How do 3D printers work? And what are their limitations?



How do 3D printers work? And what are their limitations?

In: Technology

There are two main types of consumer 3d printers. The first is called PLA, and it takes a plastic like filament, which it melts a tiny bit of the PLA and builds things up one tiny drop at a time, and cooling it until it solidifies. The second called SLA and it uses a liquid resin that is hardened by a laser or a DLP projector. The main limiters of the type of printers is that they can only be made with these specific materials, which limits the strength and durability of what you can create with them. Certain shapes aren’t possible, or require extensive support structures that then need to be removed. Especially PLA is limited in how smooth or how much detail that smaller objects can have based on the smallest size of the melted substance you can extrude at a time (called layer height).

There are many other types of 3d printing but they are either at an industrial scale, or just not as popular in the consumer market.

3D printers are conceptually relatively simple.

First, you have an arm, that using a mix of gears, belts, or pulleys. Is able to be programmed to move forward, backwards, left, right, up, and down.

How big you can 3D print something is determined by how large of a frame you put this system on.

Then, you run a feed to that arm that carries your medium, usually plastic, these thing strands of plastic are then run through the arm where they are heated to the point of melting just as they get pushed out onto the item being printed. Or alternatively you might here people “3D printing a house” where instead of using plastic they use cement on a much larger scale.

That’s the basics of how it physically functions, belts moving around an arm/pointer that deposits plastic.

Now integrate that with 3D CAD software that gives the printers instructions on where to deposit plastic and where not to deposit plastic, and you can print out items.

Most of them work by extruding small dots of molten plastic onto a tray. It puts all of the dots of one layer down, moves up a touch, then puts down all of the dots of the next layer. It repeats until the object is completed.

There are five main limitations:

– It can only make things out of materials that can be printed. You can’t make objects out of metal or ceramic or other materials that can’t be heated and extruded.

– They can (generally) only print one material at a time. This means that printing complex objects (like a wire) are not possible, further limiting what can be printed.

– The printer can only make a dot so small – called its resolution – so the detail of any object you print is limited by the resolution of the printer.

– It can only print an object that fits inside of the print area. You can get around this by printing multiple small objects and putting them together, but it is still limited by the size.

– They take a _long time_ to print. Since they go dot by dot, it can take hours to print an object of any real size.

For these reasons, they are primarily used for rapid prototyping of simple objects or when a variety of plastic items might be required (for example, they have one on ISS because they don’t know what type of tools or parts the might need). For anything of scale, traditional manufacturing is still faster and cheaper.

Ones you can buy for home use melt plastic and use motors to carefully place the melted plastic on a build plate. It builds the object one layer at a time, stacking the plastic on top of the previous layer. You can almost always see each individually when you look closely at something 3d printed.

There’s another type of home 3d printer where it uses UV light to solidify a layer of resin on the build plate. It works basically the same as above, but instead of melting plastic it uses a tiny LCD screen to stack the individual layers of solidified resin.

You can scale up both of these methods – for example, you can use the first method to make 3d printed chocolate, clay, cement (even things as big as entire houses).

You can scale up the second method to create parts in metal – picture a sandbox of metal dust and a laser scans a layer of the part sintering together the metal it hits, then it spreads more metal dust on top and repeats.

The limitations is a little more difficult to answer. You are limited in materials – for the first type you need something liquid that can stack without squishing itself apart. Additionally, the strength of the thing you print is also limited and not usually as strong as other manufacturing methods. Most industrial uses of 3d printing use the printer to make a mold which then they use in traditional manufacturing.

There are several types of 3D printers all with their own advantages and limitations. The most common consumer 3D printers however are plastic extruder based. This is essentially an automated hot glue gun. A long plastic fulfillment is pushed through a heated nozzle so it comes out as molten plastic. By placing the nozzle exactly where you want the plastic when it melts you can create plastic models. It is a cheap and fast way to go from a computer model to a physical object. But there are plenty of issues which makes this hard for industrial 3D printing or large volume manufacturing.

Firstly there is issues with creating overhangs. You can not get plastic to hang in free air so you need plastic under everything. I is possible to build temporary plastic scaffolds that you remove later but it is still a limitation. Secondly the surface finish is often not the best. You can not control in detail where the molten plastic is going to end up before it hardens so the surface tends to be fairly rough and may need some finishing work. Thirdly you can not use any kind of material but you need to use plastic that can actually be melted down and recast safely. Not all plastic can do this. And lastly the strength of the finished product is often not very good. The obvious issue is that the different layers do not always fuse together perfectly so if you apply a force through the layers you can separate them. But maybe more important is that mass produced plastic products usually have a lot of glass fiber in it to act as reinforcement to bind the plastic together. This is obviously not possible for 3D printers because the different layers are made at different times and you can not get the fibers to cross between the layers and bind them together.

For industrial level 3D printers you usually use a more expensive but better method. Instead of filling air with molten plastic they can fill a sand with glue to make 3D objects. You lay a layer of sand and glue it to the layer underneath before you lay down the next layer and so on. This gives you much better control so you get much higher fidelity and surface finish. The figure does not warp or need scaffolding while you make it. You can even add dies to the glue to make colored objects very easy. You can also use various other materials instead of the sand and glue to for example make metal sintered objects or a ray of other materials.

Normal printers work by scanning row by row across a sheet of paper and either putting a dot of ink down or not, depending on of the paper is supposed to be black or white. (Let’s forget about colour for now. It’s basically the same but with more options for each ‘pixel.’)

3D printers sort of work the exact same way. Except, the ‘ink’ they use is a blob that has depth. Once they complete one layer, they lift the nozzle up slightly and do it all again. They keep going layer by layer until they reach the top of the model.

A few caveats. If something is floating in the model, they’ll print some thinner pixels below it. These are just to support it during the printing process until it becomes connected to the rest, and are meant to be cleaned off afterwards. Sometimes, these supports are made of a material that dissolved in water. Also, fancier printing software improves speed by only making the nozzle go where it needs to. E.g. if you were printing a cylinder, it would make more sense to go round in circles with the nozzle, rather than going across the whole ‘page.’

3D printers are fairly simple to understand, conceptually.

To illustrate how a 3D printer works, I’m going to liken it to a regular paper printer.

A printer works by laying down material (toner or ink) on a paper in lines, while a separate motor moves the paper through the printer.

A 3D printer does the same thing, but with two more axes of movement. Instead of having the head move just from side to side, it moves in and out as well as up and down. This gives the printer its “3D”.

The print head is given instructions on how to move to make the object it’s printing. What it’s basically doing is drawing in layers to form a 3D object.

There are other types of 3D printers, like resin printers, which work differently. A resin printer only has one axis of movement. When printing, the print head is dipped into a resin fluid and a screen underneath the fluid flashes light to harden the resin. This is done in layers to build up a 3D object.


Another commenter listed out a very good list of limitations. I don’t think I can do any better, so I differ you to [their comment](