The rejuvenation of sheet molding compounds highlights new reinforcing materials, new fillers, new resin matrix and new opportunities.
A significant change for the traditional SMC formulation is that it is no longer defined by chopped glass fiber, and mixers and molders are exploring the use of carbon fibers and even basalt fibers, as well as the selective use of continuous fiber reinforcements from staple fiber formulations . These continuous fiber reinforcements include not only unidirectional rovings but also biaxial and triaxial fabrics as well as non-crimp fabrics to impart greater rigidity, strength and impact resistance to the components while still having to be compared to standard SMC As an added bonus, in many cases, the part wall thickness can be made thinner.
The continuous fiber SMC can often be used in combination with a conventional discontinuous / chopped fiber reinforced SMC, a study by researchers at the Fraunhofer Institute for Manufacturing Technology ("F-ICT" in Pfinnztal, Germany) The advantage of the 'tailor-made SMC' technology is that short fiber SMC can fill the ribs while continuous fiber reinforcements tend to bridge other complex design features, but the presence of continuous fibers brings mechanical properties closer In real prepregs and at a lower cost without the need to invest in specialized processing equipment and without affecting the SMC's component integration advantages or forming complex 2.5D geometries (more than typical prepregs Shape is more complex.) An example of a continuous fiber SMC component is a battery pack for use on a Chevrolet Spark electric vehicle (EV) at General Motors, which incorporates SMC components of continuous and discontinuous reinforcements and is Structural body undercarriage provided by the American Automotive Research Council ('USCAR', Southfield, MI, USA).
Growing interest in carbon fiber SMC as carbon fiber prices fall and more suppliers offer large tow (50K and 25K, primarily in the automotive and industrial sectors) carbon fiber SMC. Since the 2003 Dodge Viper model Daimler Chrysler, now part of FCA US LLC's structural fender support, carbon fiber reinforced SMC has been commercialized.
A pair of brackets made of 50% chopped carbon fiber and a vinyl ester resin were assembled by Meridian Automotive Systems, Inc. (Allen Park, MI, USA, no longer exists), incorporating the previous 15 to 20 metal components Reducing weight by 18 kg compared to stamping steel parts.Moreover, it has been said that carbon fiber reinforced SMC was used in locomotive covers in Europe as early as the early 1990s.
Many of today's mixers and molders are offering SMC materials reinforced with various forms of carbon fiber, but there are some challenges in matching these materials: First, find carbon fibers that are suitable for infiltration of the matrix material of vinyl ester and vinyl ester compounds, Not suitable for typical epoxy or polyurethane infiltrated carbon fibers to ensure good adhesion between the matrix material and the reinforcing material.The lack of such products has promoted the development of the Advanced Composite Materials Manufacturing Innovation Institute ("IACMI" A new project from Zoltek (a subsidiary of the Toray Group of Companies in St. Louis, Missouri, USA), Michelman (Cincinnati, OH), and Ash, a new project in Knoxville, Tenn. Orchid involved in the project.
The second challenge is to make the fibers well infiltrated in typical SMC thick paste systems. "The infiltration of carbon fibers in SMC processing is very difficult, especially with large tow carbon fibers." Fraunhofer Daniel Park, research engineer at the Composites Research Project Center ('FPC', London, Ontario, Canada), explains that 'people are therefore looking for ways to separate the fiber bundles in order to facilitate wetting and to improve their mechanical properties.Using large tow carbon fibers The enhanced SMC, though of increased rigidity, is usually only half the strength of a similar glass-reinforced SMC, which is hard to say when the carbon fiber has only a tensile strength of 100-150 MPa compared to the tensile strength of glass fibers above 300 MPa This is a reasonably priced carbon fiber that we have solved in our direct SMC (D-SMC) process by adding a fiber deployment system to reduce the fiber bundle from 50K to 3K. Fiber bundles and more uniform material come in a more exponentially more wettable form, which is why fiberglass plied yarns (twisted yarns made of very few glass fiber yarns) Used in showing a faster, more thorough infiltration, but these have not yet appeared in the carbon fiber.
'Carbon fiber SMC is our choice of materials for structural and semi-structural applications where we used prepregs in the past. "Magna Exterior Company (Troy, Michigan) Global Product Director Lightweight Composites Andrew Swikoski explains that the company currently has two carbon fiber products, all using a modified vinyl ester resin matrix, one EPICBLEND CFS-Z chopped fiber grade and one continuous fiber reinforced EPICBLEND CFS-Z 0 ° / 90 ° NCF fabric grade, which can be used together with chopped grades of materials for partial reinforcement of parts for parts that require higher mechanical properties.
