OpenSLS

From RepRap
Revision as of 10:04, 17 October 2013 by DavidCary (talk | contribs) (tweak syntax so it actually shows up on the "category: powder" page.)
Jump to: navigation, search
Crystal Clear action run.png
OpenSLS: An Open Source Laser Sintering Experimentation Platform

Release status: Experimental

OpenSLS-logo.svg
Description
This is a prototype SLS system designed to interface with a laser cutter to create a platform for experimentation with the SLS process.
License
GPL
Author
Contributors
Based-on
Categories
CAD Models
External Link


Introduction

Under active development-- stay tuned here!

OpenSLS is a platform for exploring the selective laser sintering process and a functioning prototype SLS 3D printer capable of fabricating objects in a variety of materials. It was developed at the Advanced Manufacturing Research Institute (AMRI) and is being used for research in regenerative medicine and tissue engineering at Jordan Miller's lab for microphysiological systems and advanced materials. This project is unique in that it appropriates existing, affordable laser hardware, namely laser cutters, for use as in the SLS process. This changed the nature of the challenge from one of procurement (ie, where to find a sufficiently powerful laser and optics) to one of integration, modular hardware design, and material development. The openSLS hardware is designed to be a drop-in powder management module whose sole purpose is to synchronously lay out layers of powder for the laser motion system (the laser cutter's gantry and laser) to sinter or melt. By adapting commercial and widely available laser cutters for this process, the barrier to entry for this technology can be lowered.

SAFETY

Because SLS is an inherently high-energy process involving a high-powered, invisible laser, CNC equipment, and hazardous materials, it should not be treated like the more familiar extrusion-based machines. This is a very dangerous process. Much can go wrong. If you decide to explore this technology, please exercise extreme caution: the potential for lasting bodily harm is real and ever-present and should taken very seriously.

Laser

The lasers used in laser cutters are incredibly powerful-- literally tens of thousands of times more powerful than the pocket laser pointers that already bear a eye-safety warning label. This is a whole other class of laser safety. Your eyes are no longer the only thing at risk. In addition to the danger of the laser radiation, the gas tube that emits the laser is driven by a high voltage power supply with voltages upwards of 30kV. Extreme care should be taken when working on the laser cutter electronics as the high voltage supply holds charge for some time after the laser cutter is powered down. Sam Goldwasser has prepared an excellent page on laser safety, including an extensive section on DIY lasers and the safety challenges there-in. Please read through it before attempting any of the work described here.

Materials

Finely powdered materials are often VERY FLAMMABLE or EXPLOSIVE in addition to being a serious respiratory hazard. Extreme care should be taken when handling powdered materials. Static shocks are in some cases enough energy to ignite clouds of dust, causing an explosion. Additionally, this first prototype does not incorporate inert-gas shielding of the powder-laser interface, which, for some materials, could be very dangerous, especially if any powder becomes airborne.

Process Overview

Laser-based additive manufacturing technologies are a little different from the more familiar melt-extrusion techniques. Of chief interest is the flexibiity that comes with using a powdered feedstock. Many non-thermodecomposing materials have the potential for compatibility with the laser-forming processes.

Terminology

The term SLS is used often broadly to describe a number of laser-forming processes. It is sometime distinguished from selective laser-melting (SLM) because there is a clear difference between the two solidification methods. Laser-melting involves fully melting the powdered material to its liquid phase, which results in a fully-dense part. Laser-sintering relies on very high thermal gradients to liquify (though not in all cases) the surface of the powder particles, bonding to each other, but leaving small voids between the bonded particles. In terms of bonding mechanisms, this distinction is important, but in terms of terminology, laser-sintering and laser-melting are often used somewhat interchangeably. Kruth et al, 2005 proposed 12 distinct binding mechanisms in the family of laser-sintering/laser-melting processes. The important point though is that the distinction between mechanisms is mainly material-dependent and based on particle size, geometry, packing structure, and thermal conductivity as well as the bed temperature, laser power, spot size, and atmosphere. But the implications of the finer points of laser-forming terminology with respect to this project are interesting: this system has been demonstrated successfully with the laser-melting process, but has yet to be demonstrated with the laser-sintering process.

Mechanics

Hardware

Powder Module

The SolidWorks model of the first prototype powder module.

Progress in open-source laser-sintering technology has been stymied by a lack of affordable, high-power laser sources and optics chains. Laser cutters are increasingly accessible at hackerspaces and are rapidly dropping in cost. By using an existing high-precision gantry and associated optics, laser source, and power supply, the most significant barrier to entry into this technology can be removed. I designed a relatively simple powder-handling module that drops into a laser cutter to create a platform for laser-sintering research. I specifically designed the powder module to be primarily composed of laser-cut parts to allow it to be fabricated upon the laser cutter whose functionality it will augment. I also used 3D printed parts extensively and sourced nearly all other hardware from McMaster and Adafruit in an attempt to keep procurement simple.

The powder module consists of two pistons of identical dimensions, each driven by a lead-screw of different pitch. The feed piston is driven by an M12x3.0 Acme screw while the print piston is driven by a M8x1.0 threaded steel rod. The difference in pitches allows the two pistons to be drivenb off the same motor control channel in a direction-reversed parallel configuration: they move different distance simultaneously in opposite directions. For each layer, the print piston moves down one by one layer height and the feed piston advances by three layer heights to provide ample material to counter spill-over losses during powder distribution.

The powder distributor uses a cable-driven counter-rotating anodized Aluminum rod to spread print powder during layer distributions. Tensioned nylon monofilament wrapped around static pulleys coupled to the ends of the distributor rod enforce counter-rotation during translation, which is accomplished by belt-driven bushings at each end of the Aluminum rod. This allows for a one degree drive to effect two degrees of motion. While the powder distributor uses two stepper motors (solely for the lack of an appropriately-sized GT2 timing belt), these too can be driven in parallel to reduce again the number of motor channels needed to control the system.

