Strapping Machine Not Sealing / Welding Strap

If the machine tensions but the strap won’t seal (no bond, weak weld, or melted strap), the fault is usually within the seal head condition (heater/friction surfaces, contamination, clamping pressure) or the seal settings/timing not matching the strap type. The checks below help you identify whether it’s a simple wear/cleaning issue, a setup mismatch, or a seal head problem that needs engineer attention.

Related issues: Not tensioning correctly · Cuts strap too short · Cycles but doesn’t complete a cycle
Common things we see on site
Seal head contamination or strap dust on the weld surfaces of a strapping machine
Seal head contamination (strap dust on weld faces = weak or no bond)
Worn heater or friction weld surfaces on a strapping machine seal head
Worn heater/friction surfaces (inconsistent heat transfer and poor weld quality)
Seal head clamping area on a strapping machine showing pressure or alignment issues
Low clamping pressure / misalignment (strap isn’t held square during the weld)
Melted or deformed strap caused by incorrect weld time or temperature on a strapping machine
Incorrect weld heat/time (strap melts, deforms, or scorches)

Does This Look Familiar?

Common symptoms when the strap won’t seal/weld properly:

  • Strap releases with no bond (seal phase runs but doesn’t weld)
  • Weld is weak and pulls apart by hand (insufficient heat/pressure or worn weld surfaces)
  • Strap melts, deforms, or scorches (temperature/time out of range)
  • Seal works sometimes, then fails (contamination build-up or inconsistent pressure)
  • Machine tensions correctly but stops at sealing (seal head not completing the sequence)

Typical Causes Behind This Fault

  • Weld head contamination (strap dust on heater/gripper faces reduces bond quality)
  • Worn heater/weld elements or worn friction surfaces (can’t transfer heat consistently)
  • Incorrect weld settings (heat/time/cool parameters not matched to strap type)
  • Insufficient pressure from worn springs, linkages, or pneumatic/hydraulic issues (where fitted)
  • Strap specification mismatch (PP/PET type, thickness, or embossed strap not suited to setup)
  • Seal head alignment issue (strap not clamped square, uneven weld face contact)
  • Sensor/sequence fault preventing a full weld/cool cycle completing
Quick sanity check: If the weld is weak, start with contamination and pressure. If the strap is melting, check weld heat/time and confirm the strap type/spec.

When It Needs An Engineer

If the strapping machine won’t seal/weld the strap after basic checks (correct strap type, strap path clear, and settings verified), it’s usually quicker to have it diagnosed properly rather than repeatedly re-feeding and attempting seals that waste strap.

! Weak welds that pull apart (heater/weld head or pressure issue)
! No weld at all/strap releases without bonding
! Strap melts, deforms, or scorches (temperature/time out of range)
! Intermittent sealing faults stopping production

At this stage, structured fault-finding is usually more effective than continued adjustments — especially on older or heavily used machines. Most “not sealing” faults come down to weld head wear/contamination, heater/temperature control issues, incorrect weld timing/pressure, or a sequence/sensor fault preventing the seal cycle completing correctly. Our engineers work across all major strapping machine brands and control systems.

Service context
Part of our wider Packaging Machine Repair support

This strapping machine fault page sits within our engineer-led, multi-brand Packaging Machine Repair & Breakdown Support.

Request Strapping Machine Support

Send us the machine make/model (if known) and the symptoms you’re seeing. A quick photo of the display or fault helps speed things up.

You’ll get the quickest response by including the fault symptoms and any error message.