RepRapMicron
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Release status: Early development
| Description | Micron-scale 3D Printer
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Contents
Summary
The µRepRap is intended to be a RepRap capable of micron (1/1000th of a millimetre, or 1µm) and sub-micron fabrication. This degree of accuracy has been made possible by 3D printed microscope platforms designed by The OpenFlexure Project. Expectations for the initial prototype are to demonstrate repeatable positioning to better than 3µm on a work area 10mm across, and to produce a probe tip in lieu of a print head that is suitable for manufacture in a simple home workshop.
Overview
Microelectronics and Micro-electromechanical Systems (MEMS) are essential components of most of the electronic wizardry we use in our everyday lives, whether we realise it or not. In the way that RepRap brought Open 3D fabrication to the masses, the aim of the µRepRap Project is to bring users the same capabilities on a much smaller scale and allow those components to evolve in the same way.
Background
Precision manufacturing began somewhere in the 1700's, and the first micron scale electronic devices were fabricated from silicon in the 1960’s. The Intel 4004 CPU, for example, was made with a precision of 10μm. Ever since then the technology focused largely on silicon, with fabrication systems becoming ever more complex, esoteric, and costly. The techniques used are difficult for the average hobbyist to manage, and in many cases are downright dangerous.
In biology and medicine, equipment to measure and manipulate objects on the micron scale are relatively common - though these devices tend to be large, specialized, and expensive. Recently though, microscope platforms capable of sub-micron resolution were developed by The OpenFlexure Project, and these have created an interesting opportunity for developing micron scale fabrication.
If micron scale manufacturing can be achieved by RepRap-like technology, it is likely that these fields will be advanced in the same way that manufacturing was by the RepRap. The biological sciences will gain from inexpensive, rapidly-evolving equipment. The microelectronics field will regain the potential for independent communities develop on the micron scale, and break away from its fixation on silicon as its main platform. As with 3D printing, there will certainly be new developments in fields that do not currently even exist.
There is also the possibility that this technology could replicate in the future, and do so at a yet smaller scale.
Requirements
Micron scale 3D printing has many of the same requirements that The RepRap Project developed when initially printing on the macro scale:
- A 3-axis positioning system
- CAD/CAM software
- Axis zeroing sensors
- A deposition system
- Building material
In addition there is the practical aspect that humans are unable to directly manipulate micron scale assemblies and sub-assemblies. Novel systems are therefore needed to:
- Detach printed items
- Transport items
- Rotate and position items
- Conduct micron scale maintenance tasks
While conventional optics are readily available to allow humans to initiate and inspect the fabrication processes, it is likely that some customisation of the optics will be desirable.
Adaption Of Existing RepRap Technology
The 3-axis positioning systems developed for the RepRap are largely applicable to operation on the micron scale. They are also readily available and understood by many potential collaborators. Likewise the CAD/CAM systems developed can largely describe the volumetric and control aspects on the micron scale. Early RepRap designs catered for many initial design issues experienced, such as backlash and the management of delays in the extrusion system, and these will likely have parallels.
One example would be the positioning system. Current 3D printers use microswitches, optoelectronics, and hall-effect sensors to detect the zero position of an axis. Others simply slam the axis into a physical stop. One possible solution is a light gate closing off a light source, the light minima indicating a known position. Actual probe height above the work area needs to be determined, and this initially is likely to be a manual process.
The control of the deposition process and the choice of building material will need to be reconsidered due to the practical issues of creating fine extrusion orifices and moving phase-changing materials through them. Photosensitive resins as used in resin printers do scale however, and similar materials are already used widely in the microelectronics industry. Their wide availability to the 3D printing community makes them worthy of consideration. If the work area is illuminated by UV for photosetting from underneath, the reservoir of resin can be kept in a shielded portion of the work area above. The probe can shelter in this area when exposure is in progress, and reload itself with resin at the same time.
It is likely that a number of substances with desirable physical properties will be experimented with. Conductive and electrically active materials are an obvious step. A magnetic material would allow a means of activating assemblies by means of an external magnetic field. Droplets of catalyst could be used to solidify a substrate or render it soluble etc.
