Hybrid molding technology supports polymer structural components

Designing with metals includes extra weight, excessive waste, and manufacturing efficiency challenges. As aircraft manufacturers focus on improving fuel efficiency and reducing overall costs by removing weight and improving time-to-market, the specification of metals can be counterproductive. Victrex polyarl-etherketone (PAEK)-based composites have been effective in addressing these challenges.

Continuously-reinforced PAEK-based composites deliver up to 70% weight savings and chemical resistance while providing up to 5x greater mechanical properties than metals.

In addition, the thermoplastic enables design and manufacturing improvements, matches thermal and impact resistance of typical metals, and brings thermal insulation and vibration/noise damping benefits.
 

PAEK composites vs. thermosets

Thermosets have long been the industry standard when it comes to composite solutions, but the tide is turning. A PAEK-based solution is able to capitalize on the challenges thermosets present by delivering improved chemical and impact resistance; fatigue performance; and flame, smoke, and toxicity (FST) performance in structural components. While the stiffness, strength, and thermal expansion properties may be similar, thermoplastic composites offer faster processing cycles by avoiding the use of autoclaves. It can also be recycled for use in other components. The largest challenge faced is that thermosets are the incumbent. With the demands on structural components expanding, implementing a thermoplastic PAEK-based system can be a viable solution given the range of performance benefits.

Hybrid molding

With thousands of brackets on each commercial aircraft, removing weight and providing long-term strength with material solutions is attractive to those searching to improve fuel efficiency and reduce maintenance requirements. Components can range from clips and clamps to highly-loaded structural brackets. With loaded applications, the hybrid molding process can offer a step-change to current technologies.

Hybrid molding techniques enable the combination of optimized, complex injection-molded features with the mechanical strength of a continuously-reinforced thermoplastic composite. The reinforcement can either be carbon or glass fibers, depending on which properties are needed for the application. Unlike traditional overmolded metals or composites, the new process enables the use of a pre-formed composite shape. This can be inserted into a molding tool and then over-molded with an injection-molding grade.

Historically, molded PAEK-on-PAEK composites were not practical because of the challenges associated with creating a strong bond between the two.

With the new PAEK-based polymer, the possibility is now open for the successful molding and bonding of reinforced PAEK-on-PAEK composite.

Tri-Mack Plastics Manufacturing Corp., a molder of high-temperature thermoplastic resins and composites for aerospace, collaborated in the development of a Victrex PAEK-based hybrid-molded bracket. This resulting design offers a 60% weight savings compared to stainless steel and titanium, and equivalent or better mechanical properties such as strength, stiffness, and fatigue. On the processing end, Tri-Mack could manufacture the bracket in a matter of minutes compared to the hours it could take for a metal or thermoset equivalent. Less processing time, lower energy requirements, and reduced waste help to reduce the overall part costs.
 

Comparisons with PEEK

As the requirements on materials become more demanding, a frequent question is: “What is the maximum use temperature for Victrex’s polyether ether ketone (PEEK) materials?” Often, the assumption is made that the maximum use temperature is the glass transition temperature (Tg) of the polymer. In the case of amorphous polymers or thermoset materials, this would be the upper limit. When thermosets are exposed to temperatures above the Tg, mechanical performance drops dramatically and properties such as creep become significant and could lead to failure. In the case of semi-crystalline materials, however, properties are retained above the Tg (Figure 2). The change in properties is minimal when evaluating Victrex short fiber-reinforced materials at Tg compared to the unfilled materials.

The mechanical properties of the short fiber-filled Victrex PEEK materials also compare favorably with typical aerospace grades of aluminum above 100°C (212°F) (Figure 3). At more than 100°C (212°F), the properties of the aluminum start to fall dramatically to the point where the properties are similar to those of Victrex PEEK, particularly above the Tg of the polymer.
 

Summary

Material solution providers such as Victrex understand manufacturers’ needs to improve fuel efficiency and cost, which led to the development of a new PAEK-based polymer for composites and the hybrid molding technology. With these two advancements, engineers are able to design stronger, lighter, and lower-cost components than current metal and thermoset solutions. Given the achievable performance benefits, Victrex PAEK-based composites and hybrid molding are helping shape the future of flight.

Victrex plc
www.victrex.com

Tri-Mack Plastics Manufacturing Corp.
www.trimack.com

About the author: Rob Mazzella is the aerospace business development manager at Victrex and can be reached at rmazzella@victrex.com or 610.391.0313.