Methods of force and torque testing for food packaging quality

  • Mecmesin

Packaging solutions for food are incredibly diverse; in all aspects of physical design, mechanical function and aesthetic appeal. Their purpose is more straightforward: be safe, secure, yet easy to open, close or reliably re-seal; dispense cleanly and efficiently; minimise loss and waste; maximise shelf life; be cost-effective to manufacture and ultimately recycle or dispose of. Add to that, the consumer desire for attractive and innovative new designs, and the straightforward ultimate purpose can become quite involved in terms of meeting each individual challenge.

Requirements for testing

Each aspect of the desired performance for a food packaging container can be quantified by a considered, scientific approach. The requirements may be documented by international standards, or a company's own internal quality assurance experience may indicate which value represents acceptable and which does not. What is certain is that a brand's reputation depends on how well the product performs when reaching the end customer; assuming that the upstream manufacturing process created a product that met the consumer's aesthetic and budgetary criteria; and so was actually purchased.

Testing outside the box

Throughout the lifecycle of an item of food packaging there are many steps at which a Mecmesin test solution will ensure that quality is tested, beyond simply protecting the contents:
• Characterisation of the strength of raw materials
• Optimising manufacturing machine settings by assessing friction and other process-influencing properties
• Assessing assembly processes for sub-components
• Measuring the resistance to damage during storage and transportation

Boxes, bottles, cartons and cans

The main functional testing requirements for food packaging centre on secure containment and ease of access to the contents; the way the packaging performs when in the hands of the end consumer.

The seal for modified atmosphere packaging (MAP), or for retorts and pouches, clearly must provide maximum seal but be readily opened by peel or tear. For stocking and stacking in-store, trays and cartons require crush resilience, whilst lids, closures and perforations must be user-friendly.

Meeting these primary goals is possible with the right testing equipment; a system able to grip the sample, the ability to apply the forces or displacements representative of those experienced by the packaging in service and a means to display the results and compare to target values.

The theory and practice of testing

The use of testing machines to apply force or torque, utilising grips and fixtures appropriate to each sample design and capturing every detail from load sensors, enables food packaging manufacturers to quantify every aspect of how their product performs. We may only need a simple peak value to find acceptability, but often the full profile of an action is useful; for example the smoothness of a complete peel from the corner of a square retort. This is why a full graph trace is as useful as the simple pass/fail of a single data point. Mecmesin supplies a wide range of precision equipment with software to apply the loading, acquire detailed data, and analyse test results, for both new packaging development and routine testing at the production line.

Types of test: The full package

International standards bodies publish procedures for many types of test on most designs of packaging, covering every conceivable way in which mechanical failure may occur, or operational functionality may fall short of expectation. With the inclusion of tests which an individual manufacturer may determine are uniquely applicable to their own proprietary knowledge, there is almost a bottomless package of possible examples to demonstrate. The following sections cover some of the tests on food packaging implemented by Mecmesin systems and fixtures using our off-the–shelf products or bespoke designs for specific customer requirements.

Peel testing from every angle
Peel testing measures the strength of an adhesive bond between two components, perhaps a foil-lidded container, a plastic film seal across a blister tray or a peel-to-read label. These tests are used to evaluate the ease of separation, where peel functionality is desired, or resistance to peel where it is not. Peel test methods prescribe the speed and angle of separation, since consistency in both is fundamental to repeatability. Fixturing to maintain the angle, and gripping to avoid deformation and provide consistent sample support, are essential for accuracy in this type of test.

Mecmesin designs and manufactures vacuum holders for an exact fit to the pack geometry and for positioning at the required angle. This eliminates pack distortion as a variable between tests. The lid is peeled back with a pinch grip on a flexible link to maintain a constant angle; 135 degrees (see the bacon pack example, above), 90 and 180 degrees are most commonly indicated.

Tear testing
Tear strength is measured where a product is intended to tear, e.g. along a crease, perforation or region of reduced thickness, in order to open a package. Moist food pouches and condiment sachets would be typical examples. Tear resistance is tested to ensure protective strength. A prescribed cut or notch in the sample ensures the point of tear initiation, from which it propagates as predictably as possible. Peel and tear testing are often part of a suite of tests on similar products; Mecmesin's FPT-H1 Friction Peel Tear Tester is designed as a complete dedicated solution for such a product manufacturer.

Tension and tensile strength tests
Tensile testing is the application of uniaxial force to measure the performance of a material test specimen or manufactured product sample up to the point of yield or breaking.

In the packaging industry these tests are used to measure ultimate tensile strength of a container (or closure), properties of its raw construction material, or the force required to open the package — flip a cap, pull a stopper, or the effort to override the intended method of access.

Standard tensile strength measurement procedures can compare the mechanical performance for different raw materials; if suppliers change, or legislation effects a change which requires review. Mecmesin's Universal Testing Machine, the OmniTest may be used to perform stress and strain analysis of material specimens in addition to product component performance.

Pull-off / pull-out product testing
A particularly important product test, requiring equipment which is able to put the sample in tension, involves measuring the force to pull-off / pull-out one component from another. This test of breaking strength or disassembly threshold can be performed on the same Mecmesin equipment that is able to remove a stopper or tamper-evident security cap, or flip up the protective lid from a dispenser to test designed functionality.

