Blow Mould Label vs In-Mould Label: why the two films are not interchangeable

Almost every week someone asks us for in-mould labels for a blow moulding line, or the reverse. The two sound like the same thing and the industry often uses the terms loosely. They are not the same film, and sending the wrong one to the wrong process is the single most common reason a first trial fails.

The difference is pressure

In injection moulding, molten polymer is forced into a closed cavity at very high pressure. The label is pressed hard against the cavity wall and fuses directly with the polymer. No adhesive is needed, because the pressure and heat do the work.

In extrusion blow moulding, a hot parison drops into an open mould and is inflated with air. Air pressure is a fraction of injection pressure. There is simply not enough force to fuse a plain polypropylene film to the container, so a blow mould label carries a heat-seal coating that the heat of the parison activates.

What happens when you use the wrong one

• IML film on a blow moulding line: the label sits on the surface, bonds patchily or not at all, and peels at the edge once the bottle cools.
• BML film in an injection tool: the heat-seal layer is unnecessary and can soften too early, giving wrinkles, trapped air or shade shift.

How to tell which you need

Forget the label and look at the machine. If the container is formed by injecting polymer into a closed cavity — pails, tubs, cups, closures, thin-wall containers — you need IML. If it is formed by inflating a parison — HDPE bottles, jerry cans, lubricant and agrochemical packs — you need BML.

What a supplier should ask you

A supplier who quotes without asking these is guessing:

• The moulding process, and the machine make
• The resin and the wall thickness
• Whether the mould uses vacuum ports or static pinning to hold the label
• The panel layout — front only, front and back, or wrap-around
• Annual volume and the number of SKUs

One useful side effect of BML

Because the bonded label becomes part of the container wall, it adds stiffness. Some customers use that to take weight out of the bottle and keep the same top-load performance. That is a resin saving on every unit, and it usually pays for the label several times over.

If you are unsure which applies, send us the container drawing or a sample part. We will tell you which of the two it is — including when the answer is neither.