WeaselTech - 3D printing swarm mecha

I have lots of old metal BattleMechs for my heroes to pilot but WeaselTech also requires swarms of less powerful enemy mecha.  There are lots of great BattleMechs on Thingiverse and Cults 3D and I thought I could print some of these at a reduced size.  This also seems like a good opportunity to run through the process I use for 3d printing, just in case there's anyone out there curious about the process who hasn't read up on it elsewhere yet.

My first task was to trawl through the scores of mechs available to select suitable candidates.  I wanted some of the ones with less obvious cockpits and a slightly less "human" appearance.  I reduced them all to between 45% and 60% of their original size.  Given that they were a range of sizes to start, this brought them all down to 20-25mm in height.  At this size, they will also be able to double up as combat droids for my 15mm games.  A couple of different pieces of software were used in preparing them for printing.  Models were resized and supports added using Chitubox.  I angled the models to try and reduce the print lines to some extent, and so the supports attach to the rear and underneath where they'll be less noticeable.  The supports ensure that there are no "islands" where the printer tries to print something in thin air with no attachment to the main body, for instance the bottom of a crooked elbow, the toes on a slightly lifted foot etc.  The supports hold the part in place until the layers above can connect it to the main body, avoiding melted-looking lower limbs or bits of hardened resin floating around the resin vat and sticking to a different part of the model. 

Thanatos 'Mech at 55% with supports.

I've angled the 'mechs slightly and printed them 3mm off the build plate.  This helps avoid "elephant's foot", a thickening and loss of detail in the bottom layers. This is because the printer will over-expose the bottom few layers (in my case by 10x the normal exposure length) to give a really firm base layer that is bonded tightly to the build plate.  This ensures that the print doesn't fall off into the resin vat part way through printing due to the suction as the printer lifts the model between each layer.  There tends to be some light bleed around the edges, resulting in a thin rim of additional cured resin around the base layers.  It doesn't matter with some things, I often print scenery flat on the build plate because it's easy to just sand off the rim on something like a crate, or hide it in the ground cover. Epic scale vehicles often work well flat on the build plate too, slight loss of detail on the tracks isn't noticeable on the tabletop.

Archer 'Mech at 45% with supports.

 

Once prepared, the 'Mechs were grouped together onto the build plate and "sliced" for printing using the Anycubic software that came with the printer.  Slicing is the process of the computer taking the 3D model and cutting it into the individual slices or layers that the printer will cure.  This is the point at which you can specify how thick each layer is.  Thicker layers = less detail, more obvious print lines, but faster print time.  Thinner layers = more detail, less obvious print lines but longer print times.  You also set the exposure time for the resin, too short and the resin captures detail better but is softer and may come apart during printing (the suction as the print lifts from the vat can cause the model to break off).  Too long an exposure and the UV bleed through the resin can cause additional curing that obscures detail.  This all sounds very complicated but fortunately printers can use a "RERF" (Resin Exposure Range Finder) file that usually comes pre-loaded on a USB stick.  The RERF prints 8 different exposure times on a test object with lots of different textures, angles and sizes.  Examining this makes it easy to see the best exposure time for your particular resin.

I managed to fit 10 'Mechs onto the build plate, so 2 print runs gave me a good sized enemy force.  You can enter the price of your resin into the slicer and it will estimate amount of resin used and the cost.  These 10 'Mechs used about 14ml or resin and will have cost about 50 pence - or 5p each. You always lose a bit more resin during the washing process, but they still work out incredibly cheap and you can see why companies like Games Workshop are worried about people printing a fan-sculpted model for only a few pence when they want to charge you £10 for the official version. But that's a whole different topic that I won't get into here.

Starting to print - just the supports so far.

The printer works by lowering the build plate into the resin vat until the thickness of resin between it and the UV screen is the desired thickness.  I've chosen 50 microns (1/20th of a millimetre) in this instance, though I could print at 20 microns if I wanted really fine detail. The UV LED exposes the first layer, an LCD screen blocking out areas that won't be cured. The build plate lifts, allowing more resin to flow underneath.  Temperature can affect this - too cold and the resin thickens and flows more sluggishly, potentially leading to failed prints because it hasn't completely refilled the void. Down comes the build plate for the next layer, and so on until it's finished.

 

Stirring the resin.

