Seal (Peel) Strength Testing Method

A. Introduction 

DestructiveTo measure the tensile strength required to cause separation of peelable or fused seals.Not applicableOff-line, > 30 min> $15,000

B. Operation 

Tensile tests are used as a quality assurance tool for assessing the inherent sealing qualities of flexible packaging films and are employed as an optional test. Regardless of the specifics of the technique (sample widths, equipment, and variable crosshead or jaw separation speeds), the tensile test can best be used for surveillance of the sealability of materials and as a spot check on sealing conditions and equipment operation. Tests for seal strength are basically tensile tests performed by a universal testing machine or its equivalent. A section of the package seal is gripped between jaws (one movable and one fixed) and pulled apart at a controlled rate. The force or energy required to separate the two sealed surfaces gives an indication of the seal strength.

It is recommended that the heat-sealed specimens be conditioned as the standard conditioning atmosphere, prior to carrying out the testing. Judgment should be exercised in the selection of conditioning times and procedures, which may be necessary to meet specific test objectives.

  • Sample packages
  • Sample cutting apparatus
  • Scissors (sample dimensions are critical to precision)
  • Tensile strength testing device
  • For testing samples at angles different from 180o (i.e. 90o peeling test), other fixtures are required.
  1. Obtain representative sample from production line.
  2. Cut open sample and remove contents. Do not disturb seal to be tested.
  3. Cut a segment of the seal to produce a test strip (25.4 mm wide and at least 75 mm long). The edges must be clean-cut and perpendicular to the direction of the pouch seal.
  4. Precondition test strips at 21-25oC (69.8-77oF) and 45-55% RH, for 40 hours (minimum).
  5. Secure both ends of test strip in separate clamps. With screwdriver, move one screw clamp away from the other, creating a 180o separation of the seal.
  6. Observe force required to fully separate seal. Results to be expressed in pounds per linear inch, average of sample (that is, 3 adjacent specimens cut from that seal).


Sample separates at peak tensile strength less than established standard. 


Sample separates uniformly at peak tensile strength greater than or equal to established standard.

False positive

Sample separates at peak tensile strength less than established standard because of equipment miscalibration or greater separation speed of jaws.

False negative

Sample separates at tensile strength greater than or equal to established standard. However, a different portion of the same sample failed at a tensile strength less than the standard.

C. Application 


DEFECTS Flexible Pouch Semi-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 

  • Instron Inc. 825 University Ave, Norwood, MA 02062-2643, USA . TEL (800)-877-6674 ( Inc. 2125 Barrett Park Drive, Suite 107, Kennesaw, GA 30144, USA. TEL (770)-420-6555 (, G.W. JR. 1998. Chapter 22C Examination of Flexible and Semirigid Food Containers for Integrity. FDA Bacteriological Analytical Manual (8th Ed)
  • ASTM F88 / F88M - 09 Standard Test Method for Seal Strength of Flexible Barrier Materials
  • ASTM D882 - 09 Standard Test Method for Tensile Properties of Thin Plastic Sheeting
  • Canadian Food Inspection Agency. 2002. Flexible Retort Pouch Defects Manual - Identification and Classification. (