DIY Selective Laser Sintering FAQ

Introduction

Some time ago I started research on feasibility of building a DIY Selective Laser Sintering rapid prototyping machine to work with thermoplastics. It quickly turned out that a there are quite a few enthusiasts who are already doing something in this area, as well as wealth of helpful information. However, what I found, it is very time consuming to gather enough background information to actually be able to design and build an SLS machine. So, I've started this FAQ with the aim to build easily accessible all-in-one collection of common answers to common questions so that other enthusiasts like me could save time looking for the same information. Hopefully this becomes a living document as knowledge of the community grows.

--Igor Lobanov 20:08, 27 June 2013 (UTC)

Few simple rules are followed when working with the FAQ:

• Text taken from somewhere else (e.g. forum post) is shown as a block quote with a link to the original page and, if applicable, with the credit to its author. Note, if you've found yourself quoted on this page and do not wish to be referenced, please feel free to remove the block quote and references

General Information

What is Selective Laser Sintering?

From Wikipedia[1]:

Selective laser sintering (SLS) is an additive manufacturing technique used for the low volume production of prototype models and functional components (...) An additive manufacturing layer technology, SLS involves the use of a high power laser (for example, a carbon dioxide laser) to fuse small particles of plastic, metal (direct metal laser sintering), ceramic, or glass powders into a mass that has a desired three-dimensional shape. The laser selectively fuses powdered material by scanning cross-sections generated from a 3-D digital description of the part (for example from a CAD file or scan data) on the surface of a powder bed. After each cross-section is scanned, the powder bed is lowered by one layer thickness, a new layer of material is applied on top, and the process is repeated until the part is completed.

Key process in SLS is sintering, which is basically a process when powder particles diffuse across the boundaries of the particles, fusing together and creating one solid piece. It is based on atomic diffusion, which occurs in any material above absolute zero, but it occurs much faster at higher temperatures.

Lasers

What lasers can I use for a DIY SLS machine?

Any laser of sufficient power to heat the powder particles would work. Practicalities of using a particular type of laser for a DIY SLS project are mainly influenced by two factors: size of the laser tube and the wavelength of the laser light.

What are the pros and cons of a diode laser for a DIY SLS machine?

Pros:

• Diode lasers are small and lightweight, so they could be installed directly on the moving carriage. They also do not overheat and require only a simple heat sink. This ensures simpler construction of the SLS machine
• Diode lasers work with simple, easily accessible currents/voltages, e.g. 3 Watt infrared diode laser takes 2.2 volts and 3 Amps. Consequently, drive circuits are relatively simple

Cons:

• White or clear thermoplastic powder is either highly reflective or transparent for visible or near-infrared light (< 1um), which means that one have to work only with dark powders to achieve any sensible result. This is opposed to CO₂ lasers operating on far infrared wavelengths where thermoplastic is opaque
• High-powered diode lasers (tens of Watts) tend to be more expensive than CO₂ lasers of the same power rating

What are the pros and cons of a CO₂ laser for a DIY SLS machine?

Pros:

• CO₂ lasers have wavelength of 10.4um, which is deep infrared. On this wavelength all plastics are opaque and highly absorptive, so CO₂ laser would not have any problem with white or transparent plastics
• CO₂ lasers are move powerful than diode lasers, which could mean faster processing

Cons:

• CO₂ lasers are bulky and heavy. A 40W tube is 70 cm long and weights 2 kg, so it is impossible to mount it directly on the moving carriage. It is also impossible to fiber-couple CO₂ laser, so the only option is to employ complex system of mirrors to delivery laser bean where it needs to be
• CO₂ lasers require tens of thousands volts to drive. There are special power supply units, but such voltage easily could be lethal if handled improperly
• CO₂ lasers require water cooling. Firstly, this adds to bulkiness of the assembly, and, secondly, doesn't really make a safe mix with high voltages

Where can I learn more about lasers?

Some exceptionally good resources are openly available:

• Sam's Laser FAQ: A Practical Guide to Lasers for Experimenters and Hobbyists -- Vast collection of practical and theoretical knowledge on safety, drives, construction, and application of various types of lasers. Maintained by Samuel M. Goldwasser

Are there any viable alternatives to lasers for a DIY SLS machine?

Technically speaking it wouldn't be SLS without a laser, but there are approaches which are generally agreed to be viable.

Light from a powerful halogen or incandescent lamp could be focused by a lens into a tight beam. The drawback of this method is that it is very difficult to construct very narrow beam of light, so it will considerably limit the precision of the parts produced.

An inkjet print head could be used to print a shape over a layer of powder using black ink to improve heat absorption. The layer is subsequently heated by an infrared heating element, such as halogen lamp or a resistance wire

Materials

What materials are suitable for DIY SLS processes?

