Advanced Materials for Demanding Applications: Unlocking the Value of SMC
Sheet molding compound (SMC) is a reinforced polyester material that incorporates carbon or glass fibers and is preferred for its strength and lightweight properties. SMC is a ready-to-mold mixture of thermosetting polyester resins, thermoplastic additives, fiber reinforcements, fillers, pigments, stabilizers, catalysts, release agents, etc.
Developed in the mid-20th century and known for its excellent dimensional stability, corrosion resistance, and surface finish capabilities, SMC is the preferred choice for complex parts that require consistent strength and technical advantages. Its sheet-like form allows for consistent material handling and efficient manufacturing, making it particularly suitable for compression molding.
Compression molding is an ancient yet still practiced molding method in producing composite materials. This method enables efficient production and can facilitate specialized and automated manufacturing processes. It results in high product dimensional accuracy, good repeatability, and a smooth surface with no secondary modifications required.
SMC at a Glance
- Formed in ready-to-old sheets with uniform thickness
- Typically reinforced by 20-50% glass fiber by weight
- Compression or injection molding processing
- Cured by heat and pressure initiate crosslinking into a rigid structure
- Lightweight alternative to metal
- Mostly used in the automotive industry
- High corrosion and chemical resistance
- Paintable, Class A surface finish capable
- Ideal for complex, high-strength parts
- Ideal for medium-to-high volume production
Key Properties
SMC is particularly effective in parts that require structural performance and cosmetic appearance. Its most notable characteristics include:
High Tensile & Flexural Strength: SMC exhibits excellent tensile, flexural, and impact strength, so it’s suitable for structural components in demanding environments. The typical tensile strength ranges from 50 to 100 MPa, and flexural strength can exceed 150 MPa.
Excellent Dimensional Stability: Due to its thermoset nature and fiber reinforcement, this material maintains its shape under varying loads and temperatures. It’s resistant to warping, shrinkage, and deformation over time, which is essential for precision applications.
Thermal Resistance: It can withstand continuous operating temperatures of up to 150° C (302° F) and intermittent exposure up to 200° C (392° F). Flame-retardant formulations meet various UL94 flammability ratings.
Chemical & Corrosion Resistance: SMC resists a wide range of chemicals, which include fuels, oils, salts, moistures, etc. It’s best suited for marine, automotive, and industrial environments where components need to withstand these conditions.
Electrical Insulation: With excellent dielectric strength and low electrical conductivity, SMC is ideal for electrical housings and insulators.
Lightweight with High Stiffness: About 20-30% lighter than metals with equivalent strength, SMC offers high stiffness-to-weight ratio, reducing component weight while maintaining structural integrity.
Paintable, Moldable Surface Finishes: SMC can achieve Class A finishes for various parts, offering paintability and the ability to mold-in color or texture. This is ideal for automotive components or consumer-facing parts where aesthetics are important.
Acoustic & Vibration Damping: The composite structure of SMC helps absorb sound and reduce vibration, an important feature in applications that need to withstand constant movement, such as in automotives and appliances.
Manufacturing Process
Producing SMC involves spreading long strands of fiber in a resin bath (typically unsaturated polyester or vinyl ester) with a reactive diluent (such as styrene monomer), compacting them between films, and then curing the material through compression molding.
The resin is spread uniformly onto the bottom of the polyethylene film. This film covering separates the layers to enable coiling and prevent contamination, sticking, and monomer evaporation.
Then, the chopped glass fibers are placed onto the spread. Fillers and additives are added, such as calcium carbonate, aluminum trihydrate, etc. These agents are used to enhance the performance or processing of the material.
A thickener or maturation agent is introduced to increase the viscosity of the resin mix without actually curing it, which makes it easier to handle the material during the premolding operations (such as cutting and stacking). The top film is then introduced, and the material is compacted, rolled, and kept for about 24-72 hours for aging/maturing. The actual time depends on the thickener, resin chemistry, and the temperature used for maturation.
This maturation time is required to allow the relatively low-viscosity resin to chemically thicken. The thickened SMC is easier to handle and prevents the paste from being squeezed out of the glass fiber sheet during processing. Typically, SMC requires about three to five days to reach the desired molding viscosity.
This manufacturing process allows for optimum versatility, creating parts of varying thickness, length, and complexity. Promoting superior efficiency, this production process is ideal for high-volume manufacturing with reduced waste and costs.
Formulated in-house, the material is designed to meet the performance requirements of any particular application such as tensile properties and/or Class A surface finish. Control over the type and percentage of the composition results in variations in mechanical properties and processability.
Types of SMC
SMC-R: The most common form of SMC is SMC-R, which contains randomly oriented chopped E-glass fibers in the plan of the SMC sheet. The length of the glass fibers is generally less than 5 cm. The random orientation saves time in the pre-compounding of fibers in the thermoset resin matrix, resulting in higher production volumes and lower manufacturing costs.
