Fifty years ago, automotive and aerospace parts were mainly processed by melting and shaping metals or alloys. Today, with the advent of fiber composites, individual carbon strands have become the focus of manufacturing technologies. Long before preforming can begin, individual carbon or fiberglass filaments are already being processed and combined to create a roving. While weaving such rovings into fabrics has been the norm for some time, Automated Fiber Placement processes such as the patented Compositence system have proven to be significantly more economic by working directly from pure rovings or prepregs.
Classification and Terminology
When constructing products ranging from sports equipment or high-performance tools to aerospace parts, the ideal solution would be to work on the level of individual filaments. This would allow the manufacturer the greatest amount of detail when determining the relative direction of fibers in each layer of the layup. However, working at this level would be very time-consuming and uneconomic. This is why entire bundles of fibers usually form the baseline for such composites.
A roving is normally unidirectional and unspun or otherwise shaped into patterns, making it the most simple form of raw fiber material employed in manufacture.
The term itself refers to bundles of separate filaments. These are always arranged as unidirectional continuous filaments, allowing for the production of any conceivable roving length. While the word itself is originally derived from its usage in spinning and knitting with materials such as wool, cotton or worsted, industrial fibers made from materials such as carbon, fiberglass or basalt are also referred to as such. This also explains the usage of the term “tow”, referring to particularly thick bundles frequently used in automotive applications.
As a rule of thumb, a distinction can be made between the different levels involved in carbon composite manufacture:
- Filament (individual fibers)
- Roving (UD bundles)
- Tow (thick bundles)
- Woven non-crimp fabrics
Pure rovings or tows do not already include an epoxy resin binder, as prepreg or towpreg materials do. When working with a pure roving, this makes it necessary to inject the binder during the layup procedure.
Sizes and Material Properties
There are various factors which determine the material properties of rovings. Aside from the actual material, the number of individual filaments in a roving is of great importance. This number is usually given in steps of 1000, meaning that a 24K roving, for example, consists of 24,000 individual filaments.
The filament number has a significant impact on all practical aspects of the final processed composite. A roving with a greater filament number tends to provide greater structural integrity and less elastic modulus. At the same time, the smaller the number in a specific roving, the better its drapability, making the material better suited for producing complex shapes and patterns. This is why smaller filament numbers are often preferred in industrial applications, since layups in this area are created using multiple thin layers oriented to support each other mechanically. This allows for the production of particularly thin yet robust composite parts.
The Baseline of Fiber Composite Manufacture
When planning the construction of composite materials including fiberglass or carbon fiber reinforcements, the right roving type choice is as essential as the choice of material itself. The specifics of the roving – thickness, shape and filament number – should always be determined according to the manufacturing process employed and the parameters necessitated by the planned composite part. Once this is done, technologies such as the patented Compositence system can use pure rovings to make the preforming process significantly more economic.