Types of Corrugated Flute

Types of Flute, Board and Box - From A to Z

Corrugated board is produced commercially from a variety of combinations of fluting medium that is corrugated and combined with flat liners.

Single Face Board. This is a speciality of only a few plants. It is produced in reels with a re-winder either in the corrugator wet end line or as a specialist single facer group and re-winder off-line. The material is flexible in one direction, stiff in the other and it has cushioning properties, which gave rise to its early uses in stiffening top hats and protecting glass lamps and jars or ceramics. To this day it is used as a primary protection for fragile items as well as books for postal delivery, furniture and white goods. Single face had a very healthy market for light bulb and tube packaging throughout the mid 20th century. Equipment to produce single face board has been basic and crude until the arrival of the first fully automatic continuous slitter and rewinder for single face web, which runs up to 200 metres per minute which was developed  in Sweden as late as 1995. Microflute single face is used as fashionable ‘exposed flute’ packaging of cosmetics and gifts which can be printed overall or with patterns and logos.

Conventional Corrugated Boards. Corrugated board is the most widely used packaging medium for boxes, divisions and fittings. It has also been used for specialist applications such as temporary or expedition ‘housing’. It provides a stiff, resilient, recyclable container with cushioning, puncture-resistance, stackability and has a good printing surface, making it suitable for primary, point-of-sale and transit packaging as well as for display stands. It can be specified with various flute forms and paper grades to suit the application. It  can be made waterproof, moisture resistant, suitable for freezing and cold storage. In most regions over 50 per cent of corrugated packaging is used for food and drinks – it also passes hygiene requirements.

Most common single wall board is B flute in the West (A flute in Japan and some other areas of the Far East). Double wall board gives a combination of protection, high stacking strength and good print surface. It was traditionally used for high value bottled drinks such as scotch whisky and gin, when boxes were stacked in the holds of ships for export. Bulk shipping and bottling at destination and the advent of sea containers led to a previously unthinkable downgrading to single wall C flute with heavy liners, partly because it was now sufficient and cheaper, but also it took up less space and increased the number of cases that fitted in a container. However, double wall has many applications still and represents some 10-12 per cent of board produced.

Triple wall board is ultra heavy-duty and used in the place of timber where similar strength is required together with a significant reduction in weight. In many applications, such as military ordnance, triple wall board is used in combination with timber. It is probably true to say that most triple wall board is shipped as sheet board to specialist converters. There is even a company in France producing quad board four ply. Triwall was a registered name for a particular patented triple wall board with three layers of A flute, giving some 14mm caliper board. It was one of very few patents to hold in the box industry and single companies in various countries were licensed to make and market Triwall. Using dual- or multi-flute single facers and the wide variety of papers available, there are literally thousands of combination possibilities in corrugated board from which to choose to suit any application.

Synchro-flute is a specialist form of double wall board, also produced on a normal corrugator with special control and tensioning equipment added. It requires two identical flutes produced by two conventional single facers and the means to ensure continuous accurate positioning of the flute tips of the bottom single face web with the valley tips of the top web. For the same materials this gives a substantial increase in flat crush and edge crush figures due to the structural strength and improved adhesion in the double facer. With water resistant adhesive, it is particularly advantageous for cold storage, where it keeps its strength even after being frozen. Amcor in Australia has developed Xitex, a synchro-flute without a centre liner. This has to be produced on a specialist single facer designed for the purpose but otherwise has the same advantages as already mentioned plus saving the paper of the centre liner.

Flute Forms
The table below shows the flute forms available as well as their height, pitch and take-up ratio (the length of fluting medium required to produce a unit length of board, otherwise known as draw factor). The figures can vary with different manufacturers or for different applications or requirements, so those shown should be considered as being for guidance only. When referencing the list below, users should bear in mind that our new range of corrugated, 'Rigid Performance Grade' is found in a range between B flute and E flute.  