'Our method of making chopped carbon fiber' prepregs' means we have better fiber wetting, compaction and consolidation, 'Swikoski said.' Compared with other methods, we cut, scatter and There are different ways to store the fibers.We can cut finer fibers with the cutter and change the fiber length and fiber distribution on the SMC line, which helps with orientation and flow. "In general, the smaller the diameter, the easier it is to infiltrate , The easier it is to separate and flow in the matrix resin. "Our fiber distribution is very random, unlike ribbons from other fibers, which gives us a significant cost advantage and also allows us to reduce the wall of the part Thick, and able to flow longer with greater consistency. '
Swikoski notes that the chopped fiber SMC does not produce Grade A surfaces. "It is always used for inner panels. It can be textured but already has a very 'technical' look which makes it a hood, a door or Ideal for Class B surface applications such as lift gates, interior panels, etc. "He also said that Magna switches back and forth between full carbon and carbon and glass fiber blends." We are looking for the lowest weight and thinnest Of the wall thickness, we can now reduce it to 1.2-1mm, which is on steel, but we have provided a flowable material with the ability to integrate parts that metal stamping can not provide. "
Mixer A. Schulman Ltd. (Fairlawn, OH) has several new carbon fiber SMC production methods and its subsidiary Quantum Composites Inc. (Bay City, MI, USA) has developed The chopped carbon fiber SMC was commercialized initially with an epoxy resin matrix followed by a vinyl ester resin matrix.Since Quantum Composites Limited and Callaway Golf Company (Carlsbad, CA, USA) since 2006, Began working closely with Lamborghini Motor Company shortly thereafter, using a modified vinyl ester resin matrix to develop a short cut carbon fiber SMC material, which is called "Forged Composites." Subsequently, Carla Forgras Golf used Forged Composites for its Diablo Octane service ball, which Lamborghini uses for the various concept cars and parts of the Sesto Elemento, Aventador J, Veneno and Huracan supercars. Callaway recently requested a similar product for its Big Bertha Fusion tee, which is thinner and stronger. 'A. Schulman Douglas Gries, director of market development for composite materials, recalled that 'the only way we can do this is with a continuous fiber system, and fortunately we have done some research with weaved carbon fiber fabrics so we have developed a New material to meet the structural and cosmetic requirements of Callaway, and we are now very interested in showing this material to car manufacturers.
A. Schulman Ltd. refers to this product as 'Forged Preg', which was introduced at CAMX 2016 and has now been expanded to three levels, each using a patented vinyl ester resin mixing system: The 8575 incorporates a A 3K tow triaxial braid, the 8585 uses a 12K tow biaxial fabric, and the 8595 uses a 60K tow unidirectional fabric.
Fiberglass technology is also progressing, with Chongqing International Composites Co. ("CPIC", Chongqing, China) claiming it ranks third in the world in production, and the company is said to have several SMC-ready product categories with the most interesting and unique Is a flat fiber that is easier to achieve isotropic dispersion and significantly reduces warpage in thin-walled parts. The other attributes of this flat glass fiber bundle are the equivalent of the conventional round alkali-free glass fiber The company also offers conductive paste systems and an HL level that is lower in dielectric constant than traditional non-alkali glass fibers.Recently, CPIC has commercialized its HT and TM grades, which offer higher modulus. TM grade is the most alkali-free glass fiber currently in the market with the highest tensile modulus. Tensile tests on 1700 m, 2400 tex rovings according to the ASTM D 2343 test method show that the tensile modulus of the impregnated fiber bundles is 88-92 GPa The HT grade is said to have a higher tensile modulus of 92-96 GPa, approximately midway between the values for E-glass and high-strength glass fibers but at a lower cost than high-strength glass fibers .
Fibrous infiltration of carbon fibers SMC Thorough infiltration of fibers in a typical thick paste system of SMC is a challenge and one solution to this problem is to separate the fiber bundles to help infiltrate and improve the mechanical properties especially in the automotive industry Tow carbon fiber Fraunhofer Composites Research Center uses a fiber dispersion system to reduce fiber bundles from 50K to 3K in its direct SMC process.It is reported that these smaller tow are easier to infiltrate and can form More uniform material, thereby greatly increasing the strength.The right picture shows the standard chopped carbon fiber, the left picture shows the first to get a standard dispersion of chopped carbon fiber (picture from the Fraunhofer Composite Research Center)
Distribution of fibers in carbon fiber SMC Magna Outsourcing developed a carbon fiber SMC formulation with a modified vinyl ester resin for its own use, a chopped fiber grade called 'EPICBLEND CFS-Z' Better wetting, compaction and consolidation, a unique manufacturing process achieves anisotropic fiber distribution, with the left (left) smooth and the right (right) two types of textured surfaces (images from Macquarie Satisfied that the external decoration business)
The 8575 grade uses a 3K tow triaxial braid and provides a tensile modulus of 33090 MPa at 50% by weight. The 8585 grade uses a 12K tow biaxial fabric and is available in 55 % By weight provides a tensile modulus of 55150 MPa The 8595 grade material utilizes a 60K unidirectional tow and provides a tensile modulus of 99300 MPa at 55 weight percent (Image courtesy of A. Schulman Inc./Quadum Composite Materials Co., Ltd.)
Flat fibers are used for more stable thin-walled SMC. Fiberglass technology may be a long-established reinforcing material technology, but it is not without any progress. One example is Chongqing International Composites' unique flat glass fiber, Material isotropic dispersion, thus greatly reducing the warpage of thin-walled SMC components