Fibre composites and their processing

Fibre-reinforced plastics have developed in recent years to become an important class of materials, especially in modern structural applications. Because of their outstanding potential for lightweight construction in combination with top-class mechanical performance, they are frequently used in the aviation and automotive industries, power engineering and sport. The driving force behind the attempts to substitute traditional materials like metal is the desire to save weight. Depending on the intended application, short, long or continuous fibres are used in thermoplastic or thermosetting matrices.

Carbon, glass or aramid fibres are most commonly used. The reinforcing fibres must be long as possible and best oriented in loading direction if they are to develop their full potential. The disadvantage is, however, that they generally extend cycle times and thus increase costs.

Cost-efficient production of high-performance parts based on fibre composites still presents major challenges to research and development, especially with regard to mass production. The key here is to combine shorter cycle times with maximum fibre length.

As before, there are conflicting tendencies between productivity and mechanical performance: Very good mechanical properties correlate with high fibre volume contents and long, highly oriented structures, but the manufacturing costs increases at the same time. The used materials and process are essentially governed by the future application.

Processing of fibre-reinforced thermosetting plastics

Sheet Moulding Compounds (SMC) and Bulk Moulding Compounds (BMC) consist of short and long fibres and, up to a certain level of cost performance relationship, can be considered state of the art. The matrix is frequently an unsaturated polyester resin (UP). If high impact strength is specified for the part, vinyl ester resins (VE) may also be used. The compression moulding of SMCs produces parts with superior mechanical properties to the compression moulding or injection moulding of BMCs.

Reaction Injection Moulding (RIM) or Reinforced Reaction Injection Moulding (RRIM) is another moulding process used for the production of thermosetting fibre composites. The process involves intensively mixing of two components and then injecting the reactive melt into a mould. The cycle time with this process is between one and five minutes. RIM is used predominantly for the production of moulded parts made of polyurethane, e.g. for the automotive industry.

An extremely simple, flexible and proven method for producing continuous fibre composites is known as hand lay-up. With this method, any number of dry fibre layers are placed, one after the other, on a – in the simplest case – one-part male or female tool, and then impregnated with resin using a brush or roller. The main advantages are the low cost of toolmaking and the lack of costly capital expenditures for hot presses, autoclaves, vacuum pumps etc. To ensure consistently good part quality, strict quality control is essential.

Unlike hand lay-up, the prepreg process uses semi-finished fibre products, preimpregnated with resin, to build up the layers. Curing is usually conducted in the autoclave. With this manufacturing technique, very good mechanical data can be achieved with FRP. Despite enormous efforts to automate the process and thus cutting costs, the prepreg process has remained the preserve of the aerospace industry.

The basic idea of the wet filament winding process is the laying of continuous textile reinforcements in a predefined pattern on a rotating core. This process enables high fibre volume contents and low manufacturing costs, although it is only possible to produce relatively simple geometries without any inward curvatures.

The main characteristic of the liquid composite moulding process (LCM) is the impregnation of a dry preform with a liquid matrix in the mould and subsequent curing. This is carried out either in a closed rigid mould or in an "open" mould that is only rigid on one side. These techniques allow the economical production of high-quality parts with a high fibre volume content. Processes that fall into this category include Resin Transfer Moulding (RTM), Single-Line Injection (SLI), Vacuum Infusion Process (VIP) and the Seeman Composite Resin Infusion Moulding Process (SCRIMP). LCM processes are used predominantly for the production of parts for wind power units (e.g. rotor blades) and in the automotive industry. There is certainly still plenty of potential in the preform technology in which, in recent years, several direct processes such as Tailored Fibre Placement (TFP) and Fibre Patch Placement (FPP) have been developed.

To reduce energy consumption and shorten the curing times, modern approaches are looking at fast curing cycles (Quickstep process) using heated fluids or induction heating (Roctool process), or an alternative energy input via electron beams or microwaves.

Processing of fibre-reinforced thermoplastics

Directly usable short or long fibre-reinforced thermoplastics can be effectively manufactured in large volumes by injection moulding processes. These materials are available in bulk and can be recycled in-house. This proven, inexpensive production method is widely used in the automotive industry.

