Good Afternoon,
Process improvements have occurred in the industry but there are still some difficulties that will cause variance in tube measurements. A good example is that cured epoxy continues to harden and cure several days after the product has been baked and removed from its tool, causing the part to shrink slightly. Certain manufacturing methods and tube types are better than others (see our available product here, but there are others available in the marketplace:
https://dragonplate.com/ecart/categories.asp?cID=304). For example, the common roll wrapped tubing involves wrapping sheets of material around a mandrel and compacting it during the cure cycle. The ID is closely controlled as this is the tooled surface, but any imperfections in the raw material thickness, film compaction, or resin content has a major impact on the final OD. The OD naturally also has ridges from the compaction process. It is because of this tolerance stack up and ridges on the outside that the OD has tolerances as wide as +/- 0.02”. Square tubes are a special challenge! To keep the sides flat, one would use call plates (otherwise the outer shape gets very round). Squeezing the sides flat tends to cause the thickness in the corners to increase, resulting in imperfect wall thickness and slightly bulging corners. Most of us in the industry have ways of mitigating the worst of this, some of which are proprietary, but it is extremely common.
For your application, I would recommend a roll wrapped tube. I would like to say pultruded tubing, but I don’t know of any manufacturer making square tubes that large. Most manufacturers in the US use imperial units and measure by the ID so you’ve got a challenge ahead. At DragonPlate, we have a 0.75” ID square tube mandrel and two wraps of twill material should get you close to a 20” OD. Something we could try if you’d be interested, but I’d look around the market place and see if you can find any stock items first!
We hope this helps!!