How reticulated foam is made.

Polyurethane foam begins as two liquid streams: a polyol and an isocyanate. Metered together in fixed proportion and mixed, they react — and the reaction does two things at once. It builds long polymer chains, turning liquid into solid, and it generates gas.

Because both happen together, the mixture does not simply set. It expands. Gas bubbles nucleate throughout the liquid and grow while the polymer around them is still soft enough to be pushed aside, then the polymer gels and locks the structure in place. The timing of those two curves against each other — gas generation and gelling — is the whole art of foam formulation. Gel too early and the bubbles cannot expand; too late and the structure collapses under its own weight.

The mixed liquid is poured onto a moving conveyor and allowed to rise freely into a continuous loaf several feet tall, called a bun or slabstock. The industry borrowed the bakery vocabulary and the analogy holds right through to slicing it.

This is the step most people get wrong, so it is worth stating plainly: pore size is set during foaming, not during reticulation.

Catalysts and surfactants in the chemical mix control how many bubbles nucleate and how large each one grows before the polymer gels. Many small nucleation sites give a fine, high-PPI foam; fewer, larger bubbles give a coarse, low-PPI one. A 20 PPI foam and an 80 PPI foam are therefore different formulations, not different processing of the same block.

Reticulation, which comes later, removes the membranes between cells. It does not change how many cells there are or how big they are. This is why pore size is a specification you give at the order stage rather than something that can be adjusted from stock — and why choosing a PPI is a decision worth making deliberately.

A freshly risen bun is not finished. The reaction continues to completion over hours, and it is exothermic — the centre of a large block gets genuinely hot, hot enough that curing is a monitored stage rather than simply a wait. The block is left to cure and cool to a stable solid before anything is cut.

Curing also produces a feature that has to be dealt with: the outer faces cure against air and form a denser skin, quite different in behaviour from the material inside. That skin is skived off before slicing. It is one reason foam is supplied as cut sheet rather than as whole blocks — the usable material is the inside.

At this point the foam is open-celled but not reticulated. The cells connect, but thin membranes — cell windows — still span the gaps between the strands, and those membranes are what obstruct airflow. Reticulation removes them while leaving the much thicker skeletal struts intact. What remains is a three-dimensional lattice of strands with almost nothing in between: a solid you can see through.

Thermal reticulation — zapping

The bun is loaded into a sealed pressure vessel, the air is evacuated and replaced with a precisely proportioned combustible gas mixture, and the mixture is ignited. A controlled flame front sweeps through the block. Because the membranes are vanishingly thin and the struts are comparatively massive, the flame melts the membranes and passes on before the struts absorb enough heat to be damaged.

It is a violent-sounding process that is in fact very precise, and it leaves a smooth, clean, polished strand. It works on both polyester and polyether polyurethanes, which makes it the more generally applicable of the two routes.

Chemical reticulation — quenching

The alternative is a caustic bath at controlled concentration, temperature and dwell time, which etches the membranes away chemically. It leaves a rougher, more etched strand surface — which is an advantage where the foam has to hold liquid, because surface tension grips a rough strand better than a polished one.

Quenching suits polyester-based foams and is not effective on polyethers. It also tends to produce a softer feel, particularly in the finer grades.

The reticulated block is then cut down. Horizontal slicing takes flat sheets off the block at a set thickness; rotary peeling spins the bun against a stationary blade to produce long continuous rolls. Thickness is a machine setting rather than a stock category, which is why we quote from 5 mm upwards with no fixed upper limit — see foam sheet stock.

The final step is what turns a sheet into a component: die-cutting to profile. Rings, discs, strips, gasket outlines and irregular shapes, cut to a drawing rather than trimmed by hand. Foam cuts unusually cleanly because it compresses under the blade instead of tearing, so internal features and tight radii come out sharp. That step is covered in die-cutting to your drawing.