Tester Tech Tips, Corrugated Series #002: Replacing the Concora Medium Test (CMT) with the NEW S-TEST
Manufacturers of corrugated board continuously strive to balance box strength versus material and manufacturing costs. In order to monitor the production quality, several quality control tests at different production stages are performed. For example, the linerboard and fluting (Figure 1) are both tested before the flute is made and the corrugated board glued together. These quality tests are important to reduce cost and maintain product specifications. The producer can measure the quality of the different materials and calculate the strength of the corrugated board and the box itself before producing it. Underperforming paper can be replaced before finishing the production. This reduces material cost, waste and valuable production time.
The most important test for fluting material is the Corrugating Medium Test (CMT). This test compresses a piece of fluted material of 6-inch x ½ inch. The CMT is important, but at the same time difficult to perform. The sample preparation is time consuming and can be influenced by the operator and the tape used. This will increase the variation of the test results. This higher variation makes it more difficult to adjust production and predict the performance of the corrugated board. A new test with the working name S-test can accurately predict the first CMT plateau without difficult sample preparation and variation in result.
A CMT test is performed with 10 pieces of paper with the dimensions of 12.7 mm x 150 mm (½ x 6 inches). The samples are cut in the machine direction, then fluted in a bench top laboratory fluter set to 177 °C (±8 °C). Using the rack and comb fixture, the flutes are then taped. The CMT fixture will achieve the right geometry of the flutes when manually fixated with the tape. After the sample is carefully removed from the comb, it is ready to be crushed in a compression tester. This should be completed within 30 seconds of fluting. This process can be complicated and time consuming.
The CMT test gives you two values that are used – the first plateau and the max peak (figure 2). The CMT plateau has two big advantages over the CMT max. At the CMT max, the sample is almost fully crushed. Just before the peak, the sample can delaminate. The degree to which delamination occurs will add variation to the results. Another problem is that thicker paper delaminates more and will therefore give a lower value at CMT max. The first plateau does not have this problem and clearly shows that the heavier flute is also stronger. Due to the variability of the CMT max, the CMT plateau value is the preferred measurement.
How the S-Test works: Smurfit Kappa was the forerunner to developing the new test method. The goal was to develop a test that gave an immediate result, without difficult sample preparation, and that would correlate with the CMT first plateau. The Short Span Compression test was used as the basis for the new test. To simulate the flute, the clamps are widened to 4mm and have a 1mm offset which also gives the test its name.
The S-test is a way to simulate CMT plateau measurement. To simulate the forces of the CMT test at the first plateau, the S-Test is introduced and further developed. The Cepi Container Board (a European industry association of producers of corrugated case materials) investigated the measurements from 17 different European mills. The tests were executed in cooperation with the Technical University in Darmstadt to verify the results. The results showed the validity of the S-Test. Since May 2015, the S-Test has been performed daily at the Smurfit Kappa mill in Germany to adjust production on the paper machine. Up to now, the S-Test has shown a correlation coefficient of 97.6% with the CMT first plateau value.
Since then, further investigation has been done confirming the correlation with the CMT first plateau and the relation with the corrugated board performance. In order to internationally accept this method, a draft of DIN 5014 has been published and is available in German. In 2019, Smurfit Kappa plans to complete the install of S-test units at all of their facilities in Europe.
The S-Test has proven to be a more reliable and consistent test method. It saves time due to simple sample preparation and no operator variability. This enables the user to compare results with other instruments/plants. Because of the reliable and repeatable results, production can be optimized to reduce cost, valuable production time, and material.
S-Tester Offered by TMI
Testing Machines offers Model 17-38 S-Tester as an alternative to the CMT test. The new method was developed to measure the compressive strength of the flute/medium. A 15 mm wide specimen of uncorrugated medium is placed between two clamps. The span between the clamps is 4mm and the clamps have an offset of 1mm. The offset of 1mm compresses the flute to form an “S” shape during the test. The test result is the maximum compressive force.
|Cutting Sample||1/2 inch||15 mm|
|Sample Prep||Yes, approx. 1 min per sample (10 required)||No|
|Instruments Required||Cutter/Vertical Fluter/Crush Tester/Tape||Cutter, S-Test|
|Investment Cost||36,000 Euro, yearly cost for tape 1200 Euro||22,000 Euro|
|Variation of Operator||Yes, operator can influence result||No|
|Easy Comparison of Results||No||Yes|
|Measuring Parameter||First plateau and CMT max||First plateau|