It starts as liquid, and it is a race
Flexible polyurethane foam is not moulded or extruded. It is poured as a liquid and it inflates itself.
Polyol and isocyanate are metered into a mixing head along with water, a silicone surfactant, catalysts and - depending on the specification - flame retardant and pigment. Water reacting with isocyanate generates carbon dioxide, which is what blows the foam. At the same time, a second reaction is building the polymer that has to be strong enough to hold those bubbles open.
Those two reactions are in a race. If gas is generated faster than the polymer can support it, the foam collapses or splits. If the polymer sets first, the foam stays dense and tight. Getting flexible foam at all means holding that balance steady - which is the real skill in this industry.
Continuous versus batch, and why it matters to a buyer
Foam can be made box by box - a batch process - or poured continuously onto a moving conveyor. Batch foaming is cheaper to set up and perfectly viable at small scale, but every box is a slightly different foam, because every box is a separate mix with its own temperature and timing.
A continuous line pours without stopping. The chemistry, the conveyor speed and the laydown are held steady, so the foam in the tenth metre matches the foam in the first. For a buyer, that is not a technical detail: it is the difference between a repeat order that behaves like the last one and a repeat order you have to re-test.
Laydown, rise and the fall plate
A traversing head lays the reacting liquid across the width of a moving trough lined with kraft paper. The mixture begins to rise almost immediately, expanding many times its liquid volume as it travels down the tunnel.
Left alone, a rising bun crowns - it domes in the middle. A domed block has to be trimmed flat before it can be cut, and trim is money. The fall plate is the piece of geometry that manages this: it controls how the rising foam is supported so the top comes out flat rather than curved. Yield in this business is decided at the fall plate.
Cure, and the part people get wrong
Foam coming off the line is not finished. The reaction is exothermic - the block is generating heat internally - and it continues to cure for hours after it is cut off the conveyor.
Two things follow from this. First, blocks must be cured under watch and not stacked tight while the exotherm is still running, because a large volume of foam holding its own heat is a genuine fire risk. Second, foam cut hot is foam that changes dimension later. A block that is cut before it has stabilised will not hold the size you ordered.
A supplier who is rushing cure to hit a delivery date is a supplier whose sizes will drift.
Cutting, and what happens to the offcuts
Cured blocks are cut horizontally for sheet, vertically for plan shapes, peeled for thin roll stock, and profiled where the part needs a contour.
Every one of those operations produces trim. In an integrated plant that trim does not become waste - it is sorted and fed to the rebonding plant, where it is pressed into high-density block for bases, flooring and matting. That is the practical reason foam manufacturers tend to run a rebonding plant alongside the foaming line: it turns the single largest waste stream into a saleable product.
Common questions
What is slabstock foam?
Foam poured as a continuous bun on a conveyor and then cut into blocks and sheets, as opposed to foam moulded into a finished shape.
Why does foam need to cure before cutting?
The reaction continues after the foam leaves the line and the block holds heat internally. Cutting before it stabilises gives sizes that change afterwards, and stacking hot blocks tightly is a fire risk.
What decides foam density?
Principally the water content of the formulation, which sets how much gas is generated. More gas gives lighter foam.
Is continuous foam better than batch foam?
It is more consistent, which matters most for repeat orders. For a one-off small quantity the difference may not matter to you.
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Written by the team at Plush Foams Private Limited, who manufacture flexible polyurethane foam, rebonded foam, mattresses and pillows at Jodhpur, Rajasthan. Last reviewed 2026-09-09.
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