In the last article, we mentioned different processes that can be used to manufacture CFRP parts. Here we want to give a more detailed overview of the most common manufacturing methods for CFRP parts and how they work.
In wet carbon fiber layup, the fibers often come in the form of twills, that need to be cut to size, are first laid out by hand onto a mold. The mold needs to be manufactured beforehand as its structure is what dictates the form of the part that is to be manufactured. They can be made from wood, foam, waxes, or metal. The fibers are then draped into the desired position and orientation and afterward coated with resin and a hardener. The part is then heated in a vacuum bag until the resin has hardened and the proper matrix has formed.
Wet fiber layup is usually relatively cheap but requires a lot of manual work. The parts are weaker and heavier than the parts produced by the other methods and usually have a shiny “wet” appearance. It is also commonly used for DIY projects.
The RTM process technically works like the wet carbon fiber layup but is technologically more advanced and automatable. The fibers (in the form of mats like weaves or twills) are cut to size and inserted into a two-piece mold, compromised of a lower and an upper half. Once the preformed fibers are placed correctly, the mold is closed, and the liquid matrix system (resin) is injected into the mold cavity at high pressure and temperature. The resin displaces all the remaining air in the mold and then starts to harden. The resulting part has smooth surfaces on both sides, a consistent thickness, and fiber-volume content. RTM is a process that can be integrated well into a process chain made for serial or high-volume productions since it can be easily automated. However, the initial costs for the mold and the process control system are usually high in comparison.
Figure 1: Schematic diagram of a resin transfer molding process (RTM)
Note: The terms “wet” and “dry” correspond to the condition of the carbon fibers while the part is manufactured. In wet layup, the fibers are soaked with resin, making them wet. In dry layup, the fibers are already impregnated by a partially cured matrix, which makes them easier to handle.
Dry carbon fiber layup processes often use prepreg materials such as uncured twills or weaves. Just like the wet layup, these are draped onto their respective molds. Afterward, the part must be cured in an autoclave at very high pressures and temperatures so the resin can harden properly.
Very complex parts can quickly be manufactured using more advanced machinery, such as robots or gantries with specialized end-effectors that can feed and cut tows or tapes. Many companies provide different solutions for the so-called Automated Fiber Placement (AFP) and Automated Tape Laying (ATL) technologies.
Figure 2: Broetje-Automation's STAXX Flex layup head. It's using a segmented compaction roller with 16 ¼in tows.
The big benefit that AFP processes provide is that they are flexible and highly customizable. Instead of buying entire mats that need to be cut to size, AFP makes it possible to lay only the material that is needed, because the end-effector can cut each tow individually. This functionality drastically decreases waste yet maintains high productivity. This makes AFP a very useful technology, especially for serial production.
In the next post, we will give a more detailed overview of the AFP process, advantages, and limitations and how it can be used to optimize material layup even for complex parts.
Until then, stay safe and stay tuned.