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Brookfield AMETEK

Polythene Bag Seal

Test Principle

Evaluation of the seal strength of bags using the dual grip assembly fixture.

Background

The growth in packaging across industries has necessitated the need for testing the seal strength and package integrity of packaging materials. With fixtures and accessories available to test packaging materials, Texture Analyzers are now extensively used throughout the packaging industries.

The aim of packaging a product is to be able to maintain the cleanliness and sterility of the product throughout all stages from the manufacturing plant, transport, shelf-life, and storage. To achieve this, the integrity of the sealed package and the seal strength must both be assessed.

A seal strength test measures a quality of the seal intended to provide a barrier/seal that is to some point of similar strength as the rest of the packaging. Using the tension test, seal strength can be measured by the ability of the seal to resist separation. This test is suited for assessing the strength of a seal and has the advantage of being a very sensitive technique.

The CT3 Texture Analyzer has a number of probes and fixtures designed for testing packaging materials. The instrument is therefore capable of measuring seal strength, peel strength, burst strength, and crush capabilities of packaging materials. The CT3 Texture Analyzer is a very robust and user-friendly instrument that provides accurate and reproducible results within minutes making, it adoptable for use on the production line.

Method

EQUIPMENT

CT3 with 50kg load cell
Dual Grip Assembly (TA-DGA)

SETTINGS

Test Type: Tension
Pre-Test Speed: 1.0 mm/s
Test Speed: 1.0 mm/s
Post Test Speed: Select Return at Test Speed option
Target Distance: 100 mm
Trigger Force: 10 g

Sample Preparation

Condition samples to the temperature testing conditions (ideally room temperature). Cut the packaging material vertically down the seal into strips of equal length and width

Procedure

  1. Place the lower grips on the base of the instrument and loosely tighten the thumb screws to enable some degree of mobility.
  2. Attach the upper grips to the instrument
  3. Lower the instrument arm until the upper grip is a few millimeters from the lower grip
  4. Align the lower grip by re-positioning them to the upper grips
  5. Once alignment is complete, tighten the thumbscrews of the lower grips to prevent any further movement
  6. Raise the arm of the instrument to provide enough space to slip in the sample to be tested into the grips.
  7. Clamp one end of the strip with the lower tensile grips. Ensure that the strip is inserted over the full length of the grips; this will ensure a tight grip. Clamp the other end of the strip to the upper tensile grips, inserting the strip to the full length of the tensile grip. The sealed part of the strip should now be such that it is midway between the upper and lower grips (see picture below).

Note: The sample likely to have the greatest seal strength should be tested first in order to anticipate the maximum force load range for subsequent samples.

For comparison purposes, sample strips should be of equal length and width. Sample strip orientation, when inserted into the grips, should be kept consistent throughout all tests.

Results


Figure 1

Figure 1 shows the seal strength of food bags using the dual grip assembly fixture. The maximum force value is the peak force value. This is the force required to stretch the seal. Sample 1 (data set 1) shows a drop in seal force as the seal strength is exceeded by the applied force until the seal is fully pulled apart, which indicates that load force drops to 0. The area under the graph is a measure of the work done to separate the seal. Sample 2 shows a stronger seal with a similar peak force value (force to initiate stretching the seal); however, more energy is required to break the seal. Fluctuations on the graph are a result of the stretching and tearing of weaker points in the seal. Samples 3, 4, and 5 have stronger seal strengths, as shown by the maximum force (peak force) the seal remains strong while the packaging material begins to stretch.


Figure 2

Figure 2 shows the seal strength of food bags using the dual grip assembly fixture. This is an alternative option for displaying the results. The maximum force value is the peak force value. This is the force required to initiate stretching the seal. The area under the graph from the start of the test to the target distance point (where plots ends) is a measure of the work done to separate the seal. Fluctuations on the graph are a result of the stretching and tearing of weaker points in the seal.

Discussion

The test begins when a trigger force of 10 g has been attained. At this point, the upper tensile grip pulls the sample strip over a specified distance of 100 mm. The force required to initiate the stretching of the seal is measured as the peak load. This is the force required to overcome the attractive forces between the seal and the packaging material. The energy required to overcome the strength of the internal bonds within the sample is shown as the area under the curve. As seen from the graphs, sample 1 had the weakest seal strength and became totally separated. Sample 2 had a similar peak force to sample 1, but the seal was much stronger resisting further separation. Samples 3, 4, and 5 had very strong seal strengths with a high resistance, even at the peak load.

The TexturePro CT Software can automatically calculate the average peak load and work done values as shown below (four sample strips have been tested for each sample type (n=4):


Figure 3