Practical solutions to process-related challenges in LATW have great potential in pressure vessels and storage tanks. #Troubleshooting
Image source: ambliFibre project, Fraunhofer IPT
In recent years, thermoplastic composites (TPC) have been widely promoted because of their huge potential in increasing productivity and reducing weight. One of the most discussed topics of using TPC to increase productivity is in-situ consolidation (ISC) during automated fiber and tape laying (AFP/ATP).
In fact, ISC first demonstrated with restricted structures such as pipes, and continued to provide great potential for outer packaging pressure vessels and storage tanks. These structures can be produced using laser-assisted tape winding (LATW), which includes a laser-assisted tape winding (LATP) machine and a rotating mandrel (Figure 1).
Figure 1. LATW process. Image source: Figure 1, Hosseini, Baran, van Drongelen and Akkerman. "Regarding the temperature evolution during continuous laser-assisted tape winding of multiple C/PEEK layers: the effect of roll deformation." Int J Mater table (June 2020).
The AFP/ATP head is mounted on a CNC six-axis robot and uses a laser to heat the incoming tape and substrate to melt the thermoplastic polymer. A roller is then used to press the tape onto the substrate to promote interlayer adhesion and consolidation.
As with any additive manufacturing process, the first layer in LATW is a problem that should be solved. For pipes produced on metal mandrels (aluminum is preferred), consideration should be given to taking out the mandrel and using a release agent after winding.
Another consideration is fixation. If circular winding is applied to the first layer (as opposed to spiral or axial laying, Figure 2), tape can be placed manually at the beginning and end of the route to improve stability and adhesion to the incoming tape. If spiral or axial laying is required, a complete tape loop should be used and fixed to the edge of the cylinder, using tape to prevent slippage.
Figure 2 The toroidal winding is approximately 90° with the mandrel axis, the angle of the pole winding is closer to 0° (or axial), and the angle of the spiral winding is between the two. Image source: CW.
For Type IV pressure vessels (plastic lining with carbon fiber composite outer packaging), or when packaging plastic pipes, the incoming tape will be fused with the inner lining. This eliminates the need for tape fixation, but the plastic liner and TPC tape should have the same base polymer to ensure proper bonding.
Heating the plastic lining may cause problems. For example, if it cannot absorb laser heating, or if a metal mandrel is used, consider adjusting the laser angle to sufficiently heat the incoming tape. In other words, instead of the typical positioning so that the laser is distributed between the incoming tape and the substrate (Figure 1), the laser is tilted more toward the tape. This will avoid laser reflections, ensure more uniform heating of the tape and reduce energy consumption.
As mentioned above, usually during the LATP and LATW processes, the laser is distributed between the input tape and the substrate. In the case of constant radius geometry and ply direction, the distribution is constant. However, there are also considerations. For example, during axial layup on a tubular mandrel (roughly equivalent to the flat mold in Figure 3), the incoming belt will receive the same radiation and the substrate will receive more than the annular layup. Laser projection. A basket layup. Please note that this is negligible in large diameter pipes.
Figure 3 Laser-assisted placement process: tape winding (left) and tape laying (right). Image source: Figure 1.5, "Thermal skin effect in laser-assisted tape placement of thermoplastic composites", Thomas Weiler, January 2020.
During the heating process, as the polymer viscosity decreases, the thermoplastic tape unfolds and becomes wider and thinner, depending on the temperature and the laying speed (heating time). In a system with closed-loop control-the temperature of the heating zone is kept constant-the axial lamination will eventually result in a tape that is wider than the hoop lamination. Or, in a system with constant power control, the axial laminate will be formed at a lower temperature, so the tape will become wider. It is important to understand and solve this, because uneven changes in tape size may cause undesirable gaps and overlaps, thereby increasing the void content.
When wrapping the tube, the radius geometry is constant, but due to the end dome, the radius geometry will change when wrapping the pressure vessel. When entering or leaving these domes, the robot will slow down and the size of the laser spot on the substrate will decrease. Both of these behaviors can cause a sharp increase in temperature, which can lead to thinner cross-sections, different material properties, and even damage to the polymer.
Another consideration is that the size of the wound core (the mandrel added layers so far) is changing because the fiber layers of the part accumulate during the winding process. Therefore, the ply angle should be adjusted to avoid gaps. Some suppliers may provide LATP/LATW systems with software to update the diameter of the mandrel. If not, you must check the windings manually or using an online inspection system and correct them as needed.
Although it would be helpful to solve the above problems, it is impossible to 100% eliminate defects such as gaps, overlaps, and thickness variations. Therefore, the winding process should be developed to realize the design of the parts, instead of pursuing "perfect parts", which may increase unnecessary costs.
Thermal management is a key factor in achieving sufficient interlayer adhesion and full potential crystallinity in the thermoplastic matrix. The first layer is close to the mandrel, which acts as a heat sink. This prevents inter-diffusion of molecules and promotes poor interlayer adhesion, low crystallinity and higher void content. In order to overcome this challenge, consideration should be given to reducing the layup speed, increasing the temperature and adjusting the laser angle to extend the heating time to ensure molecular diffusion.
However, this does not ensure complete crystallization. This is because molecular diffusion—re-entanglement of molecules after melting during cooling—is a much faster process than crystallization, in which molecules are arranged to form an ordered crystal structure. If the manufactured part is very thin (approximately less than 2 mm), one would expect lower crystallinity than thick parts, which accept more consolidation times and less heat loss to the mandrel. For thin and thick parts, consider the slower initial layer layup speed.
To achieve complete crystallization, please consider the following:
Tape size is a key parameter for tape winding. Obviously, obtaining a constant tape width and thickness from a supplier will achieve a repeatable and uniform product. However, tape size also defines design flexibility. For example, when laying axially on a tubular mandrel, the width of the belt should be narrow enough to accommodate the curvature of the mandrel. The larger the diameter of the mandrel, the wider the tape that can be used. A wider tape means faster throughput, and a narrower tape is more suitable for design because it is more adaptable to curvature changes and easier to handle.
Although LATW has more problems and challenges, good parts are possible, and this manufacturing method will continue to evolve as composite pipes and tanks are used for hydrogen and other gas storage applications.
Yehiel Shaham is a plastics and polymer engineer with nearly 12 years of experience in the development and manufacturing of thermoplastics for Israel's leading plastics and defense companies. During 2016-2020, he was the chief engineer of RAFAEL's Thermoplastic Composites (TPC), where he specialized in TP-AFP. Currently, his goal is to promote TPC in Israeli industry.
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