Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : December 17 , 2025

Abstract

Prepreg ply adhesion prior to cure is a governing factor in the stability and quality of Automated Fiber Placement (AFP) processes for thermoset composite structures. In industrial practice, AFP parameters such as heating, compaction force, and layup speed are often fixed at the beginning of production, while prepreg material condition evolves continuously due to room-temperature exposure and handling. This paper examines the combined influence of material heating and room-temperature aging on ply-to-ply adhesion during AFP, based on experimental observations obtained using an AFP-relevant lap shear testing approach. The results show that heating strongly increases interply adhesion by promoting resin mobility and intimate contact formation. However, adhesion behavior under heated placement does not evolve monotonically with aging: separation force increases during early aging stages before declining at extended out-time, while displacement at peak force decreases with aging, indicating a shift toward less compliant separation behavior. These findings demonstrate that maximum adhesion is not a sufficient indicator of manufacturability and that fixed-parameter AFP recipes are inadequate when material condition changes during production. The paper discusses practical implications for adaptive AFP parameter control, material handling strategies, and prepreg conversion quality, particularly for slit tape feedstocks used in aerospace manufacturing.

Keywords

Automated Fiber Placement; Prepreg Adhesion; Tack Control; Infrared Heating; Out-Time Aging; Adaptive Process Control; Slit Tape Prepreg

1. Introduction

Automated Fiber Placement (AFP) has become a core manufacturing technology for high-performance aerospace composite structures due to its ability to place material with high positional accuracy, repeatability, and reduced labor content. AFP is widely used for primary structures such as fuselage skins, wing covers, and pressure-critical components, where placement stability and defect control are essential for both structural performance and certification.

Despite the maturity of AFP hardware and software, process robustness remains strongly influenced by the behavior of uncured prepreg during placement. Defects such as tow lift, bridging, course-start instability, and local gaps often originate from insufficient or inconsistent adhesion between adjacent uncured plies at the moment of placement. These defects become more pronounced as AFP systems move toward higher deposition rates, tighter curvature capability, and thinner plies.

In thermoset AFP, ply-to-ply adhesion prior to cure is governed by resin viscosity, surface wetting, contact pressure, contact time, and thermal history. These factors are not static during production. Prepreg material evolves due to room-temperature exposure, commonly referred to as out-time or aging, even when storage limits are respected. At the same time, AFP process parameters are frequently defined as fixed recipes that do not account for changes in material condition during a build.

This mismatch between evolving material state and static process settings represents a persistent source of variability in AFP manufacturing. Operators often compensate empirically by adjusting heater output or layup speed, but such adjustments are rarely supported by quantified relationships between material aging, heating, and adhesion behavior. A clearer understanding of how heating and aging interact to influence ply adhesion is therefore required to support more robust and adaptive AFP processes.

This paper examines these interactions using experimental observations obtained at the German Aerospace Center (DLR), where the influence of heating and room-temperature aging on prepreg ply adhesion was systematically investigated under AFP-relevant conditions. The results are interpreted from a manufacturing engineering perspective and translated into practical guidance for adaptive AFP process control and feedstock preparation.

2. Literature Review

2.1 Prepreg adhesion and tack in AFP

Prepreg adhesion prior to cure is commonly described using the term “tack,” but in AFP manufacturing this adhesion represents a combined outcome of interfacial contact formation and cohesive resistance within the resin-rich surface layer. Tack behavior depends on resin formulation, degree of advancement, temperature, pressure history, and contact time. Because these factors vary during AFP placement, tack should be viewed as a process-dependent response rather than a fixed material property.

Previous studies have highlighted the difficulty of measuring prepreg tack in a manner that is representative of AFP conditions. Probe tack and peel-based methods often fail to reproduce ply-to-ply contact behavior and can become unreliable as materials age and stiffen. As a result, comparisons between different tack studies are often complicated by differences in test geometry and failure mode.

2.2 Aging and out-time effects on AFP manufacturability

Room-temperature aging alters prepreg behavior through partial resin advancement, changes in viscosity, and increased uncured stiffness. Several investigations have shown that aging can influence AFP steering stability, defect formation, and rework rates. Importantly, aging effects are material dependent and may not follow a simple monotonic trend.

While early assumptions treated aging primarily as a loss mechanism leading to reduced tack, later work has shown that aging can increase certain resistance measures while simultaneously reducing compliance and conformability. This dual behavior complicates process control strategies based solely on out-time limits or simple correction factors.

2.3 Heating as a control variable in AFP

Heating is widely used in thermoset AFP to activate adhesion and promote interply contact during placement. Infrared, hot-gas, and laser heating systems are commonly employed, often controlled using relative power settings rather than direct measurement of tow temperature. Heating primarily acts by reducing resin viscosity, enabling the tow to conform to the substrate and form intimate contact within the limited compaction time available at AFP speeds.

While heating is recognized as a powerful process lever, its interaction with material aging is less well quantified. Understanding this interaction is critical for developing adaptive AFP strategies that respond to material condition rather than relying on fixed recipes.

