Fiber matrix semi-finished products
Among fiber-matrix semi-finished products refers to semi-finished products of reinforcing fibers that are impregnated with a plastic matrix. The fiber-matrix semi-finished products are available with a thermosetting and thermoplastic matrix. Glass fibers and carbon fibers are mainly used as reinforcing fibers . Fiber-matrix semi-finished products can have a firm or dough-like consistency. Solid fiber-matrix semi-finished products are mainly in the form of strands, strips or sheets.
Fiber-matrix semi-finished products are used in automated manufacturing processes. Its best-known representative is the prepreg . The higher costs of semi-finished products compared to non-impregnated fibers are offset by the lower production costs for high quantities. In 2009, Europe accounted for 27% (216,000 tons) of processed glass fiber reinforced plastic in sheet molding compounds (SMC) or bulk molding compounds (75,000 tons) (BMC) and about 9% glass fiber mat thermoplastic (GMT) or long-fiber-reinforced thermoplastic (LFT ).
Fiber-matrix semi-finished products with a thermoset matrix
Prepreg
The prepreg (preimpregnated fibers) is a semi-finished product made of continuous fibers soaked in a corresponding matrix. The fibers either form a unidirectional layer or are in the form of woven or non- woven fabrics .
SMC and BMC
Fiber-matrix semi-finished products with fiber reinforcement are SMC (Sheet Molding Compound) and BMC (Bulk Molding Compound). They consist of a resin filled with fillers. One of the tasks of the fillers is to reduce thermal shrinkage. They are designed for hot pressing and cure at elevated temperatures. Very short cycle times can be achieved with SMC and BMC.
The properties of SMC and BMC are specified in DIN EN 14598, it replaces the older DIN 16913 and now also contains test methods.
SMC (Sheet Molding Compound)
The reinforcing fibers are in the form of mats and, less often, of woven fabrics, so SMC is plate-shaped. Due to the mat or fabric shape, relatively long fiber lengths can be used, which leads to high rigidity and strength. The typical fiber length is 25–50 mm. The longer the fibers used, the more difficult it is to shape the semi-finished product in the pressing process. 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
- CF-SMC: system with endless, directed carbon fibers (carbon).
With SMC, paintable trim parts can be manufactured for the automotive industry. Fastening parts such as threaded bushings can be inserted into the mold during the pressing process. This makes SMC particularly economical. SMC is made from all common thermoset matrix systems .
BMC (Bulk Molding Compound)
BMC is a shapeless, doughy mass and consists of conventional thermosetting matrix systems with significantly shorter, smaller fillers than with SMC. In hot-pressing technology, it is partly processed as injection molding. The fibers in BMC are so short that they can flow through the mold when the reaction resin is pressed or injected. Therefore, the mold does not have to be completely laid out with BMC at the start of the press. It is sufficient to place the right amount of BMC in the center of the mold. The short fibers usually have lower strengths than SMC.
Fiber-matrix semi-finished products with a thermoplastic matrix
Fiber-matrix semi-finished products with a thermoplastic matrix have the advantage that they can be heated and shaped almost as often as required. They can also be welded. However, their low melting point means that they can only be used at relatively low temperatures compared to thermoset semi-finished products.
Long fiber reinforced thermoplastics (LFT)
A distinction is made between two LFT processes. In the G-LFT process, long fibers in granulate form ( e.g. polypropylene matrix ) are brought from an open extruder directly into a press mold and shaped. In the D-LFT process, the matrix is plasticized in an extruder and mixed with continuous fibers shortened to length in a mixer. The fiber-containing plasticate is then pressed into shape.
Glass mat reinforced thermoplastics (GMT)
Glass mat reinforced thermoplastics are a glass fiber reinforced plastic . Although the word fiberglass appears in the name, carbon fiber reinforced thermoplastics also exist. In GMT, the reinforcement fiber comes in the form of short and long fibers. Due to the tangled, non-needled shape of the fibers, large degrees of deformation are possible.
GMT is processed in hot pressing technology. In contrast to SMC or BMC, it is not used for visible parts. The strengths are lower than those of semi-finished products with a thermoset matrix. The low price, especially in combination with polypropylene , still makes GMT attractive.
GMT components often no longer have a preferred direction with regard to their elastic properties. They are therefore a quasi-isotropic laminate .
Thermoplastic prepregs
Thermoplastic prepregs, like thermoset prepregs , consist of continuous fibers. This means that thermoplastic prepregs can only be reshaped slightly. Hybrid rovings , using heat-resistant plastics such as PEEK , are often used for their production . Since micro-impregnation is difficult due to the high viscosity of the thermoplastic melt, special processes for the production of thermoplastic prepregs are necessary.
Sheet-shaped thermoplastic prepregs are also known as organic sheets .
literature
- G. Erhard: Constructing with plastics. Hanser-Verlag, Munich 1993, ISBN 3-446-17397-8 .
- M. Flemming, G. Ziegmann, S. Roth: Fiber composite construction methods . Manufacturing process: Manufacturing process with a thermoset matrix . Springer-Verlag, 1998, ISBN 3-540-61659-4 .
- AVK-TV Working Group Reinforced Plastics - Technical Association e. V .: The AVK-TV manual: Fiber-reinforced plastics and thermosetting molding compounds. Volume II: Materials and their manufacture . Self-published, 2005, OCLC 633367207 . ( PDF )
Individual evidence
- ↑ Chokri Cherif: Textile materials for lightweight construction. Springer, Berlin / Heidelberg 2011, ISBN 978-3-642-17991-4 , pp. 384-385.