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Gelbo Flex Tester

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Gelbo Flex Tester
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Gelbo Flex Tester

Gelbo Flex Tester determines whether a certain plastic laminate material can withstand repetitive strain. It involves attaching sample pieces of plastic to the two circular clamping disks. This is via hose clamps. And then the machine twists and turns the specimen in specific movements. This process will show a visual result of pin holding. Thus, testing if the plastic can sustain the strain. If not, check plastic quality. this is usually useful within the plastic industry.

The Gelbo Flex Tester has been designed to determine the resistance of flexible barrier materials to pin holing as stated in the ASTM F 392.

For Textile/ Nonwoven Gelbo Flex tester, click here.

Summary of Plastic Flex Test Method

Flex Failure is determined by measuring the pinholes formed in a barrier structure after being subjected to the Gelbo Flex test. Pinholes are determined by use of coloured turpentine and allowing it to stain through the pinholes onto a white background. Depending on the test specimen, conditions, severity and number of test strokes can vary. The flexing is done at a rate of 45 cpm with a twisting motion repeatedly twisting and crushing the film.

Significance and Use

While being an efficient way of determining the resistance of barrier materials to pin holing, this test method does not measure any abrasion component relating to the flex failure. Physical holes completely through the test sample are the only failures measured by the colour turpentine procedure. Gas permeation or moisture vapour transmission can be used in conjunction with the flex test to measure the loss of ply integrity.

Gelbo Flex Tester - Plastic Apparatus 

The IDM Gelbo Flex Tester consists essentially of a 90 mm (3.5in.) diameter stationary mandrel and a 90 mm diameter moveable mandrel spaced at a distance of 180 mm (7in.) apart from face to face at the starting position (that is a maximum distance) of the full stroke. 

The mandrels contain vents to prevent pressurisation of the sample. The motion of the moveable mandrel is controlled by a grooved shaft, to which it is attached. The groove is designed to give a twisting motion of 440° in the first 90 mm of the stroke of the moveable mandrel, followed by a straight horizontal motion of 65 mm (2.5in.), so that at the closed position the mandrels are 25mm (1 in.) apart. The motion of the Gelbo Flex Tester is reciprocal with a full cycle consisting of the forward and return strokes.

For the partial flex test cycle, the moveable mandrel is set up to travel only 80 mm (3.25in.) of the 180 mm (7in.) spacing. Therefore, only 90% of the twisting stroke is utilised giving a twisting motion of only 400°, and none of the horizontal stoke is utilised. A stainless steel template 200 mm x 280 mm (8 x 11in.) is used for ease of preparing test samples size. Clamps are supplied to hold the sample on to the mandrels, which will need to be supplied by the user along with a paint brush, absorbent tissue paper, white paper and coloured, water free turpentine.

gelbo flex testing flex lint tester digital lint counter

Plastic Film Test Specimens – All conditions

Samples are cut into 200 x 280 mm (8 x 11in.) flat sheet with the 200 mm dimension in the direction to be tested. This will also be in the Direction of the flex test axis. Test four samples in the machine direction and four samples in the transverse direction. In addition, test a control set of four, adjacent, un-flexed, specimens (either direction) for pinholes. Do not seal or tape the sides of the specimens but leave them open. Use the double-sided tape to attach the unsealed samples to the mandrels. 

Plastic Flex Testing Procedure 

  • Condition A: Set the Gelbo Flex Tester for maximum throw and twisting action (Long Stroke). This set up gives a twisting motion of 440° in the first 90 mm (3.5in.) of stroke, and is followed by a straight horizontal motion of 65 mm (2.5in.) at 45 cpm. Attach the flexible barrier test specimen to the flex tester mandrels with the clamps provided. Turn the Flex Tester on, and allow the specimen to flex for 1 hour at 45 cpm (2700 cycles)
  • Condition B: Test conditions are the same for Condition A although the test duration is 20 mins at 45 cpm (900 cycles)
  • Condition C: Test conditions are the same for Condition A although the test duration is 6 mins at 45 cpm (270 cycles)
  • Condition D: Test conditions are the same for Condition A although the test duration is 20 cycles at 45 cpm (20 cycles at full flex and twisting action)
  • Condition E: Set the Gelbo Flex Tester for the partial flex action (Short Stroke). In this setting, the moveable head is set to travel only 80 mm (3.25 in.) of the 180 mm (7 in.) spacing (the distance between the mandrels at their maximum separation or starting position). Therefore only about 90% of the twisting stroke giving a twisting motion of 400° is utilised and none of the horizontal stroke is utilised. When the mandrels are at their closet position they will be 95 mm (3.7in) apart. The partial flex period under this “short stroke” setup will be 20 cycles at 45 cpm.

Pinhole Count:

When the test has been completed, remove the sample from the Gelbo Flex Tester and tape it to a sheet of white paper with the centre areas corresponding. Paint the specimen with the coloured turpentine, allowing drying for 1 min. after which, wiping the turpentine off with tissue paper, pressing the sample onto the white paper while wiping. Remove the sample from the paper and count each strike through on the paper as a pinhole.
Test Report
A report on the findings of the Gelbo Flex Tester procedure may include such things as:
  • Individual values and average pinholes per 300 cm² (48 in²) on quadruplicate determinations in machine direction and in transverse direction.
  • Any unusual failures such as tears.
  • On each sample under test, measure the pinholes of the four un-flexed samples. Record this as “Pinholes on Un-Flexed Samples:
  • Test method used including the number of cycles and wether a full or partial flex was used.
  • Sample conditioning and test conditions used.
  • Sample thickness and structure.


  • Laminated plastic films


  • 45 cpm
  • Stroke : 80 or 155mm
  • Twist : 400° Short stroke
  • Twist : 440° Long stroke
  • Counter for oscillations
  • Totaliser
  • Home Switch
  • Stainless Steel Cutting Template: 280mm x 200mm
  • Electrical: 220/240 VAC @ 50 HZ or 110 VAC @ 60 HZ
  • Dimensions: 300mm x 1,200mm x 350mm
  • Weight: 38kg


  • Minimise end product waste
  • Easy to use
  • Fast results
  • Accurate


  • ASTM F 392 - Standard Practice for Conditioning Flexible Barrier Materials for Flex Durability

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