Stretch film testing comprises a series of essential tests to guarantee the safety of goods in palletized systems, and that of the people involved in their shipping & handling.
There are several testing methods and machinery to perform it, but all of them ultimately have the same purpose: to improve the safety of the goods, optimizing the packaging and obtaining more economical and sustainable solutions.
What is stretch film testing?
Stretch film is a material that is mainly composed of polyethylenes (LLDPE) and is characterized by its ability to be stretched. In the packaging industry, the stretch film plays an essential role, being utilized to wrap certain products and packages. Its main function is to improve load stability during storage and transportation.
Stretch film testing includes a series of tests to evaluate the suitability of the material to protect a load during goods transportation. These tests are divided into basic tests used to characterize the critical properties of the film and tests to measure the stability of the load unit.
Benefits of performing stretch film testing
Stretch film testing allows companies to have a deeper knowledge of the characteristics of this material and its protective ability, foreseeing potential problems in a laboratory setting before its application in a real distribution cycle.
The advantages of these tests include:
- Guaranteeing that the stretch film has been correctly applied and that the load is stable.
- Determining the correct baling protocol for the products being transported.
- Determining the correct amount of film to be used, optimizing the amount of packaging material. This, in turn, translates into other benefits, such as a reduction in the carbon footprint in the transportation of products and lower material and transportation costs.
Who can benefit from stretch film testing?
The implementation of stretch film testing is necessary for any company that ships its products on palletized systems. This includes both the stretch film manufacturer and whoever uses it to ensure the safety of goods.
How to perform stretch film testing
Methods and regulations to take into account
The various testing standard organizations have included a number of tests to evaluate the ability of stretch film to protect a load.
On one hand, the tests to characterize the critical properties of the film include:
- ASTM D-5458: Peel Cling of Stretch Wrap Film. This test is used to know the adhesion strength between two layers of film, so as to know which of the two layers (external or internal) can adhere with greater or lesser strength. This is necessary so as to know how to use the roll in the baling machines, in a way that the adhesive layer always ends up in contact with the load that it is wrapped around. If this layer were to face outward, it would lead to pallet clinging issues inside the truck’s container, and the baling might break when loading or unloading.
- ASTM D-5748: Protrusion Puncture Resistance of Stretch Wrap Film. This is used to know the resistance of the stretch film against puncturing, and whether it is suitable, for example, to wrap products and packages that have pointy or sharp edges.
- ASTM D-882: Tensile Properties. Used to determine the maximum resistance to breakage of the film and its maximum elongation before breakage. The more resistant a film, the fewer rotations will be necessary to attain the required level of load stability when facing the mechanical efforts of transportation.
- Ultimate Stretch before Break. This test is performed with the film still on the roll, and is used by stretch film manufacturers to know the percentage of pre-stretching that its film and rolls can guarantee when used in baling machines. This test can be used to determine the stress-strain curve of the stretch film, and to determine when the elastic region starts before breakage – in other words, the region within which the film must be stretched for optimal use.
- ASTM D-1922: Propagation Tear Resistance. Used to know the tearing force of a film. The edges of film rolls usually suffer the most damage during their storage and transportation before use. Once placed on the baling machines, if the edge is banged or damaged, when applying a force during pre-stretching and tension on the package or load that is to be baled, a defect at the edge might propagate at a certain speed and break the film. The higher the tear resistance of a film, the less the film will break during the bailing process. Often times, due to low tear resistance, the film cannot be subject to a high level of pre-stretching to reach the optimal use region, since it constantly breaks on the baling machine, leading to higher use of the material and resulting in a less stable baling, since the film would still have room to stretch.
Additionally, there are a series of testing protocols that are performed on the entire load unit to verify whether the stretch film has been correctly applied with sights on improving the load’s stability:
- Stretch film containment force. This test is used to know the force exerted by the film on the products or packages that it is wrapped around. Ideally, the containment force should be the highest that the product or package can withstand without deforming once the film has been applied. However, this parameter alone does not guarantee that a load is stable.
- Thickness of film after wrapping. By measuring the film thickness of a layer after baling, it is possible to know whether it has been pre-stretched up to the optimal use region. For example, if a 23 micron film has been used and, on the pallet, we measure that one film layer is 16 microns thick, the film has not been correctly pre-stretched and the load stability is at risk. Conversely, if after measuring the above film we obtain a thickness of 9 or 10 microns after baling, it has been correctly pre-stretched. This parameter alone does not guarantee that a load is stable either.
- Weight of total film after wrapping. Knowing the total weight of the film in use, its density and the total number of rotations, the perimeter of the baled load and the thickness of the film after baling, it is possible to obtain an approximate value for the total stretch percentage (pre-stretching due to the pre-stretch carriage + post-stretch due to the force between the roll and the load to which the film is applied). If the total stretch percentage is close to the pre-stretch percentage specified by the manufacturer during the Ultimate Stretch Before Break test, we can confirm that the film has been taken to the optimal region. This parameter alone does not guarantee that a load is stable either.
In addition to the parameters we have seen so far, in order to guarantee that a load is stable, it is necessary to look into the pallet arrangement in use, the qualities and resistance of the primary and secondary packaging, and the distribution of the film rotations on the load.
For example, it would depend on the number of rotations performed where the pallet meets the first layers of products or packages, the overlap between film layers, the number of rotations at the upper layers of products or packages, and whether film strips are applied to regions or layers that have more or less grip.
The last test to guarantee that the stretch film has been correctly applied is called the rigidity or load stability test.
During this test, the load is placed on top of a horizontal platform, which is accelerated until a constant acceleration is reached for a specified period of time.
While the load wants to deform when subjected to this test, if this deformation does not exceed the specified limits, the load is deemed to be rigid for that level of acceleration and the film has been correctly applied to withstand the forces of horizontal acceleration of the test, which are the ones that it might be subjected to during transportation.
This test is outlined in the EUMOS 40509 standard – a testing method specified in Directive 2014/47/EU to improve road transport safety in Europe.
This regulation focuses on load securement since it is considered to be fundamental in road safety. The directive states that the load is to be packaged and secured in a way that it can withstand any acceleration or deceleration that takes place while the vehicle is moving.
The EUMOS 40509 method was created with this in mind – a dynamic testing system that can be used to evaluate the rigidity and safety of a load, which is part of the stretch film testing methods.
Stretch film testing equipment
There are two types of machines that can be used to test load rigidity or stability.
Both machines simulate the horizontal accelerations and decelerations that might take place during road transportation and have been designed to comply with the EUMOS 40509 standard.
- InnSlide Horizontal stability tester. Available in certain package and packaging testing laboratories and certain stretch film manufacturers and large corporations.
- InnSlide Horizontal stability tester-Boomerang. Smaller in size, this Safe Load TT patented solution is designed to be owned and used by any company in their facilities.
At Safe Load Testing Technologies we have a track record of two decades in the transport simulation industry, helping companies find the right equipment to optimize their packaging, including stretch film testing. Contact us and we will tell you how we can help your company.