Co-extrusion is a more recent advancement in extrusion technology which brings 2 layers of High Density Vinyl Polymers into play: The outer layer, or Cap Stock, is similar to human skin, and contains a denser concentration of the essential elements, such as the UV inhibitors (much like sunscreen), which is only doing its job when exposed to UV rays. ASTM's "Specification for Rigid PVC Exterior Profiles Used for Fencing" (F 964-94) requires that the cap stock layer be a minimum of 00.015" thick and a maximum of 20% of the overall thickness of the profile wall. It is important for the potential buyer of a High Density Vinyl Polymer fence, deck, or railing system to determine what ASTM specification is being used by his potential extrusion supplier, if in fact any is being used at all, and to ensure that compliance is being verified regularly.
The inner layer, or Substrate layer, has a lowered concentration of UV inhibitors and color pigment. This allows Nationwide to formulate a better molecular bond via the impact modifiers in the substrate compound resulting in a stronger product.
Is a co-extruded product inferior?
There is a misconception is that co-extrusion is inferior to monolayer. Co-extrusion is a newer and more capital-intensive technology offering significant benefit to the consumer by concentrating the essential materials where they do the most good. This gives the consumer a stronger, more durable product, and in turn, more overall value.
Claims of inferiority of the high-tech co-extrusion process usually come from manufacturers who are hesitant to make the significant capital investment for additional extruders and specialized profile tooling, or have built their reputation on defaming this advanced extrusion technology.
Will a co-extruded product delaminate?
Some claim that the layers in co-extrusion may delaminate, but this is false. Co-extrusion is not lamination. Lamination is a process whereby adhesives are used to bond two layers together. Co-extrusion is a molecular bonding process without the use of adhesives, it is accomplished at high pressure and temperatures in a sealed environment where no moisture or air can be entrapped to compromise the molecular bonding, making it impossible to separate the layers.