Comparing 3D printing technologies for machine builders

SLS (selective laser sintering)

SLS builds parts by using a laser to fuse particles on the surface of a vat of powder. Because the parts are built layer by layer, they are anisotropic (weaker in one axis), so care needs to be taken when selecting the build orientation. Nevertheless, parts are impressive and have functional strength as well as accuracy, with material properties similar to nylon.

Beware that SLS parts are porous, but they can be lacquered if fluid-resistance needs to be factored. We use white sintered Nylon PA2200. If required, parts can be dyed in other colours including RAL and Pantone shades, though black is the most popular and cost-effective.

Internal passageways can be incorporated but bear in mind that unfused powder needs to be removed using tools and pressurised air, especially for small diameter holes or blind holes. SLS is fantastic for parts with snap fits (eg for quick-change inserts).

A benefit of SLS printing is that the components produced are abrasion-resistant, so light-duty gears and dynamic mechanisms can benefit from a lengthy service life. Furthermore, mechanisms can be 3D printed ready-assembled, provided sufficient clearance is allowed for the removal of unfused powder from between the separate elements.

Typical applications for SLS in machine building include cases and enclosures, ducting, bracketry, lightweight parts (eg end-of-arm tooling for robots and co-bots), ready-assembled light-duty gears and mechanisms, nests, gripper jaws, inserts for kitting trays, and jigs and fixtures for semi-automated assembly and testing.

SLA (stereolithography apparatus)

Stereolithography builds parts using a laser to trace a path on the surface of a vat of photopolymer resin. The laser light accurately cures the resin to the 2-dimensional cross section of each specific layered slice.

Compared with SLS parts, SLA parts have the advantage of being non-porous, so are more suitable for fluidics or needing to withstand a vacuum. It is far easier to clear excess resin in blind holes and small diameters from SLA prints than SLS prints.

SLA also holds tighter tolerances than SLS, and SLA parts are better if secondary operations are required such as milling, drilling or tapping threads as the material is isotropic and has complementing properties to allow this.

Several different resins are available for SLA. For example, we offer materials that have characteristics similar to polycarbonate, polypropylene and ABS. The first of these, Accura ClearVue, suits parts needing good clarity and transparency.

SLA parts have good strength and are isotropic. However, as the resin is photosensitive, parts should not be used where there is excessive UV radiation, though normal indoor UV exposure levels are fine.

Machine builders might select SLA for applications such as brackets, lightweight components, jigs and fixtures, nests, gripper jaws, manifolds, and parts required for the transmission of liquids, gases or vacuum.

DLP (digital light projection or digital light processing)

DLP is similar to SLA in that light cures a photopolymer resin, but the technology differs in that a digital mask enables an entire layer to be cured in one flash of light making it a much quicker process than SLA.

Another difference between the two technologies is that a wider range of resins is available. We can 3D print parts in materials that have high temperature resistance (>300°C), flame retardance, high stiffness, elasticity (Shore 65A) and high toughness. In addition to these stocked materials, we can use many others with specific material properties (details of which you can find here).

As with SLA, parts 3D printed with DLP are non-porous and isotropic. The liquid resin can also be removed relatively easily from passageways and internal voids. In common with SLA parts, DLP parts should not be used in applications where they might be exposed to UV radiation.

Typically, machine builders choose DLP for parts with complex geometries, including those with internal passageways, and where the additional cost over SLA is justified. Depending on the material used, DLP parts can be used for snap-fit interchangeable tooling, nests, gripper fingers, robot/co-bot end-of-arm tooling, and parts needing flame retardancy or resistance to elevated temperatures. If an elastomeric material is used, parts such as seals, gaskets and buffers can be made.

PolyJet

PolyJet is an extremely versatile printing technology, as it prints multiple materials simultaneously. Print heads spray droplets of photosensitive resins in a similar way to an inkjet printer. Parts are printed layer by layer, with the droplets being instantly cured and fused to the layer below by UV light.

Base materials can be used on their own or combined in specific concentrations and microstructures to create Digital Materials. Digital Materials are a result of combining two or more PolyJet photopolymers in specific concentrations and microstructures to create a composite material with hybrid characteristics. Resultant parts can therefore be rigid and opaque, transparent, similar to polypropylene, or rubber-like with Shore A hardnesses from 30 to 95.

