DIY Selective Laser Sintering FAQ

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Contents

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.

Where can I learn more about SLS?

Alas very few good definitive resources on SLS are available online. Notable few are:

  • Design rules & detail resolution for SLS 3D printing -- Well-know guide published originally by Shapeways to serve as handy reference for those who are submitting models into their 3D printing service, but gives a general feel of what's achievable by an SLS fabrication

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 beam where it needs to be
  • CO2 lasers require tens of thousands volts to drive. There are special power supply units, and 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

How powerful does my laser need to be?

Surprisingly, not very powerful if your SLS machine is built properly. To give an idea, in order to sinter nylon powder heated to the temperature of 8 degrees below melting point with a beam focused down to 0.2 mm spot and using layer height of 0.2 mm with feed rate of 300 mm/s one needs optical power of about 140 mW, which is easily achievable with commodity laser diodes.

You can do your own math using model from here. The model takes into account heat losses due to contact heat transfer within the material (negligible, in fact), but assumes zero laser beam dissipation/reflection (i.e. black powder and visible light laser), and round laser spot (i.e. some corrective optics for the diode). Real life requirements will be higher due to imperfections in optics and beam energy dissipation, but it seems adequate lower bound estimate.

Note, that according to the model, the biggest controllable factor influencing the power requirement for laser is the temperature of the build chamber. If the chamber itself is kept with room temperature, the laser power requirement goes up to 2.6 W. But we know that such a big temperature gap is undesirable as it causes wrapping (see below), so some kind of heated volume is needed anyway.

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
  • Laser Pointers Forums -- Huge online community of laser enthusiasts building various kinds DIY things with it. Well worth exploring the archives, most likely your question has already been answered in the past
  • It can be done -- Well-known and well-regarded collection of engineering notes on building the lasers

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 collimated 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 to the sintering temperature by an infrared heating element, such as halogen lamp or a resistance wire. (see BrundleFab for an example of this in the DIY community)

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.

Where can I get PLA powder?

PLA powder is used in manufacturing of some consumer goods, so it is available in insanely bulk quantities on the websites like alibaba.com and tradeindia.com. Few China-based users on RepRap forum are know to have offered clear PLA powder in smaller quantities for sale, so worth asking around.

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].

Where can I get ABS powder?

Again, it is a commodity which is available in insanely bulk quantities on the websites like alibaba.com and tradeindia.com.

Where can I get professional-grade nylon powder?

Obvious choice is to ask around companies offering rapid prototyping services based on professional SLS machines if they can sell you some nylon powder. Biggest problem is that such companies are buying the powder in bulk packages and loading it directly into the machines, so due to Health and Safety considerations they are unwilling to deal with the powder itself.

Companies offering metal coating services are know for being able to sell reasonable amounts of nylon powder, so it worth asking around for a quote.

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