Cartonboard manufacturing processes

The Wet End

This is where the actual cartonboard manufacturing process begins. The fibre stock flows out evenly onto a continuous moving plastic mesh to form the first layer (ply). Gravity drains the excess water away. Further along the machine, additional layers are placed on top of the previous ply. Excess water is sucked upwards from the top of each new ply. Extracting the water assists in bonding the fibres within each ply and from layer to layer.

In 2004 Amcor invested $55 million in a state of the art Multifourdrinier forming section resulting in improved sheet squareness and an overall appearance improvement.

Press

The four layers of fibre are sandwiched together to promote consolidation of the sheet. In the pressing process the wet web, as it is now called, is fed through a series of roll presses. The web is squeezed between synthetic felts, which transfers excess water by absorption from the web to the felt, further reducing the moisture content of the web from 80% to 50%. Water is squeezed and sucked from the continuous felt before it re-enters the press for another cycle.

Dryers

The still very moist web then winds over 72 polished, steam heated cast iron cylinders. The surface temperature of the cylinders reaches 140 degrees Celsius. An open weave synthetic fabric presses the web against the cylinders and the remaining moisture evaporates away. The dried web is now recognisable as board. Prior to coating the board passes through a computer controlled, single nip hard calender to ensure uniform thickness across the web.

Coating

To compete with the world's best coated recycled cartonboard, Amcor invested $27 million in 1993 to upgrade the coating capabilities of Petrie's board machine. The board continues on from the dryers through various coating processes where pigmented liquid, similar to water based paint, is applied to improve the surface properties of the board for both visual appearance and reproduction on a printing press.

A pre-coat, similar to an undercoat, is applied using a blade to scrape away excess coating. This initial coating is primarily to improve smoothness and to prepare the board for subsequent coating.

The board then proceeds through the first soft calender which 'hot irons' it to further enhance surface smoothness.

A coat application that is consistent is best for visual aspects, such as opacity and brightness. This is achieved using an air knife where a roll applies the second coating and - as the name suggests - air from a very narrow slit in a tube blows off surplus coating. Hot air hoods are used for drying after each coat application.

Coating the back surface of the board improves stiffness and on some grades a different coating is used for improved printability.

The final blade coating is applied before the cartonboard passes through another soft calender to give extra gloss (particularly print gloss) and smoothness. Finally, the cartonboard is wound to form a jumbo machine roll.

The board machine at Petrie operates 24 hours a day, seven days a week and produces a 15 tonne jumbo roll about every 40 minutes.