CFSMC

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CFSMC, or Carbon Fiber Sheet Molding Compound (also known as CSMC or CF-SMC), is a ready to mold carbon fiber reinforced polymer composite material used in compression molding. While traditional SMC utilizes chopped glass fibers in a polymer resin, CFSMC utilizes chopped carbon fibers. The length and distribution of the carbon fibers is more regular, homogeneous, and constant than the standard glass SMC. CFSMC offers much higher stiffness and usually higher strength than standard SMC, but at a higher cost.

Contents

Manufacturing

Scheme of the tow-based microstructure of CF-SMC CF-SMC.png
Scheme of the tow-based microstructure of CF-SMC

CF-SMC are made up of carbon tow chunks, spread between two layers of uncured thermosetting resin. The carbon fibre tows are cut from prepreg UD tape. The originating tape can be made up of a certain number of fibres (filaments), thus affecting the properties of the final composite: values can vary from 3 to 50 thousand filaments, while typical tow lengths are within 10 to 50 mm. [1] As for the resin, thermosetting resins are used: possible choices are polyester, vinyl ester or epoxy, with the former being the cheapest and the latter being the most performant. Despite not being as strong nor stiff as epoxy, vinyl ester is often used for its properties like corrosion and higher temperature resistance. [2] The constituents are combined in sheets of prepreg material. The tows usually fall from the cutter onto one of the two layers of resin, and are then covered by the second layer. The prepreg sheets of SMC are made after the viscous assembly is compacted via rollers. In this phase, any control over the orientation of the fibres is generally impossible, and the fibres can be considered to have an equiprobable orientation in all directions.

Once the prepreg sheets are made, the material can be compression moulded into the final desired shape. Compression moulding is a manufacturing technique that requires a two part mould: the first one hosts the moulding material (charge), while the second one is mounted on a press to close the cavity while applying high pressure. Due to complex geometry, it may be necessary to cut the sheets to place them more easily in the lower mould. Then, while the upper mould cavity is closing, the material is pushed throughout the mould until closed. Pressure is maintained, together with elevated temperature, to allow the curing of the resin and low porosity. This stage has a heavy influence on the mechanical performances of the final product, as the viscous flow into the mold cavity tends to orient the fibers along the direction of the flow. By controlling the amount and direction of the flow, it is thus possible to influence the fibre orientation, having a quasi-isotropic material (low-flow moulding) or higher performances in a desired direction (high-flow moulding). [3]

During the manufacturing phase, it is also important to avoid, when possible, defects like weld- lines. Weld lines occur when two flow fronts of material meet during the filling of a mold cavity. This can sometimes result in air entrapment, inhibited crosslinking in the polymer matrix, or the clumping or absence of fibers. For these reasons weld-lines can be as weak or weaker than the neat polymer resin. [4]

Material properties

Different behaviour, in terms of stress strain curve, of CF-SMC according to the orientation of the tows. In the picture are shown the two (irrealistic) extreme cases of the tows perfectly aligned parallel or perpendicular to load direction. CF-SMC stressstrain.png
Different behaviour, in terms of stress strain curve, of CF-SMC according to the orientation of the tows. In the picture are shown the two (irrealistic) extreme cases of the tows perfectly aligned parallel or perpendicular to load direction.

Due to their heterogeneous and anisotropic microstructure, mechanical properties of CF-SMC can vary significantly within broad ranges. Parameters having profound impact on these materials performances are mainly related to the fibres and matrix neat mechanical and geometrical properties (especially those of the fibres) and the orientation and content of the reinforcement. Modulus can vary from less than 20 GPa to 60 GPa, while strength values are within 60–500 MPa. [3]

CF-SMC can also be engineered, to some extent, to have better performances in a specific direction, in a similar fashion as continuous fibres composites. This can be achieved by carefully controlling the compression moulding stage to influence fibre orientation. [5] When the fibres are mainly aligned with the loading direction, the material behaviour is mainly dominated by that of the fibres, thus resulting in stronger and stiffer, but also more brittle response. In the opposite case, if fibres tend to dispose perpendicular to the loading direction, the resin contributes more to the load bearing, and the overall composite will be less stiff, less strong and more ductile. Being based on hydrodynamic transport phenomena, however, the control over fibre orientation in CF-SMC is much more limited than in the continuous composites case, where orientation is often directly determined accurately by the manufacturer. In addition, while continuous fibres composites have a specific orientation, short fibre reinforced plastics can have a preferential orientation, meaning that, considering a generic system of axis, the majority of fibres can have a higher component along a direction and a lower component along the other two axis.

