Why milk pouches leak, and how to find the cause

A leaking pouch does damage twice. It wets every pouch in the crate below it, and it tells the customer that something went wrong at your plant.

FSSAI's guidance document for milk and milk products (first edition, February 2018) lists a "leaky pouch handling tank" among the plant equipment to clean every day. Leakers are common enough that the guidance plans a tank for them.

The short answer is this. Milk pouches leak from one of three places: the seals, the film, or the handling after filling. You find out which by marking exactly where each pouch leaks, counting leakers against the machine and the film roll, and running two simple tests from the Indian standard.

Only then should anyone touch a setting.

I make film, so let me say the uncomfortable part first.

As a film maker I would love to tell you that leaks are never the film. That would not be honest. Pinholes, gels and thin bands are real film defects, and the milk pouch standard lists them. But film is only one of three suspects, and the leak map will tell us which one to question.

Step 1: find where the milk comes out

Let us start with the leakers themselves, because they carry most of the evidence.

Take every leaker from one shift. Wipe it dry, squeeze it gently over a tray and mark the leak point with a marker pen. Then sort the pouches by where they leaked:

1. Along the long vertical seal at the back. This points to the vertical sealing jaw, the overlap of the film, or a wrinkle running into the seal.

2. At the horizontal seal, top or bottom. This points to the cross jaws: heat, pressure, time, or milk and foam caught between the film.

3. At a corner, where the vertical seal crosses a horizontal seal. Here the jaw has to squeeze through extra layers of film, so weak pressure shows up first at this spot.

4. Right beside a seal, with the seal itself intact. The film has thinned at the seal edge, usually from too high a temperature or too much pressure.

5. Through the flat face of the pouch. This is a pinhole, a gel, or a puncture from a crate, a conveyor or the cutting knife.

It is tempting to skip this step and go straight to the temperature dial. I understand why. It feels like action.

But a leak map from one shift tells us more than a week of changing settings.

Step 2: count them in a way that points to a cause

"We had a lot of leakers today" helps nobody. Let us make the count useful instead. Count leakers per 1,000 pouches, and for each shift write down six things on one line of a register:

1. Date and shift.
2. Machine and filling head.
3. Film roll number.
4. Pouches filled.
5. Leakers found.
6. Where they leaked: vertical seal, top, bottom, corner, beside a seal, or the face.

Keep two separate counts.

Leakers found at the plant are one number. Leakers reported by distributors are another. If the second number is much higher than the first, the pouches are leaving your plant whole and failing on the road.

Keep a strip from every film roll you use, labelled with its roll number.

When leakers jump with one roll and drop with the next, you will have the evidence in your hand. We film makers respect that kind of evidence far more than a phone call that says "your film is bad".

Seal problems, and how they show up

A good heat seal needs three things right at the same time: the right temperature, enough pressure and enough time. It is like slapping a roti on the inside wall of a tandoor. Too cold and it falls off. Too hot and it burns before it sticks. Press it badly and it peels away at the edge.

Let us see what each mistake looks like on a pouch line:

1. Too cold or too short: the seal peels apart cleanly when you pull it, like a sticker. Check the real jaw temperature with a probe. Do not trust the number on the panel alone.

2. Too hot or too much pressure: the seal holds but the polyethylene tears right beside it, because the edge has gone thin and brittle.

3. Milk or foam in the seal area: you see a narrow channel through an otherwise good seal. Look at the fill level, splashing from the nozzle and foaming of the milk.

4. Worn jaws, a worn PTFE (Teflon) cover or a damaged sealing wire: leaks sit at the same spot on every pouch from that head. A repeating position usually points to the machine.

5. Wrinkles: the film reaches the jaws with a fold in it, and the fold becomes a channel. Check film tension and how the film tracks over the forming collar.

There is one more seal problem worth knowing. A bottom seal has to carry the weight of the milk while it is still warm. If it opens the moment milk lands on it, the sealant layer has weak hot tack, as we film makers call it, or the seal has not had time to set. I have written more about the sealant layer in our guide to milk pouch film.

