Resin Transfer molding (RTM)is a closed mold low-pressure process that allows the fabrication of composites ranging in complexity from simple, low performance to complex, high-performance articles and in size from small to very large. The process is differentiated from other molding processes in that the dry reinforcement and the resin are combined within the mold to form the composite component. The fiber reinforcement, which may be preshaped, is placed into a tool cavity, which is then closed. A tube connects the closed tool cavity with a supply of liquid resin, which is pumped or transferred into the tool to impregnate the reinforcement, which is subsequently cured. Several similar composite fabrication processes fall into the resin transfer molding category, although there are distinct variants.
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![]() The most common use of the term RTM describes a process typified by the vacuum assisted resin injection (VARI) manufacturing process. A mold is constructed of low cost materials, such as epoxy. Reinforcement is then cut to fit the required geometric pattern and is arranged by hand in the mold. Pieces of reinforcement may be placed in the mold one at a time or preassembled and then placed in the mold as a unit of preform, after which the mold is closed and clamped with bolts or bars. A vacuum can then be applied to the mold to extract the air, and resin is injected at very low pressures, often below atmospheric pressure. Because of the low cost materials used in mold construction, mold pressures must be low, resulting in slow fill times and limited glass contents. The inability of the mold to tolerate elevated temperatures, coupled with its poor heat transfer, restrict the resin chemistry to slow cure times with minimum exotherm to prevent resin degradation or tool damage. Cycle times of this process are measured in hours and days for large, complex parts. The major benefit of the process is the ability to fabricate large, complex structures with maximum part integration at a low cost |
A preform of reinforcement is prepared before it is placed in the mold. The preform is usually made by spraying chopped reinforcement onto a perforated screen. A vacuum applied to the rear of the screen holds the reinforcement in place until the binder, which is sprayed along with the reinforcement, has time to cure. After a postcure, the preform becomes an easily handled, three dimensional sheet of reinforcement. Cores and changes in thickness are possible but not generally used ha this process. Preform materials and techniques are further discussed in the section ''Resin Transfer Molding Materials'' in this article. Tooling is normally made of steel or zinc alloy, but could also be made of epoxy. The preform is placed in the mold, and a measured quantity of resin is poured or pumped into the open tool. The tool is then closed and compressed to approximately 690 kPa ( 100 psi), causing the resin to flow and ''wet out" the preform. Heated tools are usually used, and the cycle time for large parts of uniform thickness is open 3 min or less. Parts having significant complexity and integration can be made economically using preform molding. Equipment is inexpensive because of the low pressures required. The physical properties of the molded components tend to be very consistent with uniform reinforcement content, even at part edges. Unfortunately, this is at the cost of a relatively high waste factor around the part perimeter. Also, these components do not have the optimum possible level of performance for the amount of reinforcement because of the absence of reinforcement orientation and, to a lesser extent, the presence of a binder.
This process uses a preform that is placed in the tool before the introduction of the resin system. Tooling can be made from various materials, but is typically a metallic shell in order to facilitate heat transfer. The resin is highly reactive and is contained in two separate holding tanks. Resin from each tank is injected under high pressure into an impingement mixing chamber and then directly into the tool. Although the mixing pressure is high, the overall pressure of the resin, once in the tool, is only about 340 to 690 kPa (50 to 100 psi). The resin flows into the tool and wets out the preform as the curing reaction is occurring. A suitable resin system has a viscosity plateau of 0.1 to 0.5 Pa s (1 to 5 P) for around 20 s followed by a rapid cure, resulting in cycle times of about 1 min. Reinforcement levels used in this process to date tend to be from 5-55 wt%. Resin must enter the tool at fast rate if flow distances are long because of the fast cure times. These high flow rates restrict the choice of reinforcement to those which are resistant to washing (movement caused by resin flow). Because of the rapid onset of resin cure, flow distances are limited in this process, when flow distances exceed 610 mm (24 in).multiple inlet ports are desirable.