In parallel with the injection moulding processes, use is also frequently made of compression moulding processes for the production of fibre-reinforced plastics. Compared with injection moulding, the fibres are less degraded, although no complex geometries are possible. A well-known representative of this category is that of glass mat-reinforced thermoplastics (GMT). Their production involves the processing of glass fibre fabrics or glass mats into semi-finished products in combination with thermoplastics (usually PP). After heating, these semi-finished products can be further processed by compression moulding. GMT mats are available in different fibre lengths. The assumption that a GMT component made of continuous fibres has higher strength than one containing short fibres is usually incorrect, one reason being that the continuous fibres are creased or bent during compression.

One innovative approach to lowering material and production costs is to use a direct process. The Direct SMC process – which is likely to go into mass production this year – could well make production cheaper and faster. The LFI (Long Fibre Injection) and D-LFT processes are already being used in series production. The former allows the setting of locally different fibre volume contents and fibre lengths. In both cases, the main areas of application are in the automotive industry.

The dream combination of continuous fibre reinforcement with a highly productive process is illustrated by the E-LFT process. With this promising technology, inlays with local continuous fibre reinforcement are placed in a component reinforced with long fibres. Their excellent mechanical properties combined with their suitability for series production quickly led to industrial application – for the rear flap of the Smart.

More recent approaches with continuous carbon fibre-reinforced parts such as automated tape laying (ATL) are particularly attractive from the point of view of automation (e.g. Fiberforge process). Nylon, PEEK or PPS is used as the matrix.



Innovations in the field of:


1) Automotive, aerospace

Innovative applications:

Air box for charge air cooler
Ultramid® A3W2 G10 from BASF SE: The highly filled material (50% glass fibres) was specifically developed to suit high performance egines specifications of long-term service temperatures of 190 °C and pressure changes of between 0.1 and 1.5 bar.

Resin Transfer Moulding (RTM): High-pressure hydrogen tank
From Profile Composites Inc (Canada): Production of high-pressure hydrogen tank by Resin Transfer Moulding (RTM) instead of the winding process.

Vacuum Assisted Process (VAP): Upper freight door A400M
From Premium Aerotec GmbH: Manufacture of large-sized parts from CFRP for aircraft construction by VAP. The VAP process is not only cheaper and lighter, it is also around 20% faster than conventional manufacturing methods.

Extrusion of endless circles and arches of any radius and lead
RadiusPultrusion™ from Thomas GmbH: This technology enables the continuous manufacture of curved reinforced profiles from endless fibres and webbing. This modification of the well known pultrusion process makes it possible to produce endless circles and arches of any radius and lead – for example for the design of springs.

Continuous fibre-reinforced long-fibre thermoplastic (C-LFT)
From Dieffenbacher GmbH & Co. KG: This process is a variant developed from the long-fibre thermoplastic direct process and has been supplemented by a special plant engineering for the processing of continuous fibres. Continuous fibre-reinforced thermoplastic profiles were used for local reinforcement in the rear flap of the Smart fortwo.



2) Medical technology, precision engineering, optics

CFRP cervical plate
Biomaterial Endolign™ from Invibio Ltd.: This composite material is a PEEK OPTIMA® polymer reinforced with continuous carbon fibres. Biocompatibility, high strength, X-ray transparency and repeated sterilisability are just some of the outstanding properties of Endolign implants.



3) Sport, leisure

Racing bike wheel
Cosmic Carbon Ultimate from Salomon SAS (France): Wheels of 12K carbon fabric with ultralight foam core, spokes of unidirectional carbon fibres and hub (front) of 100 % carbon.



4) Energy engineering

High-performance single-end glass roving for wind energy
WindStrand™ from Owens Corning, LLC: With this product, manufacturers of turbines can extend the blade length by up to 6 % and achieve up to 12% more performance – resulting in up to 20 % lower costs compared with competitive carbon glass hybrid solutions on the market.

Fibre composites and their processing - Vita Prof. Dr.-Ing. Volker Altstädt