Combined corrugated board is made of liner-board, or liners, on the outer surfaces, and corrugated medium in the middle. At the heart of a corrugator are two corrugator rolls. These gear-like rollers press flutes into the softened medium fed between them. There are five basic flute sizes- A.. B.. C.. E.. & F.   A-FLUTE A-flute, the original flute, is the highest flute size, and therefore, when combined with an inner and outer facing, is the thickest. With 36 flutes to the foot, A-flute makes the most of corrugated's cushioning and stacking properties for fragile and delicate items. Because A-flute offers excellent stiffness qualities and short column crush resistance, it has applications across a broad range of customer uses.   B-FLUTE B-flute, the second flute size adopted by the corrugated industry, has lower arch heights than A and more flutes (50) per foot. This means that the medium contacts and supports the liners at a greater number of points, providing a stiff, flat surface for high quality printing and die cutting and with excellent crush resistant properties. B-flute is also preferred for high speed, automatic packing lines and for pads, dividers, partitions and other forms of inner packing. Complex die cuts and beverage trays are excellent applications for B-flute as are can cases, wrap-around blanks, glass-to-glass packs and slip sheets. B-flute is generally combined with light weight liners but can be used with heavier facings if the need arises.    C-FLUTE C-flute came along next to split the difference between A- and B-flute. With 42 flutes per foot, it's thinner than A-flute, thicker than B, and offers good cushioning, stacking and printing properties. C-flute is by far the most widely used flute size. An estimated 80% of today's corrugated containers are made of C-flute board.   E-FLUTE E-flute has the greatest number of flutes per foot at 94 which gives it the greatest crush resistance and the flattest surface for high quality printing applications. The thin board profile of E-flute (it is one-fourth the thickness of C-flute) reduces box size and saves storage space. Because of its thin profile and excellent cushioning properties, E-flute can often substitute for conventional folding cartons or solid fiber containers. Examples of E-flute applications include boxes for cosmetics, fragile glass and ceramic items and delicate instruments. Another growing end-use is for pizza boxes where the retailer wants a cost effective container with good graphics and excellent product protection.   F-FLUTE F-flute, the newest flute, is just a little more than half the thickness of E-flute and is the newest growth segment in the corrugated industry. The idea behind the new flute, originally developed in Europe, is to make packages with lower fibre content. With F-flute, converters can reduce the total amount of fibre in the packaging, thereby creating a more rigid box with less solid waste going into landfills. In Europe, F-flute is being used for specialty packaging, point-of-purchase displays, jewelry and cosmetic packages and shoe boxes. In the U.S., the McDonald's Big Mac clamshell in F-flute has received great attention. Dairy Queen, too, is using the F-flute clamshell for its "Ultimate sandwich" and its hot dogs.   KRAFT, MOTTLED WHITE , BLEACHED WHITE The most obvious difference among linerboards is color. The three basic colors are kraft, mottled white, and bleached white. Less apparent, but very important, are differences in grades.   BASIS WEIGHT A key defining property of linerboard and medium grades. Basis Weight is expressed in pounds per thousand square feet, which explains why huge rolls of paper weighing several tons are called "42-pound" or "69-pound" or "26-pound."   MULLEN TEST The traditional practice in making corrugated packaging is to combine different basis weights of liner and medium to meet a Mullen or bursting test that will handle the job of containment. While the practice of combining different liners and mediums continues, increasingly, attention is turning away from basis weights and bursting strength ... to a measurement of packaging performance called ECT.   EDGE CRUSH TEST (ECT) Edge crush test is a measure of combined corrugated board's ability to withstand crushing to its columns.   STACKING STRENGTH ECT has become especially important to many buyers and makers of shipping containers because in their distribution environment, stacking strength is essential. If your concern is for a package that holds up well under a stacking load, your focus may need to be on ECT for stacking strength rather than bursting strength for containability.   HIGHER RING CRUSH LINERS In line with this, you should be aware that many of today's paper mills offer higher ring crush liners. These improved liners are the product of advanced papermaking technologies, and pound for pound, they offer stacking strength that's superior to conventional liners.   RECYCLED LINERS Another concern many buyers have is for the recycled fibre content of the boxes they order. Corrugated does a good job here.   POST-CONSUMER WASTE While some amount of reclaimed fibre is used in virtually all grades of linerboard and medium, recycled liner and recycled medium can actually contain up to 1 00% post-consumer waste. A key source of reclaimed fibre is the corrugated industry itself. Waste paper and corrugated board are routinely recycled from corrugator plants and boxmaking operations. The largest and fastest growing source of reclaimed wood fibre is post-consumer waste: old corrugated containers (OCC).