When is SLS your best option for 3D printing?

If one 3D printing technology was ideal for every application, we would use it all the time. In reality, each technology has its own advantages and limitations, which is why we have invested in five different 3D printing technologies. One of these is Selective Laser Sintering (SLS), so when is SLS your best option for 3D printing?

What is SLS?

SLS is a 3D printing technology used for both additive prototyping and additive manufacturing. It builds intricate parts with high resolution and fine details. With SLS, parts are built from a fine powder, one thin layer (slice) at a time. A high-powered laser is computer-controlled to fuse powder particles to each other and the fused layer below.

After each layer has been built, the machine bed is lowered by the layer thickness, more powder is spread across the surface, then the next layer is traced by the laser. Thanks to the good fusing in the X, Y and Z axes, material properties in the finished part are close to being isotropic.

SLS materials

When customers are selecting a 3D printing technology, we recommend that the required material properties are identified first. After that, look at which 3D printing technologies can be used with materials that provide those properties – while taking into account factors such as print resolution and cost.

Because of the nature of the fine powder used in SLS, it is difficult to switch materials. We therefore run just one material with our SLS 3D printers, namely PA 2200, which is a versatile nylon-based engineering plastic.

PA 2200 builds parts with high strength and stiffness, abrasion resistance, excellent long-term stability and good chemical resistance. In addition, the material is biocompatible in accordance with EN ISO 10993-1 and USP Class VI.

SLS finishing options

Customers often request parts are simply finished by cleaning and removal of loose powder. However, the surface finish can be improved further if necessary by secondary finishing processes such as bead-blasting, polishing or hand-sanding.

PA 2200 is a very versatile material but SLS parts are porous. If this is an issue, it can be overcome by lacquering or painting.

As built, parts are white but they can be dyed in a very wide range of colours, including RAL and Pantone colours. We find that most customers for SLS parts either want them left white because aesthetics are not significant for functional prototypes, or they request black for end use parts.

Alternatively, parts can be spray painted if they are for presentation models or functional prototypes that require a better aesthetic.

Other options include blackout/RFI/EMC coatings applied to internal surfaces for functional prototype or end use parts.

Provided the correct finish is applied, parts can be made resistant to moist or humid conditions, UV radiation (including sunlight), fading, material degradation, and certain chemicals, oils and other contaminants.

SLS parts for assemblies

Whether SLS parts are required for concept models, functional prototypes, end use parts or anything else, they are often assembled with other parts. These might be 3D printed using SLS or other technologies, or they could be CNC machined parts, customer-supplied proprietary parts, or standard off-the-shelf components such as springs or threaded inserts. SLS parts lend themselves particularly well to use in assemblies because of their material properties.

SLS parts can be drilled, tapped, milled and adhesively bonded. In addition, careful design enables SLS parts to incorporate snap-fits (living hinges are sometimes possible but not recommended).

Another option with SLS is 3D printing of pre-assembled parts. For example, an entire functional drive chain can be 3D printed, ready to use once the loose powder has been removed from between the elements.

Advantages of SLS are:

• Excellent for accurate parts with complex geometry and fine detail
• Good surface finish, depending on finishing technique
• High strength and rigidity
• Good isotropy though slightly weaker in the Z axis
• Good long-term dimensional stability, UV stability, thermal resistance and water resistance
• Suitable for both prototype and end use parts
• SLS can 3D print parts that would be difficult or impossible to injection mould (eg undercuts, negative draft, hollow parts, pre-assembled parts)
• Low-friction material, so good for 3D printing separate or ready-assembled parts that move relative to each other
• Material is biocompatible and approved for food contact
• Parts do not require support structures so there are no witness marks
• Good range of finishing options
• Relatively large parts can be built
• Equipment is not as expensive as for SLA
• Relatively low-cost material
• Versatile technology

Limitations of SLS are:

• Surfaces not as smooth as SLA or DLP
• General tolerances similar to SLS, DLP and PolyJet but not as good as PµSL
• Porous surfaces may require sealing by lacquering
• Depending on the part geometry and machine parameters, flat surfaces can distort
• Hollow parts require escape holes for removal of unsintered powder
• Care is required for storage, handling and disposal of fine powder

Typical applications for SLS parts

The following applications are typical of those for which we use SLS. However, the list is not exhaustive.

• Functional proof-of-principle models
• Prototype parts for functional testing
• Presentation models
• Concept models
• Low-volume production (batch manufacturing)
• Mass customisation
• End use parts requiring strength, stiffness, durability and abrasion-resistance
• Casings and enclosures
• Brackets
• Ducting, manifolds and parts with internal passageways for vacuum or compressed air (eg lightweight grippers for assembly systems)
• Nests and gripper jaws for automated assembly systems
• Development of assembly systems

When would we advise against SLS?

The following are examples of applications for which we would recommend a technology other than SLS.

• Elastomeric parts (we would recommend PolyJet or vacuum casting in polyurethane resin)
• Small parts requiring very high precision (PµSL is best for parts with micron-level details)
• Master patterns for vacuum casting (we recommend SLA)
• Parts for which the material properties of PA 2200 are unsuitable

SLS capability

SLS is a versatile, reliable 3D printing technology that builds high-quality parts with fine detail and a good surface finish. We currently have three EOS Formiga P1 machines with a print volume of 220 x 250 x 330 mm, plus a larger P3 with a print volume of 340 x 340 x 620 mm. These are complemented by cleaning and polishing equipment and a DM60 dyeing system.

We quote a general tolerance on SLS parts of ±0.5mm but tolerances can often be tighter, depending on the part geometry.

We operate our SLS 3D printers every day, so we can always meet the needs of our customers. If you need a fast turnaround our Express Delivery service offers shipment the next working day. On the other hand, if budgets are tight, we offer Economy Delivery with parts dispatched in eight working days. Most customers opt for Standard Delivery, with SLS 3D printed parts dispatched in three working days. We never cut corners for the Express Delivery service; part orientation and machine settings are selected for part performance and quality, not build time or material usage. Note that the choice of finishing options is restricted for the Express Delivery service.

One of the advantages of SLS 3D printing is that there are no setup or tooling costs. This works very well for one-offs and small quantities of prototype parts. Also, for low-volume manufacturers, there is no need to order a large batch and then manage the logistics of storing parts until they are required. Instead, manufacturers simply order quantities matched to their production schedules.

Conclusion

SLS builds prototype and end use parts that closely mimic parts injection moulded from nylon and similar engineering plastics. The build process is accurate, quick and cost-effective, with parts benefitting from fine detail and smooth surfaces. SLS parts can be used for almost any stage in a development project, from concept models through to functional prototypes, presentation models and low-volume end use parts. Because we only build SLS parts in PA 2200, that does sometimes limit the technology’s applicability. Nevertheless, we offer five different 3D printing technologies, as well as CNC machining and vacuum casting, so we can usually offer a technology to meet the customer’s requirements.

Talk to us

If SLS meets your needs for prototype or end use parts, talk to us about your project – or get in touch if you are unsure whether SLS is the optimum technology for your parts. Contact us on 01763 249760.