How to Test Insulated Bag Performance: Ice Retention Protocols Explained

Most ice retention numbers in this category are not comparable, and the reason is not dishonesty: it is that two laboratories made different unstated assumptions. One pre-chilled the load, one did not; one used a calibrated probe, one used a consumer thermometer; one opened the bag, one did not. This piece sets out the protocol field by field, explains what each field is worth in hours, and gives the reporting format that makes the second test comparable with the first.

What Is the Endpoint, and Why Does It Move the Answer by Hours?

Four endpoints are in circulation. Some ice present at the stated hour is the loosest and the one most marketing copy uses; it can be four hours later than the next one. Fifty percent of the ice remaining by mass is the retail comparison standard and is repeatable. Core temperature at or below 5 °C is the food safety endpoint and is the one a food business needs. Core temperature at or below 8 °C is the beverage and lunch endpoint. On the same 8 mm bag at 25 °C, those four land at roughly fourteen, eleven, nine and twelve hours.

So the first field on the protocol is the endpoint, and the second is its location: where the probe sits. The geometric centre of the load is the usual answer and it is the warmest point in a well-packed bag, which is what you want to measure. A probe against a wall reads cooler, a probe against a gel pack reads colder, and either produces a number that flatters the bag and does not predict the field.

How Must the Load and the Refrigerant Be Conditioned?

The load is conditioned to its stated starting temperature for long enough to be uniform, which for a can or a bottle is twelve hours in a controlled cabinet rather than an hour in a fridge. The refrigerant is conditioned at its stated temperature for twelve hours as well, and for a eutectic pack the conditioning temperature matters as much as the duration: a pack conditioned at -18 °C and a pack conditioned at 0 °C deliver very different results on the same bag.

The bag itself is the one everybody forgets. A bag that has been stored in a warm room starts warm and the first hour of the test is spent cooling the foam rather than the load; a bag conditioned at the test ambient starts as the customer would start it. State the bag conditioning, and where the purpose is to predict the field, condition the bag at the ambient rather than in a cool store.

What Ambient and What Chamber?

The worst ambient the product will meet in service, with a tolerance, and stated as air temperature rather than as a setting on a dial. A chamber gives control and reproducibility; a room gives realism and is what most programmes actually use. Either is acceptable if the air temperature is logged beside the bag for the whole run, and neither is acceptable if it is not.

Two further conditions belong in the same field: air movement and radiation. A bag in still air loses heat more slowly than one in a moving vehicle, and a bag under a radiant source loses faster than one in a shaded room. State whether the air is still or moving, and keep the bag out of direct radiation from a lamp or a window, because a radiant source can add ten degrees to the effective ambient without changing the air temperature at all.

How Many Samples, and What Is the Acceptance Rule?

Three production units, with two passing. That is the working rule and it is worth stating exactly, because a protocol with no acceptance rule becomes a negotiation on the day a sample misses. Three units catch a single outlier without the cost of five, and the "two of three" rule is the one most programmes can agree to. Where the third unit decides, say so: the mean of three passing with no individual result more than an hour short is a reasonable variant.

The samples have to be production units. A sample-room unit is made by a different process with different tolerances, often by a different person, and it is usually the best unit in the building. Where the programme intends to hold the result as a contract term, the units have to come off the production line, and the protocol should say that on its face.

What Instrument, and How Is the Result Made Trustworthy?

A calibrated probe with a tolerance stated — ±0.5 °C is the usual figure for this class of work — logged at intervals short enough to show the shape of the curve, which means five minutes or better. A single reading at the endpoint tells you the bag passed and tells you nothing about why; a curve tells you whether the load was pre-chilled, whether the packs were conditioned and whether the bag was opened.

The calibration status is what makes the report defensible. ISO/IEC 17025 is the general requirement for the competence of testing and calibration laboratories, and where a report is going to be filed with a retailer or used as a contract term, an accredited laboratory with thermal testing inside its scope is the answer. In-house results are adequate for development, provided the instruments have current certificates and the method is written down.

What Are the Common Protocol Errors?

Six, in order of how much they distort the result. No stated endpoint, which moves the answer by hours. A warm load, which costs four to six hours. A probe against a wall or a pack, which flatters the bag. An unlogged ambient, which makes the result unrepeatable. A sample-room unit rather than a production one. And a single sample, which cannot distinguish a good bag from a lucky one.

