Stretch Testing Method

A. Introduction 

DestructiveTo evaluate seals (in particular, transversal sealing of aseptic cartons) to observe how seal area stretches and breaks.Not applicableOff-line, > 30 min$1,000

B. Operation 

  • Stretch-pliers
  • Scissors
  • Paper knife
  • Pocket size magnifier, foldable. Magnification x6.
  1. Commence preparations with removing two consecutive packages from the packaging machine conveyor line.
  2. Empty both packages.
  3. Cut off transversal sealing (TS) from both packages to a width of 25 mm to accommodate the stretch-pliers. Cut wider when stretching by hand.
  4. Trim a narrow strip, no more than 1 mm from the ends of each sample.
  5. Rinse samples and dry. Four samples from two consecutive packages are now ready. Good light conditions are essential for evaluation.
  6. Check for plastic lumps by running a finger along the seal before stretch.
  7. When using stretch-pliers, insert sample into the gripping jaws and squeeze handles to stretch the seal. When stretching by hand, apply force and stretch perpendicular to the seal.
  8. Gently tension the seal by stretching it and observe how the seal area stretches and breaks. Stretching slowly makes evaluation easier.
  9. All four samples have to be stretched and evaluated along the entire seal. Pay special attention to the TS ends where the TS meets the longitudinal sealing (LS).
  10. Continue stretching until the sample has opened completely.

Good Seal

If there is stretched plastic film or breakage in packaging material when testing the sample, it is a good seal.

Bad Seal (Blocked)

If the seal is separated without the plastic film stretching and without breaking the packaging material, it is a Blocked seal. In a blocked area the seal is weaker than the packaging material.

Bad Seal (Plastic lumps)

If the sample has hard lumps of plastic squeezed out during sealing, it is called Plastic lumps. Plastic lumps can puncture the package from inside during handling.

C. Application 


DEFECTSFlexible PouchSemi-rigid and
Rigid plastic container
Plastic Can (Double-seam Metal End)Paperboard
Corner Dent   
Cut (Fracture)
Double Seam Defects*   
Foreign Matter Inclusion  
Label Foldover   
Leaker (Channel) 
Leaker (Corner)   
Leaker (Notch)   
Leaker (Perforation)   
Leaker (Pulltab)   
Leaker (Seal)   
Loose Flap or Ear   
Puncture (Pinhole)
Seal Defects (Blister)   
Seal Defects (Blocked)   
Seal Defects (Burnt)   
Seal Defects (Compressed)   
Seal Defects (Contaminated)  
Seal Defects (Convolution/Embossing)   
Seal Defects (Creep)   
Seal Defects (Crooked)   
Seal Defects (Incomplete)   
Seal Defects (Misaligned/Deformed)  
Seal Defects (Nonbonding/Weak)   
Seal Defects (Plastic Lumps)   
Seal Defects (Seal-width Variation)   
Seal Defects (Stringy)   
Seal Defects (Uneven Impression)   
Seal Defects (Uneven Juncture)   
Seal Defects (Wrinkle)  
Swell (Swollen Package)

D. Source 

Tetra Pak Inc. 101 Corporate Woods, Parkway, Vernon Hills, IL 60061. TEL (847)-955-6000 ( Pak Technical Bulletin. 1991. Tetra Brik Aseptic Transversal Sealing Evaluation. Issue 1. Tetra Pak, Sweden.