Cellular PVC Foam Extrusion: Process Routes, Formulation and Cooling

What cellular PVC actually is

Cellular PVC — foam PVC, sometimes called rigid cellular PVC — is PVC with gas dispersed through it. The gas comes from a chemical blowing agent that decomposes in the barrel, or from a physical blowing agent injected into the melt. The result is a material with a density of 0.45 – 0.75 g/cm³ instead of 1.4, which is why a foam PVC board is a third the weight of the solid board it replaces.

The reason it matters commercially is that solid PVC is heavy and expensive. A 20 mm solid PVC board weighs 28 kg/m² and costs accordingly. The same board in foam at 0.55 g/cm³ weighs 11 kg/m², can be nailed and screwed like timber, machines like hardwood, and takes a wood-grain surface. That combination is why foam PVC replaced timber in fascia, soffit, window surrounds and bathroom panelling.

Three ways to make it

Table 1 — The four cellular PVC routes. Celuka is the volume process; co-extrusion is the quality one.

Celuka is the dominant process. The die has an internal mandrel or "torpedo" that cools the centre of the melt as it exits, so the surface freezes into a solid skin while the core stays hot and expands. The result is a board with a hard, closed surface and a uniform cellular core — the structure that makes it usable as a timber replacement.

Free foam lets the extrudate expand freely in air before it reaches the calibrator. It is cheaper and simpler, gives a lower density, but produces a thin skin and a surface that shows the cell structure. It is normally laminated or painted.

Co-extrusion puts a solid PVC skin over a foam core in a single die. It is the best surface and the best weathering, and it costs 20 – 35 % more because it needs a second extruder and a more complex die.

Formulation: what each part does

Table 2 — A working foam PVC formulation in parts per hundred resin. The blowing agent / activator pair is the balance that sets density.

The blowing agent and its activator are the two ingredients that decide whether the board is worth making. AC (azodicarbonamide) decomposes at about 200 °C on its own, which is above the PVC processing window. Adding zinc oxide drops the decomposition temperature to 170 – 185 °C, inside the window. Get the ratio right and the gas is released exactly as the melt leaves the die; get it wrong and either the gas comes out in the barrel (making a low-density, coarse, weak board) or it comes out too late (making a dense board with a rough, over-expanded surface).

Die and calibration

The die and the calibrator on a foam line are a single system. Unlike a solid profile, the die does not define the final dimension — it defines the melt stream, and the calibrator defines the board.

Table 3 — Foam die and calibration. The vacuum is lower than for solid profile, which surprises people.

The vacuum point is worth explaining. On a solid profile, high vacuum pulls the melt hard against the calibrator and gives sharp detail. On a foam, high vacuum crushes the cells at the surface, producing a dense skin over a coarse core and a board that is heavier than specified with worse insulation. Foam lines run at 0.1 – 0.3 bar, just enough to control the surface, not enough to collapse it.

Cooling: the real bottleneck

PVC foam has a thermal conductivity of about 0.04 – 0.05 W/m·K, against 0.16 for solid PVC. It is an insulator — which is the point of the product, and also why foam lines are long.

Table 4 — Cooling length for Celuka board. The thickness-squared relationship is brutal at 30 mm.

A board that is cut before its core is cool will bow, and the bow appears hours later — often after it has been packed. The practical rule is that the core must be below 60 °C at the saw, which for a 20 mm board means a tank long enough that the line speed drops to under 1 m/min. That is why foam lines look disproportionately long relative to their output.

Surface: wood grain, lamination and painting

Table 5 — Surface options for foam board. Co-extrusion is the only finish that is genuinely exterior-grade.

Embossing works on foam because the surface skin is still thermoplastic when it leaves the calibrator. A textured roller or a moving belt presses the grain in at 90 – 110 °C, and the skin retains it. Emboss depth is 0.3 – 1.0 mm; deeper than that and the roller breaks through the skin into the cellular core, which shows as a rough patch.

Defects and causes

Table 6 — Foam board defects. Six of the ten are density or skin-thickness problems, which are the same problem seen from different angles.

Properties and where foam PVC is used

Table 7 — Foam versus solid PVC. Stiffness drops by two-thirds but so does weight, and insulation improves fourfold.

The stiffness loss is the design constraint. A foam board is roughly one-third the modulus of solid PVC, so a shelf or a fascia that worked in 10 mm solid needs 16 – 20 mm in foam. That is fine for cladding, where stiffness is not the governing requirement, and it is why foam is not used for structural profiles.

Screw pull-out is the second constraint. A foam board holds a screw at 600 – 1,200 N, which is adequate for cladding fixings but not for anything load-bearing. Fixings should be into the skin, not the core, and should be at least 20 mm from an edge.

Frequently asked questions