Packaging Tube Tolerances: Diameter, Wall Thickness and Ovality

Tolerances are set by the filling line, not by the drawing

A tube drawing usually arrives with four dimensions and four tolerances, and the tolerances were copied from a previous supplier's drawing without anyone asking why they are what they are. That is how a project ends up paying for ± 0.10 mm on a diameter that the filling nozzle does not care about, while a length tolerance that the crimping jaws absolutely do care about is left loose.

The right way round is to work backwards from the filling line:

• Bu outside diameter sets the cap fit and the print mandrel fit
• Bu inside diameter sets whether the filling nozzle enters without scoring the wall
• Bu wall sets the squeeze feel and the crimp seal
• Bu length sets whether the tube sits squarely in the tail crimper
• Bu ovality sets whether it prints evenly and caps without cross-threading
• Bu squareness of the cut sets the weld quality at the shoulder

Everything else is decoration. Tightening a dimension that does not appear in that list costs money and buys nothing.

The four dimensions that matter

Table 1 — Typical tube body tolerances. The "tight" column is achievable but roughly doubles the reject rate and needs in-line gauging.

Notice that wall thickness is expressed as a percentage, not an absolute. A ± 0.05 mm tolerance is 10 % on a 0.5 mm wall and only 5.5 % on a 0.9 mm wall — the same tolerance band means different things on different products, which is why absolute wall tolerances in drawings cause arguments at goods-in inspection.

Where each tolerance comes from

Table 2 — Root causes. Ovality and bow are the two that cannot be fixed by turning a knob while running.

Concentricity is the one that drifts. A crosshead die is centred with four screws before the run starts, and the mandrel moves as the die body heats and as the melt pressure changes. A die that was centred cold at 0.02 mm can be at 0.08 mm two hours into the run. On a 0.5 mm wall, 0.08 mm of eccentricity means the thin side is 0.42 mm and the thick side is 0.58 mm — a 32 % variation that fails most specifications while the average wall reads perfectly in the middle of the band.

Ovality is a time-dependent property

This is the single most misunderstood number in tube extrusion, and it causes more rejected batches than any other dimension.

A tube measured at the end of the line is round, because it is still held by the geometry it was cooled in. Cut it, stack it, leave it overnight, and the residual stress from cooling relaxes. The section creeps into an oval. On a thin-wall PE tube with aggressive cooling, the ovality can grow from 0.1 mm to 0.4 mm in 24 hours.

Table 3 — Ovality development after cutting. Most of the growth happens in the first four hours, but it is not finished until the next day.

The cure is in the cooling design, not in the inspection. Slower cooling, warmer water, a longer trough, or a post-cooling tunnel all reduce residual stress. A plant that measures immediately and passes the batch has not solved anything — it has measured a tube that does not exist yet.

The practical rule: hold the samples for 24 hours before the final ovality check, and do the first-article approval on aged samples. If you need to release product the same day, run the check on a sample from the previous day's production of the same setup.

Measuring, on-line and off-line

Table 4 — Measurement options. The weighing method is underrated: mass per unit length divided by circumference gives the average wall to within a few percent.

Weighing deserves a note because it is the cheapest capable method. Cut a known length, weigh it, divide by the density and by the circumference, and you have the average wall. It will not show eccentricity, but it will show a drift in output or a die gap change immediately, and it takes thirty seconds.

The in-line laser gauge is the right answer for anything under ± 0.12 mm. It closes the loop to the vacuum and haul-off, so the operator is supervising a controller rather than chasing a number. On a manual line, the operator checks every 30 minutes and the process drifts between checks — which is fine for a ± 0.20 mm tolerance and not fine for ± 0.08 mm.

Matching the tube to the filling line

The tolerances only matter because of what happens downstream. These are the interfaces where a marginal tube becomes a line stop.

Table 5 — Where tube tolerances meet the filling line. The crimper is the most sensitive interface and the least often specified.

The tail crimper is the one people forget. It closes the tail flat and applies heat or ultrasound to weld the two walls together. If the cut is not square, one side of the tail is longer than the other, the jaws grip unevenly, and the seal is thin at one corner. That corner is where the tube bursts when the consumer squeezes hard, and it will be a complaint three months after the batch shipped.

How to relax tolerances and save money

Every tightening step costs yield. On a typical line the relationship is roughly:

Table 6 — Yield against tolerance, indicative. The curve steepens because the process distribution does not narrow as fast as the tolerance band does.

Three practical ways to avoid paying for precision you do not need:

1. Specify the diameter that the cap actually sees. The cap contacts the neck of the moulded head, not the tube body. The body OD tolerance only needs to suit the print mandrel, which is usually looser than the cap.
2. Use a percentage wall tolerance, not an absolute one. It survives a product change without renegotiation and it matches how the process actually behaves.
3. Put the tight tolerance on the length, not the diameter. Length is controlled by an encoder and a cutter, and ± 0.4 mm is cheap. Diameter is controlled by vacuum, cooling and draw-down, and ± 0.08 mm is expensive.

What a good tube drawing contains

A tube drawing that can be quoted without a meeting has these on it:

• Nominal OD with tolerance, and whether it is measured over the print or the plain body
• Nominal ID, derived from OD and wall — not specified independently
• Wall with a percentage tolerance
• Length with tolerance, and the datum (cut face to cut face)
• Ovality limit and the ageing period before measurement
• Cut squareness
• Resin grade or performance requirement, plus colour
• Any co-extrusion layer structure with individual layer percentages
• Packaging format, because nesting affects ovality in transit

The one that is most often missing is the ageing period for the ovality measurement. Without it, the supplier measures at the machine, the customer measures at goods-in a week later, and both are technically correct and completely incompatible.