SLS 3D Printing Problems and Solutions

JawsTec operates with several cutting edge types of industrial nylon 3D printers. Two HP MJF 4210, an EOS P 396, and two EOS P 400’s. In this article problems and solutions will be discussed based on the SLS nylon powder printer (the EOS P series). Frequently asked questions will be addressed on consumer and producer based problems with the SLS process.

If you would like to order a print of SLS or MJF please visit JawsTec for all your 3D printing needs at: www.jawstec.com

Common Consumer Questions

• How does SLS work?

SLS means selective laser sintering. A laser melts together layers of powder to create the part in a bed of powder, free of supports and surface issues where supports would connect. SLS printing is usually done with 120 microlayers. This provides the best quality parts with the highest dimensional accuracy without causing a build to fail. SLS creates white parts that can be dyed for the desired look.

• How do I design for SLS 3D printing?

Designing for SLS is very easy. Because the parts are built in a bed of powder no thought has to go into supports and the design doesn’t have to have chamfers for injection molding. Walls are recommended to be no less than 1mm. Nylon 3D printed parts are more flexible so with small walls the parts can snap. All nylon parts are usually stronger than any FDM or SLA printers.

• What tolerances can SLS 3D printing achieve?

Tolerances depend on the material being used in the SLS build. A typical tolerance for SLS printing is +-0.1mm. For any walls or connectors, all edges of the wall should have room for +-0.1mm variance. That means, if a lid has to fit around a box, all 4 walls need room for this tolerance. EOS would recommend at least 0.4mm but this would be around the minimum size. For a hole, the tolerances are +-0.1mm as well. Holes and the connectors built and connected after need to have at least +-0.2mm play.

• What strength qualities does SLS have?

Strength depends on the material in the SLS build. Nylon 12 holds a Tensile Strength of 52MPa in the x, y, and z-direction. It holds a Tensile Modulus of 1800MPa in x and y and 1750MPa in the z-direction.

• What surface finishes can be achieved on SLS?

The surface finish on SLS machines can vary. SLS surface is usually rougher than SLA and a different type than FDM. The finish almost feels like smooth sandpaper. The finish can be helped with some types of dyeing and blasting. 3D printed parts can also be tumbled in plastic media to smooth up. Oil polishing and shot peening are also effective ways to smooth a part.

• Does orientation matter on an SLS build?

Orientation does matter on an SLS printer. The bottom side of the part usually has softer edges and the top of the part usually has sharper angles. The most accurate part of the printer is the laser. So any detail that is very important should be placed in the z-direction so that the laser determines the edge the best.

• How hot can the SLS 3D printed part withstand?

The heat an SLS part can handle depends on the material. Nylon 12 can stand heats up to 163 degrees Celsius before getting soft. The Melting temperature is rated at 176 degrees Celsius for the powder.

• What materials are offered with the SLS 3D printer?

SLS has many materials: Nylon 12, Nylon 11, Nylon 12 Glass-Filled, and Nylon 12 Carbon-Filled.

• What is the smallest wall that can be printed in the SLS?

The smallest wall that is recommended by EOS for their SLS machines is 1mm in thickness. Walls can be printed smaller, but they are more difficult to post process. If the part is going to be tumbled walls should be at least 2.5mm in thickness.

• What is the smallest hole that the SLS can print?

The smallest hole that can be printed and that can be cleared effectively is 1.5mm. If the hole goes farther than 100mm the minimum gets increased to 4mm to be cleared effectively.

• What colors can be produced on the SLS printer?

SLS produces white parts that can be dyed many colors. Black, red, blue, green, yellow, pink, purple, brown, and grey.

• How do I get my part dark black?

Dyeing parts black can be tricky. The base of black is blue so the parts can have a blue or purple tint to the black part. To get the parts darker you can shot peen or oil polish the parts to get a darker color.

• Why is SLS hard to color?

SLS is the easiest out of the Nylon powder printers to color. But dyeing in itself can be tricky. The parts dye best in boiling water with professional nylon Rit Dye.

• Why are SLS colors better than MJF?

With the white base, SLS parts can be dyed a more vibrant color than MJF which has a black base.

Consumer Problems with SLS

• Why is the surface finish rough?

The surface finish for SLS can have a rougher feel because small bits of powder get lightly sintered to the edge of the part. With a laser printer, you always have some thermal bleed. The powder around your part will harden because it is close to the heat of part.

• Why do the parts feel like sandpaper?

Thermal bleed makes SLS feel a little rough at the beginning. You can combat this will tumbling, shot peening and oil polishing.

• How do I smooth my SLS part?

You can order your part pre smoothed from Jaws Tec in the form of tumbling, polished, or oil polished. You can also sand down your part with sandpaper for a smoother finish.

• How do I change the color of my SLS part?

SLS parts can be dyed with boiling water and fabric dye. Once the part is dyed it is that color. You can spray paint the parts if the color has to be different. Ink-based sprays are also effective in coloring a part.

• Why does my part have surface defects?

Surface defects can appear during the build process. The part can be pulled by the recorder which causes some parts of your part to look like it has a growth.

• How do I fix surface defects?

Surface defects can be sanded down if there is a growth on the part. Sometimes the part will need to be reprinted if the growth is bad enough. If there is a hole in the part, it can be filled in with some glue or wall spackling. If the hole is too deep though it is best to reprint the part.

• Why don’t my parts fit?

Tolerance issues are usually designed failers. Either the parts weren’t designed for SLS printing or the tolerances are too tight.

• How do I make my parts fit?

Parts can be sanded down or pieces cut off with a razor blade. If you believe the part is not within the tolerances, contacting the producer is the best way to find out what to do.

• Why are my holes not clear?

Holes less than 4mm diameter most likely will not clear in the sandblasting stage. They will have to be manually drilled out and anything less than 1mm will usually be left to the customer to drill.

• Can I drill out my parts?

Yes, you can drill out any holes that are not clear or perfectly round.

• Can I put inserts into my 3D printed part?

Yes, nylon parts can have press inserts and heated inserts put in after they have been completed. Jaws Tec offers inserts if you contact them directly and provide efficient information on what parts need inserts.

• Is my part designed for 3D printing?

Most parts can be designed for 3D printing. With powder beds that really opens up the possibilities for what can be done. Thin walls or prongs won’t come out amazing but anything above a 2mm wall thickness will be fine for post-processing.

• Is SLS better than FDM?

Yes, SLS is of higher quality with usually better surface finish, temperature qualities, and strength qualities.

• Is SLS better than SLA?

Yes, SLS is of higher quality. The surface finish on SLA is better because the layers are usually thinner than 120 microns or 80 microns. SLA parts are very fragile so they are not as useful as SLS, MJF, or FDM.

• Is SLS better than MJF?

Both processes are very close in quality. SLS can be printed in white and is slightly more rigid. This allows the parts to be dyed bright colors and the parts will break sooner when pushed past their limit. SLS is slightly porous so the printed parts can’t hold water well. MJF comes out a raw ash gray. This means most MJF parts are dyed black to keep consistency. MJF parts also have a higher tensile strength. MJF is denser than SLS so it can hold water.

• Is DMLS like SLS?

Yes, SLS and DMLS are the exact same process but SLS is for plastic powders and DMLS is for metal powders.