Why Frozen Food Bags Leak at the Seal: Causes and Fixes

Start With Where the Bag Leaks

A pillow bag has three seals: one long seal down the back, and a cross seal at the top and at the bottom. Before you touch a setting, pull a few leaking bags and mark where the leak is. Each spot points to a different cause.

On a vertical bagger the cross jaws close the top of one bag and the bottom of the next in the same stroke. So the bottom seal takes the drop of the next dose while it's still fresh. A heavy dose, or hard frozen pieces, can stress it. If leaks show up mostly at the bottom, look at the product and the drop before blaming the jaws.

The pinhole row matters too. A shrimp tail through the film looks like a leaking bag on the shelf, but no seal setting will fix it.

Product in the Seal: the Usual Cause on Frozen Lines

Frozen products are hard on seals because they bring their own contamination. Each one does it differently:

• Breaded products like chicken nuggets shed crumbs. Fines collect in the weigher and chutes, then get carried into the seal area.
• Glazed products shed ice. Shrimp lose chips of glaze on the way through the weigher, and ice fragments and moisture in the seal cause weak or leaking seals. We cover this in how to package IQF frozen shrimp.
• Round, hard products bounce. Frozen meatballs rebound at the bag mouth, and a meatball or fragment caught in the jaws means a leaking seal and a rejected bag.
• Products that wait start to thaw. Frosted pieces that sit on the line stick together and bridge in hoppers, and the feed turns uneven.

For the first three, the fix is timing and drop. Every dose has to fall through the forming tube and clear the seal area before the cross seal closes. Long drops, pieces that hang in the tube and product that bounces back all leave something in the jaws.

What helps:

• Set the drop height and discharge timing for your product, then confirm them with the real product.
• For the fourth, match feeding, weighing and bagging rates so product doesn't wait on the line.
• Keep crumbs and ice from building up in the weigher and chutes, since what collects there ends up in a seal later.

Speed makes this worse. If the bagger runs faster than the product can settle, there's less time for the last pieces to clear the seal area.

Seal Settings, Jaws and Speed

If the leaking seals are clean, with no product in them, look at how the seal is made. Heat, pressure and time in the jaws have to suit the film. Film structure and thickness set how long the jaws need to make a good seal.

We don't publish seal temperature or pressure figures here, because the right values depend on the film, the machine and the speed. But the checks are the same everywhere:

• Check the actual jaw temperature. The setpoint on the screen and the temperature at the jaw face can drift apart. Measure the jaw face with an independent thermometer.
• Peel a seal apart. A seal that peels cleanly with little effort usually didn't get enough heat or time. Thinned or burnt film points the other way.
• Check jaw contact. Uneven pressure leaves channels in the seal that let air through. Worn or dirty jaw faces do the same.
• Watch what changed. A seal that held at a lower speed can fail after a speed increase, because each bag spends less time in the jaws. Turning the speed back down will confirm it, but treat that as a test rather than the fix.

Film Choice for Frozen Products

Some bags pass every check on the line and leak in the freezer. That usually means the seal was only just good enough, or the film wasn't specified for frozen storage. Ask your film supplier to confirm the structure for the storage temperature and the product.

Film also decides how much abuse the bag takes. Tails and frozen edges can puncture thin film, while a tougher film resists them but needs more time in the jaws. The SVC260 continuous-motion VFFS, our main bagger for frozen foods, runs laminates, foil and paper-based films from 0.04 to 0.10 mm thick. Where your film sits in that range affects both the seal and the speed.

One more trade-off: metallized or foil film limits metal detection. If you want foil for barrier, choose the inspection method together with the film.

How to Check Seals on the Line

Pulling a few bags at regular intervals catches most seal problems before a pallet of them reaches the freezer. A simple routine:

1. Look and feel. Wrinkles, trapped product and uneven seal width are visible. Squeeze the bag gently and listen for air.
2. Run a bubble test. Hold a sealed bag under water and press it. A stream of bubbles shows exactly where it leaks.
3. Log the location. Top, bottom, back seal or panel. Use the table above to point to a cause.
4. Log the timing. Start of shift, after a stop, after a film change or after a speed change. Patterns show up quickly.
5. Hold some bags in the freezer. Check them again after storage, so a marginal seal shows up in your plant instead of at your customer's.

Before you buy a machine, do the same with your real product. In a packaging test we check seal quality and product in the seal area with your product and film, and record rejects and stops.

Questions to Ask Any Supplier

• Will you run my frozen product in my film and check the seals, including product in the seal area?
• How are drop height and discharge timing set for my product?
• What output can you hold with clean seals, and how are leaking bags counted as rejects?
• Which film structures have you run for frozen products, and which inspection method works with them?
• What does FAT check on seal quality before the machine ships?

FAQ