Furthermore, a single component can have different material properties in various zones. For example, there could be two rigid areas connected by elastomeric elements. Alternatively, a rigid component can have integral elastomeric seals.

Similarly, a single component can be 3D printed with an integral transparent ‘window’ with no risk of the clear part becoming detached.

PolyJet parts are porous, but they can be sealed by lacquering. Parts are not isotropic and have minute structural weaknesses, so care is needed when choosing the build orientation. Harder grades of material can be readily drilled and tapped.

PolyJet has a higher cost than the other options, but its unique abilities grant it a solid place within the sector. Bear in mind, however, parts may also degrade if subjected to heat or humidity for extended periods. Fully understanding the working environment of the part will help engineers decide if PolyJet is the right choice.

Machine builders choose PolyJet for parts such as: gripper fingers or nests with soft elastomeric inserts to protect delicate surfaces on parts being handled; components with integral clear windows; parts with integral seals or gaskets; parts with integral living hinges and snap-fits for securing components during processing; and parts where built-in compliance can be achieved by means of elastomeric elements.

PµSL (projection micro-stereolithography)

PµSL (pronounced ‘puzzle’) is similar to DLP except that it is on a macro scale; the resolution is far higher, the tolerances are tighter, and extremely detailed features can be produced. However, the scale comes at a cost, the overall size of the print bed in far smaller than usual methodologies. Components for microfluidic systems or micro-electro-mechanical systems (MEMS) are well suited to use the PµSL process. Parts are non-porous like most resin SLA and DLPs, isotropic, and generally don’t require secondary finishing as the precision of the machine extends to surface finish capabilities.

We offer two materials, both have good strength, stiffness and dimensional stability. Depending on the application requirements, the materials we offer are suitable for parts with high temperature resistance (up to 114°C) or that need biocompatibility, suitable for non-implantable medical parts.

It is more expensive to produce parts with PµSL than other 3D printing technologies. However, the types of parts for which we use PµSL simply cannot be made using those other 3D printing technologies. PµSL produces layer thickness of 5-40µm and surface finishes of 0.04-0.8µm Ra on top surfaces and 1.5-2.5µm Ra on the sides. Our BMF microArch S240 PµSL 3D printer has a build envelope of 100 x 100 x 75 mm. The insane tolerances of this machine do come at a cost. The shear number of layers produced when slicing a part by 10µm means that the build time can seem excessive. The price you pay for accuracy!

Typical applications for PµSL parts include micro-fluidics, MEMS, fine nozzles, turbomachinery, and small plastic parts that would be difficult to CNC machine and deburr due to their size.

FDM (fused deposition modelling)

FDM uses plastic filaments and a heated nozzle that extrudes semi-molten plastic to build up the part layer by layer. It is a relatively simple, low-cost technology, which is why it is popular among hobbyists as well as professionals.

FDM is, by nature, not as accurate as the other 3D printing technologies, and the surface finish is inferior. Parts often suffer from warpage and distortion when exposed to sudden thermal changes. A range of different materials is available. The materials behave differently so it can be difficult to switch from one to another and get good quality parts first time; it is often necessary to fine-tune the printing parameters several times to get satisfactory fusing and minimal distortion.

Nevertheless, the technology can be useful for quickly producing parts such as bracketry, kitting tray inserts, assembly aids for semi-automated processes, and other parts where accuracy and appearance are less important.

SAF (selective absorption fusion)

SAF could be suitable for machine builders who manufacture in quantities but not in high enough volumes to justify investing in injection mould tooling. Alternatively, SAF can be useful when ramping-up production, bridging the gap while injection tooling is being laid down.

Similar to an inkjet printer, SAF works by using printheads that jet a high-absorption fluid (HAF) onto the powder bed to define the area being fused on that layer of the part. The surface of the powder bed is then exposed to infrared (IR) radiation. Areas with the HAF absorb more IR energy and their temperature rises sufficiently to fuse the powder particles to each other and to the layer beneath. The powder bed descends, fresh powder is spread across the top of the powder bed, and the cycle repeats.

Currently the choice of materials is limited to high-yield PA 11 and Nylon PA 12, though both have good mechanical properties and are suitable for a variety of applications. Parts also benefit from good accuracy and surface finish.

Because parts are supported by the surrounding powder during the build process, there are no support structures or witness marks to remove.

