Composite Material Technology for Aircraft Manufacturers

Thermoplastic composites have, since their advent 15 years ago, provided aircraft manufacturers with a strong, resilient structural composite. Aerospace design permittable databases are available for commercial and military applications.

The commercial side occupies the larger volume of applications, with structural applications ranging from leading edges to engine pylon doors to beams and brackets. Uses are highly variable, applied in a broad spectrum from interior galleries and overhead bins to sound suppression structure on aircraft engine nacelles.

Image Credit: Toray Advanced Composites

OEM’s have recently renewed the interest in thermoplastics. A variety of key reasons are driving this trend, all encouraging the selection of thermoplastic composites.

Primarily, the fabrication of very low void content thermoplastic structure without using an autoclave is becoming possible due to automation solutions and the development of very low void thermoplastic unitapes. Composite part production has been advanced by novel machine technologies developed by equipment and part vendors, such as ACM/Experion, Automated Dynamics, Cutting Dynamics and Fiberforge.

Secondly, the capacity of thermoplastics to be fusion welded by resistance or induction welding techniques is a feasible way to minimize cost and weight through fastener elimination. For example, the tail assembly for the G650 Business Jet by Gulfstream through Fokker Aerostructures was made using induction welding. This construction achieved 10% saved weight and 20% reduced costs over thermoset composite structure.

Finally, shifting to composite fuselages requires the removal of metallic structural brackets and brings about the related effects of galvanic corrosion. Thermoplastic composites provide structural properties and extraordinary fire retardancy. In addition to this, they offer a cost-effective procedure for the production of multiple metallic brackets through compression molding or thermoforming.

Several of the up-and-coming automation solutions need high consistency, low void content thermoplastic prepreg materials. Quick production automation tape solutions profit from low void content tapes, which leads to materials with well impregnated tow and characteristically low void content being favored as starting materials.

The materials and material forms utilized today in aerospace will be the focus of the remainder of this paper, with special focus on the production methods used.

Polymer Technology

Thermoplastic polymers can be divided into two categories: amorphous and semi-crystalline.

The amorphous thermoplastics subcategory forms no crystalline structure. Below the glass transition temperature (T_g) the polymer molecules are solids, while above T_g there is sufficient energy for the molecules to move in relation to one another.

At this temperature, the polymers can be molded. Usually, the temperatures at which the amorphous thermoplastics can be molded are closer to their T_g than semi-crystalline polymers. It should be cautioned, however, that amorphous thermoplastics can have weak resistance to certain solvents.

In a semi-crystalline polymer, there are areas where the polymer crowds closely together in a crystal lattice, but also areas where it is amorphous. The polymer type and the cooling rate used during formation of that part decides a specific part’s amount of crystallinity.

As with the amorphous thermoplastic polymer subtype, semi-crystalline polymers have a T_g where the amorphous phase has sufficient energy to begin moving. There is also a melt temperature (T_m) where the crystalline structure breaks down.

The Main Thermoplastic Used in Composites Today

• Poly-ether-ether-ketone (PEEK) – a semi-crystalline structure
• Poly-ether-ketone-ketone (PEKK) – a semi-crystalline structure
• Poly-ether-imide (PEI) – a amorphous structure
• Poly-phenylene-sulfide (PPS) – a semi-crystalline structure
• Nylons – can be either amorphous or semi-crystalline structure

Material Forms

There are three material forms of thermoplastic material types. One such are fabric prepegs, which use a common carbon or glass fiber woven material with a thermoplastic resin merged into the fabrics. Such materials, which are somewhat conformable, are used mainly on large continuous structures, such as leading edges, tail elevators, flaps, and so on.

The resin is mainly on the fabric’s surface and impregnation takes place in a high temperature autoclave, and because of this these materials are usually called “semi-pregs”. Toray, under the brand name Cetex, offer a host of materials that use carbon or fiberglass fabrics with an assortment of resins such as PPS, PEEK, PA and PEI resins.

Another material form are the reinforced thermoplastic laminates. Being multi-ply laminates, they are available in one to 20 layers ranging from 4 foot wide to 12 foot in length. A high pressure and high temperature thermoforming cycle is used to give the RTL laminate the good fiber bundle impregnation of the higher viscosity thermoplastic resins.

The RTL laminate user can move immediately to a short cycle thermoforming process when producing the part. In a matter of minutes, the laminate can be heated under ceramic heaters.

It is then transported to a place where thermoforming process complex bits can be formed (rib stiffeners) in under five minutes. OEM’s typically write their specifications with many different orientations, fabric, and resin forms, permitting users to refer by grade, class, resin and orientation. The materials are called Cetex RTL by Toray, where RLT is short for reinforced thermoplastic laminate.

The third material form is a thermoplastic unitape. This typically ranges in width from an eighth of an inch slit tape (or cut molding compound grade) to as wide as six to 12 inches. This material form is highly beneficial due to its capacity to utilize automated tape laying and the most efficient fiber placement equipment.

A broader assortment of automation solutions is available with this form through tape laying, continuous compression molding or fiber tape placement (in situ). These products are provided by Toray under the “Cetex Thermo-Lite” brand name.

The State of Thermoplastic Composites Today

For several decades thermoplastic composites have been successfully implemented. Thermoplastics have several advantages:

1. Good durability and damage resistance
2. Capability of using short cycle time thermoforming part production methods
3. Storage capable at room temperature allowing the production of large structures without the restrictions of out-time constraints
4. Limited moisture uptake
5. Fire retardancy
6. Capacity to re-form parts
7. Very low void content
8. Options to part production without using autoclaves

There are disadvantages to thermoplastic composites compared to thermosets, such as:

1. Higher temperatures during processing
2. Initial raw material cost is raw in comparison to thermosets
3. Usually higher cost tooling
4. Conventional part maker inexperience with current thermoplastic composite processing options

One of the primary worries when using composites for structural purposes is its ability to absorb energy without cracking upon impaction, or the resistance to crack propagation if there are miniscule flaws within the part. In general, thermoplastics are tougher than thermoset counterparts. This is an important advantage, given that composites often show no surface damage even if the interior is damaged.

