What is the Flat-Top Block Foaming Process?

Flat-top block foaming is a commonly used method in block foaming processes. Typically, the foam products produced through manual or mechanical block foaming have a rounded, dome-shaped top. This is primarily due to the surface tension of the foaming slurry itself and the resistance at the edges of the foam material caused by the contact with liner paper.

The domed shape increases the edge waste during the cutting of block foam products. In conventional large-scale block foaming machines, the waste edge rate can sometimes reach as high as 15% (relative to the finished product), leading to significant material loss.

To reduce edge waste and improve yield, extensive research has been conducted, focusing on minimizing the bulging top of the foam to produce block foam products with a regular rectangular cross-section. Currently, there are several flat-top foaming processes, including:

① Edge Sliding Foaming Method (Draka/Detzetakis Method)

② Top Control Foaming Method (Hennecke Method)

③ Downward Movement Foaming Method (Maxfoam Method)

Among these, the downward movement foaming method is the most ideal, reducing the edge waste rate to as low as 8%. Countries like the United States and Japan have adopted European technology and begun production. Below is a brief overview of these three methods:

1. Edge Sliding Foaming Method (Draka Method)

In this method, a layer of liner paper that can slide upward is added to the inner sides of the mold cavity during foaming. The liner paper moves upward as the foam rises, reducing or eliminating resistance on both sides of the foaming slurry. This ensures that the height of the foam at the edges matches the center, resulting in foam products with a rectangular cross-section.

2. Top Control Foaming Method (Hennecke Method)

This method involves placing a thin plate on top of the foam during the foaming process. As the foam rises, the plate moves upward with it and stops at a certain height, forcing the material to flow toward the edges of the block. This results in a flat foam top.

3. Downward Movement Foaming Method (Maxfoam Method or Foamax Method)

This is the most popular flat-top foaming process in recent years. The process involves pouring foam slurry into a rectangular mold cavity. When the foam reaches approximately 30% (0.3H) of the total height, the bottom plate of the mold is lowered, allowing the remaining 70% (0.7H) of the foam height to expand downward. This ultimately produces block foam with a rectangular cross-section.

The industrial production equipment for this method is designed based on the above principle. Compared to conventional block foaming machines, the downward movement foaming machine has the following advantages:

① Rectangular Cross-Section: Eliminates edge waste caused by the domed foam top, significantly increasing yield. For example, conventional block foaming waste is approximately 13.5%, edge sliding foaming is 12%, while the Foamax method is only 8%.

② Reduced Substrate Waste: Since the expansion begins within a trough, the liner paper absorbs fewer chemicals, and the foam at the bottom experiences less collapse. The bottom has only a thin skin layer, saving raw materials and reducing waste.

③ Uniform Density and Hardness: The density gradient and hardness variation across the product cross-section are minimal.

④ Lower Machine Height: With 70% of the foam height expanding downward, the Foamax machine height is only 4 meters, compared to 6–8 meters for conventional machines, significantly lowering plant height requirements.

⑤ Higher Foam Block Height: At a relatively low material loading rate, the Foamax machine can produce foam blocks up to 1.2 meters high, while conventional machines achieve only 1 meter.

⑥ Reduced CO2 and Agent Loss: The process minimizes CO2 and foaming agent losses, enabling lower usage of TDI, foaming agents, catalysts, and silicone foam stabilizers, reducing costs.

⑦ Simplified Equipment: The Foamax machine does not require a moving mixing head, eliminating the need for flexible hoses and reducing mechanical and control system failures.

⑧ Improved Worker Safety: The lower machine height allows for better enclosure, facilitating ventilation and improving worker safety.

Disadvantage: The product width is less adjustable in the Foamax machine. However, modifications to conventional foaming machines can allow for adjustable widths at the cost of a higher edge waste rate compared to the Foamax method.