From car-body panels and helicopter parts to electrical enclosures and sporting goods, manufacturers increasingly rely on structural-thermoset sheet-molding compound (SMC) and bulk-molding compound (BMC) to address demands for strong, light, and durable materials.
Due to intrinsic limitations with metals and thermoplastics, many engineers are turning to structural-thermoset compounds to bolster strength and corrosion resistance. The strong molecular bonds inherent in thermosets impart a tight web of inner connectivity that lets these materials maintain excellent structural properties despite prolonged exposure to chemical and temperature extremes. The materials also increase design flexibility for engineers and production efficiency for manufacturers. Here’s a closer look.
Structural thermosets are distinguished from standard thermosets by their use of more specialized resins and higher levels of reinforcement — glass, carbon, and aramid fibers, for example. The added reinforcement increases strength and stiffness, while resins protect the fibers and improve the compound’s overall physical properties.
Moreover, manufacturers can tailor these properties by varying ingredients. For example, changing fiber type, length, and mix proportion alters its flow, strength, and rigidity; varying resin concentration and type affects overall strength, along with the compound’s heat or corrosion resistance.
Heating the material as it is molded forms three-dimensional covalent bonds between polymer molecules. This process, known as cross-linking, is irreversible. Therefore, cross-linked materials cannot be melted and reshaped. The term “thermoset” accurately describes this chemistry. Cross-linking creates a rigid 3D molecular structure that lets thermosets maintain the desired physical and electrical properties during prolonged exposure to adverse conditions such as excessive heat. This distinguishes thermosets from thermoplastics, which are generally unsuitable for high-temperature environments because they can be remelted after solidification. Thermosets tolerate heat-distortion temperatures (HDT) and glass-transition temperatures (Tg) that would literally melt most thermoplastics.
Three of the most common thermoset resins are polyester, vinyl ester, and epoxy. Each has its own price and performance characteristics, so base selection on an application’s functional and cost requirements. For example, engineers might choose vinyl-ester resin for corrosion-resistant products, epoxy for high-strength applications, and polyester when good overall performance and cost are the driving factors.
As for reinforcement, many types of fibers can be used depending on the molding process and strength requirements. Glass-reinforcement options include chopped-strand, mat with random-fiber orientation, light textile fabrics, heavy woven materials, knitted materials, and unidirectional fabrics. Carbon-fiber reinforcement is used for applications that require exceptional strength coupled with severe weight restrictions.
Most structural thermosets are in the form of sheet-molding compound. SMC is a cost-effective, lower-weight alternative to many metals. Standard SMC contains 10 to 30% reinforcement, while structural grades are typically in the 40 to 65% range. Reinforcement is normally chopped-strand glass fibers 0.5 to 2.0-in. (12.7 to 50.8-mm) long.
Structural-thermoset SMC is manufactured in a continuous process that combines a viscous paste and glass fiber on specialized equipment with a continuous web. Paste containing the resin and additives is poured onto a carrier film, then cut glass fibers are added, along with a second layer of film. The paste and glass between top and bottom carrier films creates a thin “sandwich” that is run through a series of serpentine rollers. The serpentine action and resulting pressure causes the paste to coat and adhere to the glass fibers. The SMC, typically 12 to 60‑in. wide, is then packaged in continuous lengths on rolls or soft-folded into large, flat containers for curing and handling. The rolls and containers often weigh in excess of 1,000 lb.
The packaged SMC is stored for a specific period (usually 48 hr, depending on the formulation) at a controlled temperature and humidity before it is shipped. This curing or maturation step is critical because material viscosity increases with time. Proper maturation lets the finished SMC product readily peel from the carrier film, making it easier to handle. Because of this, it is important to tightly control the amount of water and chemical thickeners (such as metal oxides, metal hydroxides, and isocyanates) added to the paste during manufacturing.
Maturation is an ongoing chemical reaction, so there is an optimum molding-viscosity window for the best end results. Typically, SMC should be molded within 30 days of manufacture unless it is stored below 75°F. Many molders store structural-thermoset SMC below 32°F to extend its shelf life.
