Applications Of HexTOOL® Quasi-Isotropic Prepreg

Interview conducted by G.P. ThomasNov 19 2013

In this interview, AZoM talks to David Wallis, Product and Technical Support Manager at Hexcel, about the composition, properties and applications of the unique prepreg HexTOOL®.

Could you please provide a brief introduction to HexTOOL® and its primary mechanical and physical properties?

HexTOOL® is a quasi-isotropic form of prepreg, designed to be easily machined to produce high quality, machined tolerance tools with a high surface hardness. The long and random fiber orientation within HexTOOL® provides its quasi-isotropic properties. The high fiber volume, 55%, ensures a material modulus comparable to continuous fiber materials and ensures fine machined details and surface hardness is retained throughout the material, whatever the machined surface orientation and depth.

What is the general chemical composition of the resin that HexTOOL® is fabricated from?

HexTOOL® has two primary resin types, M61 and M81. M61 is Hexcel’s toughened, third generation bismaleimide resin developed for tooling applications and very similar to Hexcel’s M65 BMI resin as used in advanced aerospace composite structures. M61 has enhanced tack compared to other BMI resins and a working life of 30 days at room temperature. M81 is a toughened, low flow epoxy resin developed for tooling use from Hexcel’s 8552 resin which again is one of Hexcel’s primary systems widely used in composite aerospace structures throughout the world.

The high Tg that M61 provides, 275°C (527°F), makes it well-suited to high rate, long-life production tools for composite structures with cure temperatures of 180°C (355°F). M81 with a Tg of 220°C (428°F) is suited for high rate production tools at part cure temperatures of 120°C (250°F) and can also be considered for lower rate production and development tools for part cure temperatures of 180°C (355°F).

HexTOOL® Production Tools for Manufacturing Carbon Composite Engine Fan Blades for GE90 and GEnx Turbofan Engines. Image credit: Hexcel.

Could you briefly describe the production process involved in creating HexTOOL®? How does this compare to processes used to create other materials for the same applications?

HexTOOL® is manufactured using Hexcel aerospace grade unidirectional carbon fiber prepreg with Hexcel AS4 fiber. The prepreg tape is cut into consistent and uniform pieces or ‘chips’, then through a unique process the chips are randomly orientated into a high areal weight, quasi-isotropic, continuous ‘mat’ which we provide to the customer in roll form.

Conventional tooling prepregs are usually woven fiber products, or woven fiber cloth which is infused with resin. The discontinuous fiber matrix in HexTOOL® produces a very stable and consistent material and its stability is maintained through and after machining without experiencing some of the movement that can occur when machining through continuous fiber structures.

HexTOOL® Production Tooling for Turbofan Engine Cowlings. Image credit: Hexcel.

How does the density of HexTOOL® compare to other materials used in similar applications and how does this affect the composite’s performance?

HexTOOL® is 20% the weight of Invar, the metal often used for machined surface mould tools for high-tolerance, high-cure temperature carbon composite parts because of its low coefficient of thermal expansion (CTE) which is close to the CTE of the carbon composite part. HexTOOL® CTE also closely matches the CTE of carbon composite structures as they are similar materials.

The lower weight of HexTOOL® can reduce the tool weight by 75% compared to Invar, making tool movement and loading of multiple tools into autoclaves easier and reducing the need and cost of equipment to move and lift large Invar tools. The lower thermal mass of HexTOOL® allows autoclave cure cycle times to be reduced by 20% or more compared to Invar tools. An Invar tool requires 80% more energy to heat the tool compared to a similar sized HexTOOL® tool.

How is the performance of HexTOOL® unique compared to other materials used in similar applications?

The quasi-isotropic structure of HexTOOL® provides much improved performance in resistance to mechanical and thermal cycle fatigue. As the fibers are orientated in all axis, the combination of the fiber crossover and length characteristics mean any defect or damage cannot easily propagate, which maintains the strength and vacuum integrity of the tool over several hundred part production cycles.

HexTOOL® allows tools to be easily reworked or repaired if an area of the tool is damaged or needs material added for tool modifications. In the required area the damaged material can be removed before adding material only in the required area. When cured the new material can be machined and finished the same as the original tool surface. This allows tools to be easily modified with minimum downtime and cost, resulting in a tool structure of the same quality as the original tool.

