There is one time Warp Drive is not so cool, that's when the warp is driven your scrap rate up and reducing production. Warp is caused by a moisture imbalance between the top and bottom liners that make up the corrugated board. Applying the heat at different stages in the process, has become a fine art to cure or dry bonding agents to the paper surfaces. The paint brush is electronic controls and other technologies to maintain the delicate balance needed for a warp free corrugated sheet.
- The steam aspects of the Pre-heater provide moister that improve warp control.
- Web tension required to drag the newly-formed corrugated board can be as large as 3000 pounds.
- Thin film of steam created by hot plates "floats" the web in a nearly frictionless manner.
- The main reason for warp is often irregular moisture profiles in the liner webs.
- Complementary profile dryers can balance out the moisture profiles in the liner webs.
- Moisture migration from the outside liner to the inside liner and imbalance causes warp.
- Lifting of the weight rolls in response to machine speed is important to control warp.
- Excess starch increases the overall moisture content of one side of the board, increases warp.
Pre-heaters designed to use both high pressure and low pressure superheated steam. Friction and web tension are not a concern as the paper is heated by the hot steam film. The adjustable paper moisture remains the same (balanced) at around 212°F, at varying speeds The steam aspects of the Pre-heater provide moister that improve warp control.
United-container has an excellent designed pre-heater with great benefits. In order to create adequate heat transfer and bonding, a beltless double facer's fixed weight blanket needs to apply substantial downward force. United's process studies show that the overall web tension required to drag the newly-formed corrugated board out of the heating section can be as large as 3000 pounds. This extreme, high tension results in excessive waste due to frequent web breaks and end-to-end tension warp. It also creates relative motion at the fragile glue joint, weakening the bond and causing de-lamination.
However, the thin film of steam created by the Tempest's InFusion hot plates "floats" the web in a nearly frictionless manner over the hot plate surface - thus dramatically reducing the brutal tension required to transport the web through the heating section. The result is reduced incidence of web breaks, de-lamination and warp commonly experienced with other machines.
There are several reasons for warp, but the main reason is often irregular moisture profiles in the liner webs. By controlled zone drying across the web width, moisture profiles can be corrected and an acceptable product produced. Installation of a complementary profile dryer to the traditional triplex steam cylinders on the corrugator machine can balance out the moisture profiles in the liner webs. With precise controlled drying zones, a profile dryer can give better warp control while allowing higher operating speeds. The principle is that less moisture migration from the outside liner to the inside liner that is in contact with the belt. IRT-System for cross directional moisture control is a system worth checking into. The drying power can be adjusted precisely, matching the highest to lowest variations in moisture levels.
For optimum warp and heat control, lifting of the weight rolls in response to machine speed. This achieves an essentially constant heat exposure of the combined board as it passes through the double facer. To accommodate various board grades and moisture conditions, the time of heating exposure can be adjusted by selecting any one of a series of computer controlled weight vs. machine speed curves with the United-Container design. United allows adjustments to be made by the operator at the control console or remotely when the Allen Bradley PLC is interfaced with a United wet end control system or other host computer.
The TAPPI organization offers a unique manual with information on recognizing and correcting quality defects in combined board. (www.tappi.org/pressorder)
An excerpt from this manual is as follows…
"If the applicator roll picks up too much starch and applies more than is needed for a good bond, (or applies it to the flanks as well as the tips of the flutes as was often the case with earlier generation glue machines) the excess starch increases the overall moisture content of one side of the board. This moisture imbalance is a major cause of warp.
Modern fingerless single facers have reduced single facer starch consumption significantly. This change in starch application must be balanced by a similar reduction in starch applied to the bottom liner to prevent warp."