SLS test rig

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SLS test rig

Postby Jeremy » Wed Feb 24, 2016 10:13 am

Hey guys. Greetings from New Zealand. Thought I may as well do my intro and project in the same thread.

Pleasantly stumbled across this forum whilst looking for methods to produce metal powders. I've been dabbling with notion of a SLS 3D printer for the last few months and have made some progress. My current intent is to develop a test rig which will enable me to learn more about the process involved with SLS printing, starting with plastics and migrating to metals. I've been reading research papers where I can but theory can only take you so far. Eventually, I'll design a production quality rig but for now I'd like to gain a holistic understanding of what's involved.

My test rig is based upon 'Pwdr 0.1', using a revised powder spreader and some subtle modifications to the build chamber itself. Initially this will be run in an open environment without temperature or humidity stabilization. I'll look to refine this in due course.

Image

Most of my spare time over the last few months has been spent writing software for the sake of the controller. I'm in the process of designing my own and have started over a couple times now mainly due to hardware constraints (too slower clock for Gcode processing / touch screen interface / RAM interface / use of RTOS etc etc). I'm currently using a STM32F429 discovery board as this incorporates both ram and a touch screen which is handy, and is capable of 180MHz. I've coupled it with a 24 bit DAC and a SD Card interface. So far so good. I'll get around to developing a PCB for it at some point. I'm using standalone A4988 stepper drivers.

The laser + optic system I'm using utilizes a 40 watt CO2 laser, a beam expander setup in reverse (drops the beam width from 10mm down to 2mm), and a set of cheap chinese galvos retrofitted with 10mm Molybdenum CO2 grade mirrors. I'm currently in the process of setting up the optics so everything is balanced and ready to go once I acquire some powder. I've been on the hunt for some Nylon powder for a reasonable price but alibaba seems to be the only option; you either buy a small quantity for an exorbitant price (30 USD a kilo + 13 USD per kilo for shipping, MOQ 25 kilos) or a excessively large quantity for a reasonable price (1 metric ton at $3 USD per kilo). Can anyone suggest to me where I can buy some without the constraints?

Anyway, that's briefly where I'm at. I'm waiting on a focal lens to arrive at the moment. Once that has arrived I will be able to perform some testing. Following that, the plan is to build a chamber to house everything for the sake of temperature control and an inert environment. Have some work to do before I get there though. Thanks for reading.

Pwdr 0.1 rig:

Image

Laser + electronics:

Image
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Re: SLS test rig

Postby VDX » Wed Feb 24, 2016 11:10 pm

... this sounds interesting - keep us updated ;)

You can start testing with (icing) shugar - highly available and easy disposable ...

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Re: SLS test rig

Postby danielm » Thu Feb 25, 2016 10:04 am

Very interesting project. How are you generating your G-code files?
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Re: SLS test rig

Postby Jeremy » Fri Mar 04, 2016 10:13 am

Thanks guys. Will give the sugar a go. I'm using Slic3r to generate my Gcode at the moment.
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Re: SLS test rig

Postby Jeremy » Sat Mar 05, 2016 9:26 am

So today I spent some time tinkering. I've been having difficulty getting the scanner set up properly, mainly due to my own curiosity, creating problems for myself by changing settings that ought not to be changed. I'd like to gain a better understanding of the scanner controller so I can integrate it into my own design (an ebay china unit), so I've decided to take the time to begin the PCB project. I'll post some updates as I go. Wish me luck. :D

Something that has surprised me so far is the feedback mechanism on these galvanometers. I was expecting something that would measure the relative position of the scanners based on the field strength but that isn't the case. Instead it uses a LED and two LDRs with a paddle which offsets light to determine the position. Only 2 of the 7 connections between the scanner and the control board actually drive the galvanometer itself, a bipolar input and its respective ground.

The feedback mechanism:
Image

Scanner input:
Image
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Re: SLS test rig

Postby MasterBlaster » Sat Mar 05, 2016 7:37 pm

Nice work! It's good to see some active development. :)
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Re: SLS test rig

Postby Jeremy » Thu Mar 17, 2016 10:16 am

I thought I'd post a short update. The lens arrived and is now fitted. It causes the beam to converge but not at the correct focal length. I believe this is caused by the beam expander introducing divergence, extending the focal length - I assume. The beam appears to be weak at the correct focal length (127mm). What levels of attenuation can I expect from a CO2 beam passing through air? Not that much I would have thought.

