The combination of paper temperature and humidity is a delicate balance. If this can be accurately managed, well-bonded and flat board can be produced.

The process of converting paper into corrugated board requires careful control of heat and humidity. Let us examine how heat and humidity affect paper. On the single facer, corrugating medium needs to be heated to soften the fibers and improve plasticity, so that corrugation can be formed without brittleness. Using steam spray heads to add moisture often facilitates this process. Liner board also needs to be heated to promote gelatinization of the starch adhesive for strong bonding.

Moisture in the paper enhances its ability to transfer and retain heat. Cellulose (the main component of paper), like air, is an insulator — only the moisture in the paper can absorb heat. To demonstrate this, we can use two cotton towels, one dry and one wet, and place them in a 350°F oven for 10 minutes. The dry towel can be picked up directly by hand, while the wet towel will be too hot to handle. The same principle applies to paper, since cotton is also cellulose.

When the moisture content of paper decreases, the paper shrinks. When humidity increases, it relaxes and expands again, but cannot fully return to its original state. With each cycle, it shrinks a little more, similar to how a new pair of jeans shrinks with each trip through the dryer.

Heating the paper affects the moisture contained within it, and also affects the paper's receptivity to adhesive. When paper is first heated, its pores open and fibers swell, making it more receptive to adhesive. As the temperature rises, the moisture in the paper becomes active. When the paper temperature exceeds the boiling point of water (212°F or 100°C), its moisture evaporates rapidly. Overheated paper loses moisture and cannot accept adhesive penetration, resulting in poor bonding. Therefore, controlling the paper temperature to prevent excessive moisture loss is very important.

Overheated paper loses moisture and cannot accept adhesive penetration, resulting in poor bonding.

The application of heat requires balancing two factors: reaching sufficient temperature to form a bond, while not losing too much moisture. For humidity management, the general rule of thumb is that the moisture content of the board leaving the corrugator should be approximately the same as when it entered (assuming a paper run recovery rate of around 6%–8%).

Using preheaters to manage paper temperature is a dynamic process. As speed increases, the paper spends less time in the preheater, reducing heat transfer, which requires a larger wrap angle to increase heat delivery.

Tension also plays a role in heat transfer. The paper must be pressed firmly against the preheater surface, driving out air beneath the paper for effective heating. Air, acting as an insulator, impedes heat transfer.