Sheet molding compound
Sheet Molding Compound (SMC) refers to sheet -like, dough-like molding compounds made from thermosetting reactive resins and glass fibers for the production of fiber-plastic composites . In the SMC, all the necessary components are completely premixed and ready for processing. Fiber-matrix compounds that are not supplied as sheets are also called bulk molding compounds .
Usually polyester or vinyl ester resins are used. The reinforcing fibers are in the form of chopped fibers, more rarely in mat or fabric form, the typical fiber length of which is 25.4 mm (1 inch). The properties and names of SMC are regulated in EN 14598.
This fiber-matrix semi-finished product has been known since around 1960. In the early 1960s, the USA succeeded in producing SMC with low shrinkage by adding polyethylene powder. At the end of the 1960s , the shrinkage of the polyester resin could be partially compensated for by adding styrene and thermoplastic components.
A distinction is made between the following SMC types:
- SMC-LP low profile : System with high surface quality
- SMC-LS low shrink : system with low reaction shrinkage
- C-SMC: system with endless, directed fibers.
In LS systems ( low shrink ) the shrinkage must be between 0.05% and 0.15%. For LP systems ( low profile ) a shrinkage between 0.05% and −0.5% is required. Paintable surfaces in the automotive industry must be of Class A quality. SMC systems for this application have a shrinkage of less than −0.05%.
The SMC mass contains approximately:
- 12% cross-linkable resin
- 45% mineral fillers, e.g. B. chalk or rock flour
- 30% glass fibers (with appropriate size )
- Additives to reduce shrinkage
- inert release agents
- Peroxides as accelerators or reactants
- depending on requirements: color pastes, flame retardants , conductive additives
The fully automatic mixing of the resin filler mixture with the glass fibers creates the SMC semi-finished product in film form. This can then be cut to size and processed into the finished component by means of extrusion. SMC is used to manufacture body parts for cars, sports equipment, and parts for the electrical, sanitary and aerospace industries. When pressing, complex shapes can be filled in one work step and fastening elements can be inserted into the pressing mold. This makes SMC particularly economical. The fillers are primarily used to reduce costs, depending on the filler, also to reduce weight or to change other physical properties.
- Reinforced plastics eV (Ed.): Manual fiber composite plastics: Basics processing applications. 3rd edition (2010), pp. 237-259 ( google books ). 4th edition 2013, ISBN 978-3658027544 .
- AVK market data (PDF; 104kB)