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The edge crush test (ECT) using a short-column specimen is an important test for the box maker.  Procedures for making the test are discussed in TAPPI T 811 “Edgewise Compressive Strength of Corrugated Fiberboard (Short Column Test).” For accurate and reproducible results, these procedures should be followed in detail. A critical point of the method is cutting the test specimen with sharp parallel cuts exactly perpendicular to the faces. If cutting is by means of a circular saws as prescribed, the saw blade must be small toothed, no-set, and hollow or taper-ground. To improve the saw cut, the saw blade opening (throat plate) in the table saw bed should be as small as possible. This reduces chatter of the corrugated board as it is being cut. Sharp-knife cutters in use in Europe and Japan have also been found to give satisfactory cuts for single wall and lightweight double wall. Single-bevel knife blades are necessary in these cutters.

For referee purposes, the test specimen should have its edges reinforced with paraffin to prevent edge failures per T 811. However, for routine control, other types of sample holders or those used in the Japanese standard method may be adequate. (Ref. TAPPI UM 817).

But why should anyone want to make this test? Well, it may be the most useful test for corrugated board.

The first reason is fairly obvious. If the corrugated is being made to conform with Alternate Rule 41/Item 222, in which the ECT is the specification test, the box maker should perform the ECT to determine compliance to the regulation.

The second reason is not quite so obvious. Edge crush is the single most important property of corrugated board in determining box compression strength. The most widely adopted formula for predicting box compression is the McKee equation:

C=k₁ x P_m x h⁵⁰⁸ xZ⁴⁹²

Where:

C= box compression strength

k₁= a constant 5.874

P_m= edge crush

h= caliper of the corrugated board

z= box perimeter 2 (L+W).

The above is the modified form of the McKee equation. The original involved the use of flexural stiffness instead of caliper. Since flexural stiffness is not frequently performed test, caliper is substituted. The substitution is only appropriate when used to analyze materials for which the caliper has not been damaged.

A “short” version of the McKee formula is:

C= 5.87 x P_m x hZ

The “short” version yields predicted compression approximately 5% higher than the “modified” version. Thus, for any box Z (perimeter) is already determined and the only two controllable variables are P_m (edge crush) and h (caliper). P_m is three times as important as h.

NOTE:

When using any of the versions of the McKee predictive formulas for the box compression, caution is advised. Several factors in the addition to ECT can affect box compression. Moderate crushing (caliper loss) during fabrication may not affect ECT, but will affect compression. Deep slotting, poor corrugator or press scoring, box squareness (score line-up), box skewness (fishtailing), and bundle strap damage are additional manufacturing defects that will affect box compression.

The third reason for performing the ECT test is that it is basically a material test. It depends upon the quality of the raw materials: the linerboard and the corrugating medium. Of course good quality fabrication is required. However, if adhesion is adequate, there are no excessively wide dry finger lines, and the board has not been crushed more than 20%; edge crush is determined by the stiffness of the liners and medium. The relationship has been well known for a long time.

ECT= k₃  [k₂(RC_L1 + RC_L2 + tRC_M)] + 1  

Or     

 ECT= k4 [(S_L1 + S_L2 + tS_M)] + b

Where:

RC_L= the ring crush of the liners

RC_M= the ring crush of the medium

S_L= the short span compression (STFI) of the liner

S_M= the short span compression (STFI) of the medium

t= take-up factor

k₂= a constant; 1/6, because ring crush is measured in pounds per 6-in. stripe while ECT is expressed in lb/in.

k₃, k₄, a & b = constants.

NOTE:

Several versions of the ECT formulas have been published (Ref. 1), individual companies, API, FBA, and the IPST all have formulas. The various published formulas all have different constants (k) and some have constants a & b as well. The situation as of the date of the revision to this TIP is in a state of flux. An industry committee is working to resolve the issue and to come up with one agreed-upon formula for ETC vs. Ring Crush, and ECT vs. Short Span Compression (STFI).

Edge crush is a measure of the edge stiffness qualities of the liners and medium used. If adhesion is adequate and there has not been excessive crush, and there are no finger lines in the sample, the box maker can get a good estimate of the quality of his raw materials by measuring edge crush.

Finally, because of this interrelationship, ECT is an excellent test to measure corrugating efficiency. This technique was described years ago by Gartaganis and Ostrowski in Tappi Journal as a report of a Committee Assignment of the Process and Quality control committee. By relating the actual short column or ECT to that which is predicted from ring and fluted crush test of the liner and medium, a convertibility index is calculated. This number gives a measure of how the box maker is doing in getting the most out of his raw materials.