Novel Requirements
The simplest form of deposition system is an old-fashioned dip pen. It requires no more than being dipped in an inkwell, and then to be touched to a surface. If the ink can be persuaded to change phase by thermal cycling, photosetting, or application of electricity etc., the print head itself need not have any complex or moving parts.
A sufficiently sharp tip, in the sub-micron range, can be easily made on the workbench from fine wire. 22 gauge (0.12mm) titanium or nichrome wire work well. Place a large electrode in the bottom of a container of 5% sodium chloride solution and connect this to the negative side of 3 AA cells in series. Suspend a length of wire vertically in the salt solution with about 8mm immersed, and apply +4.5V to it. Electrochemical erosion occurs, and when the end of the wire falls of, cut the current. The process takes a few minutes and can be automated or done manually.
Combined, these items allow the formation of a test system for deposition, operated by conventional CAD/CAM systems attached to an OpenFlexure stage.
Assembly Manipulation
Seeing the object being fabricated during development is crucial. As micron scale optics tend to be very 2D and have a limited depth of focus, gauging the height of things is particularly difficult. There are some tricks that can be applied such as creating shadows, and illuminating the object with different coloured LEDs from several angles. These techniques are also useful for observing completed objects.
Once an object has been fabricated, one way to detach and manoeuvre it would be to simply use the “ink” to glue the probe tip to the assembly. While adequate for initial experimentation, eventual re-use of the probe is desirable and a release mechanism such as heating the probe tip could be implemented.
To rotate parts does not necessarily require a rotating manipulator. Parts could be made to rotate around built-in pivots when moved or operated with the probe. To move in the vertical plane, an assembly could contain joints that allow it to erect itself at the desired angle by manipulation of hold points with the probe. Once the assembly is in the required orientation the probe can be glued to the angled assembly. By use of multiple probes, each attached to an OpenFlexure stage, and the ability to apply glue, assemblies can be combined arbitrarily to produce macro-scale items and either positioned with the probe or more conventional manipulation. Another possibility is electroadhesion or plain old magnetism, but this only works with some materials.
The maintenance tasks are currently unknown. Likely more convenient tools – grippers, rotating devices, probe recovery systems, ink well fillers – will need to be manufactured. The early stage of development will likely have a high attrition rate.
Future Advances
As with the original RepRap, it is hoped that once the first μRepRap is operational, it will be able to replicate. Unlike the original, the replicant will not be identical to its parent but will be a much smaller functional equivalent. Control of the replicant will be difficult, but not impractical. Options include perpendicular magnetic fields, inducing vibrations to operate ratchet mechanisms or tuned structures, or at its most basic cranking the mechanism with the tip of a probe until something better can be worked out. It should be possible to run more than one printing mechanism simultaneously.
It might also be possible to fabricate a μRepRap that is scaled down further. Assume that a RepRap has a precision of approximately 0.2mm on a 200x200mm work area, and a μRepRap has potentially a 1 micron resolution on a 10x10mm work area. We see roughly two to three magnitudes in reduction in scale. If flexure systems also scale, then it might be possible to use a μRepRap to create a smaller printer with a resolution on the nanometre scale. Unlike current attempts to operate with nanometre precision with macro scale hardware, errors in the device due to thermal expansion and other material-related noise would be much reduced. It would at least be interesting to see how far this limit could be pushed.
Practical Progress - Block Stage
Initially a modified Delta Stage was used. After sufficient experience was gained with the larger working volume, the Delta Stage was abandoned in favour of the Block Stage.
The Block Stage was modified to have a slightly larger working volume roughly 4mm x 4mm x 2mm (about a quarter of the Delta Stage), reinforced stepper motor lugs, a gap for UV illumination to be added under the stage, and a hole added to the platform so that a fixing screw underneath it could be accessed. NEMA17 motors were fitted to mounting plates previously used with the Delta Stage. Block Stage Mounting lugs seemed to be shorter causing some interference with the NEMA adaptor plates, so stand-off collars were added to the plate. The corners were rounded off to prevent wiring etc. catching on them.