Comparison of the accurate fibre orientation in a laminate of UD plies (a) and the preferential orientation achievable with CF-SMC (b) UD laminate vs CFSMC.png
Comparison of the accurate fibre orientation in a laminate of UD plies (a) and the preferential orientation achievable with CF-SMC (b)

The discontinuous tow-based microstructure of these materials makes is even more heterogeneous than standard composites: fibre ends themselves acts as stress concentration areas for both the resin and the neighbouring tows; moreover, especially for complex shaped parts, it is impossible to prevent some local spots with badly aligned tows (e.g. perpendicular to the direction of axial stress) or with low fibre volume content, like resin pockets. Although making the material weaker and the structural design more complex, this feature makes these materials quite notch-insensitive. [3]

When moulded, CFSMC has a very different appearance than traditional carbon fibre fabric composites, which traditionally appear with a woven checkerboard pattern. CFSMC has the appearance of black and grey marble or burl.

Industrial use

CF SMC combines the lightweight properties of carbon composites with a manufacturing process, as compression moulding, that allows fast manufacturing and thus is suitable for high volume industrial applications. For these reasons, the automotive industry is one of the best candidates for this technology.

Car manufacturers have used standard glass SMC for over 30 years as a material for body panels in select sport cars such as the Chevrolet Corvette. [6] Substituting glass fibres with carbon is a recent development, having been used for significant structural components of the 2003 Dodge Viper, [7] the multifunctional spare wheel pan of Mercedes-AMG E-Class, [8] the Mercedes-Benz SLR McLaren, the 2009 Lexus LFA, [9] 2015 Lamborghini Huracán, the 2017 BMW 7 series [10] and 2017 McLaren chassis. [11] Lamborghini (together with Callaway Golf Company) patented an advanced version of CF-SMC called Forged Composite. [12] They first introduced it in the Sesto Elemento concept car, and since then, Forged Composite has been a distinctive mark for Lamborghini cars, used both in structural and aesthetical purposes. CF-SMC use is recently spreading also to the much broader non-high performance automotive sector as for the 2017 Toyota Prius PHV. [13]

CF-SMC has also been used in the aeronautic industry by Boeing, for the 787 Dreamliner window frames, while producers suggest that the use of these materials will grow in this sector as well. [14] [15] [16]

Related Research Articles

<span class="mw-page-title-main">Composite material</span> Material made from a combination of two or more unlike substances

A composite material is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions. Composite materials with more than one distinct layer are called composite laminates.

Fiberglass or fibreglass is a common type of fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened into a sheet called a chopped strand mat, or woven into glass cloth. The plastic matrix may be a thermoset polymer matrix—most often based on thermosetting polymers such as epoxy, polyester resin, or vinyl ester resin—or a thermoplastic.

Fibre-reinforced plastic is a composite material made of a polymer matrix reinforced with fibres. The fibres are usually glass, carbon, aramid, or basalt. Rarely, other fibres such as paper, wood, boron, or asbestos have been used. The polymer is usually an epoxy, vinyl ester, or polyester thermosetting plastic, though phenol formaldehyde resins are still in use.

Pre-preg is a composite material made from "pre-impregnated" fibers and a partially cured polymer matrix, such as epoxy or phenolic resin, or even thermoplastic mixed with liquid rubbers or resins. The fibers often take the form of a weave and the matrix is used to bond them together and to other components during manufacture. The thermoset matrix is only partially cured to allow easy handling; this B-Stage material requires cold storage to prevent complete curing. B-Stage pre-preg is always stored in cooled areas since heat accelerates complete polymerization. Hence, composite structures built of pre-pregs will mostly require an oven or autoclave to cure. The main idea behind a pre-preg material is the use of anisotropic mechanical properties along the fibers, while the polymer matrix provides filling properties, keeping the fibers in a single system.

<span class="mw-page-title-main">Compression molding</span> Method of molding

Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured; this process is known as compression molding method and in case of rubber it is also known as 'Vulcanisation'. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms.

