STFI EDGEWISE COMPRESSIVE STRENGTH OF PAPERBOARD
Introduction
A corrugated box or any other paperboard box used for shipping purposes must perform several functions: the box must contain its contents; it must protect them; and the box may have a display or advertising function.
Here, we are concerned with the protection the box provides for the contents, that is to say with the strength of the box. During transport or in a warehouse, boxes are often stacked one atop the other, and the box at the bottom is made to carry the weight of all boxes on top of it. Two hazards are inherent in this situation that threaten the integrity of the box and therefore the safety of the contents. First, the side walls of the box may buckle. This type of box failure can be prevented by making the board stiff enough. Accordingly, the bending stiffness of the board is an important figure of merit, and in a previous section we have encountered two testing methods for this property (Taber stiffness test and Lhomargy method). Second, the walls of the box may be crushed by compressive forces acting in the plane of the board. Whereas buckling does not destroy the structural integrity of the board as such, crushing does imply irreversible damage to the board’s internal structure. The STFI method of testing for edgewise compressive strength is specifically designed to characterize a board’s resistance to such crushing forces.
The stacking height of a paperboard box is the property of immediate practical importance. It can be improved by increasing the bending stiffness and/or the edgewise compressive strength of the board; this section is concerned with the latter strength property.
Significance
A board’s resistance to crushing can be measured by several methods. The two methods most widely used in the pulp and paper industry are the ring crush test and the STFI edgewise compressive strength test. The latter is generally acknowledged to be the best method available today.
Ring crush data fall well below STFI edgewise compressive strength when both are measured on the same board. This is because the ring crush method does not completely eliminate
5 - 51
buckling of the specimen; especially at low basis weights, a buckling mode of failure increasingly replaces the crushing mode.
The chief virtue of the STFI edgewise compressive strength test is that for all practical purposes, buckling is eliminated. A pure crushing mode of failure is thus assured. This is achieved mainly by a very small sample span, 0.7 mm, and by specimen clamps machined to very close tolerances.
Of the two specimen clamps, one is stationary, the other movable. In a test, the instrument pushes the movable clamp toward the stationary one with steadily increasing force until the specimen is crushed. The instrument records the maximum force developed in this process and indicates it as Fmax in the digital display window in units of lbf. The edgewise compressive strength printed out by the instrument is defined as Fmax per unit specimen width divided by the basis weight and is given in units of klbf ´ ft/lbf. (This definition is analogous to the definition of breaking length in a tensile test.)
Test Specimen
Cut test specimens 15 mm wide (critical dimension) and at least 70 mm long (not critical). To test a board in machine direction (MD), let MD coincide with the long axis of the specimen. Similarly, to test in the cross direction (CD), let CD coincide with the long axis.
TAPPI T 826 requires that 10 specimens be tested in each principal direction to obtain the desired average STFI edgewise compressive strength in MD and in CD. Here, we are satisfied with testing only five specimens in each principal direction.
This test is extremely sensitive to the specimen’s moisture content. Careful conditioning is therefore essential. Even more important, the specimen should be handled very carefully: Never touch the compression area with your fingers. Handle the specimens at the edges and at the end.
Test Method and Operating Instructions
5 - 51
The instrument available for the STFI edgewise compressive strength test is that of Lorentzen and Wettre. It accepts specimens in the grammage range from 100 to 400 g/m2, corresponding to 20.4813 to 81.9251 lb/1000 ft2. As the basis weight of the sample must be supplied to the instrument in units of lb/1000 ft2, it is useful to remember that
Make sure that the valve to the pressurized air supply line is open. This valve is located to the left and above the nearest Lorentzen & Wettre tensile tester.
Turn on the power to the STFI tester by pressing the toggle switch on the rear panel, located directly above the power plug, from “0" (off-position) to “1" (on-position). Allow 30 minutes for warm up.
Make sure the clamping pressure, indicated on the outer black scale of the pressure gage located on the left-hand side of the front panel, reads approximately 0.5 M Pa; adjust if necessary. At this point, the display window should indicate “Program rev. 1.5". To proceed, the operator presses “YES” on the keyboard. The entire measurement procedure consists of a dialog between instrument and operator. The instrument poses questions that are displayed in the window, and the operator responds by pressing appropriate keys and by occasionally inserting specimens. Use Program # 100 stored in the instrument’s computer by following the steps as outlined on the next page. If the instrument responds in unexpected ways and you wish to start over, you can always abort a test series by simply pushing the small red button located at the top of the back panel and to the left of the on/off switch.
5 - 51
Step / Window display / Press key / Comment1 / Program rev 1.5_ / YES
2 / PAPER IN PRINTER?_ / Yes
3 / CHOOSE PROGRAM_ / PF2
4 / SPEC No?_ / 100
5 / SPEC No? 100_ / YES
6 / No OF TEST PIECES?_ / 5 / Here, you can choose any number of samples you want. In the present case, 5 was arbitrarily chosen.
7 / No OF TEST PIECES? 5_ / YES
8 / WEIGHT lb/1kft2?_ / 10 / Here, the instrument is asking for the basis weight of the sample in units of lb/1000 ft2.
9 / WEIGHT lb/1kft2? 10_ / YES
10 / SAMPLE No?_ / 123 / The operator assigns an identifying number to his sample and inputs it here.
11 / SAMPLE No? 123_ / YES
12 / * MACHINE DIRECTION? / YES
13 / TEST PIECE No 1?_
14 / TEST PIECE No 1 FIX TEST PIECE! READY?_ / YES / Before you press YES, place a sample between the clamps. The machine direction must be horizontal. Upon pressing YES, the instrument carries out a test.
15 / Fmax 8.466 lbf OK?_ / YES/NO / The result of the first test is here displayed in units of lb-force. OK? asks whether or not the operator accepts the test as valid. Accept by pressing YES; reject by pressing NO. In either case, the result is printed out in terms of
Fmax/width/(basis weight). Rejected tests are not used for determining the mean.
Continue until 5 accepted tests have been run in MD.
16 / * CROSS DIRECTION?_ / YES
17 / TEST PIECE No 1?_ / YES
18 / TEST PIECE No 1 FIX TEST PIECE! READY?_ / YES / Before you pree Yes, place a sample between the clamps. The cross direction must be horizontal.
19 / Fmax 9.438 lbf OK?_ / YES/NO
Continue until 5 accepted tests have been run in CD.
20 / No OF TEST PIECES?_ / Having completed all tests in MD and CD, the instrument returns to step 7, and a new sample can be run.
After completion of all tests on a sample, the window displays the message “No of Test Pieces?”. This means the instrument has gone back to step 6 and is ready for testing a new sample. In this way, the operator can continue testing an arbitrary number of additional samples.
5 - 51
Report
Report the average value of STFI edgewise compressive strength in units of klbf ´ ft/lbf for both principal directions. Also, report a 95% confidence interval for each mean.
References
TAPPI T 826
Chr. Fellers, Edgewise compression strength of paper, in Handbook of Physical and Mechanical Testing of Paper and Paperboard, ed. R. E. Mark; Marcel Dekker, Inc., New York, 1983; Vol. 1, p.349
5 - 51
5 - 51