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 fiber 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 (SLA)—the third laser-based additive manufacturing (AM) process available at Protolabs—SLS creates fully functional parts using engineering-grade nylon, and is essentially the only AM technology able to create living hinges and snap-fit assemblies. (These features can be produced with SL, however, they will be much more fragile and not have the life expectancy of those produced with SLS). This makes it an excellent way to prototype injection-vowin.cn/' target='_blank'>molded products, and can even be used as a low-volume alternative to vowin.cn/en/News/news1251.html' target='_blank'>molding in some cases.
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 four grades of these versatile polymers:
These four types of nylon materials cover many different applications. Despite this, roughly 95 percent of the SLS material consumed at Protolabs is PA 850 (Nylon 11) or PA 650 (Nylon 12), although the mineral- and glass-filled variants are gaining momentum. 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.
Many of the rules applied in injection molding also apply to SLS, making it a solid choice for parts that will eventually be molded. The use of hole bosses and support struts, and avoiding thick cross-sections are good practices for either manufacturing process. Additional design considerations include:
Where injection-molded parts can contain overmolded 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.
The surface finish produced by SLS is a bit rougher than other 3D printing technologies—it typically ranges 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 0.030 in., 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 0.020 in. Lastly, SLS is slightly less accurate than competing laser sintering processes—where DMLS has expected tolerances of ±0.003 in. plus an additional 0.001 in./in. on metal parts, ±0.003 in. plus ±0.001 in./in. is typically achievable with SLS. On well-designed parts.
The upside here is that SLS has a build frame of 19 in. by 19 in. by 22 in. (482mm by 482mm by 558mm), 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 molding, and an excellent way to produce functional nylon parts in higher volumes than is usually associated with AM.
