It’s easy to think of a container as a simple geometric shape, but things get complicated when you want to create an exact duplicate of that shape. You might measure the outer length of one side of a box, and replicate that exact measurement on a blank piece of cardboard, and the inside of the resulting container might not be the same. This is due to the fact that a container has two distinct sets of dimensions, those that exist on the interior and those that exist on the exterior of the container itself. Understanding which set of dimensions you’re dealing with before you start to draw out panel dimensions, score lines, or whether you’ve successfully created a sample of a particular container, is vital.
Internal dimensions of a container are the measurements of the space available within the container. These become relevant when you need to ensure that the container can hold an item of a particular length, width, or height. External dimensions of a container are the measurements of the space the container itself will occupy. These become relevant when you need to ensure that a container can be shelved, stacked, or placed within a given overall volume. While the differences between internal and external dimensions may be negligible in thin materials like paperboard, they may become quite apparent in thick materials such as corrugated board, or other rigid materials.
You can conduct an experiment by finding a container, and measuring both the outside dimensions and the inside dimensions using a ruler or measuring tape. Start by measuring the outside length, width, and height of the container, then open the container and measure the inside length, width, and height. If possible, also view the cross-section of the container, and measure the thickness of the material being used. The results may not be immediately obvious, because the way in which the container is constructed and folded can impact the results. However, the point is not necessarily to develop a universal rule, but to demonstrate that materials have substance, and that the act of folding affects the placement of various surfaces.
This is particularly true when dealing with a flat blank. A score line on a flat blank is not an infinitely thin hinge. Instead, the material that makes up corrugated board must bend over a small region, and the liner and flute must flex during the process. Without accounting for both the material and the effects of bending, the resulting container can easily be tight, loose, or distorted. Novice designers often find themselves with this situation after constructing a container when they realize that their last flap doesn’t align properly with the seam. Rather than realizing that this was caused by inaccurate measurements, they blame the construction itself, and conclude that there is something wrong with the fold itself.
To help illustrate these points, you can disassemble a small container, and compare the original flat blank to the resulting container. Identify the main panels, flaps, tabs, score lines, and joints in the container. Take a measurement of one panel while the container is flat, then compare that measurement to the resulting internal or external measurements once the container is formed. You can also use a square to check if the formed container is square, which helps connect the three steps that can otherwise be confusing: creating accurate panel dimensions, forming the material correctly, and ensuring that the resulting container is accurate.
You should also perform dimensional checks after the container is fully assembled, and shouldn’t stop checking dimensions when you begin the cutting phase. A seam that has shifted while gluing, a fold that hasn’t been made properly along the score line, or a panel that isn’t perfectly square can affect the resulting measurements of the container. Quality control should include not only ensuring that the container looks professional, but also comparing the measurements of the container to the desired measurements, inspecting how well the flaps overlap, checking that the seams are properly aligned, and noting whether or not opposite sides of the container remain parallel. Having a basic checklist of these criteria can help make your quality control process more consistent.
One of the best ways to simplify dimensional analysis of containers is to clearly label any measurements you take or include in your drawings. Rather than thinking about whether a particular measurement is an internal, external, or flat panel dimension, write it down. This way, if you discover that the finished container isn’t the right size, you can go back to your drawings and figure out where the problem originated: whether the material wasn’t thick enough, the score line was in the wrong place, the panel was cut incorrectly, the seams were glued together improperly, or the fold was misaligned. This allows you to shift from an abstract problem to a concrete production error.