Bulk moulding compound (BMC), bulk moulding composite, or dough moulding compound (DMC), is a ready-to-mold, glass-fiber reinforced thermoset polymer material primarily used in compression moulding, as well as in injection moulding and transfer moulding. Typical applications include demanding electrical applications, corrosion resistant needs, appliance, automotive, and transit.

Sheet moulding compound (SMC) or sheet moulding composite is a ready to mould glass-fibre reinforced polyester material primarily used in compression moulding. The sheet is provided in rolls weighing up to 1000 kg. Alternatively the resin and related materials may be mixed on site when a producer wants greater control over the chemistry and filler.

A thermoset polymer matrix is a synthetic polymer reinforcement where polymers act as binder or matrix to secure in place incorporated particulates, fibres or other reinforcements. They were first developed for structural applications, such as glass-reinforced plastic radar domes on aircraft and graphite-epoxy payload bay doors on the Space Shuttle.

Carbon fiber-reinforced polymers, carbon-fibre-reinforced polymers, carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic, also known as carbon fiber, carbon composite, or just carbon, are extremely strong and light fiber-reinforced plastics that contain carbon fibers. CFRPs can be expensive to produce, but are commonly used wherever high strength-to-weight ratio and stiffness (rigidity) are required, such as aerospace, superstructures of ships, automotive, civil engineering, sports equipment, and an increasing number of consumer and technical applications.

<span class="mw-page-title-main">Z-pinning</span>

Z-pinning is a technique to insert reinforcing fibres along the Z-direction of continuous fibre-reinforced plastics. Z-pins can be made of metal or precured unidirectional composite fibres. It is designed for use within pre-preg technology; there is extensive experimental evidence that Z-pinning dramatically improves the resistance of the composite structure to delamination. The figure on the right shows a Z-pin inserted in between the fibres of the material. The pin spreads the fibres and creates an oval shaped gap that is filled with resin. The Z-pin prevents the composite from delamination. When a load is applied the cracks will typically form along the line of the opening.

Three-dimensional composites use fiber preforms constructed from yarns or tows arranged into complex three-dimensional structures. These can be created from a 3D weaving process, a 3D knitting process, a 3D braiding process, or a 3D lay of short fibers. A resin is applied to the 3D preform to create the composite material. Three-dimensional composites are used in highly engineered and highly technical applications in order to achieve complex mechanical properties. Three-dimensional composites are engineered to react to stresses and strains in ways that are not possible with traditional composite materials composed of single direction tows, or 2D woven composites, sandwich composites or stacked laminate materials.

Forged composite, commonly referred to as forged carbon, is a type of carbon fiber SMC material composed of small pieces of carbon fiber composite material that are pressed into shape as the resin sets. This is in contrast to most carbon fiber composites, which are made of larger continuous layers that are 'laid up' one at a time, often manually. Forged composite allows for a higher range of shapes to be formed with precision, relative to traditional carbon fiber. It was originally developed jointly between Lamborghini, Callaway Golf Company, and the Lamborghini Lab. It was unveiled at the 2010 Paris Motor Show in a Lamborghini concept car, the Sesto Elemento. The United States trademark for forged composite was filed on July 13, 2010, in the category Toys and Sporting Goods Products by Callaway Golf, while the trademark for Forged Composites was registered in 2018 in the automotive category by Lamborghini.

Out of autoclave composite manufacturing is an alternative to the traditional high pressure autoclave (industrial) curing process commonly used by the aerospace manufacturers for manufacturing composite material. Out of autoclave (OOA) is a process that achieves the same quality as an autoclave but through a different process. OOA curing achieves the desired fiber content and elimination of voids by placing the layup within a closed mold and applying vacuum, pressure, and heat by means other than an autoclave. An RTM press is the typical method of applying heat and pressure to the closed mold. There are several out of autoclave technologies in current use including resin transfer molding (RTM), Same Qualified Resin Transfer Molding (SQRTM), vacuum-assisted resin transfer molding (VARTM), and balanced pressure fluid molding. The most advanced of these processes can produce high-tech net shape aircraft components.

<span class="mw-page-title-main">Tailored fiber placement</span>

Tailored fiber placement (TFP) is a textile manufacturing technique based on the principle of sewing for a continuous placement of fibrous material for composite components. The fibrous material is fixed with an upper and lower stitching thread on a base material. Compared to other textile manufacturing processes fiber material can be placed near net-shape in curvilinear patterns upon a base material in order to create stress adapted composite parts.

