Sintered Nylon: First Prints

OpenSLS was funded by Dr. Jordan Miller’s Lab for Microphysiological Systems and Advanced Materials.  It is an exciting example of what can happen when the open source hardware community and academia collaborate and it is important to understand that openSLS was made possible through innovative new funding and development strategies.  In December 2013, openSLS began producing laser sintered nylon parts using both industrial and low-volume commercial nylon powders. Below are some of the early prints.

The final layer of the Nervous System "Regular Bracelet"
The final layer of  Nervous System’s Regular Bracelet (scaled down to ring size here).
Extracting the print.
Extracting the print.
Excavating a completed engine block from the print bed.
Excavating a completed engine block from the print bed.
The cleaned part.
The cleaned part.
The cleaned engine block.  Some subtle delamination issues to address with better sintering parameters.
The cleaned engine block. Some subtle delamination issues to address with better sintering parameters.

Nervous System Ring

Nervous System Ring

Tom Martz from Taulman3D kindly donated several pounds of his 618s powdered Nylon to my project.  It required slightly higher power to sinter, but sintered well.
Tom Martz from Taulman3D kindly donated several pounds of his 618s powdered Nylon to my project. It required slightly higher power to sinter, but sintered well.
The thin walls have a nice translucence and the laser was able to define a lot of the fine detail in this model.
While the laser was able to define many of the fine features, there is a marked roughness to the thin walls.

Exciting Results, New Project: DMLS

I concluded development and testing of the R2 power module in late December.  During 2014 the Miller Lab will conduct research with machine and later in the year we can share more of its capabilities and exciting results.

The completed R2 module incorporated into the laser cutter.  There is room for an additional two more modules if needed.
The  R2 module incorporated into the laser cutter, powder ducting removed to the left of the piston assembly. There is room for an additional two more modules if needed.

In January, I started a residency at Autodesk, in their phenomenal Pier 9 facility.  I am continuing my research into low cost SLS technology there and am focusing specifically on laser sintering steel.  I will be sharing my designs soon.

Improved Powder Distribution on R2 Hardware

Early in the week I bolted the module to the laser cutter’s z-stage, focused the laser, and started running my testing scripts. At first I disappointed in the new powder distributor’s performance. I had put a lot of thought into improving upon my previous design. It rides on two linear rails to ensure that it doesn’t deviate from the print plane during distribution and the blade is adjustable to ensure that it deposits powder plane-parallel to the gantry. After carefully observing the distribution cycle for the morning, I began to see a pattern: freshly distributed powder’s pristine surface was marred only when distributor blade returned to its resting position behind the feed piston. Even though I had mounted two slim brushes to wipe the blade clean of any compacted powder both before and after distribution, the powder still was affected by this very light contact with the blade. By changing the powder distribution sequence and docking location of the blade, I was eliminated this problem and was able to get completely repeatable and perfect layers.  The old powder change sequence:

  1. Raise feed piston k*[layer height], where is a multiplier sufficient to account for spill-over losses during powder transfer.
  2. Drop the print piston [layer height].
  3. Transfer powder from the feed piston to the print piston with the distributor blade.
  4. Return the distributor blade to its docked position behind the feed piston.

New powder distribution sequence:

  1. Drop the print piston [layer height].
  2. Move the powder distributor blade from its docked position behind the print piston to behind the feed piston.
  3. Raise the feed piston k*[layer height].
  4. Transfer the powder from the feed piston to the print piston and leave the distributor blade behind the print piston.
The new powder module installed in the laser cutter.
The new powder module installed in the laser cutter.
This photo was taken immediately after the powder distributor deposited fresh powder over a series of test blocks.  While not perfect, the surface is quite smooth.
This photo was taken immediately after the powder distributor deposited fresh powder over a series of test blocks. While not perfect, the surface is quite smooth.
This photo shows the powder surface disruption that is a result of the distributor returning to its "docking" position.
This photo shows the powder surface disruption that is a result of the distributor returning to its “docking” position.  The deviations from the target powder plane lead to sintered regions that protrude from the powder plane, creating further disruptions and distortions over multiple layers of powder.
A new layer using the improved distribution sequence. Still not perfect, but definitely functional.
I though a Teflon coating would reduce powder adhesion to the distributor blade, but it actually increased adhesion, leading to very poor surface quality on freshly distributed layers.
I thought a Teflon coating would reduce powder adhesion to the distributor blade, but it actually increased adhesion, leading to very poor surface quality on freshly distributed layers.  A plain stainless steel surface performed much better.
Testing 3D sinteirng parameters.
Six blocks printed successfully with the new distribution sequence.

