Manufacture of Corrugated Board
Corrugated board is manufactured on large high-precision machinery lines called corrugators, usually running at about 500 feet per minute (2.5 m/s) or more. These machines, over time, have become very complex with the objective of avoiding some common problems in corrugated board production, such as warp and washboarding.
The key raw material in corrugating is paper, different grades for each layer making up the corrugated box. Due to supply chain and scale considerations, paper is produced in separate plants called paper mills. Most corrugating plants keep an inventory of paper reels.
In the classical corrugator, the paper is softened with high-pressure steam. After the board is formed it is dried in the so-called dry-end. Here the newly formed corrugated board is heated from the bottom by hot plates. On the top, various pressures are applied by a load system on the belt.
The corrugated medium is often 0.026 pounds per square foot (0.13 kg/m²) basis weight in the U.S.; in the UK, a 90 grams per square metre (0.018 lb/sq ft) fluting paper is common. At the single-facer, it is heated, moistened, and formed into a fluted pattern on geared wheels. This is joined to a flat linerboard with a starch based adhesive to form single face board. At the double-backer, a second flat linerboard is adhered to the other side of the fluted medium to form single wall corrugated board. Linerboards are test liners (recycled paper) or kraft paperboard (of various grades). The liner may be bleached white, mottled white, colored, or preprinted.
Common flute sizes are "A", "B", "C", "E" and "F" or microflute. The letter designation relates to the order that the flutes were invented, not the relative sizes. Flute size refers to the number of flutes per linear foot, although the actual flute dimensions for different corrugator manufacturers may vary slightly. Measuring the number of flutes per linear foot is a more reliable method of identifying flute size than measuring board thickness, which can vary due to manufacturing conditions. The most common flute size in corrugated boxes is "C" flute.
Standard US Corrugated Flutes
| Flute Designation | Flutes per linear foot | Flute thickness (in) | Flutes per linear meter | Flute thickness (mm) |
|---|---|---|---|---|
| A flute | 33 +/− 3 | 3/16 | 108 +/− 10 | 4.8 |
| B flute | 47 +/− 3 | 1/8 | 154 +/− 10 | 3.2 |
| C flute | 39 +/− 3 | 5/32 | 128 +/− 10 | 4.0 |
| E flute | 90 +/− 4 | 1/16 | 295 +/− 13 | 1.6 |
| F flute | 128 +/− 4 | 1/32 | 420 +/− 13 | 0.8 |
Corrugated fiberboard can be specified by the construction (single face, singlewall, doublewall, etc.), flute size, burst strength, edge crush strength, flat crush, basis weights of components (pounds per thousand square feet, grams per square meter, etc.), surface treatments and coatings, etc. TAPPI and ASTM test methods for these are standardized.
The choice of corrugated medium, flute size, combining adhesive, and linerboards can be varied to engineer a corrugated board with specific properties to match a wide variety of potential uses. Double and triple-wall corrugated board is also produced for high stacking strength and puncture resistance.
Most corrugators are two knife corrugators, which means that they can produce two different sheet lengths side-by-side. This leads to an optimisation problem, known as the cutting stock problem.
Read more about this topic: Corrugated Fiberboard
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