Though it can be used in transfer and injection-molding processes, structural-thermoset SMC is best suited for compression molding. Structural SMC can be molded into complex shapes with little scrap. Ease of handling and sheet size often makes structural-thermoset SMC the only reasonable choice for larger parts.
For structural-thermoset bulk-molding compound, manufacturers blend resin, fiber reinforcement, and several other ingredients to form a viscous, puttylike material. By weight, structural BMC normally includes 25 to 40% reinforcement, usually chopped-strand glass fibers measuring 0.03 to 0.5 in. (0.75 to 12.7 mm) in length.
BMC is suitable for compression, transfer, and injection molding. It can be injection molded at cycles as fast as 10 sec/mm of part thickness. Depending on the application and formulation, structural BMC provides tight dimensional control, flame and track resistance, excellent mechanical properties and dielectric strength, corrosion and stain resistance, minimal shrink, and color stability. Available in numerous colors, BMC surfaces are also receptive to powder coating, painting, and similar finishing processes.
Structural-thermoset compounds hold a number of important advantages over other commonly used materials. And just as important, identifying required attributes and material properties early in the design process lets manufacturers create custom formulations tailored for specific applications. Core advantages include:
Tensile and flexural strength. Structural-thermoset compounds have higher tensile and flexural strength per unit weight than do most metals. And compared to thermoplastic blends such as polycarbonate/ABS (acrylonitrile butadiene styrene), nylon 6/PPO (polyphenylene oxide), and polycarbonate/PBT (polybutylene terepththalate), structural-thermoset SMC has significantly higher flexural and tensile strength and tensile and flex modulus.
Structural-thermoset compounds can also be custom designed to meet the strength requirements of a particular application. Unlike metals, which have equal strength in all directions, structural thermosets are anisotropic and can be tailored for extra strength in a specific direction. For example, if a thermoset part must resist bending in one direction, most fibers can be oriented at 90° to the bending force for structural stiffness in that direction.
Thanks to their molecular structure, thermosets maintain strength and other physical properties despite prolonged exposure to extreme temperatures. By contrast, metals and thermoplastics exposed to high temperatures may bend under applied loads. In addition, thermoplastics become brittle at low temperatures. Some engineered thermoplastics offer physical properties close to those of structural thermosets, but these materials are expensive and cannot always replace structural-thermoset SMC.
Dimensional stability. Besides strength, the cross-linked molecules in structural-thermoset compounds are dimensionally stable at high temperatures. A thermoset part is far less susceptible to relaxation or creep failure than a thermoplastic alternative. The material’s high fiber content reduces structural variations and makes thermosets ideal for low-shrink applications.
The dimensional difference between structural thermosets and thermoplastics can be seen during tensile and flexural tests at elevated temperatures. Thermoplastics may stretch several inches, while structural thermosets stretch just thousandths of an inch. In addition, tensile loads applied at high temperatures cause molded holes in thermoplastic parts to elongate over time, while holes in thermoset parts retain their original shape.
Molded structural thermosets typically shrink from 0.2 to 0% and, if needed, some thermoset materials can expand after cooling. Minimal shrinkage helps ensure close tolerances in molded parts, which often eliminates the need for secondary operations such as drilling and machining. For many applications, structural thermosets mimic the coefficient of linear thermal expansion (CLTE) of metals. This lets engineers readily combine thermosets with other materials in a single design.
Corrosion resistance. Unlike common metals, structural-thermoset SMC won’t rust or corrode when used outdoors or in harsh environments and holds up well over the long term. For example, thermoset ductwork in chemical-manufacturing plants routinely lasts more than 25 years, and thermoset compounds provide long service life in underground chemical-storage tanks. The corrosion resistance of structural SMC also make it suited for applications subject to strict sanitary requirements. Frequent exposure to harsh cleaning chemicals will not damage the material.