Any kind of thermoplastic powder could be processed in an SLS machine.

Practicality of using a particular kind of thermoplastic powder is dependent on several factors:

1. Whether the powder releases bad fumes when heated, because due to the nature of sintering process plastic powder is kept very close to its melting point
2. Whether the powder easily absorbs laser light of a particular wavelength
3. Whether you have a supply :)

Metal powders are possible in principle, but there are some practical limitations for doing it in DIY environment. See below

Is it safe to work with thermoplastics in an SLS machine?

Currently no hard data exists on the safety of SLS in a DIY environment. However, there's a study that suggests that 3D printing with ABS and nylon (and even a trimmer line!) is safe according to the limits set by safety Authorities (OSHA NIOSH ACGIH) as far as the most toxic carbon monooxide and hydrogen cyanide are concerned [2].

On the other hand, there's clearly unpleasant smell normally associated with 3D printing in ABS and nylon, which means something is volatilizing from the melted plastic. Therefore, it is advisable to vent all the fumes through some sort of air filter, e.g. activated carbon, or by airing out the room the very least.

How to produce PLA powder?

Some people report limited success in mechanically grinding PLA in pellets or filament form to fine powder.

Alternative approach is to dissolve PLA in dichloromethane and then spraying it into water medium mixed with polyvinyl alcohol surfactant to facilitate the dispersion of the PLA solution droplets. The powder precipitant was then filtered and dried [3].

Can I use metal powder in a DIY SLS machine?

In theory, yes. SLS for metal is called Direct Metal Laser Sintering (DMLS). However, there are practical considerations which perhaps make it less viable option for a DIY SLS machine:

• Most metals have higher melting point compared to thermoplastics, which means the machine has to maintain high temperature for a long period of time without breaking down.
• DMLS requires inert atmosphere, as heated fine metal powder can quickly oxidize in an uncontrolled manner, i.e. explode. This implies more or less airtight enclosure and a steady supply of an inert gas like argon.

Can I reuse powder left in the build chamber after the job in finished?

In professional SLS applications the powder that remains in the build chamber is thrown away. The reason is that the powder is kept in the build chamber for the duration of the print job with the temperature just below the melting point, which could mean that some powder particles end up fused together even if they are not touched by the laser, after all sintering is a molecular diffusion which has probabilistic nature. As a consequence the powder tends to become more coarse, which will affect the precision of the parts produced afterwards.

This may or may not be an issue for a DIY SLS machine, no experimental data is currently available.

Hardware

Do I need a heated build chamber in a DIY SLS machine?

You do. Thermoplastic expands when heated and shrink when cooled, so whe the laser brings the plastic powder from room temperature to just above melting point it causes considerable expansion. As a result precision is quickly lost. This could be avoided if the powder in the build volume is preheated to temperature just below the melting point to avoid any additional temperature expansion.

As proto points out [4]:

Laser power itself isn't nearly as important as the heating of the powder bed. I'll give you an example. Our nylon melts at around 188C. The machine itself heats the build area to 183C. The laser itself is only responsible for the last 5C temperature difference, just enough to push the temperature past the glass transition point. While its possible to do some LS work with just laser heating, it will be extremely prone to warping/curling/fouling etc. By doing most of the heating externally, you're also able to speed the laser up significantly. I've done successful SLS Nylon-11 prints with Laser Powers down in the 3-4 watt range, at the same speed that I usually use 12 watts at - around 10m/s laser travel.

What are the limitation on the build chamber volume?

In theory, nothing stops one from building very big chambers. In practice, the size of the chamber is a balance of the following factors:

1. Energy required to maintain the temperature in the chamber
2. Build time
3. Volume that needs to be filled with the powder that is not processed in the end

Electronics and Firmware

Can I use RepRap electronics and firmware for a DIY SLS machine?

The mechanical movements involved in the SLS process are very similar to those for FDM process, with the exception of extruder commands being replaced by laser pulses and the need to apply a layer of powder after advancing along Z axis. It it possible, therefore, to use traditional RepRap electronics and firmware for a DIY SLS machine with little modifications.

Projects

Are there any projects for building DIY SLS machines?

Known projects and their statuses. Please add your own.

Known DIY SLS Projects

Project	Author	Overview	Current Status
3dp	Andreas Bastian	Open Source SLS 3D Printer for Rapid Manufacturing in Wax	Inactive, last update in 2011
Focus SLS printer	dragonator	MDF frame, 3d printed parts, no heated bed	Inactive? Last update May 2013
Pwdr	?	Acrylic frame, mechanics, but no lasers yet. Build size 125mm x 125mm x 125mm	Inactive? Last update Aug 2012

Are there any kits on sale to build a DIY SLS machine?

At the moment, none exists. Someone needs to do something about it