Foamed Molding Process

The foamed molding process, also known as the secondary foaming process, is characterized by the addition of low-boiling-point solvents or liquid CO₂ into the foaming materials as a source of foaming gas. In earlier times, CFC-12 (dichlorodifluoromethane) was used, but since CFCs have been banned, environmentally friendly solvents such as HFC-134a are now utilized. The foamed molding process has the following features:

Low-Boiling-Point Blowing Agents: The blowing agents have low boiling points and exist as gases at normal pressure. To keep the blowing agents in liquid form for precise metering in the material tank, the equipment operates under pressure before the mixing head. When the material is ejected from the mixing head, the pressure drops suddenly to normal atmospheric pressure. This causes the low-boiling-point blowing agents to vaporize first, forming a foam-like material — the primary foaming stage, with a foam expansion ratio of 3 to 11 times.

Secondary Foaming: After the material is sprayed onto the surface, the chemical reaction generates heat, vaporizing components like HCFC-141b or cyclopentane or producing CO₂, leading to secondary foaming. The expansion ratio during this stage ranges from 3 to 6 times. After foaming is complete and curing occurs, the product is formed.

1．Comparison to Conventional Foaming

Compared with conventional foaming processes, the foamed molding process offers the following advantages:

• Higher Total Expansion Ratio: This results in lower product density, which can be adjusted by changing the ratio of HFC-134a, HCFC-141b, cyclopentane, or water in the formulation.
• Improved Compression Strength Uniformity: Foam products produced with this process exhibit smaller differences in compressive strength between the longitudinal and transverse directions.
• Density Consistency: The difference between the core density and the skin density of the foam product is smaller.
• Lower Mold Pressure Requirements: This process places less demand on the pressure resistance of molds.
• Stable Process Performance: The process maintains consistent operational stability.

2．Process Details

During the foaming process:

• Polyether Polyol, Surfactant, Amine Catalyst, and Water are combined as one component (at ~40°C) and pumped into the mixing head using a gear pump.
• Organotin Catalyst and Polyether Polyol form another component and are continuously fed into the mixing head via a flow meter (at ~21–32°C).
• TDI and Low-Boiling-Point Blowing Agents are fed as separate components into the mixing head using a pump or nitrogen pressure through a flow meter.

All materials are rapidly mixed in the mixing head and sprayed onto the working surface. The pressure inside the mixing head is controlled at 345–689 kPa to keep the low-boiling-point blowing agents in a liquid state. After two stages of foaming and curing, the product is complete.

3．Typical Formulations, Process Conditions, and Product Properties