Strapping/Banding Machine Not Sealing or Welding Strap
The machine tensions and cycles correctly but the strap falls away or separates under load. Seal head wear, contamination and temperature faults are the usual causes - and they tend to come on gradually rather than all at once.
Common symptoms
Seal faults can look different depending on whether the head is worn, contaminated or running at the wrong temperature. These are the most reliable signs.
Strap falls off the load after the machine completes its cycle
The machine finished normally but no viable seal was made. Often the first sign that something is going wrong with the seal head.
Seal looks formed but separates easily by hand
A seal that looks intact but pulls apart under low force is a cold weld. The heat wasn't high enough or contact time was too short for the bond to form properly.
One seal point fails consistently but the other holds
On machines with two seal positions, a fault at one point only usually means a problem with that specific heater element or jaw face rather than a settings issue.
Seals vary between strong and weak on the same shift
Inconsistency like this often points to a failing heater element cycling in and out, or contamination on the jaw that builds up and periodically affects contact.
No seal formed at all - strap just has a cut end
The cut cycle ran but the weld didn't initiate. Check whether the seal head is actually reaching temperature before the cycle starts, not just showing "ready" on the display.
Cycle time has shortened and seals are now failing
A faster cycle reduces dwell time at the seal head. If dwell drops below the minimum needed for the strap spec, the weld won't form reliably.
Typical causes
Seal head wear
The jaws that press the strap together for welding wear over time. Worn jaw faces can't maintain consistent contact pressure, which directly affects seal strength. This is a gradual deterioration - seals get progressively weaker rather than failing suddenly.
Contamination on the seal head
Strap dust, grease or plastic residue on the jaw faces prevents proper contact between the strap layers. It accumulates gradually and is easy to miss on a visual check when the machine is running.
Heater element fault or incorrect temperature
If the element is failing or the temperature setting is too low for the strap specification, the weld won't reach the strength needed. A machine that shows "ready" can still be running below the correct temperature.
Strap specification mismatch
Different strap grades have different melt temperatures and bond characteristics. A strap that seals well at one temperature may seal poorly if the grade changes slightly, even when ordering from the same supplier.
What to check first
Work through these in order. The pull test takes seconds and tells you a lot.
Pull test immediately after the next cycle
Before adjusting anything, pull the strap by hand after the next cycle completes. If it separates easily, confirm whether a seal was actually formed or just a surface impression. A seal that looks intact but fails under 10kg of pull is a cold weld.
Check the seal head temperature against spec
Compare the actual temperature reading against the specification for the strap you're running. If the display shows "ready" but the temperature reading is below spec, the heater element may be failing.
Inspect the jaw faces with the machine isolated and cooled
With power off and the seal head fully cooled, check the jaw contact surfaces. Any buildup, pitting or uneven wear on the faces that touch the strap will affect seal quality. Contamination is often visible as a brown or grey residue.
Try a different strap reel
If you have a second confirmed reel, try it. A strap quality issue from the supplier can present exactly like a seal fault. This test costs nothing and either confirms a strap issue or rules it out.
If any of these apply, don't wait
- Pull tests are consistently failing across multiple load types and strap reels
- The seal head temperature won't reach target or drops during a run
- Jaw faces show visible wear, pitting or contamination that won't clean off
- Seals were consistently good until recently and nothing obvious has changed
A machine making seals that look acceptable but aren't is harder to catch than one that's clearly failing. If you're seeing any inconsistency in seal strength, get it checked before a load comes apart in transit.