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What is Structural Foam Molding?

Sometimes, you are required to manufacture parts of a size larger than can be produced using traditional materials. How can these parts be created without sacrificing strength, stiffness and overall quality? Luckily, there is a simple, affordable solution: structural foam.

Structural foam is a manufacturing material which pairs a foamed core with a solid outer “skin,” rather than remaining solid all the way through. It is lightweight and flexible while also maintaining the strength and stiffness associated with traditional polymers such as polyurethane, polyester or epoxy. The ability of structural foam to produce parts of unprecedented size has led to revolutions in the production of vehicle roofs, interior and exterior body panels, housing for medical equipment, and even skis.

What is Structural Foam?

Structural foam is a composite material produced when a polymer, usually thermoplastic, is combined with either an inert physical gas, such as nitrogen, or a chemical blowing agent during the molding process. The result is not a solid material but a low-density, microcellular “core” combined with a high density outer “skin”. The core decreases the overall weight of the material while the solid skin allows it to remain strong and impact resistant.

The structure of the material is like that of a sandwich, with the low density core completely surrounded by the higher density skin. The texture of the core has been frequently described as resembling a “sponge” or a “honeycomb”. Because of its unique core texture, structural foam is typically anywhere from 20% to 40% lighter than solid polymers, aluminum, steel, sheet molding compound or other commonly used materials.

Due to its not quite solid nature, the base material used in the creation of structural foam is typically a thermoplastic polymer rather than a thermoset. Commonly used thermoplastics include but are not limited to: polyurethane, polycarbonate, polyphenylene oxide (Noryl), polybutylene terephthalate (Valox), and acrylonitrile butadiene styrene.

The Structural Foam Molding Process

The process of molding parts from structural foam is very similar to the traditional method of reaction injection molding. Two components, for example polyol and isocyanate (which are combined in order to create polyurethane) are stored in liquid form in separate containers. They are mixed together to form a resin which is injected into a pre-prepared mold and cures via the means of a chemical reaction.

However, there is one important difference necessary for the creation of structural foam rather than a solid polymer. In addition to the polyol and isocyanate, an inert gas or chemical blowing agent is added to the mix during the injection process. This changes the nature of the chemical reaction which ultimately occurs.

Less polyol and isocyanate are used than in standard reaction injection molding, so that the resin does not completely fill the mold. Instead, the gas or blowing agent is activated by the reaction between the two components. It expands, filling the empty space of the mold with foam. This creates the “honeycomb” texture for which the interior core of structural foam is known. Surface cells collapse when they come in contact with the walls of the mold, resulting in the creation of the solid skin which surrounds the outside of the material.

A subtype of structural foam molding known as gas counter pressure molding also exists. In this process, the mold is pressurized prior to the injection of the components and gas or blowing agent. This creates a flat, dimensionally stable product with a high gloss, textured finish. It is often used for products which do not need to be painted or require very specific textures.

Benefits of Structural Foam Molding

The use of structural foam as a manufacturing material comes with a large number of advantages and almost nonexistent drawbacks. Of course, structural foam initially became known for its ability to create parts of unparalleled size, such as the roofs or body panels of vehicles. However, it soon became apparent that structural foam promised a number of additional benefits.

Structural foam parts are extremely lightweight – typical weights vary from 55 pounds per cubic foot to a mere 2 pounds per cubic foot. In addition to this decrease in weight, structural foam is strong, durable, and extremely stiff – in fact, up to 8 times stiffer than similar materials such as solid polymers. The modulus of the material increases along with its density, up to a fantastic 250,000 psi.

Structural foam is also highly moldable; it is able to produce variant wall thicknesses across a single part. In some cases, structural foam has even been able to generate wall thicknesses greater than the previous maximum of ½ inch (13mm). It also allows for thick ribs and bosses and reduces the risk of “sinks” over ribs or heavy cross-sections due to the lower viscosity of the mixture. Structural foam products experience less stresses overall during the molding process, and are highly resistant to warpage or deformation.

In addition, structural foam retains the impact, element and temperature resistance common to thermoplastic and thermoset polymers. It experiences minimal to no thermal expansion, making it perfect for products which will be utilized in a wide variety of climates or temperatures. The acoustic and sound dampening properties of structural foam are also superior to those of standard solid polymers.

Structural foam molding is also a highly cost-efficient process. Lower pressures and clamping forces are required during the injection and curing periods, allowing the tooling and molds to be made from lower cost materials such as aluminum. Cycle times are competitive with those of injection and reaction injection molding, making this a time-saving process as well as a money-saving one.

Structural foam products have a lower than average risk of damage in the mold or during the removal process, making it a highly efficient and easily repeatable method. In addition, structural foam produces no potentially harmful styrene fumes, lowering the health risks for workers involved in the molding process.

Lastly, structural foam also features highly paintable, aesthetically pleasing surfaces with a distinctive “swirly” appearance. In-mold painting (IMP) is possible for all structural foam products, further saving time and money. Multiple colors can also be applied to a single part in-mold. The surface is easy to clean and can be sawn, screwed, nailed or stapled without experiencing any problems. Structural foam is even more environmentally friendly than similar materials, as it is highly recyclable and reusable.

If you need large parts manufactured without sacrificing quality, structural foam is the way to go. Contact Romeo RIM today to learn more about this amazing material and its myriad benefits!