For the past fifteen years, aircraft manufacturers have used thermoplastic composites as structural composites to improve their toughness and durability. Structural applications for thermoplastic composites range from engine hanger doors to leading edges to beams and brackets.

This article details the key factors driving the application of thermoplastic composites in today's aerospace industry, the common thermoplastic composite forms, the advantages and disadvantages of thermoplastic composites, and common processing techniques.

01 Driver Analysis

Original Equipment Manufacturers - OEMs have recently shown increased interest in thermoplastics. While there are many reasons to choose thermoplastic composites, a few key requirements fuel this trend, outlined in three main reasons:

One, thanks to automated solutions and the production of very low void thermoplastic units, it is possible to produce thermoplastic structures with very low void content without the use of autoclaves. Parts and equipment suppliers such as Automated Dynamics, ACM/Experion, Fiberforge and Cutting Dynamics have developed unique machine technologies to optimize the manufacture of thermoplastic composite parts.

Second, since thermoplastics can be fusion welded by induction or resistance welding methods, this provides an effective way to reduce weight and cost by eliminating fasteners, as Fokker Aerostructures produces Gulfstream G650 business jets by using induction welding The rear wing assembly is 20% less expensive and 10% lighter than a thermoset composite structure.

Third, changes to the composite fuselage resulted in the removal of metal structural supports and the corresponding galvanic corrosion effects. Thermoplastic composites have excellent flame retardancy and structural properties, and thermoplastics have also introduced cost-effective techniques (thermoforming or compression molding) to produce various metal stents.

Additionally, most new automation solutions require low void content thermoplastic prepregs, and tapes with low void content are useful in efficient manufacturing of automated tape solutions. Therefore, virgin materials with inherently low porosity and well-impregnated tows are favored in this process.

02 Polymer Technology

Semi-crystalline and amorphous (amorphous) are two types of thermoplastic polymers. Amorphous thermoplastics do not form crystalline structures. Below the glass transition temperature (Tg), the polymer molecules are solid. Polymers can be formed at temperatures above Tg because there is enough energy to move the molecules around each other.

In contrast to semi-crystalline polymers, amorphous thermoplastic polymers can generally be molded at temperatures close to their Tg. One limitation of amorphous thermoplastics is that they are less resistant to certain solvents.

Semi-crystalline polymers have regions that are amorphous and regions where the polymer is tightly packed in a crystal lattice. The rate of cooling and the type of polymer used during part molding can affect the crystallinity of an individual part.

Currently, the main thermoplastics used in composite materials include:

Poly-ether-ketone-ketone (PEKK) – semi-crystalline

Poly-ether-ether-ketone (PEEK) – semi-crystalline

Poly-phenylene-sulfide (PPS) – semi-crystalline

Nylons – amorphous or semi-crystalline

Poly-ether-imide (PEI) - Amorphous

3 material forms

Thermoplastic composites mainly come in three material forms.

The first type is fabric prepreg woven with ordinary glass fibers or carbon fibers, and the fabric contains thermoplastic resin. These materials are mainly used for very large continuous structures such as tail lifts, flaps and leading edges. For example, Toray Advanced Composites (TAC) provides glass fiber or carbon fiber fabric, and selects PEEK, PEI, PA and PPS resin as the matrix. UD Tape

The second category is reinforced thermoplastic laminates. These materials are multi-layer oriented laminates with 1 to 20 plies and lengths ranging from 12 feet to 4 feet. RTL laminates undergo high temperature and high pressure thermoforming procedures to obtain strong fiber bundle impregnation with high viscosity thermoplastic resin. Heating for a few minutes under a ceramic heater enables rapid heating of the laminate and then moves to a thermoforming press to produce complex parts in less than five minutes.

The third category is Thermoplastic unitapes, which typically range in width from 1/8 inch slit tape (or chopped molding compound grades) to 6 to 12 inches. The advantage of this material form is the ability to use fiber placement equipment and automated tape placement for optimum efficiency. This material form offers a wider range of automation solutions through continuous compression molding, tape laying or in-situ placement of fiber tapes.

04 Performance advantages of thermoplastic composites

Thermoplastic composites have been used for decades, and the series of advantages of thermoplastic composites are as follows:

The thermoforming of components can be achieved in short cycle times.

Has excellent damage resistance and toughness.

Low moisture absorption rate.

It can be stored at room temperature, which allows for the production of larger structures without time constraints.

Parts can be reshaped.

Flame retardancy.

Part manufacturing alternatives exist that avoid the use of autoclaves.

The void content is extremely low.

Some of the limitations of thermoplastic composites compared to thermoset composites include the following:

The initial cost of raw materials tends to be higher.

The processing temperature is high.

Tooling tools usually have a higher cost.

Traditional part manufacturers may be unfamiliar with modern thermoplastic composite processing techniques.

05 Typical properties of thermoplastic composites

One of the risks of using composites in structural applications is the ability of composites to resist crack propagation and absorb energy without cracking on impact if there are small defects inside the part.

Thermoplastic composites generally have higher toughness than thermosets. This is an added benefit, as composites typically show no surface damage even if there is internal damage.

High Toughness - Thermoplastic composites have been shown to exhibit higher toughness in typical aerospace tests. For composite materials, the surface can remain unchanged when internal cracks occur. Therefore, having a matrix system that is resistant to crack propagation and less susceptible to damage is a great benefit.

Room temperature storage - Thermoplastic prepregs can be kept at room temperature without performance degradation, as thermoplastic composites do not need to worry about chemical reactions. This eliminates the need for refrigerated transport and refrigeration, which tends to complicate the logistics of thermoset composites. More complex parts can also be used since timeouts are not a concern.

Remolding - Thermoplastics can be reshaped and processed because thermoplastic resins can be cooled and heated multiple times without affecting performance. Used parts can be disassembled and used as feedstock for alternative processes such as compression molding or injection molding.

Higher Processing Temperatures - A consideration when working with thermoplastic composites is the requirement to process polymers at temperatures significantly above the glass transition temperature. The glass transition temperature is generally regarded as the service temperature of the polymer. The table below summarizes the glass transition and process temperatures for various thermoplastic polymers.

06 Common molding process

New developments in automated processes are reducing the cost of manufacturing thermoplastic parts. These developments meet the needs of the composites industry to reduce the cost of machining composite parts. Many of these techniques are currently available for efficient and economical production of parts due to the nature of thermoplastics, which form rapidly at processing temperatures.

Since no chemical reaction takes place, a standard thermoplastic curing cycle requires the material to be held at processing temperature until the entire part reaches processing temperature. A single press can be used to shape many thermoplastics with efficient cycle times. Different companies are producing several automated processes to optimize the quality and manufacturing speed of thermoplastic parts.

06 Conclusion

Currently, there are a variety of materials to choose from in the structural composites space, but the composites industry is working to find a cost-effective manufacturing method that can help reduce the overall cost of a part. Parts are manufactured using an autoclave.

Composites reinforced with thermoplastic polymers offer manufacturing engineers and designers a range of different processes that allow parts to be produced reliably and quickly. With advantages such as high toughness, room temperature storage, virtually unlimited shelf life, the ability to bond structures through welding, and modern automation technology, thermoplastic composites will continue to play an important role in a growing number of applications.

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