, metal support structures must be removed after a print is complete. For plastics, this is sometimes as easy as dropping the part into a liquid bath for the soluble support material to dissolve. At worst, it may be using pliers and die cutters to clip off supporting materials. With metals, post-processing requires tools like band saws, machining centers, or even wire EDM. For this reason, owning or using a metal 3D printer means you may need to own or have access to a CNC machine shop. Plastics do not require nearly the equipment or expertise as their metal counterparts. It is easier for thousands of consumers and companies to own and use plastic in-house printers since the post-processing requirements are far less complex and do not require machining knowledge or equipment.
Simply put, it is harder to print in metals than to print in plastics. Metal 3D printing like DMLS requires specialized knowledge, especially that of processing control, machine requirements, the energy required, and post-processing secondary equipment. That said, these extra complexities are often worthwhile since DMLS can make some very complex geometries out of materials like stainless steel, aluminum, titanium, Inconel, and more. Metals are often orders of magnitude higher in performance than plastic on their stats, and with the design freedom that 3D printing allows, DMLS can build some incredible results.
Industrial 3D printing platforms are engineered for consistent and repeatable construction of parts using a variety of plastics and metals. They are commonly used for 3D printing services where high reliability is key to achieving quality products and consistent lead times. Services like Xometry have lowered the barrier to entry by allowing users to upload 3D models and get instant pricing and lead times immediately on its website, with live pricing updates by changing processes and materials. With no need to own or operate the equipment, Xometry becomes a free shop extension with an ever-growing number of capabilities, materials, and finishing options for custom manufacturing. 3D printing is a great way to prototype before moving on to injection molding.
About the Author:This is a guest post by Greg Paulsen, Director of Application Engineering at Xometry (xometry.com).
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