Acrylic Rod Sizes, Tolerances and Straightness

1. What sizes are actually available

Acrylic rod is nominally available from about 2 mm to 200 mm, but the range is not uniform across processes, and availability in a given diameter and colour is a stock question rather than a capability question.

"Standard" means commonly held or routinely produced. Non-standard diameters are possible but usually carry a minimum quantity, because a cast mould tube or an extrusion die is dedicated to that size.

Two practical points. First, an odd diameter like 17.5 mm will cost more than 18 mm in almost every case, because it is not a size anyone holds tooling for. Second, if your drawing calls for a nominal that nobody makes, the supplier will either machine it from the next size up — which changes the price entirely — or quietly supply the nearest standard and hope you do not measure.

2. Diameter tolerance: how to write it

This is where most acrylic rod disputes start, because "20 mm" is not a specification and the two processes hold very different tolerances.

Indicative for standard commercial production. Tighter figures are negotiable on extruded rod with in-line gauging; cast rod cannot be brought to extruded tolerance because the mould tube, not a die, sets the size.

How to write it. Give the nominal and a symmetric tolerance — 20.0 ± 0.2 mm — rather than a limit range, unless you specifically need an asymmetric band. State the measurement temperature if the part is large, and state how many points along the length are measured. A tolerance without a sampling rule is not enforceable.

The trap. A drawing that specifies ±0.1 mm on a 40 mm cast rod is asking for something the process cannot do. The supplier has three options: decline, machine the rod down from a larger size (expensive), or accept the order and ship out-of-tolerance. Only the first two are honest, and the second changes the price significantly. If you need that tolerance, say so at the quoting stage.

3. Ovality — the dimension nobody specifies

Ovality is the difference between the largest and smallest diameter measured across the same cross-section, usually expressed as a percentage of nominal. It is almost never on a drawing and it is the reason a rod that "measures 20 mm" will not fit a 20 mm hole.

• Extruded: typically 1–2 % of diameter. On a 20 mm rod that is 0.2–0.4 mm — the same order as the diameter tolerance itself.
• Cast: up to about 3 % of diameter, driven by tube roundness and cooling symmetry.

If the rod goes into a close-fitting hole, a bearing, or a collet, specify ovality explicitly. If it is decorative, it does not matter and specifying it only adds cost.

4. Straightness and bow

Straightness is expressed as maximum deviation from a straight line, in millimetres per metre of length. It is a separate requirement from diameter and it is the one most often left out.

Two things to state alongside the number. First, the length over which it is measured — a rod that is straight over 500 mm can be visibly bowed over 2 metres. Second, whether the rod is measured as supplied or after conditioning, because a rod packed while still warm can bow in the box.

For long decorative lengths where straightness is a design feature, the practical fix is often not a tighter specification but better packing — a rigid tube or a supported crate rather than a bundle.

5. Length and cut tolerance

Standard supply is 1000 mm or 2000 mm straight lengths; some producers offer 3000 mm in smaller diameters. Cut-to-length is normal and priced per cut.

• Cut length tolerance: ±1 mm is standard. Tighter needs a different cut-off arrangement and is priced accordingly.
• Squareness of cut: a saw cut is rarely perfectly square. If the rod has to butt against a surface, specify squareness or specify a machined end.
• Length and temperature: acrylic expands roughly 0.07 mm per metre per °C. A 2 m rod measured at 30 °C in a warehouse and again at 20 °C in a lab differs by about 1.4 mm — more than the cut tolerance. State the measurement temperature for long lengths.

6. End finish

The end of the rod is usually specified last and matters most, because it is the face people look through.

Polished ends are a separate operation and a separate line item. A quotation that says "cut to length" does not include them, and the difference in cost is not trivial on short lengths.

7. Surface finish and optical grade

Standard rod has a glossy as-produced surface — from the mould tube in casting, from the die and cooling in extrusion. Beyond that there are three grades worth distinguishing.

• Standard: glossy, minor surface marks acceptable, no optical requirement. Most decorative and structural applications.
• Optical: specified light transmission (clear PMMA is around 92 % over 3 mm), no visible internal bubbles or haze, ends polished. Light guides, lenses, display components.
• Protected: standard or optical rod with a peelable film on the cylindrical surface, for parts that must arrive scratch-free and be handled before assembly.

Specifying "optical" without defining a transmission value and a bubble limit gives the supplier nothing to work to. State the transmission, the measurement thickness, and the maximum acceptable defect with a reference sample.

8. Annealed or unannealed

Annealing relieves internal stress. It matters if the rod will be machined, drilled, tapped, cemented or exposed to solvents.

• Unannealed is normal for rod sold as stock and cut into decorative lengths. Cheaper, and adequate if nothing further happens to it.
• Annealed is required before machining or solvent cementing. It costs slightly more and prevents the most common field failure in acrylic components — a crack that appears a day after assembly.

Ask for a craze test result rather than a statement that the rod is annealed. It is a short test and it is the only objective evidence.

9. Packing and protection

Acrylic scratches if you look at it wrongly, and packing is where a good rod becomes a rejected rod.

• Interleaving. Rods packed in direct contact rub in transit. Paper interleaf or individual sleeving prevents it.
• End protection. Ends take the impact in a dropped carton. End caps or foam are cheap insurance on polished ends.
• Support. Long lengths need a rigid crate or tube. A bundle strapped in the middle sags and bows.
• Film. Peelable film on optical grades, applied clean and removed within the stated shelf life — an old film leaves adhesive residue that is worse than the scratches it prevented.

10. A specification you can copy

This is the level of detail that produces a quotation you can compare with another supplier's. Everything below is a decision you have to make anyway; writing it down costs nothing and prevents most disputes.

11. Frequently asked questions