An L-bar sealer cuts and seals film around the product, producing a loose sealed bag. A shrink tunnel then applies controlled heat so the film contracts onto the product. They are sequential stages, not alternatives — and knowing which stage caused a defect is how you fix it.
Two stages, two different failure modes
If the pack is loose but the seal is clean, the tunnel is under-shrinking: temperature, dwell or air flow. If the seal itself is weak, wavy or open, the problem is at the sealer: blade temperature, pressure, or film tension. Treating a sealing fault by raising tunnel temperature burns film and never fixes the seal.
Which stage owns which defect
| Symptom | Stage responsible | Typical cause |
|---|---|---|
| Loose, baggy wrap | Tunnel | Temperature or dwell too low |
| Burn holes or scorching | Tunnel | Temperature too high or air flow uneven |
| Seal opens under handling | Sealer | Blade temperature or dwell too low |
| Wavy, wrinkled seal | Sealer | Film tension or blade pressure |
| Dog ears at corners | Both | Film size oversized for product |
Sleeve sealing and side sealing
Sleeve sealing wraps film around the product and seals it into a sleeve, leaving the ends open — common for multipacks. Side sealing runs a continuous seal along the product edge and suits long or irregular items. Both feed a tunnel afterwards; the choice is about product shape, not shrink quality.
Across the 16 shrink and seal-cut series we list, tunnel and sealer models are published separately as well as in combined semi-automatic units, so a line can be built stage by stage.
See the shrink and seal-cut range for sealers, tunnels and combined units.
Figures in this article are drawn from the 137 machine series SilverSeal currently supplies, not from generic industry averages. Specifications vary by model — check the individual series page before specifying.
Frequently asked questions
Can I use a shrink tunnel without a sealer?
Only with pre-made shrink bags or sleeves. The tunnel supplies heat; something has to close the film first.
Why does one side of my pack shrink more than the other?
Uneven air flow or product sitting off-centre in the tunnel. Check air distribution and conveyor centring before changing temperature.
Is a combined seal-and-shrink machine a compromise?
For steady single-product runs it works well. For mixed products, separate stages give more control because each can be set independently.
