Corrugated Board

Corrugated Board Structure

Corrugated board is a made from a number of layers of paper including one or more layers where the paper is “corrugated” into a ridges pattern (the “fluting”) sandwiched between flat paper layers (“liners”) glued to either side of the corrugated fluting.

This physical structure with corrugations makes the resulting board thicker and stronger than the sheet paper would otherwise be. In particular the corrugations effectively become “columns” running in one direction which make the board resistant to bending or crush deformation along the direction of those “flutes” – which is why most boxes are designed so the corrugations in the board run from top to bottom of the box (rather than from side to side).

The profile of the “fluting” (ie the height and frequency of the corrugations) is categorised by letters: eg. A, B, C and E flute and the board is also so specified according to the weight and type pf paper used both for the corrugated fluting but more particualrly for the liner paper top and bottom.

Board may also have double or triple layers of fluting (“Double Wall” & “Triple Wall”). The most common is “BC flute” indicating that the board consists of both a layer of B flute and a layer of C flute. Each flute layer is separated by a flat liner paper and, of course, a top and bottom liner paper will also be present.

The strength of corrugated board is a function of the weight and type/quality of the paper used and how that is structured within the board. The specification of a corrugated board was traditionally defined by the fluting profile and then the type and nominal weight of the paper liners used (eg  “150Kraft/150Test B  flute”). However as paper industry standard weights have altered over the years and new ways of processing paper have evolved across the industry, the traditional grade specfications still being quoted have become “performance equivalent” codes for board rather than a factual codification of the indivdual paper weights concerned.

The minimum performance characteristics of corrugated board are now commonly specified; most importantly the “ECT” value (“Edge Crush Test”) – which indicates the amount downward force the board can withstand when stood on edge with it’s flutes running upright (ie like “columns” within the board). Whilst ECT is usually the most significant in determining the practical strength of the corrugated board to make a suitable box, there is also a Burst strength test (ie the board’s resitance to penetration) and a Cobb test (a measure of the board’s ability to resist water absorbtion), the importance of which may depend upon the end use application.

“Performance Packaging” is a term commonly now used to reflect advances being made to achieve the equivalent required strength of corrugated board with a lower overall paper weight and higher recycled content. Environmental benefit and cost savings are the attraction but bring compromises so that the board needs to be handled and processed in more precise ways.

Packaging Box Strength

The board material used is of course only one part of what makes up the “strength” of a final packaging solution. The packaging structural design is equally important.

A “BCT” (Box Crush Test) measurement is one objective measure of the strength of a made up, but empty, box and it’s ability to withstand a top load before buckling. Precisely measuring this requires testing equipment large enough to take the maximum size made up box. However, for a standard 0201 style box the BCT can be estimated using a long established Mckee’s given the box dimensions and ECT value of the board to be used.

However packaging in use will not be empty boxes and will need to protect product against rough handling as well as top loads.  Therefore in practice packaging designers generally use experience and judgement to propose the most suitable design and specification for a new product pack and then trial samples can undergo practical stacking, drop testing and other practical simulations of real life potential damage situations rather than a specific crush test that will only measures a single and limited aspect of protection. Any necessary refinement of the initial designs and material specification are best made as a result of realistic storage and handling simulation testing and product transit trials.

Smithpack Technical and Design Team

Smithpack’s Technical and Design team have a wealth of experience to expertly assess and propose solutions for complex packaging projects. CAD software with integrated computerised sample making equipment is used to produce design samples for customer evaluation and trials.

Equally they have the knowledge to propose practical alterations to existing packaging, be that protective inserts to protect vulnerable product damage points, or a better combination of pack design and board specification to improve protection, reduce material costs or help rationise the range of packaging required.