Plastic Tubes - Steel tubing and stainless steel tubing when compared to plastic tubing in terms of tensile strength will win out when you crunch the numbers, but not in the arena of material costs. For the same amount of money that it takes to process and create half a dozen pieces of steel or stainless steel tubing to fit one section of a large manufacturing process, you could probably create several hundred cut to length or flexible plastic tubing that would have the same amount of durability for the intended process.
Plastic Tubes With Cap - The key to durability in plastic is two-fold: wall thickness and the resin blend. Whether the finished product is blown, extruded or molded, what matters is the overall strength of the tubing to be used, when compared to the stress of the operation. When placed within moving parts, used to pass water or other lubricants through the machinery, it may get pressed, crimped, bumped or slammed from time to time. Steel may be tough, but even that material can only take so much damage before it begins to give out. Plastic, on the other hand, especially if the wall thickness lends it tensile strength, can take a lot more damage before it even begins to show any signs of stress fatigue.