Ask any acrylic tube manufacturer what they would like to know first, and the good ones say wall thickness. Not diameter, not length, not clarity. The wall decides whether the tube stays round under its own weight, and it decides the price more than the diameter does.
How an acrylic tube is actually made
Two routes dominate. Cast tube is poured into a rotating mould, which gives optical clarity and no directional stress. Extruded tube is pushed through a die, which is faster and cheaper per metre but leaves a faint flow line and a memory of that direction.
The route decides what you can do with the tube afterwards. A cast tube can be machined, bonded and polished without crazing. An extruded tube is fine for a straight visual element and poor for a threaded end.
Diameter, wall and the ovality trade
- 20 to 100 mm outside diameter: wall from 2 to 5 mm, ovality rarely an issue
- 100 to 300 mm: wall 3 to 10 mm, and wall uniformity becomes the visible defect
- 300 to 600 mm: wall 5 to 20 mm, usually cast on a scheduled run
- Wall under 1.5 mm at large diameter: expect ovality and a short useful life
The ratio that matters is wall to diameter. Below roughly 3 per cent, a large tube sags into an oval in storage and never recovers. Buyers who specify a 400 mm tube with a 5 mm wall are usually trying to save weight, and they usually pay for it in rejections.
Roundness is a measurement, not an adjective
A tube that looks round on a pallet can be 3 mm out of round at the mid-span. Ask for a two-axis measurement, one at the end and one at the middle, and for the acceptance band in millimetres rather than percentages.
For aquarium and exhibition work, that band is usually generous because the tube is viewed from outside. For a sleeve, a bearing or an optical cylinder, it is tight and the tube is machined after casting.
The base detail that prevents cracking
A standing tube carries its load on a ring of material a few millimetres wide. A collar bonded to the base, a radiused transition or a rubber pad spreads that load and removes the stress concentration where cracks start.
On a 500 mm tube standing 1.8 m tall in an exhibition hall, we bond a 30 mm collar and specify a 2 mm elastomer pad. That detail costs a few dollars and saves the tube.
Cutting, bonding and packing
Tubes are cut on a lathe or a saw with a rotating fixture; a hand saw on a large tube produces a wavy end that no collar will hide. Ends are then faced square within about 0.5 mm unless a decorative edge is specified.
Bonding uses a solvent cement applied to a closely fitted joint, and the joint needs 24 hours at room temperature before load. Packing matters too: tubes ship in cradles, not in a flat carton, because a large tube stored on its side deforms under its own weight. The per-metre pricing note shows how wall and diameter move the number.
Where tubes are actually used
Aquarium cylinders, exhibition columns, light guides, furniture legs, machine guards for rotating shafts, and increasingly architectural balustrade posts. Each duty sets a different priority: aquariums need thickness and joint integrity, columns need roundness, light guides need optical clarity. The exhibition display note covers the column case in detail.
State the duty in the enquiry and the specification writes itself: diameter, wall, length, end condition, joint type, and the handling that follows.
Send outside diameter, wall thickness, length and quantity to oversales@rpplexiglass.com; we will confirm roundness tolerance and lead time before you commit.
Frequently Asked Questions
How round is a cast acrylic tube?
Cast tube typically holds ovality within 1 to 2 per cent of outside diameter, and wall thickness within plus or minus 10 per cent unless machined. For a tube used as a rotating sleeve, specify a tighter band and expect turning rather than casting.
Which sizes count as large diameter?
Above roughly 300 mm outside diameter the tooling and the cooling cycle change, and 600 mm is a common ceiling for standard cast tube. Above that, tubes are usually fabricated from rolled and bonded sheet rather than cast.
Why do tube ends crack at the base?
A tube standing on a hard floor carries the whole load on a thin annulus. A collar, a radiused base or a rubber pad spreads it. Cracking at the bottom inch is almost always a base detail, not a material defect.