Sealing the deal: save time and money, while boosting quality
Advanced materials such as liquid resins, fiber cloths impregnated with resin (prepreg) and bonding adhesives are replacing sheet-metal parts and fasteners to decrease product weight and increase performance. In many instances, resins and adhesives will harden at room temperature. However, heat may be required to complete or speed the chemical reactions necessary to cure the materials.
Vacuum curing and debulking tables are unique equipment that allow companies to process composite materials as well as cure adhesives when joining components. This out-of-autoclave (OOA) solution is an alternative to the traditional high-pressure autoclave curing process commonly used by manufacturers for curing parts made from composite materials. Electrical heating elements incorporated into the table design provide heat to one or more sides of the component for curing while using vacuum pressure to debulk. These features can be used separately, but more often part processing requires both.
Processing composites requires using the correct material for the application, precise placement of it within a mold or as a patch, correct time at temperature as recommended by the manufacturer and typically pressure for debulking. This is the same whether the material will be heated in an autoclave, with a silicone rubber heating blanket controlled by a hot bonder or molded with a heated hydraulic press.
Temperature uniformity for composite curing is an important consideration, and manufacturers of heated vacuum tables accomplish this in various ways. Several tables have aluminum platens with heating elements underneath. These elements are arranged to maximize temperature transfer and uniformity. Heating blankets can often be incorporated under or on top of the sealing membrane for heating on two or more sides.
Some tables are designed for vacuum only and use separate silicone heating blankets for heat. Due to the size and heat requirements, three-phase power is often required with amperage draws of 30 or more. Heating elements can be customized to provide higher or lower watt densities, which impact temperature ramp rates and maximum temperatures. Many autoclave systems use natural gas burners to produce the heat needed for curing. Use of electrical heating eliminates the cost of fuel gas delivery systems required by some autoclave systems and contributes to decarbonization.
Depending on the table size, voltage and heat requirements, more than one heating zone may be required (See Figure 1). Each zone should have a separate set of sensors and temperature controllers to provide heat as required for the part. Temperature controllers should be programmable, allowing for specific temperature ramps, soaks and cool-downs as required by the material used. Controllers should also have memory to store several ramp/soak recipes for curing varied materials. Additionally, the inclusion of monitoring thermocouples can provide temperature measurements at different locations on the part being cured. Ideally, the data recorder will have an easily accessible data port for downloading sensor readings.
Vacuum bagging is a skill into itself, ensuring there are no leaks in the seal. Vacuum tables have the advantage of using a highly flexible silicone rubber membrane capable of 800% elongation which will seal itself to the contour of the part. Curing of thermoplastic materials at temperatures above 204° C require high-temperature film to be used and sealed to the base of the table as it exceeds the use temperature of most silicone rubber materials. Tables may include an integrated vacuum pump to draw air out from under the membrane or film at atmospheric pressure of up to 29 inHG or 14 psi at sea level (See Figure 2). The self-sealing membrane has the advantage of sealing in about a minute, rather than an estimated 20 minutes to hours, to properly bag and form a seal. This reduces set up time, increases throughput and provides consistent quality. Cost of operation is reduced through labor expenses and with consumables such as film and sealing tape.
Advanced materials are engineered to have specific curing regimes required to maximize strength and durability. Thermoset (polymer) resins are used when products require high structural strength which is created by molecular cross-linking the material. They will typically withstand higher temperatures than thermoplastic materials, because once cured they will not melt or flow. Examples of thermoset resins are polyester, epoxies, bismaleimides and phenolics. Thermoplastics, such as polyether ether ketone, require consolidation temperatures in excess of 350° C (662° F), making processing on a vacuum table prohibitive. Several customers specifically mentioned processing phenolic resins as part of a prepreg, wet layup or molded piece.
Nanocomposite materials have multiphase structures that can be engineered to have specific physical properties. Vacuum tables can be used by material manufacturers to create prepreg sheets using specific fibers, resins and nanoparticles. Simplifying the process, the same table can be used to mold and cure. A special vertical lift vacuum table may be ideal for the quick loading and unloading of custom manufactured prepreg sheets or low-profile cured production parts (See Figure 3).
