Making wood-plastic composite (WPC) and polyvinyl chloride (PVC) products is a lot like cooking barbecue: It's mainly about meat and heat, but arguments rage over what's the best combination of core ingredients, spices, and techniques. It's much the same here: Recipes are closely guarded secrets, and you can't even visit some factories. But all share some basic principles.
The core ingredient for both WPCs and PVC products is plastic. Typically it's one of two things. The first is virgin PVC, a byproduct of the refining of oil and natural gas. The second is polyethylene. It's created by recycling just about any plastic product you can imagine: shrink wrap, plastic shopping bags, soda bottles, even the nearly invisible film that's put on Styrofoam cups at your favorite fast-food place and used to imprint an ad. This recycled plastic gets cleaned and processed into granules the size of short-grain rice. Companies also use plastic from factory rejects that were reground.
Wood (or a cellulose equivalent, such as rice hulls or wheat straw) provides the other major component of WPCs. It's ground into what's known as flour–particles so small that you can put 100 in a line and not cover an inch. Technically, any type of wood can be used in WPCs, but manufacturers prefer hardwoods over softwoods and look for woods free of tannins (e.g., maple rather than oak). Some of the first WPC was made from ground-up pallets. Today, companies that specialize in wood flour procure their raw material from flooring factories, furniture makers, and cabinet shops; they only go direct to the forest as a last resort. Because sourcing companies use varying species of wood, and because wood harvested at different times of year has different qualities, WPC deck firms constantly have to test the raw materials and adjust their recipes.
Then come the additives. They include stabilizers (which help the material mix together), waxes and lubricants (they help the product move smoothly through the extruder), stiffening agents (keep the end product from acting like wet spaghetti), impact modifiers (reduce brittleness), "blowing agents" (cut heat levels caused by the extruder and help the product expand), and colorants. PVC but not WPC gets titanium dioxide, which removes color and is what makes PVC trim so white. You can make WPCs and PVCs without additives, but the process would be hard and the end product pretty ugly.
1-Prepare and Mix Ingredients
The wood flour used in WPCs has to contain far less moisture than one would find in nature, so before going into the hopper, it gets heated and dried until it's almost free of water. At the same time, the plastic–which looks like soap flakes–is heated to a liquid state. (That's why it's typically called "resin.") Those ingredients are mixed along with additives until the plastic is considered to have thoroughly covered (or "encapsulated") the wood flour. If you're making PVC products, there's no wood flour used, so only plastics and additives get mixed. Ingredients for a top coat, or capstock, on WPCs are mixed separately. Capstocks don't have wood flour, but exactly what each company puts in is a closely guarded secret. Basically, it's a plastic with additives like colorants.
2-Extrude the Mixture
The heated mix is pushed through a die that typically contains a series of four to eight computer-cut metal plates. These progressively shape the product into the desired configuration: a deck, a piece of trim, a railing component. The capstock mixture gets introduced into the die near the final plates, after the core's shape has been formed. WPCs and PVCs get bigger as they leave the die. This expansion is known as freefoaming. At this point, some manufacturers move the extruded product immediately into a vacuum chamber. That process keeps the freefoam expansion from taking place and instead yields a hard crust on the surface and softer material inside. This Celuka process is often used to produce products that have to meet tight standards on size variations.
3-Cool What's Produced
The extruded product has the right shape but is still so hot it's barely stiffer than a licorice whip. Stiffness comes as the product cools. Most manufacturers do this by spraying water on the product as it's pulled down the line, but it's a fine art; a misdirected spray can cause unreached sections to bulge. At least one manufacturer cools its decking with air alone, sending the product on a circuitous trip up and down several adjacent lanes, like going through an airport security line. Another applies giant cooling wheels to each side of the product. By this point, manufacturers have a good idea of how the end product will look. Often they're unhappy; it's not unusual to reject as much as one-eighth of the output. The discards are set aside to be reground and put back into the mix.
4-Imprint, Treat, Cut, Stack
By now, the product has cooled to the point where its surface can get imprinted with a faux-wood finish. Typically this is applied with a big wheel that contains a grain pattern. Depending on the manufacturer, this pattern can be set up to repeat after so many feet or–by moving the board left or right under a wide wheel–can make the grain on one board in a pallet nearly unique from others. Here also is where manufacturers apply more additives, particularly those designed to keep the product from fading when it is put out in the sun. After that, the product is reinspected, cut to desired length, and stacked. Often it's still warm. Problems have been known to crop up if it hasn't been given time to completely cool before it's put on a truck and heads out the door to you.