3. Methodology

3.1 Experimental basis

The experimental observations discussed in this paper are based on a controlled study conducted at DLR, in which slit tape prepreg specimens were manufactured using AFP and evaluated for ply-to-ply adhesion. The experimental design focused on isolating the effects of material heating and room-temperature aging while maintaining consistent placement conditions.

3.2 Material and specimen preparation

An aerospace-grade epoxy-based slit tape prepreg was used to manufacture overlapping ply specimens via AFP. The overlap geometry was selected to produce a defined ply-to-ply contact area representative of AFP placement conditions. Using AFP to fabricate the specimens ensured that contact pressure, thermal exposure, and placement kinematics were consistent with industrial practice.

3.3 Process parameters

Layup speed and compaction force were held constant throughout the study to isolate heating and aging effects. Infrared heater output was varied across a defined range, expressed as a percentage of maximum power. Prepreg material was stored at room temperature and tested at regular intervals over an extended aging period.

Heating was controlled using relative power settings rather than direct tow temperature measurement. While this limits absolute thermal interpretation, it reflects common industrial AFP practice and allows the results to be discussed in a production-relevant context.

3.4 Adhesion measurement

Adhesion was evaluated using a lap shear configuration in which the overlapped plies were separated under controlled displacement. Two primary response variables were recorded: the maximum force required to separate the plies and the displacement at which this maximum force occurred. This approach provides a repeatable, AFP-relevant proxy for ply-to-ply adhesion that remains workable for aged materials.

4. Results

4.1 Effect of heating on ply adhesion

Increasing heater output produced a strong increase in the force required to separate adjacent plies under otherwise identical placement conditions. This increase was observed across all aging states examined. The result indicates that heating significantly enhances resin mobility and contact formation during placement.

4.2 Aging behavior without heating

When specimens were placed without additional heating, the measured separation force remained relatively stable over the aging period examined. This suggests that shear-mode adhesion alone may not be highly sensitive to aging over short-to-moderate out-time when no heating is applied.

4.3 Combined effects of heating and aging

Under heated placement conditions, adhesion behavior evolved in a non-linear manner. Separation force increased during early aging stages, reached a maximum after approximately one to two weeks of room-temperature exposure, and then declined with further aging. This behavior indicates the presence of competing mechanisms influencing adhesion.

4.4 Displacement trends

Displacement at peak separation force decreased progressively with aging, particularly under heated conditions. This trend indicates a reduction in compliance and a transition toward more abrupt separation behavior as the material ages.

5. Discussion

5.1 Interpretation of heating effects

The results confirm that heating is the dominant control variable influencing prepreg ply adhesion during AFP under fixed speed and compaction conditions. Heating reduces resin viscosity and accelerates intimate contact formation within the limited placement window.

5.2 Non-monotonic aging behavior

The observed increase in adhesion during early aging under heating challenges the assumption that aging uniformly degrades tack. Instead, aging alters both cohesive resistance and interfacial wetting. Early aging may increase resistance to shear separation, while extended aging reduces contact quality and compliance.

5.3 Manufacturability versus maximum adhesion

High separation force alone is not a sufficient indicator of good placement behavior. Reduced displacement at peak force and more abrupt separation indicate increased brittleness, which can increase sensitivity to curvature, peel stresses, and steering-induced defects. Effective AFP placement requires adhesion within a defined operating window rather than maximization.

5.4 Implications for adaptive AFP control

From a manufacturing perspective, the results support adaptive AFP strategies that account for material state. Practical approaches include tracking material exposure history, adjusting heater output based on expected adhesion behavior, and using placement stability indicators as feedback to avoid brittle operating regimes.

5.5 Implications for prepreg conversion and slitting quality

Although the experimental focus is on heating and aging, the governing mechanism repeatedly returns to intimate contact formation. For slit tape prepreg, conversion quality—including edge condition, width consistency, and roll build—directly influences pressure distribution and contact formation during placement. Consistent feedstock quality is therefore essential for effective adaptive process control.

6. Conclusion

This paper demonstrates that heating is the dominant variable influencing prepreg ply adhesion during AFP, while room-temperature aging introduces complex, non-monotonic behavior under heated conditions. Separation force alone does not adequately describe manufacturability, as increased adhesion can coincide with reduced compliance and increased defect sensitivity.

The key conclusions are:

• Fixed AFP parameter recipes are inadequate when material condition evolves during production
• Adhesion should be controlled within a defined operating window rather than maximized
• Heating should be treated as the primary lever for stabilizing placement behavior
• Material exposure history and feedstock conversion quality must be integrated into process planning

By treating prepreg adhesion as a managed process outcome rather than a static material property, AFP manufacturers can improve placement stability, reduce defects, and enhance overall process robustness.

References

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Budelmann, D., Schmidt, C., and Meiners, D. Prepreg tack: mechanisms, measurement methods, and implications for manufacturing. Polymer Composites, 2020.

Heller, K., Ghabchi, A., Centea, T., and Nutt, S. Quantifying the influence of out-time on prepreg material properties and out-of-plane steering defects during automated fiber placement. Frontiers in Materials, 2022.

Brasington, A., Sacco, E., and Koutsos, V. Automated fiber placement: history, current technologies, and future developments. Composites Part C, 2021.