21 Nov 2024

3D Printing Functional Parts with Selective Laser Sintering

SLS builds precise plastic parts for prototyping and low-volume production

Sintering is the process of applying heat and/or pressure to fuse bits of metal, ceramic, and other materials into a solid mass. It’s nothing new. Nature has been fusing sedimentary minerals into slate and quartzite for eons, and humans began using similar methods to make bricks and porcelain millennia ago. Today, sintering is used to produce everything from gears and connecting rods to sprockets and bearings. It’s also used to 3D print parts.

Selective laser sintering (SLS) is a close cousin to direct metal laser sintering (DMLS), but builds parts made of plastic rather than metal. SLS uses a computer-controlled CO2 laser versus an ND: YAG fibre laser for DMLS, but both “draw” slices of a CAD model in a bed of material, fusing micron-sized particles of material one layer at a time.

SLS needs none of the support structures typical with DMLS, however, and unlike stereolithography (SL)—the third laser-based additive manufacturing (AM) process available at Protolabs—SLS creates fully functional parts using engineering-grade nylon. This makes it an excellent way to prototype injection-moulded products, and can even be used as a low-volume alternative to moulding in some cases.

Rooted in Nylon

As with any additive process, it’s important to understand the many design considerations applicable to SLS. One of these is the material. Despite their wide-ranging uses, all SLS parts are currently limited to nylon materials—the same thermoplastics used in fasteners, flak jackets, frying pans, and thousands of other everyday items. Protolabs offers six different materials (five different polyamides and one TPU):

PA 12 Smooth White/ Vapour Smooth White is an economical material choice for functional prototypes and end-use parts. It offers high impact and temperature resistance, is very durable, and remains stable under a range of environmental conditions. The nylon material exhibits a white finish with a slightly rougher surface texture compared to other nylons.

PA 12 Glass Filled Smooth White is a polyamide powder loaded with glass spheres that add stiffness and dimensional stability. The material possesses higher thermal resistance than unfilled polyamides and exhibits excellent long-term wear resistance. Due to the glass additive, it has decreased impact and tensile strengths compared to other nylons.

PAx Smooth Natural/ Vapour Smooth Natural is a versatile polyamide which is characterised by excellent toughness and flexibility in any direction, including the z plane. The material possesses great long term stability and durability and can be used to produce both prototyping as well as end-use parts.

The material is very versatile, so applications range from living hinges and snap fits to fixtures, enclosures as well as orthotics.

PA 12 Carbon Filled Smooth Black is an anthracite grey nylon characterised by extreme stiffness and high-temperature resistance, coupled with electric conductivity properties and lightweight. It can be used for both functional prototypes and end-use parts. The carbon-fibre filler provides different mechanical properties based on the considered three-axis direction. This material exhibits a good surface quality and smoother finish compared to other SLS nylons.

PA 12 Flex Flex Pure Black PA 12 Flex Black is a black / anthracite nylon characterised by excellent flexibility and impact resistance. PA 12 Flex Black combines positives properties of PA12 and PP. Strength and stiffness is similar to PA 12. The elongation is comparable to that of unfilled PP. Its high durability makes it an excellent choice not only for prototyping, but also for end-use parts.

TPU-88A Pure Black is an anthracite grey nylon characterized by extreme stiffness and high-temperature resistance, coupled with electric conductivity properties and lightweight. It can be used for both functional prototypes and end-use parts. The carbon-fibre filler provides different mechanical properties based on the considered three-axis direction. This material exhibits a good surface quality and smoother finish compared to other SLS nylons.

Managing the Build

These nylon materials cover many different applications. Most of the SLS material consumed at Protolabs is Nylon 12, however many customers are selecting bstiffer materials (the carbon-and glass-filled variants) or more flexible ones (TPU) based on their unique part requirements. There’s far more to effective part design than material selection, however, and controlling the in-build curl and post-build warping common with AM is paramount to good part quality.

Much of this control falls to Protolabs. To keep parts straight and true, our technicians will often tip parts slightly in the build chamber. If you’re designing a case for a handheld video game, for example, a compound incline of 10 to 15 degrees in the X and Y axes during the build is probably all that’s needed to keep the walls square and the box lid fitting smoothly. It’s important to point out that some “stair stepping” may occur as a result of this technique, so it’s important to identify cosmetic surfaces when submitting your design to Protolabs for quoting and analysis.

For especially challenging parts, ribs can be used to strengthen large, flat surfaces—if your handheld game design requires a thin lid, a honeycomb or checkerboard pattern on the inside surface will not only strengthen the lid, but also reduce material cost and potential warping.

It’s Never Too Early to Improve Mouldability

Many of the rules applied in injection moulding also apply to SLS, making it a solid choice for parts that will eventually be moulded. The use of hole bosses and support struts, and avoiding thick cross-sections are good practices for either manufacturing process. Additional design considerations include:

  • Adding corner radii where walls meet to reduce stress
  • Uniform wall thickness—between 1.5mm and 3.8mm is recommended to reduce in-build curl and potential for warping
  • Integrating ribs to reduce warping

Example of radiused corners

Radiused corners, shown here, can be added to reduce stress.

Where injection-moulded parts can contain overmoulded metal bushings or threaded inserts, SLS parts achieve comparable functionality via heat-stake inserts—in our handheld game example, threaded inserts can be heat-staked as a secondary process at each corner of the housing for strong assembly purposes.

Look and Feel

The surface finish produced by SLS is a bit rougher than other AM technologies—anywhere from 100-250 RMS—but it still works reasonably well for most functional prototypes. Protolabs also bead blasts the majority of customers’ parts to remove loose powder and create a smooth matte finish. Very fine text is another consideration—since the minimum feature size with SLS is 1mm, very small fonts tend to get jammed with powder, making letters and numbers less legible. Moving to inset text provides better results, but is still limited to features no smaller than approximately 0.5mm. Lastly, SLS is slightly less accurate than competing laser sintering processes—where DMLS has expected tolerances of ±0.01mm plus an additional 0.001 mm/mm on metal parts, ±0.25mm plus ±0.0015mm is typically achievable with SLS. On well-designed parts.

The upside here is that SLS has a build frame of 700mm by 380mm by 580mm, far larger than its metal-making sidekick. And because there are no support structures involved, the entire powder bed can be utilized, making it easy to nest multiple parts into a single build. This makes SLS a solid alternative to machined plastic, a logical stepping-stone to injection moulding, and an excellent way to produce functional nylon parts in higher volumes than is usually associated with AM.

For more information on SLS, explore Protolabs’ design guidelines, and feel free to contact an Application Engineer at customerservice@protolabs.co.uk or 44 (0) 1952 683047 with any questions.