In contrast, corrosive substances and environments can weaken thermoplastics. And metals are notoriously susceptible to corrosion by water and common chemicals. Metals used in corrosive environments must first be coated, or designers must opt for expensive corrosion-resistant alloys.
Cost effective. Structural-thermoset compounds have long lives. Many thermoset structures built in the 1950s are still in use. In addition, structural thermosets require little maintenance.
They can also reduce manufacturing costs. Complex metal designs often require several parts. Individual pieces are made in a series of progressive dies or costly stamping stations, then assembled into the final product. With structural-thermoset SMC or BMC, on the other hand, complex parts can be made as a single piece in a single step. They generally require little final finishing, if any, and benefit from molded-in color and an attractive, durable surface. The simpler process translates into faster, more-efficient production with fewer secondary operations, fewer errors, and lower costs.
Design flexibility. Finally, structural-thermoset compounds give designers more freedom than they have with metals. Normal thermoset-molding processes permit complex shapes and intricate details that are impractical or even impossible with metals. And unlike metals, thermosets allow for a wide range of material combinations. Various resins and reinforcements can be combined to give a product unique properties. In some cases, structural thermosets can be molded on the most basic systems for R&D and quick prototyping.
Applications for structural thermosets
Military and aerospace. Commercial and military aircraft benefit from structural thermosets’ ability to reduce weight, cost, and production time, offer FST (fire, smoke, and toxicity) retardance, and prevent galvanic corrosion. Applications include military radomes, ammo-handling guides and containers, helicopter components, rifle hand guards, and other weapon components.
Transportation. A large and growing number of exterior automotive components are now made of structural thermosets instead of metals. Thermoset SMC is also gaining popularity among designers of interior parts. Reasons include:
Weight: Thermosets are 25 to 35% lighter than steel parts of equal strength.
Dimensional stability: Thermoset SMC has a low CLTE and holds up well to engine heat and summer temperatures, making it suitable for vehicle hoods, deck lids, and roof panels.
Memory: While metal panels permanently deform on impact, structural thermoset SMC panels deform and spring back to their original shape.
Cycle time: By reducing the number of parts in finished assemblies, structural thermosets shorten design and production times.
Temperature resistance. Ability to maintain strength at high temperatures makes it ideal for heat shields and skid plates.
Safety equipment. Heat resistance, fire-retardant properties, and high strength-to-weight ratios are critical considerations in the safety market. Applications include firemen’s helmets, firefighting-equipment components, and composite toe caps.
Medical. Corrosion resistance, antimicrobial properties, dimensional control, thermal insulation, and dielectric strength make structural thermosets well suited for the medical market. X-ray equipment components, instrumentation covers and bases, biohazard receptacles, and prosthetics are just a few applications.
Electrical. Structural thermosets hold up well during electrical arcing and tracking, with no significant changes to shape or performance. Parts molded from thermoplastic materials, on the other hand, will often carbonize or melt. Structural thermosets offer comparative tracking index values exceeding 600 V and dielectric strength above 15 kV/mm. In the electrical industry, structural-thermoset SMC and BMC are used for parts with track resistance >600 min and arc resistance >180 sec.
Industrial. Excellent load-bearing capabilities, in addition to corrosion resistance and dielectric strength, make structural thermosets a good choice for heavy industrial applications such as load bearings and valve bodies, as well as downhole plugs and other components for the oil and gas industry.
Alternative energy. Structural thermosets are widely used to make fuel-cell end caps. They provide heat and corrosion resistance without the shrinkage or residual stresses that might compromise a thin thermoplastic part. Structural-thermoset SMC and BMC are also suitable for solar-power tiles and wind-turbine components such as electrical control panels. These applications require materials that won’t warp or deteriorate during long-term exposure to the sun and other natural elements.
Marine. Structural thermosets’ material properties, including corrosion resistance and high strength-to-weight ratio, suit the marine market well. Applications include gimbal rings and cowlings, out-drive gimbal housings, and power-boat seat shells.