After gaining extensive experience in the field, we felt compelled to write a comprehensive and insightful LinkedIn article on how to choose the right foil.

In product development, especially in industries using sealed containers, getting the details right at the beginning can mean the difference between success and costly delays. One of the most overlooked but crucial decisions in packaging is choosing the right sealing material for your container. Here’s what you need to know.

1. Start with first principles

Many people believe that heat-sealing foil works like baking foil: heat it up and it melts into the plastic. But that’s not how it works.

What actually makes heat-sealing foil effective is the heat seal layer, a special coating applied to the foil, either as a lacquer or a co-extruded polymer, depending on the application. This layer is designed to melt at a specific temperature and bond to the container. In some cases, the seal behaves like a hot melt adhesive, especially when bonding to glass or metal. Materials like surlyn are often used for sealing to glass, acting much like a glue. But in most cases, particularly when sealing to plastic, the heat seal layer is a polymer chosen for compatibility with the material from which the container is made.

The most common container materials are polypropylene (PP), polyethylene (PE), and (polyethylene terephthalate) PET. Other materials like polycarbonate or COC may appear in medical applications, but the principle is the same: the heat seal layer must be chemically compatible with the container material to form a reliable bond. If they’re not, no amount of temperature or pressure will create a reliable seal.

2. Peelable or Permanent? Know your end goal

Before you choose a foil, ask: Do you want the seal to peel off cleanly or you need to form a permanent weld?

• Permanent welds require the heat seal layer and container material to be the same, e.g., polypropylene on polypropylene.
• Peelable seals are either made from a different but compatible polymer or include a release agent to reduce adhesion.

Always consult technical data sheets provided by material suppliers and verify compatibility. Universal heat-seal coatings exist, but they may come with trade-offs. The key is knowing your container material, your required seal strength, and what performance you need: peelability, durability, chemical resistance, and so on.

Here’s an example of a container seal welded using conduction sealing and the wrong foil (left) vs induction sealing and the correct foil (right):

3. Don’t overcook it!

4. R&D shortcuts can backfire

5. Two real-world examples

Case 1: Validation can lock you in

A customer in a development lab in Germany wanted to make a welded seal to their PP container.  They had some foil-seal material in house which unbeknown to them was designed for retortable metal pet food trays. It sealed, but only at temperatures over 180°C, far above the plastic container’s melting point. The result? Excessive sealing times and significant melt-down on the container sealing surface

We were later asked to improve the sealing performance.  Using a polypropylene alternative with a lower activation temperature it delivered perfect seals in a third of the weld-time. Unfortunately, at this stage our customer couldn’t switch materials due to their validation documents already having been filed. An expensive lesson in why early material selection matters.

Case 2: A seal that wasn’t really a seal

Another customer wanting to form a welded seal tried using polyethylene (PE) foil on polypropylene (PP) containers. They were able to make a leak-proof seal using this material but as soon as any significant negative pressure was applied, the seal failed. Instead of asking why, they used some prior knowledge of ultrasonic welding tools to redesign the contact surface on their heat-sealing head to press the foil in mechanically at different angles, forcing a seal which is mechanically keyed-in to the container.

This approach did form a significantly stronger seal but it was solving the wrong problem and failed to use the heat-seal coating in the way it was intended to work. The risk? Microscopic damage to the seal layer as it is stretched and over-heated, which could let aggressive chemicals attack the aluminum beneath along with irregular melting of the container surface.

Final Thought: Expertise saves time and cost

Heat sealing might appear straightforward…until it isn’t. Whether you’re developing packaging for food, pharmaceuticals, or consumer products, getting the right seal depends on:

• Knowing your container material
• Understanding the sealing process
• Selecting the right foil, from the start

From CE marking to FDA compliance, or just ensuring long-term product integrity, early engagement with material experts can save you time, cost, and complexity later on.

In packaging, “it works for now” can lead to bigger problems later. The best time to get it right is at the beginning. If you’re working on a sealing challenge or developing a new product line, don’t hesitate to reach out. We’ve seen the good, the bad, and the melty. And we’re here to help.