How Milk Pouch Film Is Made and Tested at a Film Plant
Every milk pouch starts as plastic granules, each about the size of a small lentil. By the time it reaches a dairy, those granules have become a printed roll of film thinner than a sheet of paper, made of three separate layers fused together.
We have offered milk pouch film to dairies for years. So this is how I would explain the process to a dairy owner standing next to me.
In short, milk film is made by three-layer blown film co-extrusion. Three extruders melt three different recipes, one circular die joins them into a single tube, and air blows that tube into a tall bubble. After cooling, the film is surface treated, wound, printed, slit to the dairy's machine width and tested lot by lot.
Here is each step. I have added what can go wrong at each one, because that is what a buyer really needs.
Step 1: Choosing the resin
The recipe decides most of the film's behaviour.
A typical milk film uses a blend of LDPE, LLDPE and metallocene LLDPE. LDPE helps the bubble stay stable and gives clarity and softness. LLDPE adds toughness. Metallocene LLDPE is the expensive one. Film makers often put it in the sealing layer, because it seals at a lower temperature and still makes a strong seal when a drop of milk sits in the seal area. On a fast FFS machine, that happens all day.
A good plant does not save money here.
For anything that touches milk, the resin must be virgin and food grade. India's food packaging rules ask for polyethylene conforming to IS 10146 and do not allow recycled plastic in contact with food. White and black masterbatch go in at this stage too. The black blocks light, and the white hides the black and gives a clean printing surface.
Step 2: Blowing the bubble
This is the part visitors remember.
Molten plastic comes up through a ring-shaped die as a thin tube. Air is blown inside, and the tube swells into a bubble that rises up a tower, sometimes several floors high. It reminds me of a phulka puffing up on the tawa, only this one keeps rising for hours.
An air ring cools the bubble from outside. At the top, nip rollers flatten it into a two-ply sheet. Two numbers set at this stage control the film's strength:
- The blow-up ratio, i.e. how much wider the bubble is than the die. It decides how evenly strength is shared between the length and the width of the film.
- The haul-off speed. Pull faster and the film gets thinner.
The cooling has to be even all the way round. If one side of the bubble cools faster, the film comes out thicker on one side and thinner on the other. Uneven gauge like this is a common reason for pouches leaking along one edge.
The bubble does not forgive.
Step 3: Surface treatment and winding
Polyethylene does not hold ink on its own.
Its surface is too slippery and greasy for ink to grip. So the outer face is passed under a corona treater, which uses an electrical discharge to make the surface accept ink. Plants check the result with dyne pens, which show whether a liquid spreads or beads on the film.
Respect this machine. A weak corona treatment gives you print that looks perfect on the day it is printed and rubs off later, when nobody at the plant is watching. Treatment also fades with time in storage, so a good plant checks it again just before printing, not only when the film is made.
Step 4: Printing and slitting
Printing goes on the outer face only. FSSAI rules say the printed surface must not touch food, and inks must meet IS 15495. After printing, the wide film is slit into narrow rolls that match the dairy's form-fill-seal machine, wound on the right core and labelled with lot number, micron, width and weight.
Step 5: The tests that matter
A good plant tests at three points: while the film is running, after each roll comes off, and on the finished lot.
These are the checks that tell you whether a milk film is fit:
- Thickness profile across the full width, not one average figure. Thin spots leak.
- Dart impact, where a weighted dart is dropped on the film (ASTM D1709). It shows how the pouch will take a fall.
- Tensile strength and elongation (ASTM D882). These show how much load the film takes and how far it stretches before it breaks.
- Heat seal strength, tested by sealing strips and pulling them apart.
- Slip, measured as coefficient of friction. Film that is too sticky jams the FFS machine, and film that is too slippery tracks badly.
- Ink adhesion, with a tape pull on the printed area.
- Filled pouch tests: fill, seal, then drop and squeeze, and count the leakers.
- Overall migration as per IS 9845 at an NABL-accredited lab. The FSSAI limit is 60 mg/kg or 10 mg/dm².
Ask your supplier which of these they run in-house on every lot, and which go to an outside lab. Migration is a lab test, so also ask which lab, how often, and for the date on the last report.
What a film plant cannot test
I want to be honest about one limit. A plant can pass every test above and the pouch can still leak at the dairy. Sealing jaws that run too hot, worn teflon on the jaws, or rolls stored for months next to a hot wall will undo good film. So when a dairy sees leakers, the first three things to collect are the lot number, the jaw temperature and a few failed pouches. The failed pouch usually shows where it gave way. A split along the seal points to the machine or the seal layer. A pinhole or a tear in the body points to the film.
Why this matters to you
If you buy milk film, ask your supplier for the lot test report with every delivery. A plant that runs these tests will send it without any fuss. If you want to know what to check on your own side when the rolls arrive, our dairy buyer's checklist for milk pouch film covers it.
The same blown film process also makes general purpose films like the LDPE film on our PackSmith store, though milk film itself is made to order for each dairy. For milk film, send us your pouch size and machine details and we will suggest a structure to trial.
