Automated Fiber Placement (AFP) Prepress Guide

1. Fundamentals of Automated Fiber Placement (AFP) & Tow Steering

Automated Fiber Placement (AFP) is the pinnacle manufacturing technology for high-performance aerospace structures, including the Boeing 787 and Airbus A350 fuselage barrels, wing skins, fan cowls, and space launch vehicle interstages. AFP robotic end-effectors collimate and deposit multiple narrow slit tape prepreg tows (typically 8, 16, or 32 tows of width $b = 3.175\text{ mm}$ to $12.7\text{ mm}$) under heat and pressure onto complex 3D tooling mandrels.

By independently controlling individual tow feed, cut, and restart capabilities, AFP enables fiber steering—depositing fibers along curved trajectories to create variable-stiffness tailored laminates that redirect primary load paths around window cutouts, door hatches, and fastener rows, dramatically reducing structural weight.

2. Mathematical Formulations & Wrinkling Physics

Steering a flat, straight prepreg tow along an in-plane curve of radius of curvature $R$ creates differential path lengths across the tape width $b$:

• Differential Edge Strain Distribution ($\epsilon(y)$):

  \epsilon(y) = \frac{y}{R} \quad \text{for } -\frac{b}{2} \le y \le \frac{b}{2}

  The outer edge ($y = +b/2$) experiences tensile elongation $\epsilon_{tensile} = +b / (2R)$, while the inner edge ($y = -b/2$) is subjected to compressive strain $\epsilon_{comp} = -b / (2R)$.
• Critical Steering Radius ($R_{min}$) & Out-of-Plane Buckling: Because continuous carbon fibers cannot accommodate large in-plane compression, the inner edge buckles out-of-plane, causing tow wrinkling, tow folding, or tow peeling from the substrate. The theoretical critical buckling limit is:

  R_{min} = \frac{b}{2 \cdot \epsilon_{crit}} = \frac{b}{2} \cdot \left( \frac{\pi^2 E_{11} t^2}{12 G_{12} b^2} + \frac{F_N \mu_{tack}}{b \cdot t} \right)^{-1/2}

  where $E_{11}$ is longitudinal fiber modulus, $G_{12}$ is in-plane shear modulus, $t$ is single-ply thickness, and $F_N$ is compaction roller normal force.
• Tow Gaps & Overlaps: When steering a parallel band of multiple tows (a course), constant course width geometry dictates that tows must either gap or overlap along curved paths:

  \Delta_{gap/overlap} \approx \frac{b^2}{8 R} \cdot N_{tows}

3. Vector Prepress & CAM Trajectory Optimization

Generating defect-free AFP robot trajectories from CAD solid models requires rigorous vector prepress rules:

1. Spline Curvature Clamping: Prepress algorithms must analyze the curvature profile $\kappa(s) = 1/R(s)$ along every toolpath vector spline. Any section where $\kappa(s) > 1/R_{min}$ must be automatically re-parameterized or split into narrower slit tape widths ($3.175\text{ mm}$ vs $6.35\text{ mm}$).
2. Tow Staggering & Step Cuts: At course boundaries and ply drop-offs, individual tows must be cut with trapezoidal angle compensation ($45^\circ$ or $90^\circ$ stepped offsets) to prevent stress concentrations and thick resin pockets.
3. Mesh-to-Vector Surface Projection: 3D NURBS mold surfaces must be flattened into 2D UV coordinate space with isometric surface metric tensors ($g_{ij}$), ensuring zero geodesic distortion during robot kinematic conversion.
4. Dynamic Heating & Compaction Control: Vector files should carry auxiliary metadata tagging high-curvature zones ($\kappa > 0.001\text{ mm}^{-1}$) to automatically increase laser/infrared heating power by $15\% - 25\%$ and reduce head deposition velocity to promote prepreg tack adhesion.