Release Films for Composites: Engineering Strength
Today, engineering composites are a critical design element in such diverse industries as aerospace, wind energy and automotive. These application areas require high strength, chemical and corrosion resistance, and lightweight materials to deliver practical, efficient, high-performance, and cost-effective engineering solutions. Composite use is on the rise to replace standard metal structural components across these industries. The molding of composite structures is the most important step in their delivery.
Composites, a Quick Primer
Composite parts consist of two or more materials whose properties combine to achieve performance or strength greater than those attributable to the individual materials. The materials that are joined are distinct but structurally compatible. A polymer matrix, or binder, serves to connect the individual fibers of reinforcement material. The reinforcement material, often glass or carbon fibers, provide mechanical strength and stiffness. Tertiary materials may include fillers, foams, or other bulk materials that enable the creation of forms or complicated shapes.
Engineered composites must be formed into net shape by placing the reinforcement material into a mold in a process called the "lay-up". The polymer matrix may be included (pre-impregnated) with the reinforcing material before the layup, or it may be added in a process called resin infusion. Any entrapped air within the composite is removed at this point via a process of degassing or debulking, and then the part is cured, creating the composite. During the process of molding a composite part, several different process materials are utilized. To ensure an easy release of the part from the mold and bagging materials, release films made from specialty materials such as fluoropolymers are utilized. The traditional vacuum bag may be replaced with higher-performing PTFE and polyimide-based films for higher temperature cures. Specially designed tapes are used to assist in the layup and to break resin flash after curing. New advancements in-process materials are improving the process through an innovative design and materials approach.
Process Materials -- New Jobs to be Done:
- Reduce or eliminate post-cure surface conditioning
- Enable efficient debulking
- Increase design-to-manufacture flexibility
Two industries, aerospace, and alternative energy show where and why these tasks are important.
Aerospace applications for composite molding are found in commercial and military airplane and helicopter manufacturing. Here, the traditional job to be done by the composite part is improving the ratio of strength-to-weight and resistance to corrosion and fatigue. Composite parts are superior to traditional metals, including aluminum, due to higher strength, lower weight, and excellent resistance to flex fatigue and exposure to environmental extremes such as heat, cold, humidity, and pressurization.
Composites are commonplace now in structural sections of an aircraft such as the fuselage, wing, tail, and nose sections and even complete airplanes. In the air, the failure of a structural part is catastrophic. Designers of composite parts require materials that provide the highest levels of performance in the composite molding industry. Advances in process materials include specially-designed rubber adhesive tapes that break the flash overage created during vacuum component bonding and conform intimately to a variety of surfaces, without leaving a silicone oil residue that may contaminate and potentially introduce rework or scrap to a molding operation. New to composite molding is textured release films that assist in effective debulking of the composite layup before curing. The texture on the surface of these films allows pathways for air to travel in order to achieve maximum part density more efficiently. Also when these films are utilized in the final curing, the texture of the film is imparted perfectly to the surface of the composite part, which reduces the need for secondary operations in preparation for painting or bonding.
In wind-turbine applications, again the focus is on weight. In this case, the ability to produce very long blade arms at an extremely low weight, with the required stiffness, strength, and damage resistance. Since the purpose of a wind turbine is to convert wind energy to electrical energy, the drag of the blades through the air and the friction in the bearings and supports reduce the turbine's efficiency. The use of advanced composite materials delivers lightweight aerodynamic blades that reduce these areas of energy loss and are capable of working in harsh environments for years.
The challenges addressed by composites in this application are especially visible when considering the high wind, rotational, and gravitational forces that these large and extra-large turbines will encounter, as well as the difficulty to repair and maintain them. The only materials with sufficient strength, fatigue resistance, and stiffness are composites. Wind blades use a variety of composites. For the blade shell, strong and lightweight composites assist in maintaining blade shape and resisting wind and gravitational loads. Unsupported parts of the shell that must resist the buckling load use thickened sandwich structures of light core materials and multidirectional laminates. Adhesive layers between the web and blade shell provide strength and stiffness to the blades and are composed of a strong and highly adhesive matrix. Typically, the industry uses thermoset plastics such as epoxies, polyesters, or thermoplastics as matrixes in wind-blade composites. As complicated as these blades are, their molding process is equally difficult. Here process materials such as ultrawide and durable tapes are used to completely line the mold surfaces and provide long-lasting, repeatable release characteristics that prevent damage to the blade when removed from the mold after the cure cycle. These tapes are more consistent in performance than the other products which may be used, such as a liquid mold release agent.
Partnerships and Products
No matter the application, modern product development is a collaborative effort between the supplier and end user. For composite molding, suppliers such as Saint-Gobain Composite Solutions readily provide their expertise in the design, prototyping, and manufacturing of process materials. By collaborating with the supplier, a manufacturing or design engineer will be able to select the optimal materials to maximize the value of their advanced composite designs. There are a variety of products available that can reliably, effectively, and efficiently be used when designing composite solutions. Non-stick, disposable surfaces created by PTFE adhesive tapes (2255 and HM) as well as PTFE coated fabrics (SGB6 and SG25) ensure smooth and certain release from component molds and tools, while the tape is used for sealing, flash breaking, and mold lining across all temperature ranges and resin system requirements.
For the engineer looking for collaboration, VERSIV™ offers a complete product line of films and CHR® Tape offers industry-leading tapes for molding and mold release applications.
VERSIV™ High-Performance Films provides the broadest line of performance polymeric films available, based on more than 40 years of experience in selecting and processing premium grade polymers into specialty films. The range of polymers includes FEP, PFA, PTFE, ETFE, ECTFE, PI, and UHMW PE, and thin films are available in thicknesses from 0.0005" (12.5 μm) to .010" (254 μm) at widths up to 60" (1524 mm).
CHR® Tape is a complete line of wide and narrow width tapes for use in composite molding as tool tapes, hold-down tapes, and flash tapes, offering the right combination of polymer, substrate, and adhesive type to meet the needs of any application. They are available with silicone, acrylic, and rubber adhesives, or with no adhesive for certain applications. Substrates such as glass fiber provide dimensional stability, high tensile strength, and edge tear resistance, and can operate over a temperature range from -100° to 500° F (-73° to 260° C). Polymers such as PTFE offer quick release and chemical resistance characteristics and support better abrasion resistance when coated with glass cloth, which increases the product lifespan. Polyester tapes with rubber adhesive are specifically designed for flash mask protection in composite molding applications where silicone contamination is of the utmost concern; these products are designed to provide a clean release from the mold surface at temperatures up to 350º F. Ultrawide tape is available specifically to produce high-performance wind blades.
Applications for composite materials are many and are rapidly growing. In automotive applications, for example, body parts, dashboard components, hood, and trunk lids are often redesigned as molded composites. In maritime applications, hulls molded from advanced engineering composites have long enjoyed popularity. Given their substantial weight savings, anti-corrosive performance, lifetime cost, and other advantages, composite materials are here to stay. Composite technology continues to expand with new composite combinations that will accelerate usage well into the future.
This article was produced by IEEE GlobalSpec. Visit the Saint-Gobain Composite Solutions' Engineering360 page on IEEE GlobalSpec.