Foams are cellular structures created by gas bubbles during the polymerization of polyurethane. Flexible polyurethane (PU) foams have limited resistance to applied loads, are permeable to air, and are deformable. There are two main types of flexible foams, hot cure and cold cure, which differ in composition and processing temperature.

Hot cure polyurethane foam is widely used and represents the actual foam composition, while cold cure polyurethane foam has various advantages in processing and properties, such as faster mold time, better moisture tolerance, and greater versatility.

Cold cure foam processing is notable for its low energy consumption (mold temperature ranging from 30 to 65 degrees Celsius) compared to hot cure foam (mold temperature ranging from 30 to 250 degrees Celsius). Another advantage is the high variety of soft materials for processing at low temperatures. Cold cure foam is made based on diphenylmethane diisocyanate (MDI), while hot cure foam is based on toluene diisocyanate (TDI).

Cold Cure Polyurethane Foam Production Methods

Producing molded foams is a common technology, and the variety of molded foam formulas is at least as diverse as different types of stones. The concept of producing molded foam is simple: foam components are mixed and injected or poured into a pre-made mold that fills the foam during formation. The foam has a specific residence time and is removed from the mold after an appropriate period of residence.

After removal from the mold, the foam is set aside to fully cure. As commonly practiced in the industry, molded foam technology has been highly optimized in terms of processing repeatability and properties. Unlike producing block foams, where pieces need to be cut from a block, molded foams incorporate the final part as part of the foaming process.

While this method reduces waste rates and eliminates the need for cutting operations for finished parts, the operational capacity of a molded foam factory is lower and requires the presence of molds, which can be costly. In typical molded foam operations, molds are placed on rotating or oval equipment that rotates in a way that the foam residence time aligns with the time needed for rotation and refilling of the mold.

In addition to operational details, manufacturers choose molded foams due to their ability to produce complex shapes and mold special objects beyond typical parts, such as functional components, decorative skins, or reinforcing elements. This structure is designed to provide the correct appearance, performance, and shape as a composite part.

Reactive components of isocyanates (polyols, chain extenders, surfactants, catalysts, water, etc.) are placed in a common tank or separate tanks and then mixed with isocyanates in a high-energy mixer. The mixed components are injected into the mold, the mold is closed, and the components are allowed to cure.

Due to the need to optimize the efficiency of producing molded foams while also producing durable, easy-to-process, and versatile foams, the formulation of complex molded foams is intricate and can be described as an art to some extent. The molding speed, pouring/injection, and curing process are all controlled by the selected foam formula. Similarly, operational details can significantly impact the formulation design. For example, if hot cure molds are used, a less reactive system can be used, while a different formula is chosen if the molds are not heated or heated at a lower temperature.

In some cases, a mold is partially filled with a polyurethane formula to create a specific characteristic and then subsequently filled with another formulation to achieve desired properties for the rest of the part production. This flexibility in molded foam operations makes it highly desirable for producing specialized parts but also adds complexity to the art of formulation.

As mentioned earlier, examples of molded foam formulas cover a wide range of options, but the main categories of components used in molded foams are similar to those used in cold block foams. If a manufacturer actively heats their mold by passing it through a hot oven, the resulting foam is called hot molded foam.

In hot foam formulation, polyols with high molecular weight are used, where a three-dimensional polyol is surrounded by isocyanates. A producer may use TDI, a mixture of TDI with polymerized MDI (pMDI). The choice depends on price, manufacturer preferences, and sometimes geographical factors that may favor a specific isocyanate based on a contract.

Cold cure polyurethane molded foams, in contrast to hot cure foams as previously described, represent a new industrial innovation and are produced with a cooler mold and distinct oven temperature. These foams generally exhibit over 60 to 70% flexibility.

Similar to cold block foams, cold molded foams use polyols with higher molecular weight. The lower temperature often results in cold molded foams having a higher number of untouched cell network foam cells. Protocols for mechanically crushing foam after removal from the mold to break cells and increase airflow and foam breathability are often in place. While initially, formulations can be designed to avoid additional crushing operations to ensure completion of the gelation stage. Some foam producers only use pMDI as the isocyanate to produce molded foam.

In fact, the choice of isocyanate type is primarily a preference issue influenced by various factors. However, there are several differences in the properties of foams produced between those made with TDI and pMDI. One difference is that pMDI foams generally cure faster due to the uniform reaction performance of MDI isocyanates (the second reaction of isocyanates in TDI is usually significantly slower than the first). While curing speed is a preferred feature in molded foam operations, a rapid increase in viscosity can interfere with complete mold filling.

However, when formulated effectively, pMDI foams can be made with a shorter cycle. This feature can also allow the producer to lower their mold and oven temperatures to balance the chemical kinetics and process further. The higher molecular volume of pMDI compared to TDI also means that the volume of the resulting hard piece for MDI foam is larger than TDI foam. This different molecular volume leads to MDI-based foams having a potentially higher potential charge in water and isocyanate indexes compared to TDI.

However, it has been noted that foams with higher charge may be destructive in some dynamic properties while showing greater hardness.

The complex aspect of polyurethane foam formulation, as previously mentioned, is the common aspect of using molded foams. This is largely influenced by a rough understanding of the comfort that leads to foam firmness, flexibility, and breathability (related to airflow). Another foam feature dictated to some extent by regional preferences is the extent of resistance change with increasing compression, sometimes referred to as "support" or "comfort," and is quantitatively defined by foam hardness at two different pressures.

Factors that make up foam that a foam designer may consider to meet customer preferences include:

Foam density (affected by water content and percentage of hard parts)

Cell structure and foam openness (partially dictated by surfactants and catalysts)

Type and index of isocyanates

Polyol performance and equivalent weight

Polyol copolymer and solid percentage

Additives for design

Methods of producing cold foam

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