How to measure a box.
Corrugated box sizes are measured from the inside. The size of your contents determines the inner box dimensions. In the United States, the dimensions are listed in the following sequence: Length (L) x Width (W) x Depth (D). Dimensions are based on the opening of an assembled box. Looking at the opening, the longer of the two sides is considered the "length." The shorter of the two sides is the "width." The side perpendicular to the length and width is the "depth" of the box.
The "Manufacturer's Joint" is where the two ends of the vertical side panels meet to form the box. We also call this a glue tab; larger RSC joints are sometimes secured with tape or staples. The side panel thickness and content weight will determine the type of seal used for the manufacturer's joint. For example, glue is used for most single wall RSC boxes. Staples are found in some double wall and in most triple wall boxes.
The strength of a corrugated box starts with its material. A corrugated sheet consists of two major components – the linerboard and the medium (the flutes). The linerboard is the flat paper that covers both sides of the sheet and the medium is the "fluted" paper found between both liners. The flute is anchored to the linerboard with a starched-based adhesive. When placed vertically on its ends, the flutes form vertical columns, capable of supporting considerable amounts of weight. If the flutes are positioned horizontally, the box will crush easily and not adequately protect the contents.
Although corrugated sheets come in five basic flute heights and shapes, these 3 are the most popular. C-Flute is the most common flute style and is used for RSCs. C-flute is thicker, with more air space, and offers enhanced stacking strength. "B-Flute" (used for die-cut boxes) is compressed and appears thinner, however it is made with more paper to provide stronger side wall protection from blows and punctures. E-flute is thinner, flatter and provides a better surface for graphics.
The amount of virgin pulp fibers and the length of those fibers in a corrugated sheet substantially contribute to box strength. For example, the difference between a 200# test box and a 275# test box is that the latter has more pulp fibers in its corrugated linerboard. The 200# test box is rated to hold up to 65 lbs. of box and contents while the 275# box can hold up to 95 lbs. of box and contents. A 350# test box is rated to hold up to 120 lbs. of box and contents.
RSCs are normally manufactured with kraft colored fiber. The corrugated sheet can be ordered with white one-side (very common), white two-sides (not so common) and triple white. Triple white is a combination of a white liner on both sides, and white flutes. Triple white is rather expensive and minimum orders apply. Triple black is also available but it’s primarily used for short run custom die cut boxes.
The Box Maker’s Certificate
One way to make sure that the material of the box you're purchasing meets industry standards is to review the Box Maker’s Certification Stamp, usually printed on one of the bottom flaps of the box. The stamp identifies the material as "singlewall," "doublewall" or "triplewall." It also indicates some rather dry technical strength information that I won’t go into at this time.
If the box and contents weigh less than 65 lbs, you can consider a single wall 200# test . This weight limit works well for multiple light-weight items in a box or for a masterpack box where individual boxes are packed tightly in organized rows and stacks. The more valuable your contents, the more conservative your box requirements need to be.
If a single heavy item is to be shipped in a single wall 200# test box, we recommend a weight limit of 45 lbs. If it’s heavier you should upgrade to a 275# test single wall box or consider a custom box that utilizes “full flaps”. Simply apply common sense and always err on the conservative side.
Additional ways to insure your contents is protected.
If you have experienced damage claims or your shipment have arrived looking less than satisfactory, here are some simple ways to improve the quality of your packaging. I’ve listed them in steps from the least expensive solution to a more secure complete solution.
1) Use the “H” pattern sealing method. The diagram shows how to properly seal a box for cross country transportation. If you are sourcing poorly made boxes and cheap sealing tape, this method can still improve the safety of your contents. Be sure to tape the top and bottom using this method. If you find you need additional protection, go to the next step.
2) Invest in a quality sealing tape. You can find cheap sealing tape, but the adhesives are weak and during the summer months, as your package travels across a warm mid-west, you may find the weak adhesive releases prematurely. Polyester sealing tape with a strong adhesive will provide you with the best sealing strength. You don’t need to upgrade to the expensive 3M tape. Remember to use the “H” pattern sealing method.
3) We’re seeing more and more boxes with poorly formed, weak kraft primarily from boxes purchased on-line. The fibers are overly recycled (too much gray newsprint) and the fiber direction is not-uniform. If you use a stock size box, ask for a sample of the kraft you’ll be purchasing. The cost of a custom high quality singlewall RSC will provide you more protection for your product.
4) Still having problems or you ship valuable merchandise? Upgrade to a 275# test box.
5) The next step would be to upgrade to a double wall style corrugated box. It’s C-flute married to a sheet of B-flute. You’ll need to be able to order about 500 units or more. The double wall style provides significantly higher stacking strength and a more durable corner.
6) Use kraft tape with a water based adhesive. It adheres to corrugated linerboard much better than plastic sealing tape. The tape is available in plain or re-enforced fiber. This type of tape is my favorite no matter what weight I’m shipping. Applied via the “H” pattern, this tape provides a very sealed environment and keeps the flaps very snug.
My BEST suggestion for improving the strength of your home shipment:
When I ship a package from home and the strength of an existing box needs to be improved (usually because the box was previously used), I cut 4 panels of corrugated from an old box to match each inside panel in height and width. I make sure I place the flutes in a vertical position and then pack the contents inside the box. I add extra paper to take up any space. I call this the poor man’s double wall box. The extra layer of corrugated improves the vertical stacking strength. The box is then sealed in an H pattern using gummed paper tape. I use un-reinforced paper tape at home because most of my shipments are relatively light.
