If you have ever worked with fiber cement board production, you know that the smooth operation of the entire line-and the flatness and density of the boards-depend not on the cutting or curing stages at the back end, but on two key components in the middle: the forming cylinder and the vacuum box.
The forming cylinder looks like a giant steel roller with a precision-ground surface. However, its function goes far beyond simply flattening the slurry.
When the mixture of fiber, cement, sand, and water flows from the headbox onto the bottom conveyor belt, it is in a semi-fluid state. At this stage, the fibers are randomly distributed-thicker in some areas and thinner in others. The forming cylinder uses precisely controlled linear pressure to "roll" the slurry into a board with uniform thickness.
It is important to note that this is not heavy pressing. Excessive pressure can cause delamination, while insufficient pressure leads to low density. The hydraulic system of the forming cylinder automatically adjusts according to the feed rate to maintain a dynamic balance.
In our exported equipment, the forming cylinder is equipped with independent pressure-regulating valves at both ends. This is critical because the slurry may be unevenly distributed across the conveyor belt. Independent adjustment ensures a truly flat slab.
In addition, the surface material of the forming cylinder directly affects slab quality. Ordinary carbon steel wears over time and can leave dark marks on the slab surface. For high-end production lines, we use hard chrome-plated or rubber-coated forming cylinders, which are more wear-resistant and less prone to sticking.
Vacuum Box: Removing Excess Water
While forming into slabs, they are shaped by passing through a cylindrical container; however, because the slabs still contain a significant amount of free water, if that free water enters into the curing kiln then they may either crack during curing or not be able to attain the required strength.
This is where the vacuum box plays its role. It is installed behind the forming cylinder, beneath the conveyor belt, and positioned close to it. A system of vacuum pumps generates negative pressure, drawing excess water out of the slab.
The principle is straightforward: the upper surface of the vacuum box consists of a grooved stainless steel panel. As the slab passes over it, water is pulled downward by suction, passes through the conveyor belt into the vacuum box, and is then discharged.
However, there is a critical operational detail-the vacuum level should not be uniform.
Based on our testing, the first section of the vacuum chamber (closest to the forming cylinder) should operate at a lower vacuum level to remove surface free water. The vacuum level should then gradually increase in subsequent sections to extract deeper capillary water. If the sequence is reversed, or if the vacuum level remains constant throughout, the board may either appear dry on the surface while remaining wet inside, or develop cracking due to excessive suction.
Another commonly overlooked issue is the sealing strip of the vacuum chamber. If the sealing strip is worn, the vacuum level cannot be maintained, regardless of pump performance. Based on our experience, rubber sealing strips should be inspected every two months, while high-molecular-weight polyethylene sealing strips can last more than six months. For customers in South America and Southeast Asia, we generally recommend the latter, as it performs better in high-temperature and high-humidity environments.
The True "Core Brain": Coordinated Operation
The combination of these two components is the true "brain" of the production line.
The forming cylinder is responsible for shaping, while the vacuum box handles dewatering. However, stable board quality depends on the precise coordination between these two processes.
The slurry first passes through the forming cylinder, where it is compacted to a uniform thickness. It then enters the vacuum box, where moisture is removed, transforming the slab from "soft and wet" to "firm and wet," giving it initial structural strength.
If vacuum is applied too early, water is removed before proper compaction, causing fibers to shift with the water flow. If it is applied too late, the structure formed by the cylinder may be weakened by excess moisture.
This coordination is one of the most time-consuming aspects of production line commissioning. The pressure curve of the forming cylinder and the segmented vacuum levels must be adjusted according to the customer's raw materials-finer sand and longer fibers require different parameters. This is why we insist on remote commissioning or sending technicians on-site.
Routine Maintenance Is Simple
Many customers worry about the complexity of the equipment. In reality, routine maintenance of the forming cylinder and vacuum box is quite straightforward:
Cylinder Formation: Inspect both ends of the cylinder bearing for temperature daily and evaluate weekly for scratches.
Vacuum box: Before startup, ensure the sealing strip is not deformed; after shutdown, flush the interior of the chamber to prevent cement slurry from clogging the suction ports.
If these steps are followed, both components can operate reliably for eight to ten years without major issues.