If you want numbers, our advice is to send samples to a laboratory that tests seal strength by ASTM F88/F88M (ASTM International, 2023 edition). This method measures the force needed to pull a seal apart and also records how it failed, peel or tear. I find the failure mode more useful than the force figure on its own.

Film problems, and how to spot them

Now let us be fair to the film, since that is our side of the business. The milk pouch standard, IS 11805:2007 (BIS, October 2007), says the film must be free from pinholes, streaks, particles of foreign matter, bands of uneven thickness and undispersed raw material. Each of these can cause a leak:

1. Pinholes and gels. Hold a sheet against a tube light for inspection. A gel is a small hard speck that does not stretch with the film, and the film can split around it.

2. Thin bands. If leakers sit at the same position across the width on pouches from one roll, measure the thickness across that roll with a micrometer.

3. A sealant that does not suit your machine. Some films need a higher temperature than your jaws can give at your line speed. Ask your supplier for the sealing temperature range they tested.

4. Film that is simply too thin for your crates and our Indian roads. IS 11805 asks the buyer to fix the thickness by the leak and drop tests, with 50 micron as the floor for 1 litre pouches and 40 micron for 500 ml.

5. Wrong slip. Film that grips too much or slides too freely tracks badly and spoils the overlap of the vertical seal. IS 11805 sets the kinetic coefficient of friction at 0.25 or less.

Handling problems after the pouch leaves the jaws

Let us now follow the pouch out of the machine. Some pouches leave the jaws perfect and are ruined ten minutes later. I would look at these four places before any of us blames the film or the machine:

1. Crates. Milk crates have their own Indian standard, IS 11584:1986 for HDPE crates for milk sachets, listed in the FSSAI guidance. Pull out crates with broken ribs or sharp edges. Our milk travels in these crates every single day, so a damaged one does damage every single day.

2. Loading. Overfilled crates squeeze the top layer of pouches when the next crate is stacked on them.

3. Throwing and dragging. Watch one full loading of a delivery vehicle. You will learn more there than in any meeting room.

4. Conveyors and the cutting knife. A burr or a sharp guide can scratch every pouch at the same spot.

Two tests from IS 11805 you can run on your own line

These come straight from the standard and need almost no equipment. We like them because an operator can run both in under an hour, next to the machine:

1. Leak test. Take filled pouches 10 minutes after filling. Lay each one flat with the side seal on top. Spread an even load of 5 kgf on a 500 ml pouch or 10 kgf on a 1 litre pouch, for example with a flat board and weights, and leave it for 10 minutes. The standard allows no leak at all, and I would not accept even one.

2. Drop test. Take 16 filled pouches at random from the line and keep them within 2°C of the milk's filling temperature. After 15 minutes, drop each pouch from 1.2 m onto a flat, smooth floor, four times: on its flat side, on the opposite side, on one long edge and on the other long edge. The sample passes if not more than 2 pouches burst.

The standard leaves the frequency of the drop test to the buyer and supplier. My suggestion is simple.

Run it at every film roll change, after any jaw maintenance, and whenever the leak count rises.

A laboratory can go further with ASTM D3078 (ASTM International, current edition D3078-02(2021)e1), a bubble test for flexible packs with some gas inside. Its own scope warns that very small leaks may not show, because the product inside can plug a tiny hole.

So a pouch can pass at the plant and still weep later in the crate. This is one more reason we suggest counting leakers at the distributor too.

A root-cause checklist

We would use this table together with the leak map:

My last rule sounds obvious. Follow it anyway.

Change one thing at a time, and write down what you changed and when. If you raise the jaw temperature, change the film roll and replace a cover in the same hour, you will never know which one worked.

If your leak log points at the film, send us the roll number and clear photos of a few failed pouches, with the seal area in close-up.

We will look at them and tell you plainly what we see, even if the answer is your machine.