The high areal weight of HexTOOL® materials, available as 2000 gsm and 4000 gsm, enable shorter tool fabrication times and minimal waste. As HexTOOL® has no specific material direction, the material can be cut and spliced as required to minimize and eliminate waste, material cut-loss is usually less than 5%.

HexTOOL® Tooling for Aircraft Rudder Parts. Image credit: Hexcel

Could you describe the main differences between HexTOOL® M61 and M81?

As discussed earlier, the difference between M61 and M81 is the resin system. The fiber type, fiber volume and length and prepreg manufacturing processes are the same for both materials. Tool fabrication and machining processes are also the same, M61 and M81 have different cure cycles based on the resin systems.

The recommended cure cycles and processes for tool fabrication are detailed in the HexTOOL® User Guide, available online http://www.hexcel.com/resources/technology-manuals

Could you tell us a little bit about the recent work with CFAN?

We were aware that CFAN was studying various tooling materials during its development of composite tooling. As a customer, CFAN uses Hexcel’s HexPly® materials to manufacture composite fan blades. Since we understood CFAN’s part and production rate requirements we proposed HexTOOL® M61 as a solution that could offer the high tolerances and production life that CFAN was looking to achieve with composite tooling.

Following an extensive 22-month internal evaluation of composite tool technologies, CFAN selected HexTOOL® M61 for its fan blade composite tools. As commented by Tom Lednicky, Sr. Manufacturing Engineer with CFAN, “Over multiple cure cycles HexTOOL® materials demonstrated high levels of tool dimensional stability, excellent vacuum integrity and tool durability, using standard CFAN operational practices.”

Why is HexTOOL® such a beneficial material to use for tooling for engine fan blades?

The high tolerances, complex geometry and high production rates of commercial engine fan blades require a similarly high tolerance and stable tool. HexTOOL® provides a machined tolerance surface with high hardness which maintains dimensional stability, surface finish and vacuum integrity through hundreds of part layup and cure cycles.

How else can HexTOOL® be utilised by the aerospace industry?

As aerospace composite structures have grown in size, so have the tools required to manufacture these structures. Where automated fiber placement (AFP) is used to fabricate composite structures, often the tool needs to be accurately rotated during the fiber placement process.

The weight of a metal tool can be prohibitive to automated processes and equipment, sometimes a lower weight composite tool is used for the layup process with the part then transferred to a separate metal tool for cure.

The reason for the separate cure tool is that a conventional composite mandrel may not be suited to the hundreds of cure cycles expected during the life of the tool. HexTOOL’s machined surface tolerances, low weight and long production life allow a single tool to be used in place of two tools. This also removes the potential risk to the quality of the composite part caused by transferring parts from a layup tool to a separate cure tool.

HexTOOL® Omega and Spar section demonstration tool. Image credit: Hexcel.

What further application areas do you envision for HexTOOL® in the future and are there any further variations currently being worked on?

For HexTOOL® M81, we are developing a non-autoclave cure cycle for low rate and development tools. With M61 and M81 we continually develop, internally and with our customers, new methods for the design and manufacturing of composite tools to control and reduce the total tool cost.

Such projects include lower cost tool master materials as well as tool fabrication methods that remove the requirement for a master tool, which can be a significant time and cost savings of a composite tool project. We understand that management of cost, time and delivering proven tooling technologies are important requirements for all of our customers.

About David Wallis

David Wallis is Hexcel’s Product and Technical Support Manager responsible for Hexcel’s tooling materials. David has 20 years of experience in aerospace and composites manufacturing in a variety of roles from Manufacturing Engineer to Operations Manager and Product Management.

In his current role, David leads the Matrix Technical Support Team for the Americas including product development of Hexcel’s tooling materials. He works from Hexcel’s US carbon fiber, resin and prepreg manufacturing site in Salt Lake City, USA.

Originally from the United Kingdom, David moved to the USA in 2001. In his spare time, he enjoys time with his family and skiing. Football and mountain biking are some of his favorite sports.