Biohazard Coasters — raised trefoil that lets go of a wet glass, plus caddy

Ø90 mm drink coasters carrying the biohazard trefoil — and the trefoil is what the glass stands on. It is raised 0.8 mm above the well floor, so a cold, wet glass rests on three lobes with open air between them and never suctions to the coaster. The mark still sits 0.4 mm below the rim, so the coasters nest on the rim and the ink never touches the one above it. Four coasters stack into a drum caddy with a thumb scoop.

What you print

5 prints from 2 files. Nothing to assemble — you stack the coasters into the caddy.

Every part prints flat on the bed as exported, marked face up. No supports anywhere — the caddy's thumb scoop cuts down from the rim, so nothing overhangs.

Two colours, one part

The real biohazard symbol is six separate pieces — three crescents and the three short arcs floating inside them. Imported naively, that is six things to assign a filament to, one at a time. Here the six islands are welded to each other under the skin — a 1.7 mm ring buried 0.6 mm below the drink face — so in Bambu Studio you import biohazard_2c.stl, right-click → Split to Parts, and get exactly two parts: coaster_2c and inlay_2c. Assign the body colour to the first and the ink to the second. Done.

Print dark ink on a light body: it is the cheap purge direction, and it hides its own residue. The ink is what the glass rests on, so use the same material for both — a flexible filament here would squash and let the floor touch again.

Purge cost: the ink spans 11 bi-colour layers, about 4 g of purge. Put all four coasters on one plate — the ink band sits at the same height on every coaster, so the whole set shares those eleven changes and costs the same as one.

The mark is constructed, not traced

The symbol is the published compass-and-straightedge construction (Charles Baldwin, Dow Chemical, 1966) drawn as circles and rectangles, so it stays exact at any diameter: three lobe circles, three eccentric holes, a notch at each tip, a slit to the open centre, and the three thick bands underneath. One deliberate deviation: the construction leaves 0.7 mm of wall at each lobe tip, under one nozzle width, so the inner circle is opened by 0.2 % to carry 0.85 mm of ink there. It is not visible against the reference.

The caddy

Ø97 × 19 mm drum, 3 mm wall, 0.6 mm clearance per side. Four coasters stack inside at a 5.2 mm pitch with the top one standing 4 mm proud so you can pinch it out; the scoop exposes the second one as well. The sill under the scoop sits above the bottom coaster's full slab, so the stack stays captured if you knock it over.

Print settings

• 0.2 mm layers, 2–3 walls, 15 % infill (the caddy is nearly all wall anyway)

• PLA is fine — a coaster never sees more than a warm mug. PETG if you want it dishwasher-safe

• Ironing OFF on the coaster top face — it drags one colour across the other

• Filament: ~19 g per coaster (or ~17 g body + ~3 g ink), ~34 g caddy — about 115 g for the set

Dimensions

• Coaster Ø90 × 5.2 mm (4 mm slab + 1.2 mm rim), well Ø85, rim 2.4 mm wide

• Mark Ø74, seated 0.8 mm into the floor and standing 0.8 mm above it; thinnest ink 0.85 mm

• Caddy Ø97.2 × 19.2 mm, bore Ø91.2, thumb scoop 32 × 8 mm

Parametric

The included .scad exposes everything: coaster diameter, mark diameter, how far the trefoil stands proud, rim height, caddy clearance and capacity. openscad -D 'part="coaster"' -o coaster.stl biohazard_coasters.scad for a single-colour coaster; part="caddy" for the drum.

Bambu Lab P2S, 0.4 mm nozzle, 0.2 mm layer, PLA. Printed and confirmed.

The biohazard symbol is a public-domain mark, deliberately never trademarked by its designers so it could be used freely.