Dimensional Stability Testing for Knit Bags: Setting Shrinkage and Growth Limits on a Loop Structure That Moves Under Load

The Decision Tree: Five Choices Before You Set a Percentage

A dimensional change figure is the output of five decisions, and the percentage everyone argues about is the last of them. In order: what gets measured, which washing and drying procedure applies, whether laundering is the right exposure at all, how many cycles and which measurement points, and finally the limit itself.

Skipping to the fifth decision is the standard error and it produces a limit nobody can test against. A limit of plus or minus 3 percent is meaningless until someone has decided whether that is on a flat panel or on the assembled bag, after one wash or after five, tumble dried or line dried.

The ordering also settles the argument about whether a result is a failure. Most dimensional disputes are not disagreements about the number; they are two people measuring different things and both being correct about what they measured.

Decision 1

Measure a flat panel, the assembled bag, or both

Decision 2

Choose the washing and drying procedure from ISO 6330

Decision 3

Confirm laundering applies, or use a spot clean or water exposure route

Decision 4

Set the cycle count and the measurement points, recorded to ISO 3759

Decision 5

Set the limit per direction, with separate figures for length, width and depth

Step One: Decide Whether the Bag Is Measured Flat or as a Volume

ISO 3759 covers the preparation, marking and measuring of specimens and garments for dimensional change tests, and it works on marked distances. For a flat panel that is straightforward: mark two distances, wash, measure, express the change as a percentage.

A bag is not flat. It has a base, a depth and a shaped opening, and a shopper that loses three percent in height and gains three percent in width still holds roughly the same volume while looking wrong on a shelf. Measuring only flat distances can therefore report a pass on a bag a customer would notice.

The workable answer is both. Use marked distances on the panel for the comparable laboratory figure, and add a simple volume check on the assembled bag, measured before and after by a repeatable method such as filling with a known quantity of a uniform medium. Two numbers, and the second is the one customers actually perceive.

Step Two: Choose the Washing and Drying Procedure

ISO 5077 determines dimensional change in washing and drying but does not itself define the wash; it takes the procedure from ISO 6330. The 2021 edition of ISO 6330 defines 16 washing procedures on a Type A front-loading horizontal axis machine, 12 on a Type B top-loading agitator machine and 7 on a Type C top-loading pulsator machine.

It also defines six drying procedures: line dry, line drip dry, flat dry, flat drip dry, flat press and tumble dry. Drying is where a large share of dimensional change actually happens, and a buyer who specifies the wash and leaves the drying to the laboratory has specified half the test.

Choose the pair that matches the care instruction. A bag labelled tumble dry low should be tested tumble dried, and a bag labelled dry flat should be tested flat dried. The two produce different results on the same knit, and a specification that names only the wash will get whatever drying the laboratory defaults to.

Step Three: Confirm That Laundering Is the Right Exposure

Some bags are washed and some are not, and testing the wrong exposure wastes the allowance and can fail a perfectly good bag. A structured tote with a wipe-clean instruction does not need a laundering dimensional test; it needs to be checked for what happens when it gets wet and dries, which is a different and gentler question.

Where a bag is genuinely laundered, ISO 5077 with an ISO 6330 procedure is the right route. AATCC 135 covers dimensional changes of fabrics after home laundering and is the common North American alternative. The two share a purpose and differ in machine, detergent and drying detail, so results should not be converted.

There is also the relaxation case, which applies to every knitted bag whether or not it is ever washed. A knit relaxes after knitting and again after packing and shipping, so a bag can change dimension with no exposure at all. That is why conditioning and resting before measurement matter more here than in any other textile test.

Step Four: Set the Cycle Count and the Measurement Points

One cycle tells a buyer what happens once. Three or five cycles tell a buyer where the change settles, which is the more useful figure because most dimensional movement in a knit is front-loaded and then flattens. Where a bag is sold as repeatedly washable, three cycles with measurement after each is a defensible protocol.

Measurement points should be recorded on the approved sample so the test is reproducible, and the same points must be used before and after. ISO 3759 exists precisely to make that discipline standard, and a laboratory asked to measure a bag without marked points will choose its own, which is how two reports on one bag come to disagree.

Conditioning before each measurement is not optional. ISO 139 sets the standard atmosphere for conditioning and testing, and a knit that has not been conditioned and rested will report a change that is partly moisture and partly recovery rather than genuinely dimensional.

Step Five: Set the Limit, and Set It Per Direction

This is the decision the whole tree exists to support, and the rule is simple: never one number. A knitted panel is anisotropic, so length, width and depth need separate limits, and on a knit the two in-plane directions often move in opposite senses rather than both shrinking.

Commercially workable starting points for a knitted bag are plus or minus 3 percent where the bag is structured and must hold a precise shape, and plus or minus 5 percent where it is a soft shopper and some movement is acceptable. Both are starting points rather than standards, and both should be replaced by a measured figure from a bag the buyer already sells.

State whether growth is permitted. Many specifications write shrinkage limits only, and a bag that grows four percent passes a shrinkage limit while no longer fitting the display fixture it was designed for. Writing the limit as a range with both signs is one extra word and removes the ambiguity.

Shrinkage, Growth and Relaxation: Three Ways a Knit Moves

Shrinkage is the fabric contracting, and on a knit it is usually driven by the loop relaxing toward a lower energy configuration when it is wetted and agitated. Growth is the opposite, and on a knit it is usually driven by load: a loop structure extends under weight and does not fully recover, so a bag that has carried heavy contents gets longer.

Relaxation is the third and the one buyers never specify. A knitted panel comes off the machine under tension and continues to relax for a period afterwards, then relaxes again after packing and shipping. A bag measured the day it is made will read differently from the same bag measured a week later, with no washing involved at all.

That is why the practical protocol is to condition and rest before the first measurement, and to record the interval. Where a buyer and a laboratory disagree on a result, the resting interval is one of the first three things to check, alongside the drying procedure and the measurement points.

What a Dimensional Change Report Should Show

A usable report names the method and edition, the washing procedure and the drying procedure, the number of cycles, the conditioning statement, the result per direction with its sign, and the tested construction. Six fields, and the fifth is the one that turns a pass into an argument when it is missing.

The sign matters as much as the size. A result of minus 2 percent and a result of plus 2 percent are both 2 percent and they are not the same bag, and a summary that reports only the magnitude has thrown away the half of the result that tells a buyer what will happen on the fixture.

Individual specimen results matter here too, for the same reason they do everywhere. Five specimens all moving about 2 percent describe a stable construction; five moving between minus 1 and plus 6 describe a construction that cannot be specified at all.

Writing the Dimensional Stability Clause

The clause needs the method and edition, the washing procedure and the drying procedure from ISO 6330, the cycle count, the conditioning reference to ISO 139, and the limit per direction with both signs stated. Six fields, and each one prevents a conversation that would otherwise happen after delivery.

Add the volume check and the clause covers the customer's perception as well as the laboratory's number. Requiring the assembled bag to hold a stated proportion of its original volume after the same procedure is one line, and it catches the case where length and width move in opposite senses and the flat figures both pass.

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