To build a truss or panel bigger than the working surface, it is necessary to add new material onto already made pieces that have been moved to extend of the edge of the bed (secured by movable arms/clamps and positioned with registration pins). This means that the extruder has to build right up to the side of something taller than the current piece under construction. Normally, the extruder head would bump into the higher piece and prevent getting very close, so a second extruder head is needed. This one would have a long thin extension or pipe, like a hypodermic needle, to add plastic in the tight spaces between the existing piece and the new material. To prevent the pipe from plugging up, it will need to be heated. This can be done by having an inner core that is somewhat conductive, like nicrome wire, surrounded by an insulating layer, and then a good conductor on the outside like copper, and run a current through to heat the interior of the needle. Or needle is made of conductive metal and a nicrome wire runs up the middle to heat the plastic. Since this will be filling in between a previously made piece and the bulk of the new material, this needle would also be fairly large in diameter so that plastic can be extruded quickly. The length of the needle will set an upper bound to the thickness of a previously made piece that is being extended. Even though fresh hot plastic sticks well to previously laid layers, to get a good structural bond between the old piece and the new, the interface between them should interleaved or dovetailed. This will provide more surface area between the old part and the new, and it will allow better heating of the old part in the thin fingers between freshly laid down plastic to allow it melt together better.
If circuit boards are to be made from scratch, they need to be stronger than just fused ABS. Current circuit boards are FRP, fiberglass reinforced plastic. To make this we need to add another extruder to supply fiberglass thread or yarn. This will be harder to extrude with a pinch roller, but with rubber rollers it should still work. To tack it down, something sticky has to extruded at same time. The best choice would be a monomer of the plastic being used so that it will bond with the rest. It might also work to extrude a little bit of melted ABS along with the fiberglass yarn to hold it down. At the end of each length of fiberglass yarn, a knife will pass over the extruder to cut/break off the piece just laid down. It will be easy to lay down alternating layers in X and Y, but I see no way to easily add fiberglass fibers in the Z direction. The best bet will likely be to make other pieces rotated 90 degrees and glue them on after fabrication. It may also be possible to make tufted fiberglass that is only secured at the bottom to make fiberglass insulation. This would be laid down like a carpet, then a layer of plastic to make a bed for the next layer of fiberglass.
If a fiberglass extruder has been added, it also becomes possible to make belts. The belt would be laying down on its side and the fiberglass would be wrapped around and around to provide the tensile strength of the belt, and the plastic/rubber built up around it to hold it together and provide traction. And of course if the belt is toothed, harringboned, or otherwise structured, that can be laid done by the 3D process as well. Raw, unvulcanized rubber can be extruded with pump or auger, and the heat cure it as it is laid down.
One obvious solution to the conductive material extruder is to use solder paste. This is already commonly used for surface mount devices. The paste would once again be extruded by a pump or auger. Once on the surface, a separate heater would be used to locally cure the solder paste. This could either be a resistively heated metal surface on the extruder, a separate head with a heated metal surface, or an intense infrared lamp and reflector to concentrate the heat in one spot. If a heated bed is being used, that would also help to cure the solder paste if it is heated up to the safe working temperature limit of the plastic. A higher working temperature plastic would be good to use for the board body so that the bed can be heated up to closer to the 220C needed to reflow the solder paste. High temperature epoxy, requiring an extruder that delivers the two components in liquid form, then using the bed heater to post cure, would make an almost identical PC board substrate to that of normal PC boards. Using this approach, SMD ICs can be added to the board after the solder paste is laid down but before it is set. If an additional head (getting quite crowded on the top!) used to pick and place the SMDs with the precision of the 3D prototyper. A tape of parts in order that they are placed would still need to be made separately. However, such tapes may become popular 'vitamins' for standard builds such as making another reprap.
To automate the building of parts bigger than the reprap, one or more robotic arms can be added to the open sides of the reprap to move the completed part and hold it in place while the extension is being made. This also allows for more flexible placement of the built piece, such as holding it at a 45 degree angle to the bed. Since the arms could also remove finished parts and place them near the reprap, and the grab the pieces later to do some assembly, a useful first build would be a parts bin with a motor to rotate a carousel of parts. Designing this to hold all the parts to make another reprap, the reprap can first build and fill the carousel, rotating it as needed to fill all the bins, then pick the parts back out of it to assemble a new reprap. This would require drill/driver and socket wrench like functions on the arms in addition to the grasping ability. Vitamins needed for the next reprap would have to be placed in the carousel by hand by the operator, but this would still make the assembly of a new reprap much easier for the non technical, even if only sub assemblies could be made this way. Since the robot arms would be made mostly of reprap buildable parts, the accuracy and repeatability would not be great, especially if the reach was long enough to build sub assemblies. To improve on the accurate positioning of the arms, LEDs would be placed at each joint, and 2 or more CCD cameras added to the reprap. When the arm reached a critical position, each LED would be flashed in turn, the cameras would take two exposures, a bright with the LED on and a dark with it off to subtract off the room lighting, and then find the angles from each camera to each LED to compute the arms actual position. The servos in the arms would then receive correction signals until the camera tracking placed each one exactly where needed.
Adding copper to a plastic part can be done in one of two ways. Hot copper wire will remelt the plastic and stick in place. The traces of a circuit board can be built up from multiple wires. Like the fiberglass, a cutter will be needed on the end of the copper wire extruder to break off the current piece without pulling it off the board. Since the wires will not be well connected electrically, especially where a trace breaks up into several lines, it would like still need the solder paste to be added and heated to connect all the wire together. Or as each piece contacts another trace, the outer ring of the extruder can run an electric current through the exiting wire to where the new wire just touches it to weld them together. Another possible approach is to pass a very thin wire through an electric arc (there goes the 200 watts limit!) to melt the wire and deposit the droplets on the surface. This may suffer from lack of control and produce a very blobby. This might be improved by going over the copper after it is deposited with a milling or polishing bit to smooth it out. Another possibility is to build a layer of copper like a copper clad board by pressing hot ribbons of copper over the surface, then welding the edges together. Then the layer would be miller out with a milling tool. This would be easier to get the complex lines and geometry of a PCB than trying to place, bend, and attach copper wire ribbons of individual trace dimension.
The current stepper motors used in extruder is likely over kill. To get the required torque to push the plastic filament through the heater and out the tiny orifice with a direct coupled motor you need a large motor. But unlike the X, Y, and Z stage motors, the extruder motor never turns very fast, the plastic is extruded quite slowly. This is a case where a much smaller motor that is geared down to the low rotational speed but high torque of extruding plastic would work well. There seem to be few stepper motors in this smaller size available, compared to the ubiquitous NEMA 17 motorS, so it might be necessary to switch to a normal, small DC motor. To control the rate at which the plastic is extruded, a current sensor to maintain constant torque should work well. Even with the gear reduction, this should allow for much smaller extruders, and the quad-color extruder I mentioned above. This would also further decrease the cost and mass of non-RP parts.
Mike
Edited 1 time(s). Last edit at 03/03/2010 09:24PM by rocket_scientist.
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