Although the build process involves heat, SAF parts are said to be less susceptible to warpage and distortion than those built with other 3D printing technologies. Stratasys, the company behind SAF, also claims that parts have material properties that are near-isotropic.

As with SLS, it is possible to 3D print parts with internal passageways but there needs to be sufficient access for the removal of unfused powder and such manual processes are unattractive for volume manufacturing.

Typical machine building applications for SAF include ducting, bracketry, cases and enclosures, and other higher-volume plastic parts with complex geometries. Prototype Projects does not have this 3D printing technology in-house but we can work with subcontractors on behalf of customers if required.

QuickCast

QuickCast is a specific 3D printing process undertaken with stereolithography apparatus and specialist materials. It produces patterns for investment casting in metals, including stainless steel, aluminium, copper and bronze. Although the final parts are not 3D printed, QuickCast is included here because it is a way in which 3D printing plays a key role in creating highly accurate metal components.

Investment cast metal parts are much stronger than 3D printed plastic parts and have far better thermal and electrical conductivity. Compared with 3D printing in metal, investment casting generally produces parts with tighter tolerances and smoother surfaces. Parts can also be investment cast in a much wider range of materials than is available for metal 3D printing. In addition, investment cast parts are not porous, so the mechanical properties are superior to those of metal 3D printed parts.

Machine builders might use QuickCast and investment casting for replacing worn or damaged parts that are NLA, producing metal parts that are difficult or impossible to CNC machine, and as a stopgap when ramping-up production before die-cast tooling is ready.

Metal 3D printing

There are several types of 3D printing technology for use with metals, including PBF (powder bed fusion), DED (direct energy deposition), binder jetting and BPE (bound powder extrusion). Each has its pros and cons, and some are offered in different variants by various suppliers. If required, we can work with metal 3D printing specialists on behalf of customers.

Machine builders need to be aware that metal 3D printing is not a solution to all limitations found in conventional machining methods. Parts often have a degree of porosity, which can compromise strength, toughness and fatigue life. Porosity might also be an issue if the part incorporates passageways for gases, liquids or vacuum.

Because of the need for a feedstock, which is often a powder, the choice of material grades is limited. If 3D printed parts need to be sintered, there is a risk of warpage and distortion at the elevated temperatures required, so the final accuracy might be subject to over-tolerance. The surface finish of 3D printed parts will always be inferior to that of CNC machined or investment cast parts. If a 3D printed part requires secondary finishing operations, this adds to the lead time, labour and cost.

Once the printing parameters have been established, parts can be built with good repeatability. However, it can take several attempts, and much fine-tuning of parameters, to get parts right, so the lead time – even for a one-off part – could be relatively long. There is also a considerable amount of post processing to reach the desired properties in the materials post print.

Machine builders choose metal 3D printing when parts cannot be made using traditional machining techniques (eg due to internal passageways) and where plastic 3D printed parts are not strong enough. It can be quicker than investment casting, cost-effective for one-offs or small quantities, and the technology lends itself to relatively small parts. Example applications include robot end-of-arm tooling, lightweight parts on highly dynamic machinery, and the reproduction of obsolete parts.

Ceramic 3D printing

Ceramic 3D printing, also known as ceramic additive manufacturing (CAM), presents both unique advantages and challenges. This method utilises specialised ceramic materials that are extruded.

One of the notable advantages of ceramic 3D printing is its ability to produce components with excellent thermal and electrical insulation properties, making it ideal for applications requiring resistance to extreme temperatures or harsh environments. Additionally, ceramic parts offer high chemical stability, making them suitable for applications in corrosive environments.

However, ceramic 3D printing comes with its own set of challenges.

The process can be slower and more complex compared to other 3D printing methods due to the brittle nature of ceramic materials and the need for precise control over printing conditions to prevent cracking or warping during printing and post-processing stages. Moreover, the availability of ceramic printing materials may be limited compared to other types of 3D printing, potentially restricting the range of applications for this technology.

Despite these challenges, ceramic 3D printing holds great promise for various industries, including aerospace, electronics, and biomedical, where the unique properties of ceramics can offer significant performance advantages.
As the technology continues to advance, addressing current limitations, ceramic 3D printing is poised to revolutionize the manufacturing landscape by enabling the production of complex ceramic parts with unprecedented levels of precision and customisation.

Talk to us

If you are a machine builder and need 3D printed parts or want to discuss the various technologies, please contact us on 01763 249760 or request a quote.