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Resin Transfer Molding

The RTM process is capable of consistently producing produce composite parts with high strength, complex geometries, tight dimensional tolerances, and part quality typically required of aerospace applications.

RTM Process Steps

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.

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.).

Source: Radius Engineering

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Fiberglass (composite)

Glass Fiber Properties

The glass fibers used in composite structures are of a much higher structural quality than the common "fiberglass insulation" used in homes. An individual structural glass fiber is both stiff and strong in tension and compression along its axis (consider the strength of a very short fiber).

Although the fiber may seem weak in compression, it is actually only the long aspect ratio of the fiber which makes it seem so; i.e., because a typical fiber is long and narrow, it buckles easily. A glass fiber, however, is weak in shear (across its axis). Therefore if a collection of fibers can be arranged permanently in a preferred direction within a material, and if the fibers can be prevented from buckling in compression, then that material will become preferentially strong in that direction.

In fiberglass composites, it is the plastic matrix that permanently constrains the structural glass fibers to give strength in compression. Fibers within a composite structure can be arranged with multiple layers and orientations in a way that will maximize strength or flex for a given part structure and orientation.

With chopped strand mat, this directionality is essentially an entire two dimensional plane; with woven fabrics or unidirectional layers, directionality of stiffness and strength can be more precisely controlled within the layup. Fiberglass components can be created in a "shell" type construction, a sandwich structure with a rigid structural foam interior, or built up into very thick and strong components with many layers of glass fiber.

Fiberglass components can be created in a "shell" type construction, a sandwich structure with a rigid structural foam interior, or built up into very thick and strong components with many layers of glass fiber.

Components can be created in a wide variety of shapes, limited only by the complexity and tolerances of the mold required for manufacturing the component.

Fiberglass Composite Properties

(table of information)