Monolithic vs. two-piece

OpenChip can build your chip two ways.

Monolithic — one sealed block, channels as internal voids.

Two-piece — split at a cut plane into a channel plate and a lid, bonded after printing. This is the default.

The short version

	Monolithic	Two-piece
Drainage	Hard, sometimes impossible	Trivial — open grooves
Leaks	Nothing to leak	A bond that can
Assembly	None	Align and glue
Optical access	Through the block	Can use a glass lid
Minimum channel	~0.5 mm realistically	Down to the printer’s limit
Print time	One part	Two, but both thinner

If you are not sure, use two-piece. It is the default for good reason.

Why two-piece is usually right

The channels are open until the lid goes on. Uncured resin rinses straight out, which removes the single biggest failure mode in resin microfluidics — see Draining uncured resin.

It also lets you go smaller. Below about 0.5 mm, draining a sealed channel stops being reliably possible; as an open groove, the limit is just the printer’s resolution.

And it gives you options a sealed block does not: a glass lid for optical work, a membrane between two layers, or a lid you remove to clean and re-bond.

The cost is a bond, and a bond is a seam that can leak. Getting a good one is its own skill — see Bonding and sealing a two-piece chip.

When monolithic is right

Large channels, short paths. A chip with 1 mm channels and a direct route between two ports drains fine. If you can flush it clear in a minute, the argument against monolithic mostly evaporates.

High pressure. No seam means no seam to fail. If you are running near what the material will take, monolithic is stronger.

Complex 3D routing. Channels crossing over each other at different heights cannot be expressed as a single split plane. (OpenChip’s channel graph is planar at a single Z, so this is more a note about where you would need another tool.)

Nothing to align. No jig, no clamp, no cure time. For a quick one-off, it can be genuinely faster.

The split height

Where the cut plane sits, from the bottom of the block. 2 mm by default, the mid-plane of the default 4 mm chip.

Rectangular channels are placed so their ceiling sits at the cut plane, which gives you a flat lid with no features on it — the easiest thing to bond and the easiest thing to replace with glass.

Raise the split height for a thicker channel plate and a thinner lid; lower it for the reverse.

Caution

A split height outside the block means no cut happens and the chip prints as a sealed solid, even though the split type still says two-piece. This is easy to do by accident — set the split height above the base thickness and the geometry silently becomes monolithic.

enclosed-cavity catches this specific case, and the warning says why.

Checking it in the preview

Explode in the CAD toolbar separates the two halves so you can see the bonding faces. Disabled on a monolithic chip, which has nothing to separate.

Worth doing before every print. It is where you notice that a channel is open at the split plane where you did not mean it to be, or that the lid has a feature on it you did not expect.

What the checks do differently

The drainage checks — enclosed-cavity and undrained-region — only run on an enclosed chip. A two-piece split is open by construction, so they stay quiet.

That is not the checks being lenient. It is that the problem does not exist.

Switching later

Change Split type in the CAD panel at any time. The channel network is unaffected — it is the same graph either way — and the solid rebuilds.

Nothing is lost by trying both and looking at them.

Next

• Bonding and sealing a two-piece chip
• Draining uncured resin from channels