WPC Extrusion: Formulation, Drying and Why the Vent Is Not a Dryer

What WPC actually is

Wood plastic composite is a thermoplastic — usually HDPE, sometimes PP or PVC — loaded with 40 to 70 % wood flour by weight, plus a coupling agent, a lubricant package and a colorant. Nothing about it is exotic. The difficulty is that wood and plastic want opposite things from the process.

Wood flour is hydrophilic: it holds 4 – 8 % moisture as delivered, and that moisture has to be gone before the die or it flashes to steam in the melt and leaves voids, bubbles and a rough surface. Plastic is hydrophobic and melts at 150 – 200 °C. Wood starts to degrade — darkening, then volatile release, then char — at around 200 °C, and the rate rises steeply above 220 °C. So you have a melt that must be hot enough to flow and cold enough not to burn the filler inside it.

Formulation: what each component does

Table 1 — A working WPC formulation and what each part contributes. Loadings are weight percent.

The coupling agent deserves its own paragraph because it is where most cost-cutting goes wrong. Wood flour is polar; polyethylene is not. Without a maleic anhydride grafted polyolefin bridging the two, the wood particles sit in the matrix as unbonded inclusions — the board looks fine, then fails at the screw or in a cold bend because the failure runs along the wood-matrix interface. Two percent MAPE at around €2.50/kg adds roughly €0.05/kg to a compound that sells for €0.80/kg. Removing it to save six percent of material cost cuts tensile strength by a third. Buyers who specify "cheapest WPC decking" are specifying exactly this.

Drying: the step that decides whether the line runs

Wood flour at 6 % moisture contains 60 g of water per kilogram. In a 500 kg/h line that is 30 kg of water per hour, all of which must leave the melt before the die. There are three places it can go, and only one of them is acceptable.

Table 2 — The three exits for moisture. Only the first two are viable.

A vacuum vent on a twin screw can pull the last one or two percent out of the melt, and many WPC lines rely on exactly that with a vent operating at 50 – 200 mbar. But a vent is not a dryer. If the feed arrives at 6 %, the vent cannot remove it fast enough, the melt foams in the vent, material backs up into the vent port, and the line stops to be cleaned. The rule of thumb is that a vent handles 1 – 2 % moisture; anything above that needs a dryer.

Table 3 — Drying options. Note that all of them produce a colour shift: over-dried wood flour darkens.

The machine: why a twin screw, and which one

WPC needs: powder and fibre feeding, dispersive mixing of a very high filler loading, devolatilisation, and a low-shear pump to the die without over-heating. That combination is a co-rotating intermeshing twin screw, usually with a 32:1 to 44:1 L/D and at least one vent, often two.

Table 4 — WPC machine configurations. The two-stage arrangement is used where output and pressure stability both matter.

One configuration detail that surprises people: the die and adapter must be designed for a low-viscosity, highly elastic melt at moderate pressure. WPC at 175 °C flows more like a filled paste than like molten HDPE, and its melt strength is low because the wood particles interrupt the polymer chain network. That means the die land needs to be short, the draw-down low, and the calibrator close to the die.

Die and calibration for WPC

Table 5 — WPC's low die swell and low draw-down are the two numbers that most affect die design.

Low die swell is a gift. It means the die orifice is close to the profile dimension, which makes the die easier to cut and the start-up quicker. It also means the melt has little elastic recovery to help it fill a calibrator, so the calibrator must be positioned close and the vacuum applied early.

Cooling is the bottleneck on most WPC lines. Wood flour is a poor conductor — around 0.1 W/m·K against 0.4 for HDPE — so a 25 mm thick decking board cools from the outside in, and the core stays hot long after the surface is rigid. Pull it too fast and the board bows as the core contracts after it leaves the tank.

Cooling, and why WPC lines are long

Cooling length can be estimated from the heat to be removed and the achievable heat transfer coefficient:

L = (ṁ · c_p · ΔT) / (h · P · ΔT_lm)

For a 140 × 25 mm decking board at 400 kg/h, c_p ≈ 1.6 kJ/kg·K, ΔT from 180 to 40 °C, h ≈ 350 W/m²·K in a spray tank, P ≈ 0.33 m:

Q = 400 × 1.6 × 140 / 3600 = 24.9 kW
L ≈ 24,900 / (350 × 0.33 × 55) ≈ 3.9 m

That is the theoretical minimum for the surface. In practice a 25 mm thick WPC board needs 6 – 9 m of tank because the core must be below about 70 °C before the board can be cut without the saw pulling it out of shape. Add the spray tank, a soak section and a second tank, and a WPC line's cooling section is often twice as long as the extruder.

Table 6 — Cooling length. Hollow sections cool faster per unit mass but need internal support to hold shape.

Surface: embossing, brushing, co-extrusion

A WPC board out of the calibrator has a smooth, slightly waxy surface that looks nothing like wood. The wood appearance is applied afterwards, in three ways:

• Embossing. A textured roller or a moving belt presses a grain into the surface while it is still at 90 – 110 °C. Depth 0.3 – 1.2 mm. This is the cheapest and most common method.
• Brushing. Rotating wire brushes abrade the surface to expose wood fibres, giving a raised, tactile grain. Requires the compound to have enough fibre near the surface.
• Co-extrusion. A thin cap layer — 0.3 – 0.8 mm — of unfilled or lightly filled compound, pigmented and often with a higher UV package, extruded over the core. The cap takes the embossing better and carries the weather resistance, which lets the core be a cheaper, more highly filled formulation.

Table 7 — Surface options. Co-extrusion is the only one that decouples appearance from core formulation.

Defects specific to WPC

Table 8 — WPC defects, all of them traceable to moisture, temperature or formulation.

Design rules for WPC profiles

1. Keep wall thickness uniform. WPC's low melt strength means thick-to-thin transitions cause sink and internal voids. Aim for a 1:1.3 ratio maximum.
2. Round every internal corner. Minimum internal radius 1.5 mm; 2.5 mm preferred. Sharp corners are where the material pulls away from the calibrator.
3. Design for 2 – 3× the thermal expansion of PVC. A 4 m WPC board moves about 10 mm over a 40 K temperature swing. Fixings must allow it.
4. Do not put a weld line where the part will be loaded. In a hollow section, the weld line is at the far side from the die entry. If that is where a screw fixing goes, redesign.
5. Allow for a cap layer in the die. If co-extrusion is even a possibility, design the die with the cap channel from the start. Retrofitting is a new die.
6. Specify the embossing depth before the calibrator is built. Embossing changes the outer dimension by up to 1 mm and the calibrator has to be sized for the embossed surface, not the smooth one.

Frequently asked questions