- Properties:
- Moderate strength and stiffness
- Good surface finish
- Cost-effective
- Applications:
- Automotive exterior panels
- Electrical housings
- Light structural parts
SMC-C: Continuous fiber SMC contains alternating layers of unidirectional continuous long glass fibers arranged in a certain direction. The molding fluidity in the direction of the plastic fiber is small, making it a little harder to mold, but the mechanical strength is higher in this type of compound.
- Properties:
- Electrically conductive
- EMI/RFI shielding capability
- Moderate mechanical strength
- Applications:
- Battery enclosures
- Electronic device casings
- Shielded panels in automotive or telecom
SMC-CR: This compound is a combination of alternating layers of unidirectional continuous and randomly oriented discontinuous fibers. It is designed to resist chemical corrosion, moisture intrusion, and environmental degradation.
- Properties:
- Excellent resistance to acids, bases, and solvents
- High moisture and UV resistance
- Non-conductive and thermally stable
- Applications:
- Wastewater treatment equipment
- Chemical processing components
- HVAC ducting
- Flue gas scrubber housings
SMC-D: Oriented glass fiber SMC features a combination of highly crosslinked resin and long (more than 10 cm) glass fiber reinforcement, which provides high physical properties and low density that’s suitable for large interior and exterior applications. Since the fibers are non-continuous, the molding fluidity along the fiber arrangement direction is improved.
- Properties:
- High dielectric strength
- Flame retardant
- Excellent arc resistance
- Applications:
- Electrical switchgear components
- Transformer housings
- Busbar supports and terminal boards
XMC: Continuous fibers are arranged in an X-pattern intermingled with randomly oriented discontinuous fibers. It features a fiber content of 70% to 80% and contains no filler. It’s manufactured in a filament winding machine and transformed into the sheet form by slitting the filament-wound XMC roll along its length.
- Properties:
- Higher impact resistance
- Improved mechanical strength
- Better dimensional stability
- Applications:
- Truck and bus body panels
- Load-bearing automotive structures
- Large, complex moldings with tight tolerances
HMC: Featuring a high percentage of fiber glass (up to 80% by weight), incorporating into thickenable thermosetting resin systems, HMC utilizes chopped fiber glass strands and is produced by the same processing methods as are used in manufacturing regular SMC.
- Properties:
- Very high strength and stiffness
- Superior heat and chemical resistance
- Precision moldability
- Applications:
- Aerospace interior components
- High-performance automotive parts
- Structural panels and supports in demanding environments
SMC vs. BMC vs. Phenolic Comparison
SMC is similar to BMC as they’re both flow molding materials that can be used for manufacturing relatively complex-shaped parts, although the level of complexity is subject to change. SMC has longer glass fibers and higher mechanical properties while BMC has shorter glass fibers with lower relative mechanical properties but is more cost-effective.
| Material | SMC | BMC | Phenolic |
|---|---|---|---|
| Form | Sheet-like, Semi-solid | Putty-like, Bulk form | Powder or granules |
| Common Fillers | Glass fibers, Calcium carbonate | Short glass fibers, Calcium carbonate, Clay | Wood flour, Glass fibers, Mineral fillers |
| Typical Applications | Automotive, Electrical, Marine, Oil & Gas | Electrical, Appliances, Construction | Automotive, Electrical, Industrial |
| Molding Process | Compression molding | Injection or compression molding | Compression or transfer molding |
| Surface Finish | Good surface finish, Paintable | Moderate surface finish | Poor to fair (requires secondary finishing) |
| Mechanical Strength | High | Moderate to high | Very high |
| Heat Resistance | Good | Good | Excellent |
| Electrical Insulation | Good | Excellent | Excellent |
| Cost | Moderate | Moderate to low | Low |
| Recyclability | Non-recyclable (thermoset) | Non-recyclable (thermoset) | Non-recyclable (thermoset) |
| Reinforcement Type | Long glass fibers | Shorter glass fibers | Varies (short or none) |
| Dimensional Stability | High | Medium | Lower |
Compared to other compounds, SMC generally offers the best combination of strength, surface finish, and thermal stability, especially in applications in which high-performance composite parts are required.
SMC vs. Fiberglass
SMC is heavier and sturdier than fiberglass. It can withstand more significant stress and impact. Fiberglass may be more cost-effective and lightweight, but it doesn’t match up with SMC’s structural integrity under high stress conditions. Compared to fiberglass, SMC also has excellent chemical resistance, which is crucial in certain applications.
Fiberglass may have lower initial material costs, but its production process can be more labor-intensive and time-consuming, which leads to higher overall expenses in high-volume productions. On the other hand, SMC’s efficient manufacturing process reduces waste in high-volume productions, which lowers the cost per unit significantly.
Applications Across Industries
SMC is mainly found in the automotive industry where its balance of strength and lightweight characteristics. It’s found in nearly every component, from the body to the engine. Offering good dent resistance, it helps reduce assembly costs through part integration and has significantly lower tooling costs compared to steel. However, the benefits of SMC can be found in various other industries.
Automotive: SMC is widely used due to its impact strength, lightweight nature, and Class A surface finish.
