Two types of extrusion processes are used in the vinyl deck industry in the U.S.A.; single material extrusion and co-extrusion.
Monolayer extrusion is traditionally used to produce all types of extrusions such as pipe, fence, window & door frames and other structural shapes. In this process, a single plastic material is processed on a single extruder into the desired shape.
Co-extrusion represents newer technology, in a process where two or more polymers are extruded by two or more extruders into a common die. This allows the manufacturer more freedom in designing performance into his final product. Material selection can be tailored specifically to the requirements for the outer and inner layers.
For successful co-extrusion, it is necessary that the polymers (PVC in the case of vinyl decking) be compatible in material properties (i.e. melt viscosity, co-efficient of expansion, and lubricity). The capstock (or top layer) contains the same basic resins and ingredients as the substrate (or inner layer); and the two layers remain permanently bonded as function of die design, heat and pressure. By custom designing the correct properties in the capstock and substrate layers, a product superior to monolayer extrusions is produced.
Monolayer extruders may have to compromise on the level of Ti02 in their materials due to cost. This is because this expensive UV inhibitor must be used throughout the entire thickness of the profile. Customers should receive assurances that the Ti02 content of their purchase contains 10 parts per hundred resin of this additive, instead of 7 to 8 parts per hundred which are used by some monolayer producers. This inhibitor directly affects the weatherability of the product.
The vinyl siding industry analogy:
As industry leaders, who were typically better capitalized companies, began to shift to co- extrusion, there were many critics of the new technology. Today it is difficult to find siding panel which is not co-extruded. The market demands a superior, more cost effective product.
About 750 million pounds of vinyl are used in the siding industry annually. This never could have happened if there was anything inherently wrong with co-extrusion technology.
Consumers in the vinyl fence and deck industry are periodically subjected to sketchy criticism of co-extruded products (usually initiated by producers of monolayer products). The reality is that co-extrusion is proven technology which allows the producer the capability to engineer a superior product. The only reason there is controversy between co-extrusion and monolayer extrusion is because some manufacturers have refused to spend the capital required to obtain the most advanced technology.