Cost/Benefit Issues   The manufacturing process and application process have a number of positive and negative aspects. Within the manufacturing process, the obvious positive feature is the use of recycled materials. The waste products of lumber construction can be reused in the plastic lumber mixture. The normal flaws of wood, such as knots or bowing do no prevent the wood from being useless. This greater capacity to use more of the tree cut for lumber is paired with the use of post-consumer recycled plastics.       The challenge of using recycled materials is that of contamination. The chemical differences and lingering substances on plastics make it hard to efficiently reuse them. Fortunately, materials like plastic soda bottles are becoming readily available. These bottles provide a constant, uniform source of plastic without any hazardous residue.       The application cost to purchase recycled plastic lumber products averages 20% to 50% more than the price of wood. Payback is typically two to four years when you factor in maintenance and replacement costs. Many recycled plastic lumber manufacturers guarantee their product to last up to 50 years and will offer free replacement for planks that may crack or splinter. While the initial cost of plastic lumber may be hard to sell to owners, the long-term effect can be emphasized as a money saver.