I had some issues with SD card connectivity but they should hopefully be resolved with a proper card dock. Also had a surprising issue which caused my scanner calibration to go askew; the USB cable on the rig itself, supplying microcontroller and DAC, was dropping a volt and thereby limiting the effective range of the scanners. Minor issues but time consuming to identify nevertheless.

I've also discovered that the scanner resets its offset upon power up after being left overnight. This is potentially due to internal damage or bad settings. It remains consistent once setup and then powercycled, but if left to power down overnight the offset of at least 1 axis will change upon the next power up. Temptation to build my own is growing but I'd rather not spread myself too thinly at the moment. Do you guys think a FPGA/CPLD solution would be better over a conventional analogue circuit?

I also bothered to melt some icing sugar just to observe its behaviour. I noticed it tends to bead. I presume this is because I was melting it rather than sintering it. Temperature and humidity must play a role however, I'm not sure how significant they are though.
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Re: SLS test rig

Postby VDX » Thu Mar 17, 2016 5:17 pm

... the cheap chinese galvo-scanners and drivers can have all sorts of misbehave ... had several working good for some days, then starting with random errors - some from bad/weak caps, others for not indentified causes.
Have two left, which are working without problems, but not done anything with them since a year now (other projects are more urgent).

'Blobbing' while lasermelting is mostly caused by too much power over time - you should drastically reduce the pulse-time and maybe the layer height too ...

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Re: SLS test rig

Postby Jeremy » Thu Apr 07, 2016 5:10 am

Thanks again for your comments Viktor.

Time for another update. Work has slumped over the Easter period while I've been preoccupied with other things. There's a slew of challenges that only seem to be ever increasing, I guess you could say the fun is over and now only work remains. I think this is the point where most projects fall flat on their face. Nevertheless... ;)

I've spent some time setting up the stepper motor wiring and manual override on the controller, giving the user the ability to set a distance and home the hoppers/roller; works great. It's become apparent that the roller is flawed in a couple of ways though. The amount of play introduced by the lead screw, bearings, and linear rail, amount to a roller that jumps and skips instead of gliding smoothly. I plan to address this by replacing the linear rail with another screw drive + stepper motor, balancing out the load. The roller's drive mechanism also suffers from inconsistent belt tension and too smaller teeth on the belt/gear. I was aware of this being a potential issue while designing it but thought I may be able to get away with it for the sake of cost/complexity, it really needs to go however. I've ordered a couple more steppers + other parts. Hopefully I'll see them sooner rather than later.

I still have the issue of powder leakage. I'm not sure how I will approach this as the wall/piston profiles aren't consistent across the length of the bore. The walls have about 1mm of play due to the interlocking slots, which hold the design together, not being perfect. I've tried to account for it by aligning the walls as perfectly as I can, anything more seems to be in vain. I need something fine and compressible to make up for the clearances. Perhaps binding food wrap around each piston might suffice. It'll be compressible so variations in clearance will tolerable. It'll also be easy to define the width by the number of layers. Heat from the laser beam can be mitigated with an additional layer of metal/plastic, if not, the medium being printed itself. My only concern would be wear. I'm thinking aloud here so any suggestions are welcome.

Meanwhile regarding electronics; my controller became damaged while I was inspecting the connection between the LCD and the PCB so development on the software / printing process is on hiatus. I plan to take this opportunity to complete the first controller, consisting of the core components (MCU + Touchscreen) plus the stepper drivers and a DC - DC power supply to cater for everything. Later I'll revise it with an isolated powersupply. The scanner will have to wait as it will require more time than I'm willing to commit at the moment.

That's it for now.
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Re: SLS test rig

Postby VDX » Thu Apr 07, 2016 4:45 pm

... this powder leakage and even wear is normal for this sort of powder handling.

Better design the parts so, that the 'leaking' powder will fall down in a collector box and won't hinder/block the rails, belts and gears.

When driving with belts, you need to 'overdesign' the motor powers and belt stiffness to overcome all the friction and stick-n-slip of the rails -- I'm building this types of mechanics normally with NEMA-23 / NEMA-36 motor sizes (steppers or servos) with 2Nm or more torque or gear-boxes to enhance the torques to >5Nm ...

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