This process encompasses portions of all three at the aforementioned processes.The conceptual process, uses three dimensional preform with foam attachment inserts. Glass content is in 35-60 wt% range and can be a mixture of continuos and random material. Tooling for high production volumes should be made of steels in order to contain moderate molding pressure(690 to 3450 kPa, or 100 to 500 psi) and exhibit good heat transfer characteristics. For limited production, aluminum or zinc tooling would be acceptable. Molding is carried out at elevated temperature to minimize the cure time and expand the number of potential resin system. The preform is placed in the mold and following minimal hand arrangement of the preform, the mold is closed and resin is injected. At higher reinforcement levels, the mold may be left open slightly during resin injection and then closed completely to promote rapid filling in the mold. Cure should be accomplished in the mold such that the final part will require the post cure and will have acceptable dimension stability. For some complex components and components having critical tolerance requirements, a fixtured postcure is required to yield dimensional stability. Cycle times range from 1 min for small components to 8 to 10 longer longer for large, complex structures.
As the degree of complexity, level of performance, and size of composite components increase, labor-intensive processes, such as hand lay up, may increasingly be chosen to make a number of components. Because of its cost, it is unlikely that this technique can be used to make a significant quantity of parts. At the other end of the spectrum are the compression molding and thermoplastic stamping processes. The equipment required to produce parts with these technologies is expensive, thus there is a threshold of number of parts required before these process become financially viable.
The level at which this occurs, when the automotive industry considers the replacement of metal components with composites, depends on the cost of the component being replaced. As the cost of the steel component or system being Replaced increases, the number of parts to be manufactured will decrease. There is, however, an upper limit to the amount of replacement possible using these processes. Because of the limited mechanical properties of materials used with these processes and the inability to make large, integrated structures without assembling multiple small components, these process may only economically replace steel as where the degree of part integration possible is already limited by function. Examples of such areas include grille opening panels, hoods, deck lids, and doors. The high pressures required (7 to 17 MPa, or 1 to 2.5 ksi) also limit the size of the components fabricated by compression molding and thermoplastic stamping.
The RTM process allows placement of preforms of variable thickness containing a variety of fiber types in the mold cavity with minimal subsequent movement of reinforcement during further processing. This contributes to optimum performance at minimum weight. The low pressure required for the process allows the use of less-expensive presses and may slightly reduce the cost of high-volume production tooling, compared to compression molding or thermoplastic stamping. There should, of course, still be the large reduction in tooling expense, when compared to steel components, given a sufficient degree of part integration. The low pressures will also allow much larger structures to be molded. Current compression molding processes are limited by the availability of very large presses. The RTM process allows the incorporation of cores and inserts in the component design. This ability to produce three dimensional structures with deep sections and cores at low pressures allows the fabrication of large, highly integrated structures. In the automotive industry, these are not surface-quality parts but, rather, unseen structures. A superior surface quality currently requires a high pressure process.
The presses and resin control equipment required for resin transfer molding are readily available from a number of suppliers. Lower-tonnage presses can be used because of the lower pressures required for RTM, compared to processes such as compression molding. Computer control is desirable to sequence the press closing and resin management for applications in which high speed resin injection and cure are required
Tooling
Computer simulations are being developed to allow the calculation of the pressure required to fill a component ( 8). One point to consider when sizing a press is the force required to compact the lofted reinforcement preform. For glass preforms in the range of 50 wt%, with a majority of the glass being random material, a closing pressure as high as 690 kPa(100 psi) can be required to close the die.
In the case of a large, highly integrated, composite automotive structure, the cost differential between low cost tooling and standard production steel molding tools may not be significant. There should, however, be large savings in tooling cost over current tooling for fabricating steel structures because of the reduced number of required tools resulting from part consolidation. To realize short cycle times, tooling must be capable of being uniformly heated to about 90 to 150 °C (200 to 300 °F). It must also be rigid enough to compress the lofted reinforcement of the preform as the mold closes, without tool distortion. Hardened shear edges to trim excess reinforcement from the preform in pinch off areas as the mold is closed will reduce post molding finishing time and provide a good seal to contain the resin. Because of the abrasive nature of the reinforcement, and the likelihood of molding a large number of components, tooling surfaces should be chrome plated. All of these requirements limit tool choices for high volume production to materials such as cast aluminum, chemical vapor deposited nickel, and steel. For a production tool, steel will most likely be the best material based on its durability and ability to be easily modified. For low volume and prototype production, however, lower cost epoxy and zinc alloy cast tools are an acceptable alternative. Longer cycle times, reduced dimensional accuracy due to mold compliance, and reduced tool life will likely result from this tooling choice.
