Solvent in the channel nobody can see
How bonding fluid gets into a flow path, and the escape channels used to catch it.
Solvent applied at a bond line can wick sideways along the thin gap between two not-yet-closed plastic surfaces and travel straight into a nearby channel before the pieces are fully clamped, because a channel is simply the widest, lowest-resistance gap available for the solvent to migrate toward, and applying it carefully at the edges does not keep it there.
Paint that bleeds under masking tape
Painter's tape pressed along a line is meant to give a crisp edge, but if it is not pressed firmly along its whole length, wet paint finds the narrow gap under its edge and bleeds sideways by capillary action, blurring the line exactly where the sharpest edge was wanted. Nobody poured paint under the tape; it followed the lowest-resistance path once a gap existed. Pressing harder reduces the bleed but rarely eliminates it if the gap was ever open, even briefly.
Solvent bonding has the same weak moment. The solvent wets both surfaces just before they are pressed together, and until clamping pressure closes the gap it is a free liquid in a narrow slot, the geometry that drives capillary movement. A channel groove near the bond line is a much larger opening than the thin film gap, so the solvent tends to migrate toward it and arrives inside the passage the manifold exists to keep clean. From outside, the plastic looks and feels the same either way, whether or not solvent has made its way into a channel several millimetres from where it was applied.
What solvent does once it is inside
Solvent that reaches a working channel can locally soften and slightly reflow the wall, distorting the geometry exactly where dimensional accuracy matters most. It can leave a hazy, crazed surface behind as it evaporates, weakening the plastic at the surface. Any solvent trapped in a corner becomes the kind of chemical residue the leachables discussion earlier in this set warned would interfere with a sensitive measurement. None of this can be seen afterward, because the channel is sealed shut by the very bonding process that caused the damage.
Distance, time, and a groove to catch the excess
A diffusion-bonded manifold avoids the problem entirely, since no liquid is present at the interface to migrate anywhere. Within solvent bonding, the risk shrinks sharply once a design keeps a generous width of solid bonding surface, called the land, between the channel and the outer bond line, because capillary movement has a limited reach in the short clamping window and a channel beyond that reach is out of danger.
Designs prone to the problem also add a dedicated escape channel: a shallow relief groove cut around the working channel to give migrating solvent a preferred, lower-resistance path, sometimes connected to a vent at the part's perimeter where pooled excess can be seen. The solvent still moves toward whatever gap offers least resistance, and the groove gives it a harmless one, turning an invisible defect into a visible one at the edge of the part. Combined with a generous land and a tightly controlled clamping window, an escape channel usually makes solvent bonding trustworthy even beside a channel that cannot be moved further away, which is why the technique remains in wide use.