Laser Cutter

The laser cutter was procured through SeeMeCNC. It is a nameless Chinese import system with an 80W tube, a 600mm x 900mm work envelope, and surprisingly capable accompanying software.

BOM

I am in the process of compiling the BOM and will post it soon.

Print Materials

One of the powerful aspects of laser sintering is its ability to fabricate parts out of many different materials. I have tried a relatively narrow set of materials here, but I

Methods of Fabrication

Isomalt and Sucrose Powders

Wax-based Powder

Sucrose

Detail of laser-melted crystalline sucrose.
Melted region at higher magnification.

I've had limited success with sucrose (baker's sugar) under ambient atmosphere. It throws off a lot of smoke, caramelizes, and balls instead of forming continuous traces.

Isomalt

Beading of laser traces in fine crystalline isomalt.
Melted bridges and cavities in laser trace in isomalt glass.


Candelilla Wax

The wax and carbon mixture at low magnification.
At higher magnification, the individual carbon and wax particles can be clearly seen.

Nylon

Laser trace of Nylon powder illustrating melting.
(placeholder).

Iron

Four passes of the laser show promising sintering.
A very early attempt as sintering (no process optimization).
Sintering.
Two layers sintered together, each done with four passes of the laser.
Attempt to sinter a recognizable geometry.
Very brittle and porous results.

Steel

Electronics

The electronics that drive this system are a mix of native laser-cutter modules, 3D printer electronics, and 3rd party components. The main modules are as follows:

  1. Rambo Board
  2. Laser cutter stepper drivers
  3. Laser Power supply
  4. Arduino Uno (any Arduino will work though)
  5. Powder module stepper drivers

RAMBo Board

A full board pinout can be found here

The Rambo is extremely well-documented board that can be adapted to drive diverse CNC equipment and with the appropriate firmware, is an excellent tool for hardware development. Here, the X and Y step, direction, and enable pins are re-mapped to the motor ext pins (see right) and routed to the step, direction, and enable pins on the Leadshine stepper drivers (see below) used in the laser cutter.

Native Laser Cutter Electronics

Wiring for the 40W laser power supply.

The laser cutter's electronics stack is very modular and very hackable. Everything is connected via Molex connectors and, with the exception of the laser power supply, is clearly labelled. Due to the low melting point of the materials that I am targeting for research in tissue engineering and regenerative medicine, I replaced the 80W tube and power supply that shipped with the laser cutter with a lower power 40W tube and 40W power supply to provide higher power resolution at the lower end of the power spectrum. Additionally, the 80W tube would not laze below 2.5 Watts, which was right in the middle of the power range that proved to be best for isomalt and wax materials. The 40W tube and power supply can deliver power as low as 200mW, which is useful for the wax material.

The laser cutter power supply has a decent manual. The wiring wiring diagram for PWM control is shown at left. While the manual specifies a minimum pulse width of 1ms and datasheets for similar low-cost CO2 laser power supplies spec a 20kHz PWM frequency, the power supply can be driven with the native ~490Hz PWM frequency of the RAMBo. Please exercise extreme caution when working with the laser power supply. It holds a high voltage charge even after it has been powered down. I have verified this by coaxing a high-power beam out of the tube after shutting power off to the laser power supply.

Powder Management Arduino

This Arduino is entirely not necessary and is solely a reflection of time constraints and my difficulties in refactoring Marlin into a more generic 5-axis state. The RAMBo has 5 fully functional motor channels, but I found it difficult to untangle the two channels used for extruder control from the many optimization and safety features in Marlin. I encourage those with more experience in firmware development to tackle porting a stable RepRap firmware to run SLS hardware. The powder management Arduino runs relatively simple code to increment the Z-axis and distribute new layers of powder, which can be found here. The Arduino uses two stepper drivers to run the four stepper motors used in the powder module.

Integration

The most important part of integrating the RAMBo board with the laser cutter electronics is to make sure that all connections share a common ground. I chose not to rig an interface with the laser cutter's native inductance-based transistor endstops and instead mounted mechanical endstops that were compatible with the RAMBo. I also maintained the laser cutter's control over its Z-axis, which was useful for auto-focusing on the powder bed. Once focused however, there is little need for the laser cutter's native microcontroller.

Software

Videos

<videoflash type="vimeo">73432689</videoflash>

Source Files

Source files will be hosted on github and Thingiverse (solid models only) by mid-October.

Future Work

The next prototype is under active development and features heated Aluminum pistons and basic positive pressure inert-gas shielding. Progress can be tracked on Andreas Bastian's dev blog.

See Also

  1. The Focus SLS Printer is an impressive machine that can be assembled out of MDF and printed parts.
  2. Andreas Bastian's original SLS printer on which this project is based.
  3. Peter Jensen's early explorations of SLS printing that included the development of a low-cost reciprocating laser cutter.
  4. While not strictly an SLS machine, the PWDR printer is a nice powder handler.

Further Reading

  • Simchi, A. (2006). Direct laser sintering of metal powders: Mechanism, kinetics and microstructural features. Materials Science and Engineering: A, 428(1-2), 148–158. doi:10.1016/j.msea.2006.04.117
  • Kruth, J.-P., Mercelis, P., Vaerenbergh, J. Van, Froyen, L., & Rombouts, M. (2005). Binding mechanisms in selective laser sintering and selective laser melting. Rapid Prototyping Journal, 11(1), 26–36. doi:10.1108/13552540510573365