This blog post is ranslated from German with DeepL.
The correct term is actually not fiber composite, but fiber composite material. These are different types of fabric, which in turn are bonded together with different composites, a so-called matrix. The matrix can be, for example, resins of different hardnesses (depending on the application), from low to high temperature resistant.
The work, or rather the processing of fiber composites is a delicate matter, since the material, as the name suggests, is a composite of several sub-materials.
These fabrics can be made of the following materials, for example:
– Carbon fabric (high electrical and thermal conductivity, stable, easy to process, lightweight compared to many other fabrics).
– Glass fiber fabric (low electrical conductivity, insulator, high light conductivity, resistance to chemicals as well as other environmental influences, non-flammable)
– Aramid fabric (= “Kevlar”; insulator, light, strong, impact resistant)
– Natural fiber fabric (flexible and tear-resistant, e.g. flax fiber)
– Basalt fiber fabric (brittle, otherwise very similar to glass fiber)
We would like to discuss in more detail the reinforcement of plastics by carbon fabric, which results in a very strong, at the same time light material and is colloquially called carbon (also carbon). Carbon/carbon fabric is a carbon fiber reinforced plastic (CFRP) in which the carbon fibers are embedded in a plastic matrix, usually epoxy resin, which also produces the typical fabric appearance.
The stiffness resulting from subsequent curing with the ability to absorb high acceleration forces in combination with low weight has made carbon very dominant in many product categories. From fishing rods, bicycle frames and sporting goods to medical technology products, vehicle construction and aerospace technology – carbon as a material has become indispensable.
In terms of processing, there are again different variants, the “main pillars” of which are the following:
Prepreg is a fabric that has already been pre-impregnated with resin at the factory and is therefore very sticky during lamination, depending on the ambient temperature. To maintain its properties until processing, prepreg must be stored at approximately -18°C. Used primarily for exposed components.
2. Wet laminate
Dry fabric manually impregnated with resin during processing. Suitable for small to medium production quantities and large components.
Industrial process in which resin is injected into the fabric. Usually used in mass production for large quantities.The processing requires some attention, because in order to make the best use of the properties of these amazing materials it is necessary to follow some rules in each process.
In addition to the storage challenges already mentioned, there are a few things to consider during processing:
the less resin = the lighter the laminate
the less resin = the higher quality fabric, guide value: Ø 60% fiber content, 40% resin
Material build-up takes place with pre-laminated fabrics and pressure/temperature processes – curing e.g. in an autoclave, whereby the higher the pressure, the more compact and stable the component, at the same time it is also of higher quality. We explain an example of this later in this article.
2 Wet laminate
The dry laminate is “resin-coated” with the aid of brushes, rollers or spray guns. Due to the precise and fine processing by hand, the components are often used in the visible area, just like prepreg components: they are therefore also called directly visible components.
To be noted:
- The difficulty lies in the dosage, or rather the even application of the resin, in order to obtain an optimum component.
- The fabric is sometimes very stiff and therefore more difficult to handle than prepreg.
- Since it is often not possible to work as precisely as with prepreg, this is more a process for quick tests and individual parts.
3 Injection process
As this is a rather complex and demanding process, which requires both the appropriate machinery for automation and the corresponding infrastructure, this type of production is primarily suitable for mass production.
The following conditions and circumstances are crucial:
- The production is a closed process (low emission).
- The components can be very complex
- The components should correspond as closely as possible to the final shape and require little post-processing
- The surface quality must be high
- High piece count capacity with consistent quality; little to no manufacturing tolerances
- Processing of dry blanks (semi-finished fiber products) and resins with low viscosity, high fiber volume fractions possible
- Cycle duration is determined by the injection and curing times
Graphic setup vacuum infusion: courtesy of www.hp-textiles.com
Our project: Luggage
We would like to take a closer look at Prepreg – Gimelli Engineering AG develops and builds luggage shells with a focus on Prepreg for the company Swiss Luggage SL AG.
We would like to bring this work closer to you.
The manufacturing requirements and the process
A master pattern must be produced (preliminary stage: tool production)
An impression (negative mold), if necessary with a stitch line, i.e. with a guide line for later cuts, must be created (= laminating tool)
A silicone bag (actually a silicone mat) for compression/vacuuming/curing (autoclave) must be made; alternatively, depending on the component, a vacuum film can be used.
The most critical point is now the laying of the laminate/prepreg, because here it is essential to comply with the ply build-up definition, i.e. the fiber direction:
0/90° = fixed ply, +/- 45° = formable, flexible ply (see graph below).
Depending on the component, a corresponding number of layers of fiber fabric are required. In addition, a symmetrical layer structure (outside = inside) is absolutely recommended to prevent warpage of the component.
The structure in cross-section is from the inside to the outside (see graphic “Layer structure cross-section”).
5. Autoclave preparation
The laminate is now placed in the mold, release film and tear-off fabric (used or omitted depending on the component) are applied, non-woven fabric is applied as an air-dissipating material, perforated (perforated) film and absorbent non-woven fabric are applied, and finally the whole is enclosed with a packaging/vacuum film or covered with the silicone bag specially produced for this mold (reusable and for higher quantities) (see graphic). The component is now de-aerated via the vacuum pump.
Packaging layer structure of a suitcase shell (corner).
Insertion of the packaging layers into the mold
6. Autoclave cycle
Depending on the intended use of the component and the quality requirements in the subsequent area of application, curing takes place in the autoclave under heat, pressure and vacuum. The more layers are built up, the more the interaction of these 3 components is decisive for the result. Please also refer to the manufacturer’s data sheets on curing time.
Top: Autoclave cycle (temperature curve) schematic.
Bottom: smaller and larger autoclave (image source: Lu-Chen-Composite)
After curing/cooling, the part is carefully demolded from the mold and released for further processing (contour cutting with special tools, e.g. diamond grinder and drill, deburring, painting, assembling, etc.).
Image: Processing of a case shell finished in the autoclave by a robot.
So there are various potential “stumbling blocks” that can arise in the course of processing, and their avoidance must be thought about and planned for in advance.
Laminates, in our case specifically the carbon laminates, are interesting and stable materials that can be processed very well with appropriate knowledge and preparation.
The functional and optical requirements of the product are decisive for the choice of the manufacturing process and the layer structure.