Continuous fiber-reinforced thermoplastic composites (TPCs) offer several advantages, including high inherent toughness, weldable assembly, recyclability, and rapid molding capabilities, resulting in lower part costs. For instance, stamping forming can produce TPC parts within minutes, whereas thermoset composites require hours. Over 5,000 stamped TPC clips and brackets are used on each Airbus A350 aircraft, with suppliers such as ATC Manufacturing, Collins Aerospace Almere (formerly Dutch Thermoplastic Components), Airbus (formerly Premium Aerotec), and Daher collectively producing over one million parts annually for various aircraft.
Short-cycle and automated methods are crucial for meeting the high productivity demands of next-generation narrow-body commercial aircraft, advanced air mobility (AAM) systems, unmanned aerial vehicles (UAVs), and drones. These methods also help reduce the cost of composite parts and assemblies.
Figure 1. Stamping forming process and typical thermoplastic composite (TPC) aerospace components. Source: ATC Manufacturing
The rapid molding of TPC materials can be described as thermoforming, compression molding, stamping forming, or simply stamping. The stamping forming process begins with a pre-compacted blank, which is rapidly heated and then transferred to a set of rapidly closing forming dies that shape and cool the part. Cycle times can be as short as 90 seconds, with even large, complex parts molded within 15 minutes.
Key steps in the stamping forming process include:
Material Preparation
Blank Consolidation
Blank Handling
Blank Heating and Transfer
Part Molding and Cooling
Tooling Considerations
Figure 2. Blank assembly and consolidation methods. Source: David Leach
The component is made from a pre-compacted blank, whose quality is critical to the performance of the finished part. For rapid molding, the blank must be compacted before forming to ensure rapid heat transfer into the material and high-quality compaction of the plies. Part manufacturers can cut blanks from fabric laminates (also known as organosheets) supplied by various material manufacturers in sizes up to 12 × 4 feet (3.7 × 1.2 meters). For more structural applications, unidirectional (UD) tapes with custom ply orientations are typically used. These blanks are often non-rectangular and may have varying thicknesses. Most TPC UD tapes are available in widths up to only 12 inches (305 mm), and edges must be joined at seams without gaps or overlaps. For parts with variable thickness, the location of ply edges in the blank is critical to meeting design tolerances. Initially, it was believed that blanks must meet the same quality requirements as finished parts, but in recent years, it has been proven that highly consolidated (but not 100%) blanks are sufficient. This has opened up exploration of other blank preparation and consolidation methods.
Various methods exist for preparing UD tape blanks (Figure 2), including manual and automated layup assembly, automated tape laying (ATL), and automated fiber placement (AFP). Strips of UD tape in the desired orientations (e.g., 0°, 45°, 90°) can be prepared manually or using commercially available equipment.
TPC materials are non-tacky, so laminates must be locally heated to bond them together in the appropriate direction. This can be achieved through manual or automated hot-melt or ultrasonic welding methods, joining laminates in the same direction along seams and bonding adjacent laminates thickness-wise. In thermoset prepregs, ATL is used with wide tapes to form flat or slightly curved layups, while AFP uses narrow tapes for contour layups. In contrast, TPC blanks are typically flat. Low energy can be used to form loosely bonded layups, which must then be compacted in a subsequent operation; high energy can be used to fully compact the blank, which can then be directly used for stamping forming. Therefore, the term "automated tape placement (ATP)" is used here to distinguish between high-energy or low-energy layup methods, regardless of prepreg width. Continuous compression molding (CCM) is also used to manufacture blanks, aligning tapes in the desired directions to form stacked layups, which are then immediately compacted in the same process.
There are numerous methods for consolidating layups into laminates for use as stamping forming blanks:
Single Pressing
Double Pressing (Hot/Cold Press)
Continuous Casting and Rolling
Autoclave
Vacuum Bag Only (VBO) in an Oven
High-Energy Deposition (ATP)
During stamping forming, the blank will be reheated to its melting temperature, so the polymer microstructure in the blank does not affect the final part, even for semi-crystalline polymers.
Pressing methods have evolved to be highly automated. Pressing techniques are used to manufacture laminates of constant thickness, with the chosen method depending on the required production volume, capital expenditure, and recurring costs. Single pressing has longer cycle times but allows for the simultaneous consolidation of multiple laminates using spacer plates between each ply. In double pressing, a "hot" press and a "cold" press are maintained at constant temperatures corresponding to the process and cure temperatures, respectively, with blanks automatically shuttled between the two presses. CCM enables continuous automated manufacturing, with layups automatically fed through a mold with hot and cold zones in the appropriate directions, producing very long laminates.
Using an autoclave for TPC consolidation may seem counterintuitive, but it allows for the simultaneous consolidation of multiple laminates and the manufacture of laminates with variable thickness. Vacuum bag only (VBO) oven consolidation is a similar method but uses only vacuum pressure (14.7 psi/0.101 MPa) instead of full autoclave pressure (typically 100 psi/0.7 MPa), offering the advantage of not requiring a pressure vessel. Large-area, high-temperature ovens are inexpensive, significantly reducing investment costs compared to autoclaves or presses while enabling the economical consolidation of oversized laminates. VBO consolidation has proven to produce high-quality blanks, but this depends on the UD tapes used. The final option is high-energy UD tape (ATP), which achieves high consolidation levels, typically exceeding 90%. This is a good choice for large, non-rectangular blanks with variable thickness.
