Printing the mold
Open the 3MF in Bambu Studio (or the STLs in any slicer) at 100 % scale. Every part arrives in its printing position: flat side on the plate, cavity facing up.
- Filament. PLA is fine for concrete, plaster and wax, and for a handful of resin casts. PETG survives more pours and a hot summer car. For a model with undercuts (red in the preview) that you still want to cast, print the mold in TPU and it will peel off.
- Layers and walls. 0.2 mm layers, 3 walls, 15 % infill. Switch on ironing for the top surfaces if your slicer has it — the parting face seals better when it is smooth. Cavity walls are what the cast will look like, so a 0.12 mm layer height on a small resin piece is worth the time.
- Supports. None, where the preview is green. Where it shows red in the Print view the slicer will want supports; use tree supports with a 0.2 mm top gap and sand the marks.
- Seam. Set the seam to the rear, or paint it onto an outside face. A seam inside the cavity prints onto the cast.
- Smoothing. Layer lines show in a cast. A coat of XTC-3D or a few passes of filler primer and 400-grit paper inside the cavity gives a smooth surface; for concrete most people leave the lines, and they look intentional.
Casting concrete
- Mix. A fine casting mix (no gravel) or a quick-set cement picks up the most detail. About 2 g of dry mix per millilitre of cavity; the To mix figure already assumes 2.2 g/mL cured, so weigh 10 % extra.
- Release. Rigid plastic needs it: a light coat of cooking spray, wiped nearly off with a paper towel, or a paste wax buffed thin. Too much release pools in detail.
- Pour. For a two-part mold, close the halves on the pegs, band or bolt them, stand the mold spout up and pour slowly down one side of the spout so air leaves up the other. Tap the mold on the bench, or hold a sander against it, for a minute — bubbles rise out of the vents and spout. Top the spout up as it settles.
- Open molds. Overfill a little, tap, then strike the back flat with a straight edge after half an hour, when the surface has stiffened.
- Cure. Cover it with a bag so it does not dry out and leave it 24 hours (48 is better for thin pieces). Unmold on the pry slots, trim the spout flush with a file, and let it cure another few days before sealing.
- Vessels. Pour the body about two thirds full, press the core in until its collar sits on the rim, and tap; concrete rises round the core. Weigh the core down.
Casting resin
- Release is not optional. Epoxy and polyurethane bond to PLA and PETG. Use a proper mold release spray (or a silicone spray), two light coats, dried between.
- Seal the seam. Resin is thinner than concrete and finds the parting line. The rail alignment keeps most of it in; a strip of tape round the outside catches the rest.
- Pour in a thin stream from high up, into one side of the spout. Warm resin (a few minutes in a water bath, still sealed) releases bubbles more easily. If you own a pressure pot, this is what it is for.
- Exotherm. Keep single pours under the volume your resin's sheet allows; a thick block cures hot enough to warp a thin PLA wall. The To mix figure tells you the volume.
- Demold when the resin is firm but not fully hard (follow the sheet); it flexes a little and comes out cleaner. Trim the spout and vents with a flush cutter and sand.
Plaster, wax and soap
- Plaster sets in minutes and expands slightly, which is why it fills detail so well and why draft matters: a rigid mold with nearly vertical walls grips a plaster cast. Soap the mold lightly first, and unmold within the hour.
- Wax shrinks as it cools and usually drops out on its own. Pour below 80 °C — hotter wax softens PLA. A mold printed in PETG lasts longer.
- Soap (melt-and-pour) wants the mold chilled for ten minutes before unmolding; a spray of alcohol knocks surface bubbles down before it skins.
- Chocolate and food are not what these molds are for: a printed surface is porous and cannot be properly cleaned. Make a silicone mold from a printed master instead, in food-safe silicone.
Making a silicone mold
Choose Silicone master and the model prints standing in its own tray, as one piece. Fill with a 20–30 Shore A tin or platinum silicone to the top edge (the To mix figure is the volume), let it cure, peel the tray off, and cut the block open down one side with a wavy cut so it registers itself. Smooth the master first: every layer line becomes a ridge on every cast. Flexible molds release the undercuts a rigid one cannot, which is the whole point.
How Hollow works
The model is measured as a signed distance field: at every point of a fine grid, how far to the nearest triangle, negative inside. Inside and outside are decided by casting rays along all three axes and taking a vote, so a model with a few holes still comes out solid. A block, the spout, the keys and every other feature are simple shapes written as distances too, and combining shapes is just taking the smaller or larger distance. The result is traced into triangles with marching tetrahedra — which always gives a closed, printable surface — and then simplified so a flat face is a handful of triangles instead of a hundred thousand, while curves stay within a fiftieth of a millimetre.
The release map comes from the same grid: for every face of the model, a point just outside it is checked for model material above or below in its column, depending on which part of the mold it sits in. Air traps come from sweeping the cavity from the top of the pour downwards and noting every pocket that appears before joining the rest. All of it runs in your browser, in a worker, and nothing is uploaded — the page has nowhere to send it.
Hollow is open source under the MIT licence. The code is in kmay89/ABOUT.