Automated fiber placement (AFP), also known as advanced fiber placement, is an advanced method of manufacturing composite materials. These materials, which offer lighter weight with equivalent or greater strength than metals, are increasingly used in airframes and other industrial products.

Glass-filled polymer, is a mouldable composite material. It comprises short glass fibers in a matrix of a polymer material. It is used to manufacture a wide range of structural components by injection or compression moulding. It is an ideal glass alternative that offers flexibility in the part, chemical resistance, shatter resistance and overall better durability.

Transfer molding is a manufacturing process in which casting material is forced into a mold. Transfer molding is different from compression molding in that the mold is enclosed rather than open to the fill plunger resulting in higher dimensional tolerances and less environmental impact. Compared to injection molding, transfer molding uses higher pressures to uniformly fill the mold cavity. This allows thicker reinforcing fiber matrices to be more completely saturated by resin. Furthermore, unlike injection molding, the transfer mold casting material may start the process as a solid. This can reduce equipment costs and time dependency. The transfer process may have a slower fill rate than an equivalent injection molding process.

A Lay-Up process is a moulding process for composite materials, in which the final product is obtained by overlapping a specific number of different layers, usually made of continuous polymeric or ceramic fibres and a thermoset polymeric liquid matrix. It can be divided into Dry Lay-up and Wet Lay-Up, depending on whether the layers are pre-impregnated or not. Dry Lay-up is a common process in the aerospace industry, due to the possibility of obtaining complex shapes with good mechanical properties, characteristics required in this field. On the contrary, as Wet Lay-Up does not allow uni-directional fabrics, which have better mechanical properties, it is mainly adopted for all other areas, which in general have lower requirements in terms of performance.

Autoclave moulding is an advanced composite manufacturing process.

<span class="mw-page-title-main">Reinforcement (composite)</span> Constituent of a composite material which increases tensile strength

In materials science, reinforcement is a constituent of a composite material which increases the composite's stiffness and tensile strength.

References

  1. ↑ "SPE Automotive" (PDF).
  2. ↑ "Resins".
  3. 1 2 3 "Quantum resume" (PDF).
  4. ↑ "Weld lines".
  5. ↑ Advani, Suresh G.; Tucker, Charles L. (November 1987). "The Use of Tensors to Describe and Predict Fiber Orientation in Short Fiber Composites". Journal of Rheology. 31 (8): 751–784. Bibcode:1987JRheo..31..751A. doi:10.1122/1.549945.
  6. ↑ "Corvette Body Materials - Tracing the Evolution of Corvette's Use of Lightweight Materials - Vette Magazine". Super Chevy. 2011-08-01. Retrieved 2018-01-25.
  7. ↑ "APPLICATIONS OF CARBON FIBER SMC FOR THE DODGE VIPER". ACCE. Archived from the original on 2018-01-26. Retrieved 2018-01-25.
  8. ↑ "ASTAR Carbon Fibre SMC for Multifunctional Spare Wheel Pan of Mercedes-AMG E-Class".
  9. ↑ "The Making Of The Lexus LFA Supercar. An Inside Report, Chapter 2: In The Clean Room. - The Truth About Cars". The Truth About Cars. 2012-07-10. Retrieved 2018-01-25.
  10. ↑ Gardiner, Ginger. "BMW 7 Series Plant: Dingolfing, Germany". www.compositesworld.com. Retrieved 2018-01-25.
  11. ↑ "Releases - McLaren Media Site". cars.mclaren.press. Retrieved 2018-01-25.
  12. ↑ "Forged Composites®". www.lamborghini.com (in Italian). Retrieved 2018-01-25.
  13. ↑ "SMC adopted for rear door frame of Toyota's new Prius PHV". PlasticsToday. 2017-04-03. Retrieved 2018-01-25.
  14. ↑ "Astar CSMC Materials". www.astar.es.
  15. ↑ "HexMC Materials | Hexcel". www.hexcel.com. Retrieved 2018-01-25.
  16. ↑ Brosius, Dale. "Boeing 787 Update". www.compositesworld.com. Retrieved 2018-01-25.
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