The Block Stage is not symmetrical and tends to tip over with the weight of the motors, so the whole assembly was screwed to a base board which was also used to anchor the USB microscopes. A white LED with suitable current limiting resistor was placed behind the block stage connected to VMOT/GND on a spare RAMPS board stepper driver socket. A piece of white paper was put underneath the stage to provide a plain white background for observations. A switched, unbranded 4W USB UV resin-curing lamp on a gooseneck and base clip is used to set resin prints, but this is currently too large to fit under the stage.
To maintain even lighting throughout the day and night it was found necessary to cover the equipment with a sufficiently large cardboard box when observing.
Zeroing XYZ
Flexure joints do not move the Block Stage platform linearly. It was found necessary to zero the stage X, Y and Z axes for any degree of consistent manipulation on a scale of less than 10 microns. A modified Base module was designed for the Block Stage that uses simple sprung wires as endstop switches. These were connected to XMAX, YMAX, and ZMAX of the controller board.
Probe Height
The probe holder from the first prototype proved to be functional but unwieldy, so an improved version using constrained flexures rather than two springs was made as a replacement. This assists in manually setting the initial probe Z height without damaging the probe tip. In practice a low-quality USB microscope aimed horizontally is used to observe the tip as it is lowered. The shadow cast by the probe and its reflected image give an indication as to height. If the probe flexes, it is in contact!
No satisfactory way of establishing probe contact has been devised as yet, but experiments involving coating the slide with tin oxide are planned. Crude tin oxide has been deposited from homemade tin (II) chloride on hot glass, with a resistance of 800kΩ over a 10mm diameter blob (it should be 1.75 x 10^3 cm-1Ω-1 from databooks). The tin chloride was made by dissolving lead-free pewter ("Britannia Alloy") in hardware store 24% w/w hydrochloric acid.
In the meantime the probe is wiggled back and forth to see if it has made a mark on a smear of Sharpie marker on the slide, and if it has not the Z probe is lowered further.
Notes On Microscopes
Three microscopes are in use on the prototype: An inexpensive, unbranded, 2MP USB microscope to observe initial Z probe height. A 4MP Digitech USB microscope used for observing XY probe positioning. A Biolux NV turret microscope for examining the results of a run, which is fitted with a plug-in low resolution USB optic module that provides pictures for experimental results.
USB Microscopes
Magnification values given my USB microscope manufacturers are essentially meaningless. There is no optical image to compare with the original, and comparing original image size with the image displayed on a monitor is dependent on monitor dimensions and screen resolution. It is recommended that the maximum number of true pixels is used as a guide for performance when obtaining USB microscopes.
The USB microscopes provide illumination and are held in an ad hoc frame. This frame will be developed and detailed later, as stable mounting is essential. The current version involves too many unsupported ends and too much gaffer tape.
Turret Microscopes
To look at what you've made you will need a real turret microscope. Your eye will do better than any USB plugin on a real microscope - the author uses the optical USB plugin to post photos and keep records.
On the turret microscope a 4x objective and a 5x eyepiece is frequently used and a 50 micron line is quite visible. For fine detail a 10x objective and a 16x eyepiece is used. This is only 160x but if your naked eyes can resolve a 0.5mm object, at this magnification you will be able to resolve 3 micron objects. Do not use the largest magnification objective on a turret microscope. It usually requires contact with the slide and will destroy your sample when you focus on it. If you need more magnification use a "Barlow Lens" adapter.
Cats
Cat hair varies between species, but Ragdoll hair has a diameter of around 30μm. If you have a cat, it is strongly advised that you keep them off the workbench or you too will be observing the diameter of cat hairs.
Collaboration
The project will be conducted as Open Source under the terms of the GPL 3 or later licence where possible, and documentation distributed under the terms of the GFDL. Note that the OpenFelxure Project uses the CERN Open Hardware Licence v1.2, which will be respected. Progress will be blogged on the reprap.org blog, and the primary repository for technical details and conclusions will be on the reprap.org wiki. Software and configuration files are on GitHub. Participation is encouraged. There is likely to be some discussion on the Facebook RepRap page - note that due to spam you must answer the group question before posting on Facebook!
As a side note, the original Z80 processor was manufactured using an 8 micron process. Construction of assemblies on this scale seems an achievable goal.
Conclusion
Hopefully this will bring the RepRap project to smaller and smaller things. VikOlliver (talk) 22:44, 6 March 2024 (EST)
