AFP is used almost exclusively with continuous carbon fiber reinforced tapes — towpreg. It is a bundle of carbon fibers pre-impregnated with thermoset or thermoplastic, approximately 12 mm wide and 0.13 mm thick, wound on a spool.
The towpreg is fed into the headset that heats the tape by a hot gas torch or laser, etc. and presses it down. The high temperature either melts thermoplastic or makes thermoset more sticky providing the needed adhesion between the plies.
The method provides flexible control over the process while keeping a certain precision, it can work on nearly any type of surface and can repeat complex shapes and corners.
AFP emerged as a response to the demand for increased manufacturing rates and reliability of the manufacturing process: complex geometries and ability to satisfy aerospace and aviation sectors demand and some other industries that require vast production volumes such as oil and gas.
Naturally, the cost of such equipment is quite high and commonly starts from 1 mio $ for a very basic R&D machine not suitable for industrial use. Another naughty point to keep in mind is that all technologies associated with prepreg only provide a half-way product that is supposed to go to an autoclave mold. An autoclave is a special camera for sintering under high pressure close to 10 atmospheres and high temperature.
This is another costly step to add to AFP, though the quality of such composite products is very high. There are very few industries and even fewer businesses that can afford both AFP and post-processing, however it is suitable for a great number of applications.
There exists another technology that is almost similar to AFP: Automated Tape Laying (ATL).
ATL utilizes a single wide prepreg tape (around 30cm) to layup simple or flat parts, there deposition rate is even faster due to the material — wide tapes cover more area quicker but at a cost of less sophisticated contours. AFP in turn uses 12mm wide tapes and is capable of providing very complex shapes but slower compared to ATL and inevitably more expensive.
There is a whole group of technologies that strive to bridge the gap between the high precision composite manufacturing and the need to provide industrial volumes of composites while offering an affordable price. Vacuum infusion (manual layup of large pieces of carbon fiber fabric on a mold and infusing it with epoxy), for instance, does not require autoclave molding and does not imply costly machinery, though its deposition rate is a far cry from AFP and a far and desperate cry from ATL.
Fiber placement machines (FPM)
A fiber placement machine typically consists of the following components: a head with a compaction roller and a tape feeding system (sometimes holding 12-32 spools of fiber tapes), a multi-axial kinematic system holding the head, and software with a human-machine interface.
The head engine power can easily reach 1500 HP to be able to move the machine quick enough and dynamically enough to fulfill the requirements of deposition rate for aerospace, and the torque motor acceleration may be around 1200 degrees per second squared. This is some really surprising swiftness for such a heavy machine, provided that reliability and accuracy requirements are extremely tough.
AFP implies high upfront costs, a lot of experimenting and empiric knowledge, more time for developing and production, which inevitably slows down industrial implementation. So one can imagine the FPM price is sky-high — it starts from 1 mio and reaches dozens of millions of dollars.
Future of AFP
The increasing demand for faster deposition rates, the emergence of new materials and composites as well as the downward pressure on cost will fuel the growth of AFP. Pundits predict the shift from thermosets to thermoplastics and advanced head construction that will allow even more flexibility and accuracy for complex shapes and surfaces of products to be made.