Out of autoclave composite manufacturing

Out of autoclave composite manufacturing is an alternative to the traditional high pressure autoclave curing process commonly used by the aerospace industry for manufacturing composite material. Out of autoclave (OOA) is a process that achieves the same quality as an autoclave but through a different process.

Out of autoclave manufacturing processes

OOA curing achieves the desired fiber content and elimination of voids by placing the layup within a closed mold and applying vacuum, pressure, and heat by means other than an autoclave. An RTM press is the typical method of applying heat and pressure to the closed mold. There are several out of autoclave technologies in current use including resin transfer molding RTM), Same Qualified Resin Transfer Molding (SQRTM), vacuum-assisted resin transfer molding (VARTM), and balanced pressure fluid molding. The most advanced of these processes can produce high-tech net shape aircraft components.

Autoclave curing process (for comparison purposes)

In the production of composite aerospace and aircraft components, autoclave curing has traditionally been used to achieve the desired fiber content (resin-to-fiber ratio) and the absence of resin voids to produce light weight and strong components. Autoclave curing achieves this by placing the part under vacuum in an autoclave and then pressurizing the autoclave during the heated cure cycle. The high pressure on the part (within the pressurized autoclave) helps to minimize resin voids and to achieve the desired resin/fiber ratio.

Autoclave cure process details

Autoclaves are utilized where the highest of material performance standards are required such as a void content of less than 2% and high glass transition temperatures. Aerospace autoclaves normally operate from 120 to 230 degrees Celsius within a nitrogen environment at 7 bars of pressure. Liquid nitrogen is injected into the heated autoclave to create the internal pressure. Most common materials cured in an autoclave are advanced composites such as carbon fiber and epoxy resins. Curing cycles range from 90 minutes to 12 hours.

Resin transfer molding - RTM

Resin transfer molding (RTM) is a method of fabricating high-tech composite structures. The RTM process is capable of consistently producing composite parts with high strength, complex geometries, tight dimensional tolerances, and part quality typically required of aerospace applications. RTM uses a closed mold commonly made of aluminum. A fiber "layup" such as graphite is placed into the mold. The mold is closed, sealed, heated, and placed under vacuum. Heated resin is injected into the mold to impregnate the fiber layup. Having the mold heated and under vacuum assists the resin flow. The mold is then held at a temperature sufficient to cure the resin. Current RTM technology produces lightweight parts with excellent mechanical properties. With these qualities, composite materials are gaining wide use in a variety of structural and non-structural applications common in aerospace and aviation. RTM is one method of fabricating these composite structures.

RTM fabrication process

1. Precision molds are created and prepared.
2. Preforms (reinforcing graphite fiber layers) are placed into the mold. Note: SQRTM uses vacuum compacted prepreg layups.
3. Layups and internal mold components (mandrels) are placed in mold.
4. Mold is closed.
5. Mold is placed in a press and clamped to contain internal pressures. Note: Some molds are self clamping
6. Plumbing and instrumentation are connected to the mold.
   1. vacuum line
   2. resin injection line
   3. Thermocouples (TCs)
7. A very high vacuum is drawn on the mold.
8. Mold is warmed mold to injection temperature.
9. Resin is injected into the closed mold. Vacuum is maintained and resin injected at high pressure to create hydrostatic pressure and eliminate voids. Note: SQRTM process injects only a small amount of resin
10. Mold is heated to cure temperature & held for cure cycle maintain similar to the standard autoclave curing cycle (maintaining resin pressure).
11. Mold is removed from the press.
12. Part is removed from the mold.

RTM resin injectors - positive displacement injectors

RTM requires an injection apparatus capable of injecting resin at a high pressure and temperature. One method of injecting epoxy for RTM fabrication is a positive displacement injection system. In the positive displacement injection system, resin is placed into a special cylinder. The bottom of the cylinder is a movable piston. A degassing process can be conducted after loading the resin into the cylinder by sealing the cylinder and applying a vacuum. The piston can move up the cylinder to decrease cylinder volume and force resin out a tube and into the mold. The piston is connected to an actuator. The actuator pushes the piston up the cylinder forcing the resin out at high pressure through tubing and into the mold. Positive displacement injectors have the advantages of precise control over resin pressure, flow rate, and temperature control. It also allows for resin de-gassing.

Same Qualified Resin Transfer Molding (SQRTM)

SQRTM is an acronym for Same Qualified Resin Transfer Molding. SQRTM is a closed mold composites manufacturing method similar to RTM (Resin Transfer Molding). "Same Qualified" refers to this method injecting the same resin as that used in the prepreg layup. The attributes of "same qualified" are significant to a manufacturer because those who adopt this process need not re-qualify resin materials for their production process. SQRTM Process: Liquid molding + prepreg What sets SQRTM apart from standard resin transfer molding (RTM) is that, in place of a dry fiber preform, it substitutes a prepreg layup.

SQRTM Fabrication Process

SQRTM is an RTM process adapted to prepreg technology. The prepreg is placed in a closed mould and during the cure cycle, a small amount of resin is injected into the cavity through ports positioned around the part to establish hydrostatic pressure on the prepreg. The tool can either be self-clamped and self-heated or heated and clamped by a press. The equipment is composed of a tool, a press, an injector, and a vacuum pump.

Process factors that improve SQRTM quality

The following are key factors in the SQRTM process that enable this process to achieve consistent "autoclave quality" components without the autoclave.

• Precision machined closed mold tooling (RTM mold or "tool")
• Large high pressure platen type press to clamp the tool and contain the pressures within the tool
• Extremely high vacuum applied to the tool interior
• Electrically heated platens in contact with the tool for efficient heat transfer
• Precise control of heating platens
• Precise control of injected resin volume, heat, and pressure

Advantages

The advantages are:

• the use of qualified prepregs – toughened resins, UD reinforcements
• a high level of integration
• tight tolerances
• surface finish according to the moulding process.

