3D Printing and Sanding

Print Bed

If you haven’t bought one already, take into consideration the print bed size of the 3D printer you are looking to buy. This has huge implications in the way your models will be sliced up and oriented before printing. A smaller print bed means more slicing, more pieces to assemble, and more seams to hide as you finish the armor. More seams means more possibility for broken parts during inevitable drops or collisions. You should also consider the availability of nozzle sizes; more on that later.

 

Supports

Do some research to find the optimal settings for your printer when it comes to supports. It is best to minimize supports wherever you can. Supports can leave behind a lot of material that will need to be sanded off later. They also leave room for printer error if they get too tall or unstable. Orient your models in such a way to minimize overhang and eliminate the need for supports. They’re a necessary evil, but with the right settings they can snap off the print easily and cleanly.

 

Infill

Infill will affect the weight of the print. A bunch of small pieces alone may not add up to much, but assemble them together and strap it to your body for a few hours and you’ll have some regrets. Unless it’s going to be under a lot of stress, don’t print anything higher than 10%. For armor such as a forearm that will be strapped to your arm, 5% is just fine. Smaller prints like a knife or accessory could have higher infill for stability, but again keep in mind where and how it will be used. A helmet antenna attachment doesn’t look like much alone, but it will tilt your helmet to the side if the helmet isn’t balanced. 

 

Layer Height

Also consider your layer height as you print. Thinner layers take longer, but leave a smoother surface that will need less filling and sanding later. More machine time = less human time. Print your layers as thin as your machine and your patience can handle. There is a world of difference between a print at 0.2mm and 0.15mm. Machines can have smaller nozzles as well which makes these thinner layers easier to print.

 

Brim

Pieces that have a small contact area with the print bed can come loose easily and ruin a whole print. Add a brim around these pieces to stabilize it and keep it on the bed for the duration of the print. Glue on the print bed also helps with adhesion.

 

Assembly

If you had to cut up models into smaller parts to fit on the print bed, there are a couple options when it comes to reassembling them. It is helpful to use connectors when cutting them to ensure they fit together smoothly. Use super glue or E6000 between the flat surfaces. You will want to rough sand these surfaces with 80 grit if they are too smooth to give the glue something to grip. Clamp together when possible and let the adhesive cure. Lower impact pieces may be good with this alone, but to really tie them together use a soldering iron to weld them together. That’s right, melt the seams to make it one piece. This takes a gentle hand and a delicate touch so practice on less visible areas or spare pieces if you’re feeling nervous. If you press too hard, the hot tip will pierce the PLA like a well…hot knife through plastic. Get your PPE and use a flat soldering tip to melt the PLA around the seams into each other. This can create a depression along the connection, so it may need to be filled with putty/filler and sanded down. You can also take spare filament to melt it onto the seams like actual welding and then sand it down or smooth it flat with the soldering iron. CAUTION: if you have any glue overflow that comes into contact with the hot iron, it will burn and become aerosolized. The burned adhesive will smell bad and feel bad on your eyes so again, please wear PPE. Gloves, glasses, mask at minimum and work outdoors if possible.

 

Filling and Sanding

The layer height of your print will determine the roughness of your pieces and the grit you will need to efficiently sand them down to smoothness. See what I meant about machine time vs human time? If you’ve got a nice thin layered print, it will take a finer grit to remove those print lines. Your best friend for the sanding process will be Filler Primer. It comes in a spray can and is a tremendous help in filling the tinier print lines which makes that first sanding pass easier. For example, a print at 0.2mm with two coats of filler primer can be sanded to reasonable smoothness with 220 grit sandpaper. Invest in a handheld orbital sander for your bigger pieces to save your arm. It is 7th Corps policy to always wear proper Personal Protective Equipment, or PPE, so wear a dust mask or respirator so you aren’t breathing in filament or primer.

 

As an alternative to Filler Primer, you can also use Bondo Spot Putty for bigger gaps or larger print lines, especially above supports, though I have seen some people coat their entire prints with it. The spot putty is toxic so as with any chemicals, make sure you read the warning labels and take proper precautions. This is also sandable after it dries.

 

A rough print or bad supports can require 80-120 grit sandpaper. This is literally scraping plastic off so use it carefully, a couple passes at a time. As things smooth out, up your grit to 220, then 320+ depending how smooth you want it. Washable sandpaper sponges and cushioned handheld grips are great for detailed sanding work. Small thin filing tools are also very useful for those nooks and crannies, though honestly nobody will be looking that closely except for you. For mirror-like smoothness, wet sanding with 2000 grit waterproof sandpaper will be your final step, but is optional if you’re looking for a more roughcast look.

 

Once everything is sanded, rinse off the dust and you’re ready for paint!