Cost and processability often determine the choice of a resin system. Thermoset materials have been used exclusively to date, but as high performance thermoplastic materials become available in RTM type formulations. they may also be used.
Of the several resin systems that perform adequately in an RTM process, polyester is most often used because of its low cost. Epoxy has been used in both aerospace and consumer products and has been demonstrated to provide high physical properties, but at a premium price. Vinyl ester resin has also been used in a number of RTM products and provides properties between those of polyester and epoxy at a moderate price. Other resin systems, such as the acrylamate resin system family and methylmethacrylate vinyl ester, are newer systems that are proving to be very processible with RTM techniques.
In any resin system, a low viscosity plateau is required, during which the resin can provide constant flow throughout the mold, followed by a fast cure. The resin must not only gel rapidly to be acceptable for rapid RTM cycles, but must provide sufficient Barcol hardness to allow the component to be demolded without distortion. Some resins will yield lower levels of physical properties if cured rapidly, and the processing property relationships of any resin system must be evaluated for each component before it is produced. The speed of resin cure for many available resin systems is already adequate to achieve rapid cycle times. Systems that yield cycle times of 1 min or less, if desired, are available for use with small components. Developments to increase the time available for mold filling and to improve reinforcement wet out after the reactive resin components are mixed together are still needed.
PreformsPreforms are a critical aspect of the successful implementation of any high volume RTM project. Development of performing techniques and characterization of materials resulting from various preforming techniques should be carried out before finalizing component design. An optimized preform process that gives design and fabrication cost equal consideration must be developed. A structure that is optimized to use the ultimate design capabilities of composite materials, while being highly weight efficient, will likely be expensive and is therefore not appropriate in all applications.
The optimal baseline material for consumer products is typically random E glass. This material is currently being used in many conventional RTM products, including the Lotus, Avanti, and Matra vehicles. The preforming technique most often used with random glass mat is to shape a Sat sheet of this material at the time of molding. In the Lotus process, a sheet of mat is cut and formed to fit in the mold. Overlaps of several inches are made at the end of any layer of material. Foam cores are wrapped with sheets of mat before being placed in the mold. This technique is also used in many other resin-transferred components, ranging from small automotive trim to large waste treatment plant components.
A process in which the flat sheets of glass mat are preshaped before insertion into the mold represents the first level of preforming sophistication. This process uses a flat sheet of Random mat with a small amount (2 to 5%) of thermoplastic binder applied. The binder allows the sheet to retain a shape when it is heated and pressed in a forming die, imparting a gentle, three-dimensional shape to it without cutting and piecing, as in the previous mentioned process. This process, while faster cannot achieve radical three-dimensional shape changes, such as deep draws. It is possible, however to include continuous reinforcement selectively in the preform to improve its physical properties.
The most versatile and widely used preforming technique for creating the preforms with complex shapes is the spray up process. Glass rovings are chopped and sprayed on a rotating screen. A small amount resin is introduced into the stream of chopped glass, and when the glass accumulates on the Screen to the proper weight, the resin is heat-cured causing the preform to retain its shape. The vacuum applied to the back of the screen not only holds the glass on the screen as its accumulates but also helps maintain uniformity. As the holes in the screen become covered by glass, the open areas tend to attract more glass imparting a self-leveling action. This process can be fully automated, and the spray-up of a large preform, such as an automobile floor pan, would take 2 min. This process, while yielding complex shapes, tends to produce resin-transferred components at the low end of the potential physical property spectrum. The binders sprayed with the chopped reinforcement tend to cover and seal off the fiber bundles, resulting in incomplete resin impregnation of the reinforcement.
An approach to preforming that is currently used in the aerospace industry for high performance components is referred to as the engineered fiberform process. When Units of laminates, precut to shape, are stitched together in to three-dimensional structure, they become what is referred to as a fiberform. Fibers used are E-glass, S-glass, carbon, and aramid. The preforming process uses uniaxial fiber rovings that are formed into sheets of varying configurations. Some materials are reinforced solely in the 0 degree direction, while others are reinforced in the 90 degree direction. Fiber rovings can also be oriented at any intermediate angle. These materials not woven, as is cloth, but rather are stitched together with a thread of polyester or aramid. Elimination of the weaving yields improved physical properties and better resin wet-out in the final part.
