vendredi 27 décembre 2013

3D Printing

I didn't want to step on Dessi's thread so I thought we could discuss 3D printers here in more detail.



There are two kinds of plastic resins. The first are thermoset resins like epoxy. These are liquid to start with and then set like glue. Once they've set, they are rigid and can no longer be molded or formed. There is a special kind of light curing resin which includes catalysts that cause a chemical reaction under ultraviolet or laser light which then causes the hardening of the resin. These were more prominent in the early days of prototyping machines.



The second type of resins are thermoplastic resins. These act much like wax or hot glue. They are solid but can be melted with heat and then they become solid again as they cool. These are the plastics that are being used these days in low end 3D printers. So, let's go over how this works.



A regular printer uses stepper motors. Unlike a regular motor which spins when power is applied, a stepper motor only rotates a fraction of a revolution with each pulse. The amount that it turns depends on the windings so a given motor might rotate 1/8th of a revolution with each pulse. Printers use one stepper motor to move the printer head left and right, and use a second stepper motor to roll the paper forward. Since printing with ink only requires two dimensions, two stepper motors are enough.



With a 3D printer, you need a minimum of three stepper motors. Given the size of the parts, these can be the same stepper motors that are used in regular printers. Obviously, your object is going to be built on a platform. So, these can either be design to move the platform or to move the printing nozzle. However, unlike a regular printer, a third function will be required to move the nozzle or the platform up and down.



The feedstock is wound, plastic filament which looks remarkably like weed trimmer line. Basically, this line comes off of a big spool and is then carefully advanced to a heated nozzle where it melts and is applied to the project surface. This is quite similar to a hot glue gun but the nozzle is much, much smaller. A typical nozzle is 0.4 millimeters (1/64th inch) wide and the depth that is built up with each pass is around 0.2 millimeters (about 8/1000ths of an inch). In comparison, a human hair is about 0.1 mils or thousandths of an inch thick. A roughness 100x that of a hair would be pretty bad but since the plastic smooths somewhat as it is applied, this improves a bit.



Having each application adhere to the old one is a problem. It is necessary to have the plastic hot enough to partly melt the original surface. If this doesn't happen then you end up with a loose aggregate like some types of Styrofoam that will break apart into little beads. However, this also depends on the type of plastic used. Some plastics require a heated base so that the project stays warm enough to melt where the new plastic is added. This is not unusual with ABS.



ABS and PLA are the most common plastics used with low end printers. These require temperatures around 400 degrees F to melt. ABS takes about 430 degrees while PLA only needs about 360. Because of this, PLA doesn't usually need a heated base. It also puts off less odor than ABS. Since both of these cost about $20 a lb, it is easy to see why PLA would be preferred. However, neither ABS nor PLA are durable plastics. Actual weed trimmer line is made from tough nylon which has a similar cost per lb. Unfortunately, it requires about 600 degrees F to melt nylon and when used with a standard printer, it tends to pull strands off the newly applied plastic which can leave the surface spiky or hairy. PET or polyester is something of a compromise since it is stronger than PLA but only about half the strength of nylon. It also melts in between at 500 degrees F. Unfortunately, PET costs more a lot more at about $50 a lb.



The length of movement determines the size that a finished product can be. Cheap printers seem to have a span of 100mm or about 4 inches in each direction. I guess that would let you make something the size of a croquet ball which would cost about $25 in plastic. Larger, low end printers have a movement of about 200mm or about 8 inches. This would let you make something the size of a soccer ball. Of course, a plastic ball this size would weigh about 10 lbs and therefore cost about $200.



Some other things that are not necessarily mentioned are that these plastics tend to absorb moisture so you have to make sure the plastic is dry before you use it. This isn't too bad though since it could be as simple as putting the spool inside a garbage bag with drying crystals after each use. If you don't have drying crystals handy, you can use baking soda or some kinds of cat litter. Also not mentioned is that nylon is slippery so getting enough traction on the feed mechanism can be more challenging. Also, the nozzle will tend to put down a round pattern so there needs to be a slight overlap in positions to avoid gaps. Things like cleaning the nozzle head before you start may also not be mentioned.



The starting procedure typically involves putting a layer of blue, painters tape on the plate. This lets you remove the project when you are done because otherwise it would end up welded to the plate and be very difficult to remove. However, the plastic also has to partly stick to the tape since you don't want the project sliding around and you also don't want to have to scrape the tape off. Nylon will not stick to painters tape but some have suggested that you can use spray adhesive on the tape to get it to work.



A printer kit can run from $200 to $1,000. The options seem to open up a lot around $600. One of the problems though with having truly nice 3D printer work is tuning. The best way of tuning would be to video the action with a high speed camera so you can identify the problems. Then, you need to be able to make detailed adjustments to the control software to tweak the process. However, most people don't have a high speed camera and not all of the control software can be easily adjusted. I might also mention printing speed. Typical movement rates are 20 - 40 mm per second. I suppose the worst case would be a solid cube 200x200x200mm. This would take 6-12 days of nonstop printing to complete.





via JREF Forum http://forums.randi.org/showthread.php?t=271004&goto=newpost

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