The consolidated laminate must be machined to the appropriate size and shape for the finished part to allow for secure handling of the blank during heating and transfer. This is typically accomplished using fixtures or fixed frames that match the part dimensions (Figure 3). Fixation methods may include using springs to control blank movement during molding in the die. It can also involve support using polyimide film that does not impede heating. Blanks are usually dried before stamping forming to prevent porosity in the final part caused by small amounts of moisture absorbed by high-performance thermoplastic polymers during rapid preheating.
Figure 3. Blank holding methods. Source: Valeria Antonelli, Ph.D. thesis, Delft University of Technology, 2014
Blanks are typically heated to the process temperature in infrared ovens within minutes. For larger parts, multi-zone ovens can be used to ensure uniform heating across the entire blank. While rapid heating reduces cycle time, the final blank temperature must remain within a specific range that is high enough to ensure melting and polymer flow but not exceeding the polymer degradation temperature. These conditions must be met across the entire length, width, and thickness of the blank.
It is common to conduct trials using blanks with embedded thermocouples to adjust process conditions. Figure 4 shows thermocouple traces embedded in a 0.2-inch (5 mm) thick UD carbon fiber/PEKK tape blank during stamping forming. Thermocouples were placed at multiple locations on the part, including near the surface and at the center of the thickness. During heating, the traces from different locations showed minimal dispersion and stabilized within the PEKK process temperature range of 644-752°F (340-400°C). Blanks are typically heated for a fixed time, so it is important that heating is consistent from cycle to cycle and that the rate of temperature change is minimal at the end of the heating cycle, as shown in Figure 4.
Figure 4. Thermocouple traces for stamping forming of a unidirectional (UD) carbon fiber-reinforced tape component. Thickness: 0.2 inches (5 mm). Source: ATC Manufacturing
A small but critical step in the process is transferring the blank from the preheating oven to the press. This step must be completed quickly because the blank temperature drops rapidly once it leaves the oven (Figure 4). Typically, the maximum time from the preheating oven to the press is 5 seconds. Since the polymer is in a molten state, the integrity of the blank is maintained by the fiber reinforcement, causing the blank to sag and potentially even slip out of the fixture. This must be considered when designing the clamping mechanism and the transfer and placement on the forming tool.
Figure 5. Actual molding issues. Source: David Leach
To achieve the desired short cycle time, the press must close rapidly to mold the part, and the mold must maintain a constant temperature. The combination of these requirements poses challenges, as the continuous fiber reinforcement must flow quickly and consistently while the polymer must cool rapidly, leading to increased polymer viscosity.
As the polymer volume decreases significantly during cooling and solidification, the dimensions of the molded part change during cooling. Even after solidification, dimensions continue to change as the polymer cools to ambient temperature. This results in a "springback" effect, where the corner angles of the finished part are smaller than those of the mold. This can be modeled by combining the coefficient of thermal expansion (CTE) of the mold and the thermoplastic composite (TPC) as a function of temperature. Of course, the CTE of composites is highly anisotropic, so the CTE in the specific ply direction of each ply must be considered.
To mold and crystallize TPCs, mold temperatures typically exceed 400°F (204°C), and the blank temperature is even higher when it comes into contact with the mold, so metal molds are usually required. The use of elastomer molds or metal molds with elastomer surfaces on a single die offers advantages in molding complex parts by providing some compliance. The most common mold design is a matched two-part mold set, but the use of multi-part molds is increasing for more complex parts.
Figure 6. Multi-part mold for molding variable-thickness fuselage frames using UD TPC tapes. Source: Ron Jones, presentation at the 2022 ACMA Thermoplastic Composites Conference, Spirit AeroSystems
Software from companies such as AniForm (for molding) and Convergent Manufacturing (for thermal performance) can now simulate molding and thermal effects with high precision. Potential problem areas can be identified in advance, and virtual adjustments can be made to blank design, ply orientation, blank tensioning, and mold design. Simulations can now accommodate material-mold friction, variable-thickness parts, and flexible molds.
Figure 8 shows modeling of deformation strains during the molding of a complex part using UD TPC tapes. Thermal modeling can predict thermal hysteresis during melting and crystallization, thermal gradients, and thermoviscoelastic behavior during cooling. This allows for the calculation of residual stresses and the prediction of springback and warpage, enabling the design of thermally compensated molds.
Figure 7. Material strain deviation during molding of a window frame assembly using UD TPC tapes and AniForm software. Source: ATC Manufacturing
Today, the fundamentals of TPC stamping forming are well understood, and its advantages are expected to be widely applied in the aerospace and other industries. The continued development of TPCs will lead to even broader applications in the future.