Performs can be stitched in critical areas reinforcements such as aramid to increase integrity or increase interlaminar shear strength. The fiberform process is similar to assembly of a suit coat or other garment fabric building blocks. Because the process can be carried out on high speed, high volume, fabric processing equipment, the cost of large volumes of fiberforms may be low. Assembly of the fiberform may also be accomplished close to the point of use, allowing shipment of flat precut laminate sheets. Some producers are currently supplying fiberforms to the aerospace industry for fabrication trials and limited production using RTM. Layers of the oriented reinforcement sheets can be stacked and further stitched together into an engineered laminate structure, the structural efficiency of which is high because of exact reinforcement placement and orientation. For some sharply configured parts, laminates can be used as RTM preforms It is unlikely that any optimum preform could made from using one of these processes exclusively Portions of each process could be used, but because the process and the design are interdependent, they must be considered in parallel.
The many potential advantages of RTM can be summerized as the capability of rapid manufacture of large, complex high performance structures. The low pressure of the process allows very large components to be manufactured by means of low tonnage presses It also permits the use of foam cores to yield fully three dimensional parts. The ability to preplace the reinforcement where desired, and have it remain in that location, gives increased design flexibility and a subsequent optimized structure. While composites in general offer this advantage over steel, other composite molding processes are either slow and labor intensive or have some degree of movement of the reinforcement, which results in variations in physical properties throughout the finished part. In applications in which composite materials arc competing with steel or other materials on a cost basis. large amount of part integration is required to offset the increased material cost Reduced assembly cost, higher quality, and improved functionality at lower material weights are all possible advantages if a sufficient degree of integration can bc achieved. Resin transfer molding provides the capability to integrate large number of components into one part, and can provide an inexpensive means of obtaining prototype parts at low production volumes. It can also be capable of rapid production of larger volumes when high quality tooling is used. Resin transfer molding is a closed-mold process, which has advantages over open-mold processes, including low vapor emissions. Thc resulting components have both inner and outer surfaces that are dimensionally controlled, thereby requiring a minimum of hand trimming if tooling is properly designed.
Currently, the majority of RTM process limitations stem from the undeveloped nature of the higher speed versions of the process. While resin systems continue to improve, there are still problems with filling large parts with high glass content at low injection pressures. Preform Fabrication far high volume components also currently has limitations. The absence of reinforcement at part edges may be a limitation if ribs and bosses are required in a design. Ribs and bosses must be loaded individually in the tool cavity, and maintaining reinforcement at the part edge and avoiding resin richness at corners of the part can be difficult Scrap losses also may be more costly as component integration increases, and if a large component fails, replacement cost to the consumer can be significant.
There are applications of resin transferred components in the automotive, consumer products and industrial products industries. Ford Motor Company recently completed a concept study in which the entire 90 piece steel front structure of an Escort automobile was replaced by a 2 piece resin transterred front structure. The process used was HSRTM. Although only prototype tooling has been used to date, production cycle times were projected to be in the range of 6 to 9 min. The final structure was stiffer and stronger than the steel structure, at 66% of the original weight.
HIGH SPEED PRECISE LAY-DOWN of continuous reinforcement in predescribed patterns is the basis of the filament winding method. It is a process in which continuous resin -impregnated roving or tows are wound over a rotating male mandrel. The mandrel can be cylindrical, round or any shape that does not have reentrant curvature. The reinforcement may be wrapped either in adjacent bands or in repeating bands that are stepped the width of the band and which eventually cover the mandrel surface technique has the capacity to vary the winding tension, wind angle, or resin content in each layer of reinforcement until the desired mess and resin content of the composite are obtained with the required direction of strength.
