Specialized Curing Oven Design Eliminates Defects in Honeycomb Composites
The technology of composite materials continues to develop, as it strives to meet more of industry’s needs for structural lightweighting and flexibility in manufacturing. Borrowing from the symmetry used by bees to create their hives with the greatest efficiency possible, honeycomb composite materials start with an open framework of hexagonal cells to form a structure that is inherently lightweight. This material is produced from very flexible, lightweight material that undergoes specialized processing to give it strength and rigidity.
How are Honeycomb Composites Manufactured?
Honeycomb composite materials are manufactured using expanded or corrugated material, typically nomex, carbon fiber, aramid, or fiberglass. These materials are dipped in a resin (epoxy, phenolic, polyurethane, or other) and then cured in a honeycomb cure oven under very precise temperature and airflow tolerances.
An outer skin is applied to the open sides of the honeycomb, forming a “core” that is exceptionally rigid. The cores are cut to the required shape, and placed between layers of composite materials, such as carbon fiber or fiberglass. These layers are placed into molds to form the desired shape of the finished part, and sometimes vacuum bagged before being heated in a vacuum bag cure oven.
Where are Honeycomb Composites Used?
Composite honeycomb materials have a strength to weight ratio up to 30 times greater than aluminum. This makes them an ideal material wherever weight is at a premium, such as in aircraft and automobiles. The finished product can be purchased as panels to serve in a variety of end uses, and the material is commonly employed in aircraft to manufacture such items as cabin floors, wing and aileron panels, and engine nacelle components, as well as automotive body pieces, wind turbines, and spacecraft components, to name just a few.
The Role of Specialized Oven Design in Honeycomb Curing
Heat processing of honeycomb composite material requires specialized oven technology. In order to cure the honeycomb matrix after it is coated with resin, the heated air must be delivered evenly to the entire honeycomb block, such that it passes equally through all the cells.
If any of the honeycomb cells receive an insufficient flow of heated air, they will remain uncured. Specialized oven duct designs are used to deliver the air through the block in a uniform manner, and it is common to seal the supply duct to the honeycomb block prior to heating. This ensures none of the air bypasses around the block, and develops the backpressure needed to push the air evenly through the entire honeycomb matrix.
In addition to precise airflow tolerances, the oven must also meet tight temperature uniformity requirements. The air being delivered to the honeycomb block must be held within a specific tolerance of the oven temperature setpoint, +/-10° F of 350° F for example. The oven manufacturer guarantees the temperature throughout the heating chamber is within tolerance by performing a temperature uniformity test prior to shipment. A web of thermocouples is suspended inside the heating chamber, and the temperatures are validated using a temperature recorder.
Safety Features for the Honeycomb Curing Process
Some of the resins that are cured onto the honeycomb matrix can emit flammable vapors during heating. Honeycomb cure ovens are designed with special features to reduce the likelihood of fire or fume ignition. One example is an LFL (Lower Flammable Limit) monitor, which constantly samples the air in the oven during heating, and monitors its flammability. If the concentration of flammable vapors starts to approach the lower flammability limit, the oven heat turns off and an alarm sounds. These honeycomb composite curing ovens sometimes use hot oil as a heat source, which is inherently non-flammable since it can’t reach the ignition temperature like a gas burner or heating elements can. The hot oil is pumped through a heat exchanger located in the oven’s air circulation system. In all cases, the ovens must adhere to the recommendations of NFPA (National Fire Protection Association) Standard 86, which governs the design of ovens used to process material that releases flammable vapors.
As the demand for lightweight, strong materials continues to grow, honeycomb composites are proving to be a key player in industries like aerospace and automotive. Wisconsin Oven’s specialized composite curing ovens ensure the highest standards of performance, safety, and durability. At Wisconsin Oven, we are committed to providing advanced heat processing solutions that meet the precise needs of composite material manufacturing.