Extrusion and the Shape of Water Management

September 7, 2022

So spoke the Ancient Mariner in Coleridge’s Rime of the Ancient Mariner (1797). He was out there in the open sea, cursed for having killed an albatross (good luck to sailors), and had a real problem. He didn’t know why we don’t drink sea water — it’s not because of plastic pollution — but as a sailor he knew to just say no. His mysterious story is still worth reading. We still have a need for magic and mystery, which amuses as well as challenges science and, thus, supports the popular image of plastic pollution, even as our synthetic science-based materials do far more good than harm.

Extrusion helps move a lot of water, of course. Irrigation helps us grow food in areas with a lot of sun but little rain (the Mariner’s situation, too). Another water-linked (and water-resistant) extruded product is filament for fish nets, now reviled for their presence as half (?) of plastic sea waste. Years ago, we would repair them, and darn socks, too. Today, nets are abandoned when torn, but maybe the current attention will lead to economical recycling, especially of nylon (polyamide), which sinks unless buoyed (PP floats). And ergonomically, too, as people push recycling even when it takes more energy to do so than to make new polymer. Reduce waste at all costs, even at high cost to the environment? I just say, “No!”

There is much rigid pipe used in water supply, usually unplasticized PVC — no phthalates involved — and some ABS, where toughness and heat resistance are needed. Polyacetal and CPVC/PVDC have some pipe uses, too, but extruders of the high-chlorine resins must use special metals in equipment that will come into contact with hot molten polymer. For round products, here are some useful extrusion tips:

  1. Know your standards. You may be printing their numbers on the pipe itself, but compliance means enough testing to ensure that compliance, even with resin and formulation changes.
  2. With pipe used under pressure, pay attention to screening/melt filtration, as contaminant particles can cause premature failure. There are long-term hydrostatic testing machines; if you use them, understand the curves they generate, including knees.
  3. The distance between die exit and first cooling is important and may be adjustable. Differential thickness control around the pipe can be done mechanically, thermally, or by both.
  4. Cooling efficiency, related to warping and sagging, may depend on sprays and/or immersion in water. Uniform sprays or a metal tube with holes at entry may help. Length of the sizing sleeve is a balance of friction vs. cooling and may control linear speed.
  5. Vacuum over the tank water can draw the pipe out to the sleeve surface or, if applied to the sleeve, suck it directly. Remember even PVC floats when immersed, as it is empty inside. Sponge or cloth stripping of hot water in immersion tanks can also improve cooling efficiency and reduce need for refrigerated water.

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