A good design from an engineering prospective doesn't use 3d printing to print a pipe .... It implements already existing pipes and adapt the design ... It's the concept of this type of manufacturing that we are questioning Let's take as an example a standard plastic chair Yah you could 3d print it but they don't, Heck you could buy a solid 1*1m of plastic and machine it out but that's not done either, why? because stamping it in a mold is way cheaper way faster and way easier right ? The point is to find the manufacturing method that solves your problem the cheapest fastest and better way possible so if you want a straight plastic pipe for any reason you should just buy a pipe it's way cheaper than the filament it's water tight without imperfections stronger in any direction and if you want it to be close just print some caps .... 3d printing, taking in count the length of time to produce one item and the relatively high cost of production should be reserved for objects that you can't cheaply fabricate in any other way ...

I'm not saying you should throw away your already printed tube just keep in mind next time that if you need a big square plastic box maybe you should consider buying it instead of waiting 8 days to 3d print it .... (Unfortunately this is not a random example... A friend of mine insisted that I 3d printed a box for him ... )