My boss and I were discussing this, and the general feeling was that there could be, but we couldn’t think of them.
As a 3d printer prints in layers, there could be a part of it is connected from the top of the object, but not the bottom. For example, imagine printing a letter C like this, in layers, from the bottom up…
When it starts printing the top part, the end of the C wouldn’t be connected to the main body of the C until the whole thing is completed. I argued that in this case, the object can be rotated, and printed as if its flat, and this seemed to be the case for every object we thought of.
So are there any 3d shapes that couldn’t be printed in layers?
Anything with stuff floating in air.
Try to print an “i”. Well you can print it lying down.
Ok so join to the foot of the first i a second i , at 90 degrees.
Well thats not clear thats a fail, so lets join on the 3rd at 90 to the first two, and then add 4th ,5th and 6th with the feet joining to the foot of the first i, but aiming opposite to the first three
So there’s theres 6 dots, some of which would have to float in air.
Or try printing a cave, with both stalagmites and stalactites. You can’t 3d print stalactites or floaters.
Some 3D printers can print those weird shapes by laying down multiple materials. One is the material that will form the part, the other material is a supporting matrix (e.g. wax) that can later be melted or dissolved away to leave the completed part fully exposed.
In this way stalactites and/or a vertical letter “C” can be printed without deformation.
One other interim solution would be to include supports for distal portions of a part that won’t be connected to the main trunk of the part until the top layers are printed. This would be just a pedestal on the floor of the printing box along with a post that supports the unconnected portion of the part. After printing is done, these supports get cut/ground off.
Can a hollow, windowless sphere be printed, containing an enclosed but loose/unconnected object? How can the loose object be restrained during its creation?
I’m assuming the approach would be to create the inner object first, then print half the sphere, drop in the object, then continue closing the sphere - but what if the loose object had to fit in a specially-shaped cavity inside, such that it could not be dropped in?
You would support the object with a very thin support; if the object has enough weight the thin support could snap off when you shake it. Then you got yourself a rattle.
OK, so there are some permutations of this type of config that won’t be possible - for example, if the contained object is delicate, or there is a specification for no support remnants left inside.
The usual way to deal with overhangs and stalactites is with support material, that you cut away or dissolve after printing. For the delicate floating object inside a specially-shaped cavity, you’d probably have to cheat and print the outer shell in pieces to be assembled after printing.
Anything with a completely separate piece (such as a letter i) can be classed as two separate objects, so I wouldn’t include them here.
Also, printing a cave with stalactites and stalagmites could be achieved if you turned it through 90 degrees, so the back of the cave was the base of the object. As the sides of the caves were constructed, the stalactites and stalagmites could be printed horizontally.
I came up with about a dozen objects that I thought would fit the criteria, but in every case, I could find a rotation that would let it be printed. For example, two intersecting rings would be difficult (arranged as if they were two circumferences on the outside of a sphere), unless you took the base as the point where the rings crossed
Like most techniques for fabrication there are shapes that work out best being made in multiple pieces that are later attached together. It’s very difficult to make anything that doesn’t have a flat surface to serve as a base, or has interior concavities. But you can usually split these things in half and glue the completed pieces together.
Do you mean without supports? Because that would only work for a slice of a cave - turning a whole cave through 90 degrees means there would be horizontal protrusions into empty space, instead of vertical ones with no root.
How about a hollow cube with a stalactite/spike perpendicular to the middle of each inside face, or just a hollow ball that is uniformly spiky on the inside?
You could not produce such an object in a single step by traditional manufacture, either. Some assembly required.
It also depends on the sort of 3-D printer - extrusion printing allows some overhanging material, but not much. You could possibly build and enclose a sphere, but producing the inner object without it adhering to the inner surface of the sphere may be too hard. Selective Laser Sintering/Melting would be better, as you can leave layers of matrix between the sphere inner surface and the enclosed object. The problem is removing the matrix material from within the enclosed sphere once it is complete, and the matrix is very fine, and does not support overhanging.
Not everything can be 3D printed. Many 3D printing materials are not yet suitable for some tasks. Surfaces are not yet high enough resolution to be smooth. Combination devices are being developed (milling/tooling and 3D printing), and new materials/techniques are being developed, but I think that there is a long way to go.
OK, I have one that can be manufactured by conventional methods by carving from a single chunk of matter, but not by 3D printing (sans supporting structures trimmed away afterwards)…
A cube, each face of which is studded with a pyramidal arrangment of perpendicular spikes (that is, tall spikes in the centre of each face, surrounded by progressively shorter ones)
Any orientation of the cube will require the creation of downward-pointing spikes that don’t touch the ground.
It also depends on the type of 3d printer. The large (expensive) Selective Laser Sintering machines would not have a problem as the layers are built up by in layers of powder. So you have the powder there to support the structures that are temporarily ‘floating’.
Haven’t you ever taken a loop of paper, cut it, twisted one end halfway, and re-taped it? One moebius strip right there, and yes, you could 3D print one.
Objects that have significant internal tension cannot yet be printed. For example, a strung bow or a horseshoe magnet. Orientation does not matter, only the object as a whole is in equilibrium, not any subset of it.
As for the letter “C” problem, simply expand it into three dimensions. Take a cube, and attach a letters “C” to a face, so that it looks somewhat like cow horns. Attach another letter “C” to the same face, but rotated 90 degrees, so there’s two pairs of cow horns. Repeat for the remaining five faces. However you orient the shape, there’ll be a hanging bit during creation.
