Richmond Corrugated Box takes pride in delivery products of the highest quality to our customers. All of our boxes and other products are produced to the highest of industry standards. Read on to learn about the different measures of box performance that we use.
Box strength is measured in two different ways. The Mullen test measures resistance to rupture in pounds per square inch (psi). Linerboard constitutes the bulk of the bursting strength of a corrugated sheet. The basis weight of the paper as measured in lbs/1000 sq. ft. directly impacts paper strength.
When the corrugated box industry was new, most shipments were made by rail. Classification committees had control over all conditions related to shipping, including packaging. Rules regarding packaging in corrugated were incorporated in Rule 41 of the Uniform Freight Classification. When motor carriers evolved, similar rules were established in the National Motor Freight Classification Item 222 patterned after the rail classification. Rule 41 and Item 222 state minimum mullen requirements for packages based on size and weight. Although there are other rules in the Classifications that relate to corrugated boxes, most shipments are made in single wall corrugated boxes.
The Edge Crush Test (ECT) measures the ability of combined board to sustain a top-to-bottom load. The strength is directly related to the compression strength of both the liners and medium. There are two gauges of strength. Box compression strength (BCT) is the maximum load a given box can stand for a moment. Stacking strength is the maximum load a box can stand throughout the distribution cycle. This means that the bottom box must support the top load over a period of time in which it may be exposed to fluctuations in temperature and humidity, as well as other factors that impact performance such as handling, pallet patterns, pallet deck board spacing, and overhang. All these factors weaken the stacking strength of the box. Therefore the stacking strength of a box is almost always much lower than its compression strength. The most commonly used box style is the Regular Slotted Container (RSC). The McKee formula can estimate the compression strength of a given box. This is useful information when designing a package. By knowing the compression strength and carefully considering all the potential detractors encountered throughout the distribution cycle, the designer may better determine the ECT test required to achieve the desired stacking strength.
It is important to understand that whichever measurement you prefer to use to measure box strength, basis weight and flute caliper are the two most important performance indicators of raw material quality.
There is a take-up factor for medium. For C flute the take up is 1.44 and for B flute it is 1.35. The Wt/msf contains 2.5 lbs/msf of starch adhesive for single wall and 5 lbs/msf for double wall. There is a certain amount of caliper degradation throughout the converting process. This affects ECT values, box compression strength, and stacking strength. The best converters using the best raw materials have the least amount of degradation. The crush allowance ranges from .001-.006 on single wall flutes. Heavier grades are more resistant to crush. Newer machinery also helps protect against crush. The days of 26# medium are fading fast and most corrugators have switched to 23# medium. This affects the basis weights by approximately 4 lbs/msf for single wall and 8 lbs/msf for double wall. When designing a packaging solution, the best designers begin with the end in mind. Every packaged product has its own unique set of problems to solve. These include physical characteristics of the product, the mode in which the package will be shipped or stored, and the functions the package will be asked to perform. Burst strength is more relevant when product containment or puncture resistance is the main concern. ECT is more relevant when stacking strength is the main concern.