TPMS-HRV

Generates a 3D-printable counterflow core for a heat recovery ventilator (HRV). The core is a gyroid, a TPMS (triply periodic minimal surface). Its wall splits the enclosure into two interleaved channel networks, one for the supply air and one for the exhaust air. Heat passes through the thin wall between them.

The input is an enclosure volume exported from Onshape with its port faces colour-coded. The output is a single watertight mesh, ready for Bambu Studio. It is sized to print in one piece on a Bambu H2C.

Built with PicoGK (voxel geometry kernel) on .NET 9.

How it works

For each part, Program.cs runs these steps:

1. Import (ObjImporter.cs). Reads the OBJ/MTL, converts from metres to mm, and groups triangles by material colour.

2. Classify ports (PortClassifier.cs). Matches each group's diffuse colour against the port palette:

   Port	Colour	Kd
   SupplyIn	red	1.000 0.000 0.000
   SupplyOut	orange	0.973 0.529 0.004
   ExhaustIn	blue	0.231 0.380 0.706
   ExhaustOut	cyan	0.616 0.812 0.929

   Any other colour is treated as enclosure body. An off-palette port is skipped without an error, so use these exact colours.

3. Port slabs (SlabBuilder.cs). Extrudes each port's faces inward as one closed prism, fPortSealDepthMM deep.

4. Membrane (Gyroid.cs). Builds the gyroid wall inside the volume. The field is gradient-normalised with one Newton step, which keeps the wall a uniform fWallThicknessMM thick along its normal.

5. Port seals. Inside each port's slab, fills the other stream's channels solid, so each port opens only onto its own stream.

6. Skin. Adds a fSkinThicknessMM outer shell, with cut-outs at the ports.

7. Export. Meshes the voxel core, then decimates it (MeshDecimator.cs, quadric edge collapse to an RMS surface tolerance). Writes STL, 3MF or VDB.

Each step prints a numbered checkpoint. A checkpoint that fails a hard check throws and stops the run:

Checkpoint	Checks
0	Toolchain smoke test (writes out/checkpoint0_sphere.stl)
1–3	Import: group and triangle counts, bounding box, colour → port role
4	Voxelised volume vs expected (warning only)
5–7	Port slabs written, ports mapped to faces, membrane and seals built
7b	Every port has at least 5% open area for its assigned stream
8	Skin covers the body and leaves the ports open
9	Wall thickness at 200 random points: mean within 2% of nominal, every point within 5%
10	Decimated mesh has no open, non-manifold or degenerate edges and winds outward; reports volume change and surface deviation

Requirements

• Windows. It is the only platform tested so far.
• .NET 9 SDK. PicoGK 2.3.0 targets net9.0 only and is restored from NuGet.
• RAM: about 9 GB for the real volume at 0.25 mm. See Performance.

Usage

dotnet run -c Release -- [--voxel <mm>] [--only <name>]... [--export stl|3mf|vdb|none]
                          [--decimate <mm>] [--cell <mm>] [--wall <mm>] [--seal <mm>]

Option	Default	Meaning
--voxel	0.5	Voxel size in mm (production target is 0.15)
--only	all parts	Run only parts whose OBJ path contains <name>. Repeatable
--export	stl	Core output: stl, 3mf, vdb, or none
--decimate	0.02	STL/3MF surface tolerance in mm. 0 keeps the raw voxel mesh
--cell	8.0	Gyroid wavelength λ in mm
--wall	0.8	Membrane thickness in mm
--seal	0.75 × --cell	Port seal depth in mm

--cell is the main lever on output size: triangle count scales as 1/λ², and unlike --decimate it costs no geometric accuracy — the wall stays at nominal thickness. What it costs is heat-exchange area, which scales as 1/λ. Measured on the 100mm cube at --decimate 0.02: λ 8→12 gave 2.22× fewer triangles, λ 8→16 gave 4.26×.

Slicers have a practical ceiling on triangle count. Bambu Studio stalls indefinitely above roughly 1M triangles on this geometry, regardless of support and infill settings, so the real volume needs --cell 16 to export something sliceable:

dotnet run -c Release -- --only real --voxel 0.25 --export 3mf --cell 16 --decimate 0.05

To build the real enclosure for Bambu Studio:

dotnet run -c Release -- --only real --voxel 0.25 --export 3mf

Output goes to out/<part>/: the core (core.3mf, core.stl or core.vdb) plus the port slab STLs from Checkpoint 5.

Parts

The parts are listed in aParts at the top of Program.cs:

OBJ	Output
fixtures/test_cube_100mm.obj	out/test_cube_100mm
fixtures/test_cube_filleted_100mm.obj	out/test_cube_filleted_100mm
real-HRV-volume/real-HRV-volume.obj	out/real_hrv_volume

To add a part, export the enclosure from Onshape as OBJ with the port faces painted in the palette colours, then add a row to aParts.

Design parameters

The parameters are set in Params.cs:

Constant	Value	Meaning
fCellSizeMM	8.0	Gyroid cell size (λ)
fWallThicknessMM	0.8	Membrane wall thickness
fPortSealDepthMM	6.0	How far each port seal reaches into the core
fSkinThicknessMM	1.5	Outer skin thickness
fImportScale	1000	OBJ metres → mm
fColorTolerance	0.15	Maximum RGB distance for a palette match

Performance

Measured on the real volume (232 × 131 × 140 mm) with a Ryzen 7 5700U and 15 GB RAM:

Voxel	Output	Triangles	File	Deviation mean / p99 / max	Time	Peak RAM
0.25 mm	raw STL	94.0M	4.7 GB	0.003 / 0.016 / 0.093 mm	~11 min	8.1 GB
0.25 mm	3MF, 0.02 mm tolerance	11.1M	137 MB	0.014 / 0.041 / 0.127 mm	~13 min	8.4 GB
0.5 mm	3MF, 0.02 mm tolerance	11.5M	131 MB	0.018 / 0.061 / 0.214 mm	—	—

Notes on these results:

• The measured wall is 0.801 ± 0.002 mm, against a nominal 0.8 mm.
• Decimation changes the volume by less than 0.03%.
• After decimation, the output size depends little on voxel size.
• Meshing is the step that limits memory. At 0.15 mm the real volume needs about 23 GB, so it does not fit on this machine yet. The 100 mm cube does.

Status

Done:

• The full pipeline, Checkpoints 0–10.
• Decimated 3MF export.
• The real volume at 0.25 mm.

Still to do:

• Confirm the 3MF loads and slices well in Bambu Studio.
• Mesh the real volume at 0.15 mm in slabs.
• Load parameters from a config file for parameter sweeps.

See todo.md for the full list, open questions and known PicoGK issues. One known PicoGK issue is that IntersectImplicit crashes at voxel sizes ≤ 0.33 mm, which the pipeline works around.

Repository layout

File	Purpose
Program.cs	Command line, pipeline, checkpoints, export
ObjImporter.cs	OBJ/MTL parser
PortClassifier.cs	Colour → port role
PortAssertion.cs	Maps port groups to bounding-box faces
SlabBuilder.cs	Closed prism extrusion of port faces
Gyroid.cs	Normalised gyroid field and stream implicits
MeshDecimator.cs	Parallel quadric decimation, topology checks, 3MF/STL writers
Params.cs	Design constants
fixtures/	100 mm test cubes
real-HRV-volume/	The real enclosure export