Writing a Fill Weight Tolerance Into the Gel Pack Purchase Order

“They are supposed to be 500 grams and half of them are 470.” That complaint, from a distributor in Rotterdam, is the start of a negotiation that most purchase orders are not written to win. The packs were within the supplier’s own tolerance of ±5 percent. The buyer had never specified one, so the supplier’s default applied, and the default was generous.

Fill weight is the single most consequential number on a gel pack order and the least often written down properly. It sets the cold capacity, the freight cost, the freezer load and the leak risk, and it is the number a buyer can measure in thirty seconds with a kitchen scale.

What the nominal weight actually means

A 500 g nominal pack contains film, gel and air. The useful number is the net gel mass, not the gross weight, and the difference between the two is the tare of the film — typically 8-14 g for a 500 g pack. If your specification talks about gross weight, you are buying 488 g of gel and calling it 500.

Then there is the fill method. A volumetric filler set to 500 ml of a gel at 1.02 g/ml delivers 510 g; the same filler on a batch mixed 3 percent thinner delivers 495 g. The variation comes from the mix, the temperature and the filler calibration, not only from the machine’s accuracy.

Tolerance bands and what they cost

Tightening from ±5 to ±1 percent does not buy 4 percent more cold capacity in any meaningful sense. It buys a reject stream and a price increase. For most lanes, ±2 percent is the right balance, and for a pharma lane with a validated mass, ±1.5 percent with a documented check is defensible.

The counter-intuitive failure: hitting the weight the wrong way

A supplier under pressure on a ±2 percent band started adding water rather than polymer to make the low packs come up to weight. Every pack weighed exactly 500 g and every pack tested 15 percent weaker on gel strength. The result was packs that slumped in the box, pooled at one end under vibration, and leaked at the seal because the gel had no structure to hold it away from the seam.

Weight tolerance without a composition tolerance is a loophole. Specify both: net weight within ±2 percent, and gel strength or viscosity within a stated band, measured on a sample from each batch. The second number is what stops a supplier from meeting the first one cheaply.

How to write it into the order

• State net gel mass, not gross, and state the tare separately
• State the tolerance as a mass band in grams, not a percentage, so there is nothing to interpret
• Name the measurement method: calibrated scale, pack at 20 °C, 24 hours after fill
• Set the sampling plan: how many packs per batch, and the acceptance number
• Add a gel strength or viscosity band with its own sampling plan

Then verify at receipt for the first three shipments, weigh twenty packs, and plot them against the band. After that, spot-check one batch a month. The measurement takes ten minutes and it is the cheapest quality control you will ever run.

There is a freight angle as well. If your specification shifts the average fill from 500 g to 510 g across 6,000 packs a month, that is 60 kg of extra water on the outbound leg every month, and on a volumetric-billed air lane it can push a carton into the next band. Tolerance is not only a quality number; it is a freight number, and it deserves to be checked by whoever pays the freight bill rather than only by whoever pays for the polymer.

If you are rewriting a specification, keep the tolerance and the composition requirement on the same page — the logistics ice pack guide shows how the two interact for different service temperatures.