R2 Powder Module Assembly

Assembly went smoothly, though I did discover a couple minor spacing/location issues that I have fixed in the source files, which are now on github

Powder ducting pieces fresh off the laser cutter.
Powder ducting pieces fresh off the laser cutter.
Order of operations is important for both access and proper knitting of the joints.
Order of operations is important for both access and proper knitting of the joints.
Laser cutting the 1/4" acrylic for the powder module main body.
Laser cutting the 1/4″ acrylic for the powder module main body.
One of two 18"x24" panels
One of two 18″x24″ panels
I made extensive use of tapped holes in the acrylic to reduce the number of captive nuts and make some of the assembly more compact.
I used tapped holes in the acrylic to reduce the number of captive nuts and make some of the assembly more compact in several places.
The body assembled with the new print piston in the foreground.
The body assembled with the new print piston in the foreground.
This part (upside-down here) was printed with carefully modeled support material.
This part (upside-down here) was printed with carefully modeled support material.
The support rings can be easily popped out by hand or with a small tool, leaving precise cavities behind.
The support rings can be easily popped out by hand or with a small tool, leaving precise cavities behind.
Two LM8UU linear bearings and a brass M8x1.25mm captive nut are then press-fit into the cavities.
Two LM8UU linear bearings and a brass M8x1.25mm captive nut are then press-fit into the cavities.
The three parts of the feed piston linear motion system.
The three parts of the feed piston linear motion system.
The new print piston with heat-transfer conscious mounting plates to minimize (or at least distribute) the heat load to the acrylic.
The new print piston with heat-transfer conscious mounting plates to minimize (or at least distribute) the heat load to the acrylic.
All buttoned up and ready for testing!
Mostly assembled and ready for testing.

OpenSLS R2 Hardware Development Process

I started modeling the next rev of the hardware in late November and wrapped up in early December.  I took snapshots along the way to document the process a bit.  The new powder module is built around the metal print piston, which supports heating and inert gas shielding.  It has been very satisfying to take a couple weeks to design more thoughtfully.  I designed the first rev of the powder module in a weekend and its performance and functionality reflect that to some degree.  This hardware was composed with openness and accessibility in mind and it will have a very thorough BOM and assembly information.

First ideas about an internal powder ducting system.
First ideas about an internal powder ducting system.
Revised internal powder ducting system to capture powder overflow from both pistons
Revised internal powder ducting system to capture powder overflow from both pistons.  
Incorporating the pistons into the machine body.
Incorporating the pistons into the machine body.
Fleshing out the powder distribution system.
Fleshing out the powder distribution system.
Built out the feed piston linear motion hardware.
Building out the feed piston linear motion hardware.
Trying out different distributor blade frames.
Trying out different distributor blade frames.
Refining the body and incorporating electronics, power supply, and a surplus powder hopper.
Refining the body and incorporating electronics, power supply, and a surplus powder hopper.
Checking for interferences.
Checking for interferences.
Fixing minor alignment/clearance problems before laser cutting.
Fixing minor alignment/clearance problems before laser cutting.
Added integration brackets for bolting the powder module into the laser cutter frame.
Adding integration brackets for bolting the powder module into the laser cutter frame.

Catching Up: Wax Printing Results

I have been designing the next revision of the OpenSLS hardware over the past couple weeks and haven’t been doing a good job of posting developments as they occur. In November, I got working sintering parameters for Candelilla wax powder. See below for images of prints fabricated with those settings. While it is possible to sinter this material, due to its brittleness, it is very difficult to remove finished parts from the build platform. Additionally, the microscopic structure of “sintered” wax powder indicates more a fine-scale balling process of sufficient density to begin to bind individual balls to each other. It is a process that likely does fall under the umbrella term “sintering”, but I have observed binding mechanisms that are much closer to those cited in the literature with nylon materials. Consequently, I have shifted my efforts to nylon, which may serve as a better positive control for developing the laser sintering process and hardware as it is a material that has been engineered specifically for laser sintering and will hopefully present fewer fundamental processing problems.

Extracting a large gear from the powder.
Extracting a large gear from the powder.
The print failed about halfway through, so the top surface here is just dense hex infill.
The print failed about halfway through, so the top surface here is just dense hex infill.  Due in part to its low density and the brittleness of the wax print material, it fractured into many pieces when I removed it from the build platform.
Final layer of the complete gear printed smaller.
Final layer of the complete gear printed smaller.
Extracting the small gear from the powder...
Extracting the small gear from the powder…
After a touch of compressed air.
After a touch of compressed air.
While sticking hte first layer to blue tape combats warping, the adhesion force to the tape is greater than the inter-layer adhesion force of the wax.
While sticking hte first layer to blue tape combats warping, the adhesion force to the tape is greater than the inter-layer adhesion force of the wax.
It's awfully porous, but it holds its form.
The object is quite porous, but it holds its form.
As can be seen, the binding mechanism is more micro-consolidation of particles into fully-melted balls that bind on a larger scale rather than particle-scale binding between individual grains of wax.
As can be seen, the binding mechanism is more micro-consolidation of particles into fully-melted balls that bind on a larger scale rather than particle-scale binding between individual grains of wax.

 

Sintering a Wax Gear

After some parameter optimization, sintering the wax is much faster, but the material is still very brittle.  This print didn’t survive removal from the build platform.