Disadvantages

Disadvantages are:

• higher tool cost
• a lower level of flexibility to design changes

Northrop Grumman RQ-4 Global Hawk

Example SQRTM Components:

Examples of complex, one-piece components fabricated with the SQRTM method include:

• The wingtip extensions for the RQ-1B Global Hawk unmanned aerial vehicle (UAV)
• A prototype cabin roof for the Sikorsky UH-60 Black Hawk helicopter. This roof component is not only the roof of the cabin, it is also the mounting structure for the engine and transmission.

SQRTM enables net-shape parts

(description from High Performance Composites magazine)

New out-of-autoclave process combines resin transfer molding with prepregs for complex helicopter roof prototype.

UH-60L Black Hawk

The SQRTM method has been employed successfully in several aerospace projects, including the wingtip extensions for the RQ-1B Global Hawk unmanned aerial vehicle (UAV). But its toughest test, to date, was an extremely complex, one-piece prototype helicopter cabin roof, produced under the Survivable Affordable Repairable Airframe Program (SARAP), a cooperative agreement between Sikorsky Aircraft (Stratford, Conn.) and the U.S. Army Aviation Applied Technology Directorate (AATD, Ft. Eustis, Va.).

Vacuum assisted RTM (VARTM)

VARTM is one of three processing alternatives that proponents claim can achieve aerospace-grade results without resort to autoclave cure. VARTM denotes a variety of related resin infusion processes now commonly used in the marine, transportation and infrastructure markets. The processes differ radically from prepreg processing in that fiber reinforcements and core materials are layed up dry in a one-sided mold and vacuum bagged. Liquid resin then is introduced through one or more ports strategically placed in the mold, and drawn by vacuum through the reinforcements by means of a series of designed-in channels and/or carefully placed infusion media that facilitate fiber wetout. Unlike the autoclave, VARTM cure requires neither high heat nor high pressure. VARTM's comparatively low-cost tooling makes it possible to inexpensively produce large, complex parts in one shot.

Balanced pressure fluid molding

Balanced pressure molding using fluid as the heat transfer is commercially practiced as the 'quickstep' process. This process, however, lacks the high pressures of RTM or autoclave curing so does not typically achieve the high quality associated with aerospace and autoclave curing. This process allows for the curing, partial curing, and joining of composite materials. The process involves a fluid-filled, pressure balanced, heated floating mould technology. The heated floating mold technology used within the process works by rapidly applying heat to the laminate which is trapped between a free floating rigid or semi-rigid mold that floats in, and is surrounded by, a heat transfer fluid (HTF). The mold and laminate become separated from the circulating HTF by a flexible membrane. The part, typically under full vacuum, is subject to less than 3 psi (20 kPa) fluid pressure and can be rapidly heated to the desired cure temperature without risk of catastrophic exothermic reaction. The air is then removed under vacuum and the laminate is compacted and heated until the part is cured.

A flexible membrane beneath the mold is bonded into a pressure chamber creating the lower half of a 'clamshell' or 'chamber' like mold set. A second flexible membrane is bonded to a second pressure chamber creating the upper half of the clamshell. These pressure chambers are clamped together during processing, permitting the laminate to be compressed while reducing stress to the mold as it is floating in a balanced pressure environment within the HTF.

The process can use thermosetting, thermoplastic prepregs (pre-impregnated composite fibers), and wet resin with dry fiber to produce superior composite parts. This out of autoclave process can achieve aerospace grade void contents of less than 2%, with extremely fast cycle times, and at significantly lower pressures and lower labor costs than many alternative autoclave production systems using many typical autoclave qualified prepregs. The quickstep out of autoclave system is unique in that it uses fully immersed balanced pressure fluid curing and it allows the user to stop the composite cure reaction at any point in the cure cycle, and thus can halt processing on all or part of the laminate and either return to it at a later to complete cure or to co-cure, join and bond other composites to it to create larger parts.

The use of fluid to control temperature, as opposed to the gas generally used within methods such as autoclave and oven curing equates to lower energy consumption, faster cycle times and extremely accurate part temperature control.

Studies have demonstrated that the process may significantly lower overall capital costs and labor costs. Cycle times are one of the most significant differences between quickstep and autoclave processing. The process allows for more repeatability in cure cycles, and rapid heating allows for improved interlaminar properties and improved surface quality.

Studies have also shown benefits in improved productivity levels due to lower development costs, the ability to manufacture large scale volumes, lower tooling costs, and no waiting on parts to begin cures. The use of fluid heating molding processes allows small and medium composite parts to be rapidly manufactured and cured to aerospace standards without an autoclave. It also allows the marine and automotive industries to manufacture advanced composites and traditional fiberglass compositesto standards only previously seen within the aerospace industry.

Prepreg compression molding

Another Out of autoclave method for achieving external compression on prepreg based composite parts is through the use of heat shrink tape. This metdod, however, does not achieve the high quality of RTM or autoclave processes because without the autoclave or a closed mold, the part must be cured in a non-pressurized oven. These compression tapes are typically made from polyester (PET) film. Heat shrink tape is applied to a composite part prior to the heating, or curing cycle. When heated, the tape will shrink in the linear (machine direction). Heat shrink tape works best on parts that are cylindrical or semi-circular in cross section, as this allows the tape to exert even compaction forces on the part surface. Examples would be composite tubes for aerospace, wind energy, consumer sporting goods, etc. Heat shrink tape allows these parts to be processed without the need to cure with the heat and pressure of an autoclave.