- Vehicle body panels
- EV battery enclosures
- Hoods
- Truck lids
- Underbody shields
Electrical & Electronics: Manufacturers utilize the material’s non-conductive and flame-resistant properties.
- Junction boxes
- Switchgear enclosures
- Cable management systems
Industrial & Utilities: Materials meet the demands of durability and resistance to environmental degradation.
- Equipment housings
- HVAC components
- Water treatment systems
Medical: SMC offers hygienic surfaces and precision molding capabilities for equipment covers and mobility components.
- Medical imaging equipment
- Patient monitoring systems
- Equipment covers
- Mobility components
Marine: SMC’s high corrosion resistance and water absorption capabilities make it suitable for water-based applications.
- Boat hulls
- Structural panels
- Enclosures
Oil & Gas: Materials can withstand some of the harshest conditions, whether it’s wet, dusty, corrosive environments, or high pressures.
- Regulators
- Pressure switches
- Relief valves
- Relays
- Interface components
Sustainability & Performance Longevity
With concerns for plastic pollution on the rise, it’s important to note that while thermosets like SMC are not traditionally recyclable, their long service life reduces the need for constant replacement, contributing to overall sustainability.
SMC’s lightweight nature also helps reduce emissions in transportation applications by lowering fuel consumption or extending EV range. Innovations in composite recycling are improving lifecycle management and repurposing of end-of-life parts.
Design Considerations & Best Practices
Successful SMC part design requires careful attention to geometry, processing variables, and end-use requirements. Some measures can be taken to help maximize the end-product’s performance, efficiency, and longevity.
Wall Thickness: Maintain consistent wall thickness where possible. Sudden changes in wall thickness can cause sink marks, warping, and uneven material flow, which compromises mechanical performance. Typically, 2.5-6mm is the recommended nominal thickness, depending on structural needs and flow requirements. If transitions are required, use gradual tapers or fillets rather than sharp steps.
Draft Angles: Vertical walls should have 1°-3° of draft to ensure smooth part ejection from the mold. This draft reduces friction and prevents damage to the part during release. Parts with deeper draws or textured surfaces may require larger draft angles (up to 5°). Insufficient draft can result in drag marks, flash, or mold sticking, which ends up increasing cycle time and rework.
Ribbing & Bosses: Ribs can increase part stiffness without adding weight. Ribs should have a height-to-thickness ratio of ≤10:1 and shouldn’t be more than 60% of the adjacent wall thickness to avoid sink marks. Add draft and radii to ribs to improve material flow and reduce stress concentrations. Bosses should include support ribs or gussets for added strength and stability.
Fillets & Radii: Generous radii at internal corners help reduce material stress and promote better flow during molding. Avoid sharp internal corners, which can cause stress risers or result in incomplete fill. Internal radii are recommended to be at least 3mm, and external corners should be rounded for safety and flow optimization. Proper radii also help extend tooling life and reduce cracking risk.
Undercuts: Minimize or eliminate undercuts unless necessary, since they require complex tooling and increase cycle times. When unavoidable, design undercuts should be shallow, accessible, and well-vented. However, consider alternate fastening or assembly methods to avoid the need for internal undercuts.
Ventilation: Proper mold venting is critical to avoid burn marks or voids caused by trapped air. Vents should be placed at the end of material flow paths, typically at parting lines or near thin sections. Vents can be as small as 0.025mm (0.001 in) deep but must be clean and maintained to ensure consistent performance.
Parting Lines & Inserts: Design parting lines in less visible areas to preserve surface appearance. Positioning should also consider ease of demolding and flash removal. For embedded metal inserts, preheating the inserts allows proper bonding and secure anchoring during compression.
Tolerances: Achievable tolerances are based on mold precision and material flow behavior, which is typically ±0.25mm (±0.010 in) for most dimensions. Tight tolerances may require post-mold machining or the use of precision tooling. Consider material flow behavior and curing shrinkage when defining tolerance zones.
Surface Finish: Specify mold texture or polish based on cosmetic needs. For Class A surfaces (high-gloss, paintable finishes), use polished or nickel-plated tooling and ensure material cleanliness. Textures (leather grain, matte, etc.) can be molded in directly via etched tooling.
Post-Molding Operations: Design for any secondary machining, painting, or bonding processes needed after molding. Include locating features or tooling holes for trimming, drilling, or machining. Use flat, accessible areas for any required bonding or assembly operations.
Why Choose Davies Molding’s SMC
Davies Molding is the go-to choice in precision-molded components using SMC for applications that demand strength, aesthetics, and reliability. The engineering team supports each stage of the process, from design collaboration to custom tooling. We take the lengths to ensure that every SMC product meets exacting standards for dimensional accuracy and performance.
Our team of experts can help customers from concept to production with:
- Application-specific material selection
- Mold flow and structural simulation
- Precision compression and injection molding
- UL-compliant and industry-tested formulations
- Flexible production volumes, from prototyping to full-scale runs
Contact our engineering team today to learn how Davies Molding is the best partner for durable, high-performance SMC parts.