Full-Skin Molded Foaming Process

The molded products of flexible foam plastics typically have thin outer skins and are commonly used for cushions, headrests, etc. These products often require an additional outer covering, such as PVC, to form a surface layer. This makes the process relatively complex, labor-intensive, space-consuming, and less efficient. In recent years, a full-skin molding process has been developed, in which the low-density foam core and high-density smooth skin are formed simultaneously in a single step during molding. This eliminates the need for separately bonding a surface layer, simplifying the process significantly.

Typical Full-Skin Molding Process

A typical production process for full-skin molding includes the following steps:

①　Mold preparation

②　Injection molding

③　Pre-curing

④　Product demolding

⑤　Post-curing

⑥　Product cleaning

⑦　Painting

To achieve molded products with strong, durable skins and excellent appearance, appropriate formulations and molding conditions must be selected.

Key Differences in Full-Skin and Conventional Molding Formulations

The main distinction in the formulation for full-skin molding is the requirement for a smooth, pore-free surface. This prohibits the use of carbon dioxide generated by the reaction of isocyanate and water as a foaming agent. Instead, external foaming agents must be used. However, omitting water from the formulation reduces the presence of urea groups in the molecular chain, leading to decreased chain strength and stiffness. To compensate, aromatic diamines, short-chain diols, or other low-molecular-weight crosslinking agents are typically added to the formulation. Additionally, MDI is often used in place of TDI to increase the proportion of rigid chain segments and improve strength.

Process Considerations

• Mold Temperature Control: Mold temperature is critical. Lower mold temperatures result in higher skin density and thicker skin layers. However, lower mold temperatures also prolong the molding cycle, hinder curing, and negatively affect the compression set performance of the product. To address this, aluminum alloy molds with high thermal conductivity are commonly used, and flash cooling techniques are applied to the heated mold surface to quickly dissipate reaction heat and inhibit surface foaming. Once the skin is formed, the core relies on the material's internal reaction heat for foaming and curing, ensuring both high-quality products and efficient production.
• Release Agent: The choice of release agent significantly affects the smoothness and adhesion of the product's skin. Suitable release agents should ensure easy demolding without compromising the product's affinity for paint. Otherwise, additional cleaning steps may be required during the painting process.
• Material and Mold Temperatures: Mold temperatures between 30–60°C and material temperatures between 20–35°C are generally recommended.
• Filling Ratio: The material filling ratio in full-skin molding is typically higher than in conventional molding processes. Higher filling ratios lead to higher yields but also increase product density and cost. A reasonable balance must be struck.

Advantages of Full-Skin Molding

Compared to conventional molding processes, full-skin molding offers several advantages, including:

• Shorter production cycles
• Higher production output
• Simpler equipment and process
• Lower investment costs
• Higher labor productivity
• Superior product quality

However, the cost of raw materials is slightly higher, as polyurethane skin is more expensive than PVC skin.

Environmental Considerations

With the ban on CFC-11 and related products, environmentally friendly alternatives such as HCFC-141b, pentane (e.g., cyclopentane, isopentane), HFC-245fa, and HFC-365mfc have been adopted as blowing agents for full-skin polyurethane molded products.

• HCFC-141b is considered a transitional substitute and is still permitted in China for a few more years but has been banned internationally since 2003.
• Pentane-based agents are already in use domestically, being zero-DOP products worth promoting. However, safety precautions are necessary to prevent fires and explosions.
• HFC-245fa and HFC-365mfc are effective but costly alternatives.

Considering these factors, the author advocates for the development of water-based blowing agents for full-skin PU products. Although technically challenging, advancements in polyether quality and structure make the development of the next generation of water-based full-skin molded products entirely feasible. Below is a recommended low-VOC water-blown full-skin molded product formulation.

Water-Blown Full-Skin Molding Standard Formulation

• Caradol MC36-09: 100 parts by weight
• MEG: 10 parts by weight
• Polyol Tpo8: 3 parts by weight
• Water: 0.5 parts by weight
• PC CAT NP15: 0.2 parts by weight
• PC CAT T135: 0.06 parts by weight
• Isocyanate Index: 1.05