Range of Flutes Available
There are certain criteria in selecting a flute and a paper. The following offers some basic guidelines in flute selection, remembering that fine flutes have the smallest height and pitch, coarse flutes have greater flute height and pitch.
Flat crush test (FCT) is basically a measure of the ability of the board to resist forces tending to crush the flutes flat either during manufacture or in service.  It is important in holding the liners apart because the compression strength or stacking strength of the box is related to the distance the liners are apart, the flutes always being vertical where stacking strength is required.
Coarser flutes give higher ECT (edge crush test) figures, stacking strength and cushioning properties than finer flutes for the same papers.
The finer the flute, the better the quality of the surface for printing. We have all seen the flute lines in print patterns on corrugated cases, where the impression is heavier where the liner is supported by the flute tip and lighter where the liner is unsupported between tips. The effect is noticed more as the flute becomes coarser. It can be compensated to a degree by using a heavier liner, but the ultimate printing surface is solid cartonboard. The very fine flutes get closer to this ideal.
Double and triple wall cases therefore normally have the finer flute (B or E) on the outside for its printing benefits and the coarser flute (A, C or coarser) on the inside for their cushioning of the contents.
In addition to straight flutes, some specialist variations have been developed. One such is the ‘wave’ or ‘snake’ flute which, as its name suggests, varies from the straight line in a shallow wave form. This offers more design flexibility for special boxes, giving some stacking strength where flutes need to run ‘horizontally’. Corrugating rolls are more costly of course.
There are also special designs of form used with glacine pads used in cartons to cushion chocolates and upmarket biscuits, but these are a different application from the main subject of this book.

Board and Case Performance
Details of methods of testing board are covered in a later chapter (Ancillary Processes and Testing Equipment and Services), but whereas flat crush and bursting strength were the accepted major tests of board quality in the 20th century, the value of edge crush strength and bending stiffness are more to the fore today. Virgin paper producing countries in Scandinavia and North America had an interest in promoting Kraftliner and Semichem fluting papers and these materials of course gave good flat crush and burst strengths.

However, with the growing use and availability of recycled papers and recognition of the environmental benefits of its proliferation, a more rational engineering approach to corrugated board and the box have emerged. Thus it is that board performance parameters better related to the requirements of the case, economy and avoidance of over-packaging are now being adopted as more realistic criteria. Hence the increasing emphasis on edge crush strength and bending stiffness, which are the important factors wherever warehousing and transport are involved. The edge crush value is quite closely related to the square of the finished board caliper for given paper grades. Thus, if a single wall B flute board of a given make-up is 3mm and the equivalent C flute board is 4mm, then the edge crush strength ratio will be 16/9 in favour of the C flute as an approximate guide.

The objective of the box maker is to provide the most economical case fit for the application. Given that the cost of the paper represents more than half the selling price of the case, light-weighting is the easiest way to reduce its cost. There are of course limits on how far basis weight can be reduced and it depends very much on the application. For example, light contents such as savoury snacks will not require great stacking strength. Flexible heavy goods such as bags of ground coffee will need a higher stacking strength case if they are to be piled high in the warehouse. Bottles and cans lend their own strength to the filled case in terms of its stackability, so economies can be made in edge crush. But in the case of cans in this last example, the limitations in how light you can go may well be determined by the flat crush, as the cans may need to be swept mechanically on to the base, which must be strong enough to support this action without collapsing or tearing.

Double wall board is, of course stronger, but the thicker the board, the less the number of cases you can fit onto a pallet or into a container. So it becomes clear that any number of considerations have to be taken into account in selecting the material combination for the case. Performance of the case should be the basis on which it is sold and bought. The traditional attention to paper grades should be dropped, as it ignores the production ability of the supplier. Of course newer or better-maintained and operated machines make stronger board; sharper knives and scores make for stronger finished boxes, all regardless of the paper. Nonetheless, realistic basic standards are an important precursor to adopting a performance-based approach to the market. Standards have been set out for many years by FEFCO in Europe and TAPPI in the USA, but some of these are dated and there are moves under way as we write to review or replace some of these by regarding corrugated board as an ‘engineering’ material. The BSFV Institut in Hamburg is carrying out some interesting work in this area in liaison with major industry players.

International Fibreboard Case Codes
This Code has been developed by FEFCO and ASSCO as an official system to substitute long and complicated verbal descriptions of fibreboard case and packaging constructions with simple symbols internationally understood by all, regardless of language and other differences. With kind permission from FEFCO the specifications of each code are reproduced in the Appendix of this book.

Extract from 'Corrugated Industry - in Pursuit of Excellence' by Tony Pinnington
Reproduced with permission from Brunton Business Publications Ltd.