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Continuous fiber-reinforced thermoplastic composites (TPC) offer high inherent toughness, welded assembly, recyclability, rapid forming and hence lower part cost. For example, stamp forming can produce TPC parts in minutes versus many hours using thermoset composites. More than 5,000 stamp formed TPC clips and brackets are used on each Airbus A350 aircraft, and suppliers such as ATC Manufacturing, Collins Aerospace Almere (formerly Dutch Thermoplastic Components), (formerly Premium Aerotec) and have collectively manufactured over 1 million parts per year for various aircraft (see “TPC stamp forming” articles above).

Short cycle times and automated methods are key to meeting the high production rates being sought for the next generation of narrowbody commercial aircraft, advanced air mobility (AAM), unmanned aerial vehicles (UAVs) and drones. They also enable cost reduction of composite parts and assemblies.

Stamp forming overview

Rapid forming of TPC materials into parts can be described as thermoforming, compression molding, stamp forming or stamping. I prefer the term stamp forming (Fig. 1) because thermoforming, compression molding and other terms can refer to a range of processes, not all of which are rapid forming. The stamp forming process starts with a pre-consolidated blank which is heated rapidly and then transferred to a set of quick-closing forming dies which shape the blank and cool the part. Cycle times of 90 seconds are possible and even large and complex parts can be formed in less than 15 minutes. Key steps in the stamp forming process include:

• Material preparation
• Blank consolidation
• Blank handling
• Blank heating and transfer
• Part forming, cooling
• Tooling considerations.

Material preparation

The part is formed from a pre-consolidated blank and its quality is critical to finished part performance. For rapid forming, the blank must be consolidated prior to forming to ensure rapid heat transfer into the material and to ensure high-quality consolidation of the plies. Part fabricators can cut the blanks from fabric laminates, also known as organosheets, which are supplied by several material manufacturers in sizes up to 12 × 4 feet (3.7 × 1.2 meters). For more structural applications, unidirectional (UD) tapes are frequently used with tailored ply orientations. Such blanks are often non-rectangular and may be of variable thickness. Most TPC UD tapes are available only in widths up to 12 inches (305 millimeters) and the edges must be joined at the seams without gaps or overlaps. For variable-thickness parts, the location of the edge of ply in the blanks is key to meeting design tolerances. It was believed initially that the blank had to meet the same quality requirements as a finished part, and in recent years blanks with a high degree of consolidation (but not 100%) have been shown to be sufficient. This has opened other methods of blank preparation and consolidation.

Several methods can be used to prepare UD tape blanks (Fig. 2) including manual and automated ply assembly, automated tape layup (ATL) and automated fiber placement (AFP). Long lengths of individual plies in the appropriate orientations (e.g., 0°, 45°, 90°) can be prepared manually or using commercially available equipment.

TPC materials do not have tack, therefore the plies must be heated locally to tack them to each other in the appropriate orientations. This can be accomplished with manual or automated thermal or ultrasonic welding methods, joining plies of the same orientation to each other along the seams, and tacking adjacent plies to each other through the thickness. In thermoset prepregs, ATL is used with wide tapes to make flat or gently curved layups, while AFP uses narrow tapes for contoured layups. In contrast, TPC blanks are normally flat. Low energy may be used to make a loosely tacked layup which must then be consolidated in a further operation, or high energy may be used to sufficiently consolidate the blank which can be used directly for stamp forming. For this reason, I use the term automated tape placement (ATP) with high or low energy to differentiate the methods, regardless of prepreg width. Continuous compression molding (CCM) also creates blanks, collating tapes in the required orientations to create a stacked layup which it then immediately consolidates in the same process (see below).

Blank consolidation

Many methods are available to consolidate the layup into a laminate, which is then used as the stamp forming blank:

• Single press
• Dual press (hot press/cool press)
• CCM
• Autoclave
• Vacuum bag only (VBO) in oven
• High-energy laydown (ATP)

During stamp forming, the blank will be reheated to the melt temperature, and therefore the polymer microstructure in the blank does not affect the final part even for semicrystalline polymers.

Press methods are well developed and can be highly automated. The press techniques are used to manufacture a constant thickness laminate, and the method selected will depend on the volume, capital outlay and recurring cost desired. The single press method has a longer cycle time, but multiple laminates can be consolidated simultaneously using caul sheets between each layup. In the dual press method, the “hot” and “cool” presses are kept at constant temperatures corresponding to the process temperature and solidification temperature respectively, with the blank shuttled automatically into and between the two presses. CCM enables continuous, automated manufacture with the plies in the appropriate orientations moved automatically through a die with hot and cool zones to produce very long laminates.

Using an autoclave for TPC may seem counterintuitive but it can be used to consolidate multiple laminates simultaneously as well as manufacture variable-thickness laminates. VBO oven consolidation is a similar approach but uses only vacuum pressure (14.7 psi/0.101 MPa) instead of full autoclave pressure (typically 100 psi /0.7 MPa) with the advantage of not requiring a pressure vessel. Large-area, high-temperature ovens are available at low cost, which can considerably reduce the capital cost compared to autoclaves or presses while enabling very large laminates to be consolidated economically. VBO has been shown to give good quality blanks, although this depends on the UD tape, which I’ll discuss in a later Troubleshooter article. The final option is high-energy ATP, used to achieve a high degree of consolidation, typically more than 90%. This is a good option for large, non-rectangular and especially variable-thickness blanks.

Blank handling

The consolidated laminate must be machined to the appropriate size and shape for the finished part and to enable holding of the blank during heating and transfer. This is typically done using grippers or a holding frame shaped to the dimensions of the part (Fig. 3). Attachment methods may include springs to control the movement of the blank during forming in the die. It can also be supported by a polyimide film which will not block the heating. Blanks are typically dried prior to stamp forming to prevent even the small amount of moisture absorbed by the high-performance thermoplastic polymers from causing porosity in the final part during the rapid preheating of the blank.