Using adhesives to bond structures will transfer loads more efficiently compared to traditional fasteners. This exposes more surface area for bonding, opposed to traditional fasteners with unbonded surfaces. Adhesives may include a thermoset resin material, which often require heat for complete curing. Release film should be used between parts and the membrane to prevent issues with chemical compatibility or the contamination of materials.
BriskHeat Corp. is one of several companies offering vacuum heating and debulking tables for composite processing. The application information below is based on their extensive sales history, spanning more than 10 years. As with other companies offering comparable products, tables are available in a variety of sizes ranging from 56" x 42" (1.4 m x 1.1 m) up to 72" x 144" (1.8 m x 3.7 m). On average:
More than 55% of tables sales measured 60" x 66" (1.5 m x 1.7 m) or smaller.
At least 35% of all tables included a data recorder to accommodate additional temperature sensors and download recorded data.
To look at the range of applications where vacuum tables are used, each customer’s industry was determined. Customers were contacted and supplied specific information as to application, materials processed, end-product and end user. A customer may have listed more than one distinctively different industry, such as material research and composite part production. Other corporations identified as providing either aircraft, vehicles or weaponry to the military (to the United States and overseas) were 52% of total sales. For security reasons, minimal detail about applications or the materials processed were provided. The names of these companies cannot be disclosed. There was an additional 13% sold directly to the military.
The use of a table for composite part repair can be anything from large patches on structural parts to curing adhesives when repairing rotor blades. Only 14% of nonacademic industry vacuum tables are used for repairing composite parts. Those companies identified as defense contractors supplying only to the military accounted for 48% of all nonacademic applications, of which the vast majority manufactured aircraft components.
A defense contractor uses carbon-fiber prepreg material and layers it with steel reinforcements and adhesive. These are laid into a fixture and placed on the vacuum table. Once the lid is closed, the vacuum pump draws the membrane around the shape. The vehicles these panels are designed for have irregular shapes to which the membrane conforms. Prior to using a vacuum table, panels were vacuum bagged and then cured in an oven. The total time to build up the panel, then bag and cure the parts, averaged 18 hours. Productivity more than doubled as total time was reduced to only six hours. This improvement decreased shift times from two to one.
Team Aerospace, Inc. is a Federal Aviation Administration (FAA) and European Union Aviation Safety Agency (EASA) approved repair station that specializes in MRO operations on commercial aircraft. Examples of parts repaired on their vacuum table include:
A defense contractor produces a laminate comprised of several thin sheets of aluminum foil bonded with PR1829 sealant for use on its manufactured aircraft parts. Breather cloth and release film are used during the processing, which requires a four minute debulk and cure cycle between each layer of aluminum. The completed laminate is less than 0.010" (0.254 mm) thick and built up on a piece of cardboard. The ease of operation allows one employee to operate two tables.
A U.S. composite company manufactures a lightweight fiber-reinforced composite material that can replace sheet metal parts in many applications, while weighing a third of the weight. These composite laminate panels are sold to other companies or used for manufacturing of their own products. The composite is made by using proprietary resin along with Aramid fiber to produce a composite material that is highly flame and abrasion resistant, as well as antimicrobial. The durability, at a reduced weight, provides for energy savings when shipping containers are manufactured from this material. The material is used for electric vehicle battery enclosures, and other products where high durability is required. A modified version is sold to defense contractors for military armor integrated into aircraft.
Arcitell LLC developed Qora Cladding as a durable, lightweight and fire-resistant panel to use on the exterior of homes. Arcitell manufactures their own composite prepreg using a fiber matrix and phenolic resin. To manufacture Qora, a foam core is placed between two composite sheets. This material is placed in a silicone mold that can then be placed on its vacuum table or in a hydraulic press, depending on the number and shape of the pieces. The processing temperature is 190° C and cycle time is the same. A 72" x 144" (1,829 x 3,658 mm) table with top heat was a $50,000 investment in 2017, where the company’s platen press cost $250,000. Cost of operation is approximately 25% less for the table based on utility costs. The silicone forms used by both tools is the same, however the lower pressure of the table extends the life of the form. A sister company uses the same phenolic resin sheets to manufacture real brick panels using its vacuum table to form and cure the panels.
In many applications, using a vacuum table with ramp/soak heating capabilities can save time, improve quality and reduce processing costs. Additionally, the initial capital investment is lower than an autoclave or hydraulic press, deliveries are typically shorter, less space is required and it is more environmentally sustainable.
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