Choosing a forming film is often reduced to one question: how thick should it be? Go too thin and packs puncture, leak or lose their vacuum in transit. Go too thick and you pay for plastic you don’t need, slow the machine down and make forming harder.
Thermoforming thickness is not a single number you can copy from another product. It depends on how deep the pack is, what goes inside it and how the film behaves when it is heated and stretched. Here is how to work it out.
Microns, mils and gauge
Film thickness is usually given in microns (µm) in India and Europe, and in mils in the USA. One mil equals 25.4 microns, so a 150 µm film is roughly 6 mil. “Gauge” is simply the general term for thickness.
In a thermoforming pack there are two webs:
- The forming (bottom) web is heated and pulled into the mould to make the pocket.
- The non-forming (top) web is the lid. It stays flat and is usually much thinner.
Most of the decisions in this article apply to the forming web.
Why the film gets thinner when you form it
When the film is pulled into a pocket, the same amount of plastic has to cover a larger surface. The film thins out, and it does not thin evenly. The corners and the bottom edges, which stretch the most, end up thinnest.
A quick estimate shows how much this matters. Take a pocket 100 mm wide, 150 mm long and 30 mm deep:
- Flat area before forming: 100 × 150 = 15,000 mm²
- Surface after forming: base 15,000 + two long walls (2 × 150 × 30 = 9,000) + two short walls (2 × 100 × 30 = 6,000) = 30,000 mm²
The formed pocket has twice the surface area, so the average wall is about half the starting thickness. A 150 µm film averages roughly 75 µm across the pocket, and the corners will be thinner still. Double the depth and the effect becomes far stronger.
This is why deep packs need a thicker starting film, or a film with a structure designed to stretch evenly.
What decides the gauge you need
Draw depth. The deeper the pocket compared with its width, the more the film thins. Shallow packs for sliced cheese or bacon can use a much lighter web than deep packs for whole sausages or meat portions.
Sharp or hard products. Bones, shell, hard cheese edges, spices and frozen product all press on the thinnest parts of the pack. Bone-in meat often needs a dedicated bone guard forming film rather than just a thicker standard one.
Pack weight and handling. Heavier products, long transport routes and stacked shipping cases all put more stress on seals and corners.
Barrier loss. This is the one most people miss. The barrier layer stretches and thins along with the rest of the film, so a film that meets your oxygen barrier target when flat may not meet it once it is formed. Always ask for barrier figures that account for the draw.
Machine and mould. Heating time, plug assist, mould temperature and vacuum all change how evenly the film forms. The same film can behave differently on two machines.
Typical starting points
These are rough starting points for flexible forming webs. Confirm them with a trial on your own line:
- Non-forming top web: commonly 50 to 90 µm
- Shallow forming (sliced products, low-profile packs): commonly 100 to 150 µm
- Medium depth (sausages, portion cuts, cooked meats): commonly 150 to 200 µm
- Deep draw or sharp products (bone-in meat, large portions): commonly 200 µm and above, often with a puncture-resistant structure
Rigid trays are a different category. PET thermoforming and PP sheet for rigid trays are much thicker, usually several hundred microns, and are used where the pack must hold its shape.
Materials: why structure matters as much as thickness
Two films of the same thickness can perform very differently. The thermoforming materials inside the film, and the thermoforming raw materials chosen at the extrusion stage, decide how it stretches, seals and protects:
- Nylon (PA) gives formability and puncture strength, helping the film stretch evenly into corners.
- EVOH provides the oxygen barrier for fresh and cooked foods.
- Polyethylene (PE) forms the sealant layer. PE thermoforming films with good sealing layers close reliably even with some fat or moisture on the seal.
- Ionomer sealants are used where seals must hold through contamination or where extra toughness is needed.
Modern multilayer structures let you down-gauge: a well-designed 150 µm film can sometimes replace a 180 or 200 µm film made from older materials, without more leakers. Our forming and non-forming films are built on this approach.
How to test the right thermoforming thickness
- Run a trial with two or three thicknesses on your actual machine and product.
- Cut finished packs open and measure the wall at the corners and bottom edges, not just the base.
- Check the leaker rate after packing, after a day in the chiller and after a transport test.
- Confirm shelf life with real storage trials, because thinner corners can let in more oxygen.
- Pick the thinnest film that passes every check, and keep a safety margin for variation in your product.
Getting help
Thickness is one part of a balanced film design that also covers barrier, sealing and formability. If you are seeing leakers, splitting corners or rising film costs, ask our team for a trial on your line. You can also browse our full range of thermoforming films.