Blank heating and transfer

The blank is heated to the process temperature, normally in an infrared oven, in a few minutes. For larger parts, multi-zone ovens are used to ensure uniform heating across the blank. While rapid heating achieves short cycle times, it is essential that the final blank temperature is in a specific range, high enough to ensure melting and polymer flow, but not into a range where the polymer will degrade. These conditions must be met over the entire length, width and thickness of the blank.

It is normal to run trials with blanks containing embedded thermocouples to tune the process conditions. Thermal traces from embedded thermocouples in a UD carbon fiber/PEKK tape blank with a thickness of 0.2 inch (5 millimeters) are shown in Fig 4. The thermocouples are in multiple locations across the part, including close to the surface and in the center through the thickness. The dispersion of the traces from different locations during heat up is very small and the temperatures level off in the PEKK process temperature zone of 644-752°F (340-400°C). The blank is normally heated for a fixed time, so it is important that heating is consistent each cycle and that the rate of change of the blank temperature is small at the end of the heating cycle as seen in Fig 4.

A small but very critical step in the process is the transfer of the blank from the preheat oven to the stamping press. This must be done quickly because the temperature drops rapidly as soon as the blank exits the oven. A generally accepted maximum time from oven to press is 5 seconds. The polymer is in the melt state, and the integrity of the blank is maintained by the fiber reinforcement, so the blank will typically sag and may even slip from the grippers. This must be considered in the design of the holding mechanism, and the transfer and placement on the forming tool.

Part forming, cooling

To achieve the desired short cycle times, the press must close rapidly to form the part, and the tools must be maintained at constant temperature. This combination of requirements provides challenges in that the continuous fiber-reinforced materials must flow rapidly and consistently while cooling very quickly with consequent increase in polymer viscosity.

In terms of forming, there are multiple flow processes (Fig. 5). In fabric-reinforced parts, the weave limits the amount of fiber deformation. But with UD tapes, the plies can flow transversely and in shear, potentially causing ply thinning, bridging and wrinkling among other features (Fig. 6). UD tape plies in which the fibers are transverse to the direction of forming can spread around a corner causing thinning, while fibers around the outside of corners may cause bridging. Plies that go into compression during forming — for example, on the inside of corners — will wrinkle, which is not acceptable for structural parts. To control the ply deformation during forming, it is common to tension the blanks, sometimes using spring-loaded tensioners (Fig. 3).

While the blank is being formed into the finished part, it is cooling rapidly (Fig. 4). The outer plies cool to the tool temperature almost instantly on contact and even the inner plies cool rapidly to the die temperature.

For semicrystalline materials, the tool temperature must be selected to balance the ability of the material to flow and form the part, with the solidification and development of the required level of crystallinity. The viscosity of the polymer increases rapidly as the material cools, limiting the flow of the material. Semicrystalline polymers solidify at the crystallization temperature on cooling, but this temperature is dependent on cooling rate, and the degree of crystallization depends on the time and temperature under the isothermal conditions when the part temperature has equilibrated on the tool. The typical cooling cycle involves a combination of non-isothermal crystallization during the initial cooling, followed by isothermal crystallization. The volume of the polymer changes considerably upon crystallization, with an associated increase in polymer modulus, and further contraction as the material cools to the glass transition temperature (T_g) and then to ambient conditions.

Polymer and composite suppliers develop extensive data on how crystallization is affected by the cooling rate and isothermal time and temperature, and should be able to supply recommended temperatures for the fastest crystallization rate and the time required to reach maximum crystallinity.

Tooling considerations 

Because the polymer volume reduces significantly during cooling and solidification, the dimensions of the formed part changes. This causes the “spring-in” effect where the angles of corners in the finished part are smaller than that of the tooling. This can be modeled, incorporating the coefficient of thermal expansion (CTE) of the tooling and TPC material versus temperature. Of course, the CTE of the composite is highly anisotropic, so the CTE for the particular ply orientation of each layup must be considered.

The tool temperatures are usually in excess of 400°F (204°C) to allow forming and crystallization of the TPC, and the blank is at a higher temperature when it contacts the tool so metal tooling is typically required. Elastomer tooling or elastomer-faced metal tooling on one die can be an advantage in forming complex parts, providing some compliance to ensure good quality forming and consolidation of the finished part.

The most common tooling design is a matched two-part die set, but for more complex parts, the use of multipart tooling is increasing. For example, a web may be formed using two main dies, and then a movable portion may fold the flanges. This approach was demonstrated by Spirit AeroSystems in forming the curved and variable thickness fuselage frames as shown in Fig. 7.

Modeling and simulation

Large advances have been made in simulation since CW’s 2021 thermoforming article. Very accurate simulation of forming and thermal effects can now be achieved using software from organizations such as for forming and for thermal properties, among others. Potential problem areas can be identified in advance and adjustments made virtually to the blank design, ply orientations, blank tensioning and tooling design. Simulations can now accommodate material-tool friction, variable-thickness parts and compliant tooling.

Modeling of the deformation strains during the forming of a complex part using UD TPC tape is shown in Fig. 8. Thermal modeling can predict thermal lags, thermal gradients during melting and crystallization, as well as thermal viscoelastic behavior during cooling. This enables calculating residual stresses and predicting spring-in and warpage to design thermally compensated tooling.

Now that the basic science of TPC stamp forming is well understood, there is momentum to exploit its benefits for a wide range of applications in aviation and other industries. Its continued development will lead to much more extensive use of TPC in the future.