Another day another post. So I was waiting to ask someone a question at uni today so last night and today I've done a heap of research, which I've put into sections (sort of) to try and make sense.
So firstly, in the last post I was talking about whether to use slave controllers. Well I realised today on the bus, that the slave controller idea would also be far better for the extruder controller. This is because it would be far easier to upgrade that with more pins for more functions such as multiple extenders, sensors and fans compared to constantly upgrading the master controller. It also as I think I mentioned last time could do errors and monitoring to stop annoyances such as this (which as is stated at the end of the page, was caused by poor quality filament). Talking of which I found a blog for an electrical engineer in the UK who has been playing round with a MendelMax also so I got some ideas of there. The site is http://richrap.blogspot.com/. Staying with controllers, I've decided I'll go for the ATMega micro controllers for the slave controllers. They seem to work well, my mate has one and loves it, and my current MSP430 chips are annoying me to death. I've been trying to fix a problem (even directly used their sample code and wouldn't work) with one for over two weeks and still isn't working. So I've decided to give up on the MSP430 side of things for now, I'm sure other chips in the family work but the two I'm using from the G2XXX family are rubbish. I decided I also wouldn't use a PIC as it didn't support GCC which is what I would prefer to use for development as it is available on multiple platforms (I use Mac OS X, Windows 7 and XP and multiple variants of Linux).
Now for some heating. I decided early on I would prefer a sort of climate control, as in a heater to keep the ambient temperature constant and then fans to extract heat when necessary. The bed and extruder would be within an enclosure to keep the temperature constant also by reducing convection. So looking into it I found that the best type of base heater is a PCB type, basically small wires on a PCB with excess current being pushed through so it heats up. This is elegant but can be expensive if the boards are professionally made. (Reference: http://reprap.org/wiki/Heated_Bed). I think this is the best option still and can be made to have etched warnings on the other side of the PCB. The RepRap has a 300x300mm PCB heater but this is really a tad on the small size for what I would like. I also think that a glass top would be the best for the base of the work area so to spread the heat more evenly. In this post it is recommended that a mirror could be used instead as it had better evenness in the heat. A page on the UltiMaker site said that sheet metal was better than glass at evenness by a magnitude of 27 times. This is relative to pure glass though and since mirrors have a thin metallic layer, I think the mirror would work better as sheet metal has sticking problems, gets scratched and hence has to be replaced every so often. The same site also mentioned how ventilation and fans can cause localised cooling on the head bed and some of the effects. I think I would incorporate two fans one for blowing air in and the other air out. This though is just thoughts and I haven't analysed the best practises here as this is more of a feature than something that is required to print.
Moving onto items of a building nature, I think the electronics should be placed underneath the heating bed. There would obviously need to be insulation between the bed and the electronics so to not cool the electronics but I think its the best place for it. I initially had thought to make one of the walls contain all the electronics but have since decided that it would be nice if all four sides were clear with perspects or glass (perspects might melt I realised so might have to be glass). All the joints for holding things together should also be aluminium in this design as firstly I don't have a 3D printer to print plastic parts and secondly its still lightweight, cheapish and looks great.
Now something a tad more interesting and very central to the printer, the extruder or jet as I might occasionally call it. Firstly there are lots of designs and the background on simple extruder's on the RepRap site here. I will use this basic design for the plastic side of the extrusion. Qu-Bd came up with a simple idea for making the filament stay in the middle of the gearing that pushes it through so to make sure it won't mass a beat. This is done, quite simply by having the middle of the gear curved. As I've said before I want to be able to have multiple extenders so to I can get different colours during a build. So tonight I had a look round at different systems for stopping fluids. I had envisioned a pneumatic system that sort of sucked up the fluid if it needed to stop quickly but I soon realised this was complex and maybe not even practical. I kept looking round and eventually realised that in essence it was just a valve I needed. So I found three types of valves that might be useful, diaphragm, pinch and knife. The first two are two complex and expensive to use so a simple and fast moving knife valve should do the trick. The design I'll use will be slightly different from a conventional knife valve though as it needs to be able to fast move. This shouldn't be a problem though as the area that the valve will be acting on will be quite small. Just for reference on one way of mounting a three way extruder, (though this one does not use valves to stop drips) here is one from the blog I have referenced prior.
Now here is something exciting, the chocolate extruder! Firstly this extruder will have to be separate from the plastics one above due to cleanliness and safety. This means that the carriage that holds the extenders will be made for quick changes. I've decided that I'll use a separate device to get the chocolate into the correct state. This can be controlled by the master controller but has to be separate as to keep aesthetics as well as weight considerations on the structure. Due to this I've realised I should be able to pump the molten chocolate through a heated tube similar to this Freeflex Heated Tubeing straight to the jet to print. This is lightweight and means it easier to keep a constant supply of chocolate on the go for large models. With chocolate, I've found out that there is different phases or types of chocolate and that one of these is particularly favourable. This is due to the higher strength, higher melting point and crunch of the phase. This means that chocolate should be heated up to approximately 40 degrees, given time to cool then heated to between 27.3 and 33.8 so phase IV will melt but phase V (the desired phase) won't then its ready to be printed. If there are impurities such as milk in the chocolate the temperature range decreases.
So with all that in mind, I think after exams I will buy an ATmega development board, get it working with my LCD I2C screen (I2C is the thing that just won't work on my MSP430) and then buy a stepper motor and make a driver up for it. Then if that goes well I can start designing more of the electronics. On the structural side, I need to work out how rigid everything needs to be and start formalising my designs by constructing them in inventor rather than my mind.