I usually give the resin a stir with a silicone spatula before I start printing.  The pigments can settle, so a gentle stir with a soft, flexible implement ensures it's well mixed.  The transparent FEP film on the bottom of the resin vat is easily scratched, even a paper towel can damage it, so a spatula from Poundland is an ideal solution.  Waste resin is easily cured with a small UV LED torch after which it is safe to put in normal waste.

Curing waste resin.

The speed at which the build plates lifts and descends can also be adjusted, too fast a lift and the suction can cause the model to break or detach from the build plate but so far I haven't had any problems with this.  Printing took under 3 hours, then the models were rinsed in isopropyl alcohol to clean off the uncured resin left on the surface.

 

Printing complete!  Ready for rinsing.

I have a wash and cure machine for cleaning the models, I simply remove the build plate from the printer by means of a single nut and hook it onto a bracket that suspends it in the tub of isopropanol.  Appropriate safety equipment should be worn at all times when handling resin or anything that has been used for washing resin.  Safety specs and a mask protect your mouth, nose and eyes from resin splashes and nitrile gloves protect your hands.  Latex gloves shouldn't be used because they are sometimes permeable to some of the chemicals involved.  The risk of ingesting or absorbing through the skin is not something to be taken lightly but a few seconds putting on the appropriate PPE reduces the risk enormously.

Mask and safety specs for face protection from splashes.

Nitrile gloves (not latex, which is permeable to the resin).

The washing tub has a magnetic stirrer built into the bottom, I just set the timer for 2, 4 or 6 minutes and it stirs one way half the time, then reverses for the remainder.  You don't have to buy a wash & cure machine, simply dunking the build plate into a tub or isopropanol or other suitable cleaning medium and shaking it around can be enough. You can also remove the models from the build plate and drop them into a mesh basket for washing but I rarely do this because most of the things I print are too small and fall through the mesh! 

Cleaned models ready for support removal.

After washing, I remove the models from the build plate using a razor blade.  Most printers come with a steel spatula but I'm a bit nervous about using something like that for suck delicate models.  A razor blade held almost flat against the surface of the build plate can get under the edge and pop them off without too much difficulty, especially if the supports have a tapered edge.  The models are left to air dry on a square of plastic breakfast cereal bag.  I've found this is ideal, it's waterproof, free and easy to peel cured resin off afterwards.  The resin is still slightly soft and bendy at this stage, with an almost waxy feel.  This is the best time to remove the supports.  Dunking the model in very hot water for a few seconds softens the resin still further, making it easy to peel off most of the supports.  A little bit of clean up with a sharp craft knife completed the first stage.  It's best to keep a pair of nitrile gloves on at this stage, as much to protect the soft resin from your fingerprints as to keep you safe from the resin.

Archer with and without supports.

Archer with and without supports.

It's worth saving the supports, when given a suitably rusty paint scheme they make excellent industrial wreckage and can easily be mixed in with building rubble.

Removed supports.

 

With the supports removed, the models were ready for their final cure.  I water cure my models, this simply involves dropping them in jar of warm water and curing for about half the normal amount of time. In this instance, it means 4 minutes in the machine, which rotates the jar slowly to ensure even curing.

UV curing the models.

Oxygen retards the curing process, which can leave the surface of a 3d printed model feeling sticky or slimy and causes problems with paint failing to adhere to the surface. Water contains less than 1% the amount of free oxygen than air and also acts to refract the UV light around the model.  These two factors result in a much better finish, giving crisp, hard surfaces. Warm water contains even less oxygen - but I mainly use warm because it's more pleasant to put your fingers in!

Curing the models.

 After curing, the models are left on a paper towel to air dry.

Drying mecha.

Once dried, any remaining support stubs were sanded smooth. They were superglued onto bases and a smear of Wilko wood filler used to add some texture to the base.

Glued to bases.

PVA and a sprinkle of sand/brown tile grout finished the preparation work and left the models ready for painting.  A few drops of watered down PVA on the bases ensured everything was fixed firmly in place.

Base texture completed.

When painting figures, I tend to attach them to a long wooden batten using a strip of double sided tape. This makes it easy to manoeuvre them whilst spraying, and I used my usual Wilko grey primer which gives a nice fine finish and has excellent coverage.

Primed and ready for painting.

Next time I'll cover the painting and show a few of the finished models.