Scripted Traversals of the Power, Speed, and Trace-Spacing Parameter Space

Sintering square 3 of 25.
Sintering square 3 of 25.
Sintering the last square of a full traversal array.
Sintering the last square of a full traversal array (highest power and slowest speed, hence the smoking).
Power increases from bottom to top and speed decreases from left to right.
Power increases from bottom to top and speed decreases from left to right.
Sheet sintering in the lower left, balling in the middle, and full melting in the upper right.
Sheet sintering in the lower left, balling in the middle, and full melting in the upper right.
Sheets that are sintered can be removed with tweezers.
Sheets that are sintered can be removed with tweezers.

SLS is a much touchier process than either stereolithography or extrusion-based 3D printing technologies.  Material cohesion is taken for granted in the latter two– an extruded filament of of plastic is a continuous piece of material, as is a photo-cured region of photoacrylate.  In SLS, material cohesion is the fundamental challenge.  The parameter space governing cohesion includes beam speed, beam power, trace spacing, layer thickness, material temperature, particle size, laser spot size, and powder packing density.  The first four are probably the most important and I have developed scripts to traverse the four-dimensional parameter space they form.  My favorite tool so far has been a Python script to traverse the power, speed, and trace-spacing parameter space, as shown in the photos above and the video below.  It allows for rapid evaluation of parameter combinations for viable combinations, which can then be tested against several different layer heights to find a robust parameter set for full 3D printing.  All of these parameter space exploration scripts are available on my github.

New Sidewall Rails to Improve Powder Distribution

I applied tape before epoxy so that I could always remove everything without disturbing the existing acrylic edge.
I applied tape before epoxy so that I could always remove everything without disturbing the existing acrylic edge.
The straightened wire seated in epoxy.
The straightened wire seated in epoxy.
Curing the epoxy under weighted flat surfaces.
Curing the epoxy under weighted flat surfaces.
Without the flow disruption of the piston divider wall, the powder distributed with fewer problems.
Without the flow disruption of the piston divider wall, the powder distributed with fewer problems.
Powder tends to build up on either side of the distributor edge, which can cause problems.
Powder tends to build up on either side of the distributor edge, which can still cause problems.

I’ve spent a lot of time carefully observing powder flow during distribution cycles the past couple weeks.  The chief problem that I’ve been studying is the surface quality of newly deposited layers in the print piston.  To make each new layer, the printer raises the feed piston by some proportional factor to the layer height (I’m using 1.2 currently), the print piston lowers by one layer height, and then the distributor pushes the powder from the feed piston onto the waiting print piston.  For a long time I couldn’t get smooth distribution on the print piston even though the surface of the feed piston looked great.  By watching the distribution sequence and the powder behavior carefully, I saw that the acrylic wall that divides the two pistons was disrupting the stable tumble pattern that emerges as the distributor pushes the new layer of powder across the pistons.  The smooth edge of the acrylic exerts less shear on the bottom edge of the tumbling region of powder, disrupting the somewhat stable dynamics of the region (driven by shear against the static powder plane) and the leading edge of the acrylic may apply a slight compacting force to the powder.  The powder tumbling slows as the powder moves (slides) across the acrylic. As the distributor crosses over this acrylic edge, the tumbling powder encounters a slight drop to the surface of the print powder plane,  the low density leading edge of the tumbling region falls unevenly to the print plane, exerting uneven shearing forces on the remaining feed powder and preventing even distribution across the print surface.  Additionally, the friction and pressure between the sidewalls and the distributor edge cause excess powder to fuse and form large flakes that fall into the feed powder and create large tracks in the powder during distribution.

To address both of these problems, I affixed thin rails to the sidewall edges, reducing contact area between the distributor and the sidewalls and raising the distributor 1.5mm above the acrylic divider wall.  I used solid core 22 AWG hookup wire, which I measured to have a diameter variation of 0.01mm, and secured it into the epoxy using weighted flat surfaces.  This dramatically improved powder distribution quality and reliability.  I really should have followed my original design more closely, in which I solved both problems by using a sharp chamfer on the edges of the pistons.

New Powder Distributor

The new kapton-coated angle aluminum distributor.
The new kapton-coated angle aluminum distributor.
Installed on the drive belts.
Installed on the drive belts.
While significantly better than the counter-rotating roller, the results were pretty mediocre.
While significantly better than the counter-rotating roller, the results were pretty mediocre.
Traditional test print.  No optimization on print parameters, so quality is pretty bad.
Traditional test print. No optimization on print parameters, so quality is pretty bad.

The counter-rotating distributor that worked so well for sucrose didn’t work at all with the wax powder.  The wax clumped and compacted against the anodized aluminum surface of the drum, completely eliminating any chance of getting even layers.  I reverted to my old geometry of a 90 degree wedge at 45 degrees to the powder plane.  I coated a piece of angle aluminum with Kapton (I used Teflon tape last time) and found the new distributor worked far better than the counter-rotating one, but still distributed pretty uneven layers.