Extrusion is the process by which the profiles are manufactured.  In simple terms, raw ingredients in powder or chip form (including PVC resin, impact modifiers, UV stabilizers, organic fillers, heat stabilizers, and flow agents) are blended into a compound that is melted and forced through hot tooling (screw and barrel) then through shaping dies and cooling baths to obtain the final rigid profile.  These profiles are run constantly and are often cut as they come “off the line” as by a sliding chop saw.  The skill and equipment required to create high quality consistent profiles, in an efficient manner is significant.
Many of the important properties of PVC pipe are predetermined by the characteristics of the PVC compound from which the pipe is extruded.  Generally speaking larger extrusions companies blend their own compound from resins and numerous other microingredients.  Small companies, and those who make caps and accessories using the injection molding method often use compound that has already been made by major resin manufacturers.
There are two main categories of extrusion in our industry: Mono -extrusion and Co-extrusion. Mono-extrusion has one uniform layer of product in the cross cut section view of the profile (end view). Co-extrusion has a substrate (inside of the profile) and cap-stock (outside of the profile). These two layers are molecularly bonded as they are in a state of melt at the time of extrusion, and they do not delaminate when properly extruded.
There are good and bad examples of both mono-extruded and co-extruded product available. When properly made, the co-extruded product can be favorably compared to mono-extruded product, and usually costs a little less. The cost to set up for co-extrusion is higher, because two extruders and more complex tooling is required, but the end result is a product that can be run faster and more cost effectively, because the required ingredients are concentrated in the layer where they are most required. For example the UV stabilizers, which are expensive, can be concentrated in the capstock (outer layer) where they are most useful, and less can be used in the substrate where it is not as important.  Generally speaking, the only time a mono-extrusion is required is when significant portions of the substrate (inside) of the extrusion will be exposed to UV rays and the elements regularly.

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