A 40 x 40 mm PVC square pipe with a 2 mm wall weighs about 0.43 kg per meter. The same footprint in galvanized steel tube weighs 2.39 kg per meter, and the aluminium equivalent lands near 0.83 kg per meter. That gap is why a PVC square pipe shows up in drainage runs, cable routing, greenhouse frames and prefabricated housing. It is also why two quotes for the "same" 40 x 40 profile can differ by 25 percent with no visible difference in the sample.
The difference lives in the wall, the compound and the calibration that happened after the die. Buyers who learn to read those three things stop paying for filler and stop rejecting deliveries that were never going to fit the frame.
A PVC square pipe is a hollow square-section profile continuously extruded from rigid PVC-U compound through a square die and calibrated to its final outside dimension. It is not a round pipe reshaped, and it is not moulded in short lengths. The wall is formed along the entire profile, so load behaviour is set by outer size, wall thickness and compound formulation rather than by any internal reinforcement.
A PVC square pipe is a continuous extruded hollow profile with four equal outer faces and a constant wall, sold by outer dimension, wall thickness and length. Everything a buyer needs to compare is contained in those three numbers plus the compound data sheet.
Commercial stock covers 20 x 20, 25 x 25, 30 x 30, 40 x 40, 50 x 50, 60 x 60, 75 x 75 and 100 x 100 mm, with walls from 1.5 mm to 5 mm and standard lengths of 3 m, 4 m, 5.8 m and 6 m. A calibrated line normally holds the outside dimension to plus or minus 0.3 mm and the wall to plus or minus 0.15 mm. Anything tighter has to be agreed before the die is cut, not after the first container ships.
| Outer size (mm) | Wall (mm) | Weight (kg/m) | Typical duty |
| 25 x 25 | 1.9 | 0.25 | Trim and light frames |
| 40 x 40 | 2.0 | 0.43 | Conduit and panel frames |
| 50 x 50 | 3.0 | 0.79 | Partition framing |
| 75 x 75 | 4.0 | 1.59 | Pergolas and modules |
| 100 x 100 | 5.0 | 2.66 | Load-bearing frames |
The compound is where cost is quietly removed. Rigid PVC-U for exterior work usually carries 5 to 15 phr of calcium carbonate for process stability, plus titanium dioxide or carbon black for UV resistance. Economy profiles aimed at indoor use can run 25 to 40 phr of filler. Filler is cheap and heavy, which is why a 40 x 40 x 2 mm pipe that should weigh 0.43 kg per meter sometimes arrives at 0.50 kg per meter. The extra mass is chalk, and the first property it takes away is impact resistance in cold weather.
Choose square when the profile is visible, must sit flat against a surface, or has to turn a clean 90 degree corner. Choose round when flow capacity, internal pressure or cable bending radius governs the design. The two profiles are rarely interchangeable, and forcing one into the other role costs labour at installation.
The consequence is measurable. A round 50 mm pipe moves more water per kilogram of material, which is exactly why drainage and pressure work stays round. A square 50 x 50 profile gives a 50 mm flat face on all four sides, so a bracket or cladding panel lands on full contact and a corner joint can be mitred or fitted with a connector in seconds.
Wall thickness controls bending behaviour more than any other single specification. Moving a 40 x 40 profile from a 2 mm wall to a 4 mm wall raises stiffness by roughly 70 percent, because stiffness follows the second moment of area rather than the wall itself. Weight rises about 90 percent over the same change, from 0.43 to 0.81 kg per meter. That is the trade a buyer is actually making when a supplier offers a heavier pipe at the same price.
Typical wall thickness by duty for rigid PVC square pipe (mm)
Bars use a 0 to 5 mm scale. The highlighted bar marks the range where a deflection check is worth running before the frame design is approved.
Two field numbers decide whether the installation goes smoothly. Rigid PVC expands about 0.06 mm per meter for every 1 deg C of temperature change, so a 6 m length crossing a 30 deg C swing grows roughly 11 mm. Leave that gap in long runs instead of butting the pipe hard against two fixed walls. Then check the wall in the middle of a delivered length rather than at the cut end, because a die running hot thins the wall a metre in from the tip.
Cut one metre and weigh it. A 40 x 40 x 2 mm PVC square pipe should read 0.43 kg, plus or minus 3 percent. Above 0.46 kg the profile is carrying extra filler or a heavier wall, and below 0.41 kg you were quoted a wall that did not arrive.
A PVC square pipe earns its place wherever a frame must stay light, resist corrosion and arrive finished straight out of the bundle. Six applications cover most of the demand, and each one has a detail that decides whether the installation ages well.
A complete specification answers six questions. Suppliers who receive all six can quote and extrude on the first pass, while orders that arrive as a photograph usually end in a second sample round and a delayed delivery.
40 x 40 x 2.5 mm is a specification. "40 square" is not. Say whether the tolerance applies to the outside face or to the wall, and state the measured point.
Indoor trim, ten years of outdoor exposure and chemical contact each point to a different compound and UV package. Service temperature above 60 deg C rules out standard rigid PVC.
5.8 m transport lengths, plus or minus 5 mm saw tolerance, square cut and deburred, gives the factory something it can check with a tape and a square.
Solvent cement welds permanently, mechanical connectors allow disassembly, and hot-air welding suits frames that must stay aligned under load.
Co-extruded or laminated surfaces are needed if the pipe will not be painted. Bundles must be stacked so the wall is not deformed in transit.
Density exposes filler loading. A drop test on a cold sample exposes a compound that will crack on site. Both take minutes and settle the argument before the order.
Send the drawings and the service conditions together with the quantity, and the profile can be quoted in one round instead of three. Most rework on square PVC frames traces back to a specification that was never written down, not to the material itself.
For light frames, yes. A 50 x 50 x 3 mm profile handles pergola spans up to roughly 1.5 m and light panel framing when supports are spaced accordingly. Beyond that, stiffness falls away quickly, because PVC sits near 3 GPa of elastic modulus against about 200 GPa for steel. Long spans are cheaper in metal or with an internal insert.
UPVC means unplasticized PVC: no plasticizer, which gives higher stiffness and better dimensional stability in heat. Construction-grade square profiles are almost always UPVC, and the two terms are used loosely in trade listings. Ask for the compound designation rather than the marketing word.
Yes, when the compound contains a UV package, normally titanium dioxide or carbon black. Unstabilized pipe chalks and lightens within one or two summers, and impact strength drops with it. Expect a slight colour shift even with stabilization, and allow expansion gaps in long runs.
Three methods cover almost everything: solvent cement for permanent drainage and conduit joints, mechanical corner connectors and insert fittings for frames that may be moved, and hot-air welding for structural frames that must stay square. Screws need a pilot hole, because a 2 mm PVC wall cracks before it grips.
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