A Guide to Sheet Molding Compound (SMC)

There are many materials used in automotive manufacturing, offering different benefits in terms of reinforcement and protection in the event of an accident.

From fiber-reinforced plastics (FRPs) to bulk molding compound (BMC) to sheet molding compound (SMC), each has its own use in the automotive space.    

In this article, we’ll focus on SMC and what makes it suitable for the industry.    

We’ll explain what sheet molding compound is, why it’s so important and how to gain auto body repair skills with hands-on career training! Continue reading to learn more.

Key Summary Points

• Sheet molding compound (SMC) is a reinforced polyester material, incorporating carbon or glass fibers, used extensively in the automotive industry for manufacturing various car parts due to its strength and lightweight properties.
• The production of SMC involves spreading long strands of fiber in a resin bath, compacting them between films and then curing the material through compression molding. This process allows for the creation of parts with varying thickness, length and complexity.
• SMC offers several benefits, including being lightweight yet durable, which makes it an excellent replacement for metal components in vehicles. Its production process is efficient, allowing for high-volume manufacturing with reduced waste and costs.
• SMC is commonly used for external vehicle body parts such as door assemblies, body panels and bumpers. In case of damage, such as cracking, SMC can be repaired using specific adhesives and proper sanding techniques.
• The Collision Repair & Refinish Technology (CRRT) program at Universal Technical Institute (UTI) includes a module on SMC, teaching students the basics of its production process and repair techniques, preparing them for careers in the collision repair industry.¹

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What Is Sheet Molding Compound?

Sheet molding compound (SMC) is a versatile and innovative composite material primarily used in the automotive industry to create lightweight, durable parts. This reinforced polyester material, often enhanced with carbon or glass fibers, is known for its exceptional strength-to-weight ratio, making it an ideal choice for applications where reducing mass without compromising structural integrity is crucial.

SMC can be molded into intricate shapes and sizes, providing manufacturers with design options to meet specific automotive needs. This adaptability lets SMC shine in various applications, from body panels to intricate interior components.

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Comparing SMC vs. Fiberglass

Material properties

SMC is heavier and sturdier than traditional fiberglass, allowing it to withstand more significant impact and stress. While fiberglass can be cost-effective and lighter, it may not maintain the same structural integrity under high stress as SMC does, particularly in automotive applications. Sheet molded compound also has excellent chemical resistance, which is vital in environments like vehicle exteriors where exposure to fuels and oils can happen.

Cost-effectiveness

Sheet molding compound’s efficient manufacturing process not only reduces waste but also allows for high-volume production, lowering the cost per unit significantly. While fiberglass may have lower initial material costs, its production process can be more labor-intensive and time-consuming, leading to higher overall expenses in high-volume manufacturing scenarios.

Applications

SMC is predominantly used for exterior parts, including body panels, bumpers and hoods, thanks to its durability and lightweight properties. On the other hand, fiberglass is preferred for applications requiring flexibility and complex shapes, often in both automotive and marine sectors for items like custom interiors and boat hulls. Each material’s unique strengths make it suitable for varying manufacturing needs.

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Sheet Molding Compound Process

SMC is a reinforced polyester material that features carbon or glass fibers that are typically an inch or more in length. These long strands of fiber are suspended or spread out in a resin bath made of a mixture of vinyl ester, polyester or epoxy.

The combination of the resin and long fibers produces a strong final product. SMC is typically sold in rolls that can be cut into smaller pieces known as “charges.”

Production of SMC occurs in steps that ensure the fibers and resin are fully integrated. Resin gets applied to a film in the form of a paste. Fibers are then cut and added to the resin.

Next, this mixture is squeezed and compacted between two pieces of film. Once the right texture and thickness are reached, it gets stored for several days. Heat and pressure are then applied in a process called compression molding, which cures the SMC and creates the final product.

Compression molding

The sheet molding process allows SMC to be formed into a range of parts that vary in thickness, length and detail. From simple to complex parts, compression molding takes preheated SMC and uses a hydraulic press to compress it to the desired shape.

The hydraulic press typically features the upper and lower half of a mold between two heated plates. SMC is poured into one of the mold cavities, and the upper portion of the mold is lowered. Pressure of up to 2,000 pounds per square inch (PSI) is applied.

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Advantages of SMC Material

One of the advantages of SMC material is that it’s lightweight compared with other materials.

This has caused SMC to complement different metal components and be used for auto body parts. In fact, when SMC was introduced, it was seen as a revolution in parts manufacturing and shifted the industry from metals to polymer-based compounds.

Even though SMC is lightweight, it’s still a durable material that works to resist impact.

SMC can also be easily produced at a high volume. The sheet molding compound process is straightforward and provides a short production cycle. This makes it a cost-effective, reduced-waste choice for manufacturers.    

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Sheet Molding Compound FAQs

1. What car parts commonly use SMC material?

Several auto body parts use SMC—this goes for passenger vehicles and lightweight trucks. SMC parts include door assemblies, body panels, roof panels, spoilers, fenders and bumpers.

2. What damage can SMC experience in an accident?

During an accident, the most common damage to SMC is cracking. In severe situations, part of the panel might be torn off completely.

3. What does FRP mean?

FRP is an abbreviation for fiber-reinforced plastics. They are composite substances that are made of a polymer matrix and strengthened by fibers. SMC is a type of FRP.

4. How is SMC repaired?

When repairing SMC, a repair adhesive specifically designed for it can be used. It can either be a two-part urethane or epoxy adhesive. Loose or damaged fibers are removed and, in the case of a hole, a piece of scrap material can be used as a patch if it’s the same kind of material. Proper sanding techniques are then used to ensure a quality repair.

5. What’s the difference: SMC vs. fiberglass?

SMC is a combination of glass-fiber and polyester material molded into panels using a compression process. Fiberglass is glass strands that are woven into cloth or matted together. The fiberglass matting or cloth is placed in a mold, and then pure resin with a hardener is applied to form the part.

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Learn About Sheet Molding Compound With Collision Repair Training at UTI!

Accidents happen every day, which means there are plenty of opportunities for those interested in pursuing a career as a collision repair technician.

The Collision Repair & Refinish Technology (CRRT) program offered at Universal Technical Institute (UTI) teaches students the skills necessary to restore and repair a wide variety of vehicles. Courses cover a range of topics, including sheet molding compound!

The CRRT program at UTI can be completed in just 51 to 54 weeks and can provide the foundational knowledge needed for an entry-level position in the collision repair industry.¹ Find out more by requesting information online or calling us at 1-800-834-7308 today. We’ll connect you with an Admissions Representative who can help you get started!

UTI Campuses That Offer Collision Repair Training

• Long Beach, California
• Houston, Texas