Introduction: The invisible control variable
Anyone processing thermoplastic composite materials knows the rule of thumb: The temperature at the nip point — the point where incoming tape and substrate meet under the pressure roller — is the critical process parameter. Too cold: insufficient matrix melting, no adhesion, no consolidation. Too hot: degradation, pore formation, dimensional accuracy at risk.
Sounds manageable. It is not.
Three independent research studies — from Aachen, Bristol, and Delft/Enschede — together paint a significantly more complex picture: The nip point temperature is not a single, measurable quantity, but the result of at least three interacting thermal phenomena, each of which is capable of compromising component quality. Whoever has control over just one of them has not yet mastered the process.
Phenomenon 1: The surface lies — especially at high speeds
The basic principle of laser-assisted tape winding (LATW) sounds physically attractive: A laser hits the material with nearly zero thermal transition resistance (Biot number → ∞), the surface is brought to process temperature in milliseconds, a pyrometer or thermal camera measures the surface temperature — done.
The problem: At higher process speeds, the temperature distribution across the material thickness fundamentally changes. Weiler et al. (2018) analytically show that as speed increases, heat has less and less time to diffuse into the depth of the tape. The surface can be kept exactly at the target temperature — and the bonding interface on the back of the tape remains well below the melting temperature. A constant surface temperature at high speeds is no guarantee of a constant bonding interface temperature.
The consequence for practice: Simple surface temperature control is structurally inadequate in LATW processes with variable or high speeds. What is needed is an understanding of the transient temperature profile across the thickness — and a control system that takes this into account.
Phenomenon 2: Constant power is not a solution — laser power must scale with speed
Another misconception in practice: setting the laser power to a fixed value and varying the speed. Di Francesco et al. (2017) show both analytically and experimentally that the laser power required to maintain a constant nip point temperature scales proportionally to the square root of the deposition speed:
P(V) ∝ √V
This means: Doubling the deposition speed requires an increase in laser power by a factor of √2 ≈ 1.41. If this is not taken into account, the input energy per unit area — and thus the nip point temperature — fluctuates significantly. Especially during acceleration and deceleration phases, in curves, or during starts and stops of the winding head, systematic temperature inhomogeneities arise that directly affect the laminate quality.
The solution is a speed-dependent laser power control — either as an open control loop based on a pre-calibrated characteristic curve or as a closed control loop with real-time temperature measurement. Both require that the process is modeled and the coefficients of the control function are determined. A "trial-and-error" approach, as is still often encountered in industry, does not provide reproducible results.
Phenomenon 3: The hidden thermal resistance between the layers
Even if the surface temperature and laser power control are correctly designed, a third factor remains that is simply ignored in most thermal models: the inter-laminar thermal contact resistance (TCR).
Çelik et al. (2021) show that when laying down each new layer, air inclusions remain at the interface between the incoming tape and substrate as long as the intimate contact — the complete surface contact of both surfaces — has not yet been fully established. These air pockets act as thermal insulators: The thermal conductivity of air is only about 0.025 W/(m·K), compared to around 0.7 W/(m·K) for CF/PEEK in the thickness direction. The result is an increased thermal resistance at the interface, which hinders heat dissipation into the substrate and alters the temperature distribution in the freshly laid tape.
Thermal models that do not consider TCR systematically underestimate the measured temperatures. Experimentally, it also shows: The contact force of the roller affects the intimate contact — and thus the TCR — more than the laser power. This means that pressure and temperature in the LATW process cannot be optimized independently of each other. Those who only vary the laser power to improve quality are addressing the wrong control variable.
What this means for in-situ consolidation
Direct answer for practice: In-situ consolidation in LATW can only be reliably achieved if all three thermal levels are controlled simultaneously: the temperature distribution across the material thickness (transient profile), the speed-dependent laser power control, and the inter-laminar thermal contact resistance as a function of intimate contact. Anyone who only controls one of these levels risks systematic quality fluctuations — even if the surface temperature at the nip point appears constant.
Research in recent years shows that process control in LATW requires a deep integration of optical models, thermal simulations, and experimental process calibration. NASA and leading European research institutions are actively working to validate these models for industrial high-rate production — a clear sign that the technology is mature, but process expertise remains the actual bottleneck.
Process control as a true differentiating factor
For companies that procure or develop thermoplastic composite structures, this complexity has a direct consequence: The question is not only whether a supplier operates LATW machines — but whether they have understood and calibrated the process at all three thermal levels.
Alformet has direct access to this process know-how through its close connection to AFPT GmbH — one of the most experienced machine builders and process developers in the field of LATW. The combination of machine-related development knowledge and in-house manufacturing experience allows for process parameters to be designed not according to tabulated values, but specifically for the component — for reproducible quality from prototype to series.
Conclusion
The temperature in the LATW process is not a number. It is a three-dimensional, time-dependent field — shaped by process speed, laser power profile, and the state of the interface between the layers. Those who understand this understand why process control is not a marketing promise, but a technical achievement.
If you are planning a lightweight structural project with thermoplastic composites and want to know how Alformet can meet your specific requirements — contact us.
📚 Sources
Weiler, T., Emonts, M., Wollenburg, L., Janssen, H. (2018): Transient thermal analysis of laser-assisted thermoplastic tape placement at high process speeds by use of analytical solutions. Journal of Thermoplastic Composite Materials, 31(3), 311–338. DOI: 10.1177/0892705717697780
Di Francesco, M., Veldenz, L., Dell'Anno, G., Potter, K. (2017): Heater power control for multi-material, variable speed Automated Fibre Placement. Composites: Part A, 101, 408–421. DOI: 10.1016/j.compositesa.2017.06.015
Çelik, O., Hosseini, S.M.A., Baran, I., Grouve, W.J.B., Akkerman, R., Peeters, D.M.J., Teuwen, J.J.E., Dransfeld, C.A. (2021): The influence of inter-laminar thermal contact resistance on the cooling of material during laser assisted fiber placement. Composites: Part A, 145, 106367. DOI: 10.1016/j.compositesa.2021.106367
NASA (2023): Thermal Response of Thermoplastic Composite Tape During In-situ Consolidation Automated Fiber Placement Using a Laser Heat Source. SAMPE Conference Proceedings, Seattle.
NASA (2024): Thermal Modelling of the in-Situ Consolidation of Automated Fiber Placement of Thermoplastic Composites. NTRS Report 20240001488.