Consider the yogurt cup. Most of them are covered with a foil seal. Or think about bread or pastries from the shelf: many are sealed in plastic. Most sealing that you see in the food industry is heat sensitive. That means when you seal something to plastic, you’re causing a thermal bond between the plastic cup and its foil seal.
The challenge with this process is that it’s not always consistent. The temperature of the products and how they’re sealed can vary and cause problems with the actual sealing.
There are two general methods for thermal sealing. One is pure heat: a warming tool is used and pressed down on top of the foil cover. The other is an electronic method called “hypersonic sealing”. It’s like microwaving the product so that it warms up. (The net result of both is basically the same, since they both involve heat.)
But as we mentioned, these methods aren’t definitively consistent. You could make a lot of foil or plastic seal products, then have a sealing problem, but not find out about it until much later.
Often, seal leaks are due to irregularities around the seal itself. These irregularities are usually small (hence the term “micro leaks”). Micro leaks are common in the sealing process. But how seal inspection is done often varies by industry. In the tuna industry, for example, the way seal inspection is often by incubation period. You put the pouches in an incubator, and if there’s any bulging from air contamination, the tuna starts fermenting – then you know you have a seal leak.
Many times during the inspection process, you won’t see path of leakage from the inside to the outside of the product. You often see small contact areas; then when the product is handled, those already weakened contact areas break open (foil is tough, but thin).
How do we detect these conditions?
Three Ways to Inspect Seals
There are three generally types of technologies to inspect these seals.
- The “squeezer” method. If you literally squeeze the product (keep imagining a yogurt cup), the foil or plastic seal may “pop up” or bubble. That shows how much pressure or energy it can take before it broke open or went back down.
- The problem with squeezing is that products aren’t 100% uniform. The energy or pressure required to squeeze them to a certain deflection varies because the container is different. That can mean a lot of false rejects.
- Infrared seal signature evaluation. You can look at the heat signature across the seal, and if it doesn’t look consistent with what you see in other seals, you know there’s a problem.
- The challenge: this method is sensitive to the time of inspection. So if your line stops, or something goes wrong, and you don’t inspect each product at a consistent time, all the products will have different heat signatures than the ones before them. But in the inspection process, it’s difficult to ensure consistent time of inspection. You don’t want a higher risk of false rejects just because you needed to stop your line for a little while.
- X-ray Inspection. We’ll go into detail below with this method of inspection.
Seal Inspection with X-ray Systems
Imagine that you have sealed your product and five days later, want to inspect it before it’s shipped. If you use a high-resolution, top-down X-ray machine, you can take a photo of the container’s entire seal. You can focus the inspection on just the seal. You could find air gaps between the foil and the cup, or see where the thermal didn’t properly meld.
The X-ray technology perceives depth incredibly well. It’s high speed – we can do up to 100 meters a minute, which can mean thousands of products inspected per minute. And it can find foreign materials in the product. We can even electronically check weight. These are powerful benefits.
With X-ray seal inspection, you’ll get a one-stop shop for everything you want to inspect. When you talk to most people doing sealing, their loss rate is significant. An X-ray system means you don’t find out about problems a day or week later; you can stop production immediately during the inspection process.
Currently, Peco-InspX is testing all seals that are used commercially, to get a sense of how much operational variation is needed in the sealing process so we can fine-tune our software.