Paper and Cardboard Testing with a Texture Analyser – Part 1

Reliable Testing for Packaging and Print

 

Corrugated cardboard is used all over the world for strong, cheap and light recyclable packaging.

It is made up of a fluted interior sandwiched between linerboards. Variations include a linerboard on only one face, or several stacked layers. It is generally made into cardboard boxes, although occasionally used in sheet form for strengthening or insulation purposes. Most boxmakers perform a wide range of tests on their products as well as the components used to make them. Many boxes are made to certain customers’ demands, and the aim of these tests is to maximise strength (meeting the customer’s needs) while minimising costs from component materials and weight.

 

Unlike traditional crush testing equipment, which only record the maximum force during a test, Stable Micro Systems Texture Analysers, along with Exponent software, allow full graph analysis as well as optional synchronised video capture to identify failure mechanisms and tie them into points of failure on the graph. They incorporate a precise ball screw, sturdy design and high accuracy load cells for reliable, repeatable testing. Additionally, they are multi-functional – consequently, many paper, cardboard and box tests may be performed on one instrument, including crush, tensile, puncture and flexure. Specific tests will be outlined in detail in this article.

The set of tests carried out by a boxmaker are determined by the desired properties in the finished box. ‘Compression boxes’, designed to be stacked on top of one another, must always fulfil some basic criteria:

1. Professional appearance
   
   
2. High enough crush strength to allow full containers to be stacked during transit and storage
   
   
3. Robust enough to protect their contents
   
   
4. Retention of their protective properties over a time period long enough for the customer’s needs, and in the required environmental conditions

Apart from appearance, the above strength properties can be quantified using either imitative or more traditional, fundamental methods. Combinations of the methods outlined below may be used to provide a core set of tests for each type of box and storage conditions.

Box Crush Test
Example standard: BS ISO 12048

Most boxes are designed to be used in compression, and must withstand being stacked under other boxes without falling or collapsing. The box crush test can be used directly to test compression boxes to failure. In this test, a large compression platen is used to apply force on a box until it crumples. The corners of the box are usually the controlling factor in box crush strength, and ensuring corners are reliable is the best way to ensure a good stacking strength. 

A crush test will help to detect poor adhesive penetration, spotty adhesion and carton board with low bond strength.

During a crush test, a box is subjected to increasing load under a compression platen until failure occurs. Boxes can be crush tested either empty or filled with their intended contents – when full, this is termed ‘in situ testing’. Crush strength is increased when the box is filled with other boxes or sturdy items.

Additionally, made up boxes are often subjected to impact, vibration and shock tests to imitate their real world conditions.

Box Creep Test
Example standard: ASTM D7030

As boxes are usually stored for a time stacked on top of each other, the bottom package is continually subjected to a constant compression load. Consequently, some manufacturers carry out creep testing. In this type of test, a Texture Analyser compression platen subjects an empty container to a static load. Deflection-time data is used to predict the time to failure of boxes stored in this manner.

Flat Crush Test
Example standard: TAPPI T809

Rigidity of the fluted structure is essential to corrugated board, and although not necessarily visible from the outside, flute damage will weaken the finished box. Flat crush resistance is a measure of this structure’s resistance to deformation on the corrugator and other converting equipment, and will predict whether or not flutes will be damaged during processing. A low flat crush value will have an effect on the sample’s flexural stiffness and may suggest that a box will not perform as expected.

This test may be performed on a square or circular section of board, but more usually on a rectangular strip. Force is applied perpendicular to the surface of the cardboard during the test using a compression platen.

This test can be applicable to complete, corrugated board or to the corrugating medium alone. When the corrugating medium is tested before it is fabricated into combined board, it may be referred to as the ‘corrugating medium test’.

Edgewise Strength – Ring Crush and Edge Crush

Corrugated board products undergo many varying stresses during their lifetime – mainly crushing forces when they are handled, stored or transported. It is, therefore, useful to know a product’s edgewise compression strength to optimise the manufacturing process. This ‘column strength’ is often considered to be the most important factor in controlling box strength.

Additionally, edgewise compression strength is the main parameter used to determine the dynamic compression strength of a finished box made from the board in question - the edgewise strength of board along with flexural stiffness relate to the compressive strength of boxes made with vertical flutes.

There are two main methods to test the edgewise strength of a combined board – the ring crush test (which can also be used for its components alone) and the edge crush test, used for completed board only.

 Ring Crush Test
Example standard: TAPPI T822

In this test, a specimen is held in ring form in a dedicated sample holder and compressed by a platen on the test platform of a Texture Analyser. The test is carried out at constant speed until the sample collapses.

As edgewise compression strength of a board can be estimated by adding the ring crush strengths of the liners and medium, this test is especially useful for the R&D process of box making. Potential liner and medium materials can be tested in this way and catalogued to estimate the edgewise strength of different material combinations.

Edge Crush Test
Example standard: TAPPI T811

The edge crush resistance of corrugated board can also be measured using a dedicated test – the edge crush test. This test is particularly useful for quickly assessing the strength of different material combinations in a finished board, and helps the box maker to predict the performance of a box using the ‘McKee formula’. (The McKee formula estimates box compression strength – it requires the input of edge crush test value, the calliper (width) of the combined board and the finished box dimensions.)

The edge crush test is typically performed on a 2”x2” piece of corrugated board. It is either held in a lower tensile grip or between guide blocks.

... continued next week...

 

For help putting your packaging to the test, contact Stable Micro Systems today.

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For more information on how to measure texture, please visit the Texture Analysis Properties section on our website.

The TA.XTplus texture analyser is part of a family of texture analysis instruments and equipment from Stable Micro Systems. An extensive portfolio of specialist attachments is available to measure and analyse the textural properties of a huge range of food products. Our technical experts can also custom design instrument fixtures according to individual specifications.

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