What is the "bursting strength" on cardboard boxes?

Factual Questions
1

At work, we use a lot of boxes. Boxing files, boxing papers, boxing old junk, boxing boxes… etc. On the cardboard boxes we use, it has this printed on the bottom:

So what’s “bursting strength”? The point at which the box just explodes? Does that mean I could theoretically put twenty tons of material in the box before it’ll burst? Or does it refer to how much weight the box can hold up? I don’t get it. I can understand how, say, a bridge can have a load limit (that one’s obvious) or what tensile strength certain materials might have (y’know, so many pounds per square inch to cause a steel beam to buckle). So how does “bursting strength” relate?

2

I think it’s how much it can hold up.

I must say that Americans take their cardboard boxes very seriously. US boxes are bloody solid, and they come plastered in labels bragging about their bursting strength. It’s strangely cool.

3

Bursting strength defined: The strength of a material expressed in pounds per square inch as measured by the Mullen tester.

Uh-huh…

In simpler terms: It’s the amount of pressure to rupture one square inch of linerboard expressed in pounds per square inch.

4

Burst strength:
A measure of the ability of a sheet to resist rupture when pressure is applied to one of its sides by a specified instrument under specified conditions. It is largely determined by the tensile strength and extensibility of the paper or paperboard. The Cady tester and the Mullen tester are the most common burst testing devices. Testing for bursting strength is common to determine grades of corrugated and solid fiberboard.

Here’s a URL=http://www.allproducts.com.tw/machine/juyen/28-cy6103a1.html]testing device.

5

Ack! Here’s the URL…

6

[corrugated geek hat on]

TERMS:
Corrugated: what you may think of as cardboard, what shipping container are made of
Containerboard: the collective name for two basic types of paper that make corrugated boxes, liner and medium
Combined Board: three pieces of paper bonded together, two flat outer facings, and one inner squiggly corrugated medium
Basis Weight: measure of weight of containerboard, expressed in pounds per thousand square feet (#/MSF)

In the early days, as corrugated boxes attempted to replace wooden shipping crates, most good were transported by rail, and not packaged nearly as well as things are today. Since a main benefit of wood versus paper was the ability to withstand the rigors of rough handling, an early test to show the ability of corrugated to survive was the Burst Test (commonly referred to as the Mullen Test derived from a standard test device for this property). It is as described above, or at least it was. Burst is still a paper test, but it is becoming less and less applicable in today’s handling methods for shipping containers. They are not unloaded by hand, tossed to the next guy anymore. Unless you work for UPS.

If you look more closely at that stamp (Certification Stamp, or Box Maker’s Certificate), it also says “Minimum Combined Facing Weight 82 lbs” for the “200# Test” corrugated. As the actual burst testing of the corrugated faded, this CONSTRUCTION spec came into being. It states that the basis weight of the two outer facings of the combined board must add up to at least 82 #/MSF. A standard basis weight, coincidentally, was and still is 42#/MSF. The fluted medium (squiggly paper) was not a part of this construction specification, as it adds very little to the actual bursting strength of the combined board.

One main purpose of the fluted medium is to keep the two faces apart, but the curvature also serves as slightly misshapen columnar structures that give great vertical stacking strength the the corrugated board. Look closely at a cleanly cut edge of combined board, and you can see the “columns”. Since modern material handling and storage uses stacking of boxes higher in a warehouse, rather than having a lot of square feet in a warehouse and low stacks like in the old days, stacking strength has become more and more important for many shipping containers, and a (relatively) new test has been adopted, the Edge Crush Test (ECT), which is not so much a CONSTRUCTION spec as a PERFORMANCE spec. A box with an ECT stamp (look for one, they are out there) does not specify minimum combined facing weights like the Burst Test stamp does. More varied combinations can be used, high performance liners, heavier or lighter mediums, so that a small piece of combined board will withstand vertical force (parallel to the direction of the flutes) in a given set of tests, which better approximate the ability of the box to stand up to stacking conditions. It can generally be made more economically than a Burst Test box for a given handling and storage condition.

Which test to use depends on what is being shipped (at least it should). Consumer electronics, or a box of screws may go with Burst, while a shipping container for cereal boxes or light industrial goods may use ECT.

[corrugated geek hat…, aw hell, I never take it off]

7

Thanks, UncleBill. I spend about half of my working time shipping stuff in a variety of boxes. It’s interesting to know what goes into it.

So if you sealed up a spec box really well and hooked an air line to it, could it hold 200 PSI, or does it not mean it that way?

Good link, Danalan.

8

Nope, paper is air permeable, anyway. The test uses a rubber disk with oil under it, and the hydraulic pressure applied to the oil causes the rubber disk to deform in a vaguely hemispherical way, against the paper or combined board, which is held in place with a circular clamp. It is 200 psi of hyraulic pressure in that specific area (a few square inches, if I recall) that the board or paper must withstand before rupturing, or “popping”.

It is kind of a measure of how hard you have to punch the box before you punch through the wall and damage the goods.