A seventh is worth adding because it is the most common in practice: the test is run once, the number goes into marketing copy, and the bag is changed three times over the following year without the test being repeated. Every change to foam, liner or closure invalidates the result. Put a re-test trigger in the protocol — any material change, and annually in any case — and the number stays true.

One more field is worth considering where the programme will repeat: a control bag. Run a retained reference unit alongside the test samples on every occasion, and the run-to-run variation of the test itself becomes visible. A control that drifts by two hours tells you the change is in the method or the chamber rather than in the bag, and that distinction is worth a great deal when a reorder result is disputed.

An uncertainty statement belongs alongside it. Every measurement has a spread — the probe tolerance, the chamber variation, the difference between three units — and a result reported as twelve hours with a spread of plus or minus one hour is more useful and more honest than a bare twelve. Where two bags differ by less than the spread, they are the same bag, and saying so prevents a costly specification change made to chase noise.

How Should the Result Be Reported?

Sample identity

Unit reference, production date, build revision

Endpoint

Definition and the location of the probe

Load

Mass, start temperature, conditioning regime

Refrigerant

Type, mass, melt point, conditioning regime

Ambient

Mean, minimum and maximum across the run

Openings

Count, dwell and time of each

Instrument

Probe reference, calibration date, tolerance

Result

Time to endpoint for each of the three units

Curve

The full temperature curve attached, not only the endpoint

Verdict

Pass or fail against the stated acceptance rule

Two further fields are worth adding where the report will be used as evidence rather than for development: the build revision of the units tested, and the pack-out photograph. The first links the number to a specification that can be re-ordered; the second settles the argument about where the packs sat. Both cost minutes and both are the fields a reviewer asks for when a result is challenged a year later.

The curve is the field that turns a report into evidence. A curve that rises smoothly tells a different story from one that steps up at each opening, and a curve that starts by falling tells you the load was warmer than stated. Where a result is disputed, the curve is what settles it, and a report with only an endpoint value cannot be reviewed.

There is a practical question about who runs the test that matters as much as the method. A supplier-run test is faster and it uses the supplier chamber; a buyer-run or third-party test is slower and it is the one a retailer will accept. The efficient answer is to run development tests in-house or at the supplier, and to commission one accredited run at the point the specification is frozen, on production units, as the number that goes into the contract and into the marketing copy.

Whichever route is chosen, the protocol has to be written before the units are made, because the units have to be made for the test: production tooling, the agreed build-up, and the agreed pack-out. A test commissioned on whatever happens to be in the sample room produces a number about a bag that will not be produced.

How Does the Protocol Fit the Programme Calendar?

Between sampling and approval, and it is the step most often squeezed. MOQ 500 pcs per style and colourway, sampling 6-10 working days, bulk production 35-50 days from sample approval, inspection at AQL 2.5 under ISO 2859-1 General Inspection Level II, T/T 30/70, FOB Xiamen. A protocol that takes two or three days of run time plus conditioning has to be inside that plan, not after it.

Where the thermal number is a contract term, the acceptance rule belongs in the purchase order alongside the protocol reference, and the remedy belongs there too: re-work and resubmission, a buyer-signed waiver, rejection, or sort and partial release. Agreeing the remedy before the failure is what keeps a missed result from becoming a six-week delay.

A fifth practical point concerns the pack-out record. The protocol fixes the pack-out, but the pack-out has to be written on the product as well — a card in the bag, or a printed panel inside the lid — because the user is the one who decides whether the load was pre-chilled and whether the packs were conditioned. A protocol that only exists in a laboratory file governs the test and not the field.

A fourth practical point: keep the failed runs. A protocol that only files passing results gives no warning when a change starts to erode performance, and the failed curve is usually the one that explains why. File everything, with the build revision on it, and the history becomes a control chart rather than a folder of certificates.

How Do Two Laboratories End Up With Different Numbers?

Almost always through the fields in the table above rather than through the chamber. One laboratory pre-chills, one does not; one runs three units, one runs one; one logs the ambient, one assumes it. Given the same protocol, two competent laboratories should agree within an hour or two on a twelve-hour bag, and if they do not, the difference is in the fields.

Where comparability matters, send the protocol with the samples rather than a request for "an ice retention test". A laboratory will fill the gaps with its own defaults, and its defaults are not necessarily the ones your product was designed against.

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