Compression tests
Applying the opposite loading set-up to tensile testing; compression is a way of determining how something will react when it is squashed, squeezed or crushed. This family of tests enables the evaluation of the ultimate strength of the package under these loads, as well as many applications for checking the effectiveness of the dispensing element of the package.

Top-load testing
Top-load, or column-crush is one specific area of testing for which there are many international standards. The procedure involves compressing the test sample (usually downwards, from the top) to measure its resistance to deformation by recording the peak force it can withstand before buckling or collapse. This test is applicable to primary, secondary and tertiary packaging, as it evaluates the ability of the container in question to withstand the loads exerted during production, transport and storage/display. By simply turning an item of packaging on its side in the testing machine, a food tray for example can be tested in the same way to guard against damage when being handled or loaded with food. Mecmesin offers fixtures ideally suited to meeting these standards —a vented compression plate is required for plastic bottle crush; to allow the air to escape— and systems able to measure the large forces required to crush even metal packaging, for example beer kegs.

The final twist: Caps, lids and bottle tops

The loads appropriate to the testing of food packaging are not only constrained to straightforward push or pull axial forces. Torque is the application of a rotational load, via a twisting motion, and is the most common way to open and reseal primary consumer packaging—by means of its closure.

Closure torque testing involves a range of tests for events designed into anything from a screw cap to an unthreaded stopper. Most container closure systems for food and beverage packaging also include safety features to indicate breach tamper-evidence. Mecmesin's range of torque testing equipment is able to measure the effort needed to overcome these features and indicate the value separately from that needed to make the cap continue to unscrew — giving the complete picture of the user experience in opening the container.

This capability is available even in our manually operated Tornado instrument, and our computer controlled systems that deliver the ultimate in accurate and repeatable procedures to detect these events for even the most innovative closure designs. For the ultimate in accuracy and repeatability, it is essential that the closure is gripped firmly but without being deformed, which would result in higher forces being measured due to increased friction on the threads. For this reason Mecmesin usually works with customers to design and manufacture cap mandrels which are exactly mated to the closure itself.

Testing what's inside the box

The ultimate destination for a food item —in terms of the human (or possibly animal) need— is when consumed. This is the acid test as to whether the customer will repeat buy or try another brand, possibly never to return.

The texture of the food item is a major influencer on the sensory quality perceived by the consumer. Textural attributes can be correlated to force measurement. The softness of bread, for example, indicates if it is still fresh or has started to go stale. By testing the product inside the packaging in this way, the effectiveness of the packaging in extending shelf life can be evaluated.

Delivery into the package
A further benefit of texture analysis of the food product is the ability to determine the impact of its physical properties on the production equipment. Textual attributes of consistency and spreadability, which are more favoured by the consumer when manifested as thickness and creaminess in yogurts, sauces and spreads, can influence the flow though filler-line nozzles at the point of delivery to the food container. By taking a measured approach to analysing the qualities of the food itself, the producer can offer different variants, such as low fat or vegetarian alternatives, and assess the possibilities of common packaging and other cost efficiencies in the complete production process.

Wrap up

In the food sector, consumer loyalty is influenced by much more by sensory and emotional opinion than most others within the packaging industry as a whole. However, the packaging solution plays a major part in delivering that customer experience – not only through the interaction with the packaging itself, but by guaranteeing the quality of its contents. Testing plays a vital role in packaging design and development—evaluating new materials and minimising their wastage, assessing solutions for compliance and suitability (and on the production line)— ensuring optimised efficiency. Using measuring instruments and computerised testing machines eliminates human error and provides accurate audit-trailed data to keep lines running and producing packaging that meets all user expectations.

Mecmesin offers a complete range of machines and fixtures, with custom engineering services, to provide complete solutions, from manual testing to fully automated systems.

Author: Jeff West is a Technical Writer at Mecmesin Ltd. (www.mecmesin.com) and Food Technology Corporation (www.foodtechcorp.com)

See also

Mecmesin OmniTest machine demo by the Duchess of Sussex at University of Chichester Tech Park opening

The Duke and Duchess of Sussex officially opened the University of Chichester’s new Engineering and Digital Technology Park on 3rd October 2018. Part of the tour of the £35million development, based at the Bognor Regis campus, was a visit to the Department of Engineering and Design. The laboratory features several Mecmesin force measurement machines, notably a 50 kN OmniTest Universal Testing Machine (UTM); a state-of-the-art software-controlled materials testing system.

Mecmesin white paper: Force testing in medical tubing

Plastic medical tubing is central to the provision of effective health care in the 21st century. It is essential for the delivery of blood, nutrients and gases to the patient, and for the execution of minimally invasive surgical procedures such as cardiac angioplasty, which have proliferated widely over recent years. This article examines four force testing procedures for plastic medical tubing and explores the benefits and methodological considerations of each.

Universal testing machine range (0.5 - 5 kN)

Mecmesin offers a wide range of universal testing machines (UTM) for tension and compression testing. At the top end, our MultiTest-i systems are driven by the powerful Emperor™ control and analysis software on a separate PC, while our production-level testers in the MultiTest-xt series offer touch-screen ease of use and rapid processing. All of these systems give fully programmed control and accurate, repeatable testing, with test libraries for instant recall.

  • Mike Webber
  • Product Info
  • English
  • Created 19 Jun 2017
  • Modified 22 Jun 2017
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