SMC BMCMaximum design flexibility
SMC BMC materials offer exceptional flexibility to you as designer, allowing you to make highly complex parts in medium to high production series. You can integrate different functions like mechanical protection, optimized aerodynamics, electrical or thermal insulation, and superior surface aesthetics. The combination of these would not be possible to realize in metals at acceptable cost.
In order for you to create a computer aided design (CAD), and to use finite element analysis (FEM) modeling, the SMC BMC Alliance has defined multiple SMC and BMC standard product profiles. These provide you with a clear indication of the materials’ performance and physical properties for your application.
Detailed design files are available for these standard product profiles in .mtx format for easy upload into your design simulation software, with information about material flow, warpage, and PVT-C dependency, saving you time and money in prototyping and scale-up.
Design benefits of SMC BMC
When compared with components designed in steel, aluminium and thermoplastics, SMC and BMC components offer interesting benefits:
- SMC BMC components can be designed to take similar loads as steel and aluminium at much lower weight, particularly in multi-axial bending situations (e.g. truck body parts).
- At the same time, they provide excellent resistance to corrosion and water, allowing the use in harsh environments.
- The shrinkage of Low Profile Class A SMC systems can be zero or no more than 0.5 mm/m, resulting in parts with smooth and high quality surfaces.
- SMC BMC components bring great mechanical strength and durability in the final application, while resisting elevated temperatures.
- The coefficient of linear thermal expansion for SMC BMC is close to steel and aluminium, allowing the use in multi-material assemblies.
- The ‘spring-back’ behavior associated with the forming of steel or aluminium does not exist with SMC BMC. This means parts can be made with very high dimensional predictability and low tolerances.
- Read-through effects of SMC BMC are very low compared to thermoplastics.
- The sound transmission loss factor for SMC is significantly higher than for aluminium, meaning that SMC/BMC engine parts (e.g. valve covers, oil sumps) contribute to reduced engine noise and vibration.
Basic design considerations for SMC BMC components
When using SMC BMC materials for your application, it is recommended to follow best practices in design maximizing performance and minimizing cost:
- Increase moments of inertia for your design (e.g. through the use of contoured surfaces), so part deflection under load is minimized.
- Consider points of inertia e.g. styling lines, corrugations or ribs in your design for the same purpose.
- Recommended wall thickness for SMC BMC parts is in the range of 2.0-3.0 mm for exterior parts, 2.0-3.5 mm for structural parts (depending on part dimensions and material flow requirements during processing).
- Typical curing time is 30-45 seconds per mm thickness. Therefore, for maximum productivity it is recommended to design with a lower wall thickness.
- Maintain a even wall thickness for a uniform flow and curing, helping to minimize warpage and distortion.
- A uniform wall thickness also minimizes read-through.
The rigidity of SMC BMC parts can be increased by incorporating ribs and bosses into the design. However, using ribs is less desired with Class A body parts, as they can create sink marks (i.e localized depressions in the surface of molded parts) caused by the non-uniform shrinking of the SMC BMC during the cooling process in the mold. For non-Class A component design, sink marks can be minimized or eliminated by the proper design of reinforcing ribs using the right combination of draft angle, radii and rib thickness.
Bonding
SMC BMC part can be effectively combined through adhesive bonding processes. The physical properties of both the adhesive and the parts to be bonded should be reviewed already during the design phase of a part. Typically bond flanges should range from 16 to 25 mm. A minimum flange width of 6 mm may be used on smaller parts such as spoilers and a minimum of 3 mm clearance from the tangent should he allowed between the edge of the inner panel and the return flange on the outer panel. This allows for positive location in the fixture and adhesive de-roping.
Bonding for Class A components
For Class A surfaces, especially high-visibility horizontal applications such as hood assemblies, it is preferable for the bond location to be restricted to the panel perimeter and surface contour changes. This greatly reduces the risk of bond read-through.
Elastic, low modulus adhesives, can be used in such areas because, in most cases, these adhesive don’t have a high loading requirement - they are just acting as so called ‘anti-flutter- adhesive’. All closures must he vented to prevent air from being trapped in the assembly. Trapped air might expand and distort the surface when exposed to paint oven temperatures. Vents can he provided by interrupting the adhesive line or by providing a vent hole.
Coefficient of linear thermal expansion (CLTE)
The slope of the line representing dimensional change as a function of temperature is known as the CLTE. SMC components require limited attention in this respect (CLTE 12-16 x 10-6/°C), as the CLTE of SMC is very similar to the CLTE of steel (12 x 10-6/°C) and aluminium (24 x 10-6/°C).