There can be internal cracking without damage on the surface in composites. Because of this, a matrix system which is less vulnerable to damage and can resist the growth of a crack is advantageous. Thermoplastics have demonstrated a higher level of toughness in standard aerospace tests, for example in compression after impact and the amount of energy needed to spread cracks in Mode I (opening mode) and Mode II (sliding mode) testing.

Storage at room temperature – Given that there is no chemical reaction to fret about in thermoplastic composites, prepegs will not be degraded at room temperature and can therefore be stored there. Because of this, cold storage, and cold transport, which typically make logistics of thermoplastic composite use difficult, are eliminated. In addition, it eliminated out-time as a factor, which allows for more complex parts to be possible.

Re-forming – Thermoplastic materials can be re-formed since they can be repeatedly heated and cooled without an effect on their properties. It also allows for the recycling of materials, as parts can be split and used as feed materials in other processes such as injection molding or compression molding.

Higher processing temperatures – Processing polymers above the glass transition temperature, which is necessary when working with polymers, is one of the issues when working with thermoplastic composites. The glass transition temperature is often considered the service temperature when working with a polymer. The process temperatures and the glass transition temperatures for different thermoplastic polymers are summarized in the table below.

Table 1. Thermoplastic Polymers Used in Composites

Tooling – Metal tooling is commonly used for thermoplastic parts because of the higher temperatures required in processing and higher pressures utilized in some forming processes. Initially this can be costlier, however, aluminum tools can be used during prototyping. This helps mediate by lowering costs and lead time. Steel tools are typically advised for production applications.

Other Considerations Associated with Thermoplastics

Semi-crystalline thermoplastics are unique in their level of crystallinity and the effects this has on its properties. The degree of crystallinity can be measured easily using common analytical equipment, for example differential scanning calorimeter, or DSC.

For most thermoplastics, lower crystallinity is associated with increased toughness but also increased vulnerability to solvents. It is common for engineered thermoplastics to have quick crystallization rates, meaning in the range of cooling rates found in standard production process there will be little deviation in degree of crystallinity in a part.

How the composite structure is joined to other pieces of the structure is also an important issue. Thermoplastics have several methods, by being adhesively bonded in a similar manner to thermoset composites or by a number of welding techniques.

During welding, the bond line needs to be heated to processing temperature to soften the material. To prevent delamination, the heated area should remain under pressure. The heat can be applied using several techniques, such as induction, resistance, friction, ultrasonic, and laser.

Manufacturing Processes

Cost effective thermoplastic production is being facilitated by improvements in automated techniques. These improvements address the desire of the composite industry to lessen composite part processing costs.

There are several techniques available now to make parts faster and cheaper because of the nature of thermoplastics. At the processing temperature they form quickly. There is no chemical reaction, meaning a classic thermoplastic consolidation cycle necessitates the material to be at processing temperature for sufficient time to allow the entire part to reach processing temperature.

The majority of thermoplastics can be molded into shapes using a press with fast cycle times. Various companies are developing automated techniques to improve speed of manufacture and quality of thermoplastic parts. Some of the more popular techniques are:

• Automated fiber placement and tape placement
  • Thermoplastic composite tows and slit tape are used by some companies to manufacture parts. Here, the tape is placed on a mandrel or tool under pressure and temperature, consolidating the material while it is being positioned. This means no additional processing is needed.

Fiberforge. Image Credit: Toray Advanced Composites

• The unitapes that are used need to have low void content due to it being a comparatively low-pressure process. Toray technology used in production of thermoplastic unitapes inherently offers low voids in addition to exceptional polymer distribution. The photomicrograph below shows a thermoplastic unitape laminate produced under low pressure.

A cross section photo showing Toray’s TC1200 PEEK unitape laminates. Note the very even resin distribution around the carbon fiber tows and the low void content in the laminate. Image Credit: Toray Advanced Composites

• Thermoforming
  • The thermoforming processes used for thermoplastic composites and often for non-reinforced plastics are analogous. The process involves heating a consolidated thermoplastic laminate to processing temperature. This is often done using IR heaters, which can take a few seconds or minutes to warm the part to the correct temperature. The part is then moved to a stamping station, where the part is formed by a male and female mold. The thermoforming process frequently takes less than five minutes.
• Robotic lay-up
  • Laying down thermoplastic tape in the fiber orientation that is needed by design can be done quickly by machinery. Using a local heating source, the plies are fastened together resulting in blanks that can be press consolidated and thermoformed into parts.

ACM/Experion. Image Credit: Toray Advanced Composites

• Continuous compression molding (CCM)
  • CCM is an automated, semi-continuous production process that has the ability to produce highly shaped profiles or flat panels of essentially unlimited length using thermoplastic composites.

Summary

There are many material options available in the branch of structural composites. Currently, finding cheaper production techniques to help minimize part costs is the main focus of the composite industry. This is partly being realized through processes that do not require an autoclave to produce parts. When composites are reinforced by thermoplastic polymers, it gives designers and manufacturing engineers several options to make parts quickly and reliably.

Thermoplastics will carry on finding more applications with its additional advantages of toughness, the capability to join structures by welding, storage at room temperature, near unrestricted shelf life, and new advances in automation.

This information has been sourced, reviewed and adapted from materials provided by Toray Advanced Composites - Aerospace.

For more information on this source, please visit Toray Advanced Composites - Aerospace.