Furniture grade pipe is made in a large, lengthy machine about 10 or 12 metres long. As it is extruded from this machine, it is the outside diameter which is the one that is closely and tightly controlled. The inside diameter is controlled a fair amount, but the outside diameter takes priority. This is because when you start to connect your pipe and fittings, the pipe goes inside the fitting, so the outside diameter must measure correctly or it won't fit inside the fitting. It is only possible to accurately control one of the diameters due to the fact that pipe contracts - albeit a small, measured amount - when it is cooling after being manufactured.
Posted by Bob Bawden on 11/19/2018
An important note when considering furniture grade pipe is that it is manufactured in North America with a different measurement system. This means it will be incompatible with any pipe you pick up at your local DIY store in the UK, as our pipe is made differently. Another big difference between furniture grade pipe and normal PVC pipe is the strength of the PVC itself - furniture grade pipe is more ductile and impact resistant due to the formula of the PVC which has some special additives in it. This means an impact or force on the furniture grade PVC isn't going to make it shatter, unlike normal PVC.
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