The most important advantage of filament winding is the cost, which is less than the g cost for most composites. These lower costs are possible in filament winding because a relatively expensive fiber can be combined with an inexpensive resin to old inexpensive composite. Also, cost reductions accrue because of the high speed of fiber lay down, for example, for large parts such as a missile canister, 45 kg/h(100 lb/h) of low angle helical, or 320 kg/h(700 lb/h) of hoop windings
The primary advantages of filament winding are
Thermoset resins used as the binders for the reinforcements can be applied to the dry roving at the time of winding (wet winding), or can be applied previously and gelled to a B stage as prepreg. Also, rovings can be impregnated and rerolled without B staging and either used promptly or refrigerated. The filament wound composite is usually cured at elevated temperatures, without any additional step for compaction. Mandrel removal, trimming, and other finishing operations complete the process.
Fibers The most widely used fiber for filament winding is fiberglass, which has been marketed in several grades in the United States for more than 40 years. Types of glass fibers useful for filament wound structures, which gives the common designation of the fiber, the nominal tensile strength and tensile modulus of the strands, and the maximum number of filaments per strand. The latter is important in selection of a fiber for filament winding because large numbers of filaments can make handling easier. Fiber density is included in the table so that the rule of mixtures equations involving fiber volume and resin volume can be used to evaluate void volume and theoretical mechanical values. Fiberglass continues to be useful for filament winding because of low cost, dimensional stability, moderate strength and modulus, and ease of handling. Aramid fibers, which were initially useful because of their strength and modulus to-weight ratios (called specific modulus or specific strength), also show great consistency with a low coefficient of variation, enabling high design allowables. Aramid composites have relatively poor shear and compression properties, which are generally not critical for pressure vessels. A new aramid, type 981, has been developed, that has greater tensile strength with the same density (hence improved specific tensile strength).
The largest variety of strengths and moduli can be obtained with graphite fibers, which have recently been improved in terms of modulus, tensile strength, and strain to failure. Surface finish has also been improved, which facilitates handling for filament winding. Increasing tensile modulus usually lowers tensile strength; the intermediate modulus fibers have been the only exception. The amount of graphitization increases with increasing modulus, which results in greater thermal and electrical conductivity. Fiber cost also increases, primarily because there is less demand for the high modulus fibers, and large scale production economics have not yet been imposed. All fibers, except pitch fibers with a modulus of 517 GPa or greater have been filament wound.
Resin Systems The resin system in a filament wound composite serves the same functions as it does in composite structures fabricated by other means, namely:
These are some handling criteria for a wet resin system that are unique to wet filament winding:
One of the important resin properties in the cured structure is adhesive strength to the fiber which is important for most systems, although rocket motors have been filament-wound with released aramid fibers in the hoops direction. Releasing the aramids increases the performance in a biaxial strain field by eleminating transverse loading in the fiber; aramids have very low transverse strength. Another important property is heat resistance, which is critical. A high heat distortion resin system should be chosen only after a thorough study of the operating environment of the filament wound component. Also, fatigue strength, chemical resistance, and moisture resistance of the composite are key selection criteria, but should be evaluated only in relation to the required mechanical properties desired in the operating environment. In addition, high strain to failure capability of the resin system is important to allow transfer of loads s strength or higher modulus fibers.)
Manufacturing Processes A rocket motor case manufactured by filament winding will be used as a basis for addressing typical manufacturing processes and concerns. Key operations involve:
Impregnation Resin and reinforcement are joined in the impregnation process. The types of impregnation processes in common usage are preimpregnated (prepreg) roving (commercial), wet rerolled, and wet winding. Prepregs offer excellent quality control and reproducibility in resin content, uniformity, and band-width control. Many high performance resins can only be impregnated by special processes, such as the hot melt process. Many commercial prepregs can be certified to key government specifications; most use solvents or preservatives added to the resin formulation to extend storage life. These can affect the tack of the roving, making it difficult to remove the roving from the spool during winding. These same solvents can become trapped during B stage and cure. Trapped volatiles promote void incubation, which decreases mechanical strength (particularly shear and compression) in the finished composite. Open, intermediate compaction and heating operations are required to remove these solvents, particularly in thicker walled laminates. With wet rerolled prepreg, a controlled volume of resin is impregnated on a controlled length of fiber reinforcement and then respooled. Quality control can be performed away from the winding operation. Usually no preservatives or solvents are required because the roving is either used immediately or stored in a freezer for future use. This can be a very cost effective method of obtaining preimpregnated roving. Wet impregnation of the fiber can be accomplished by pulling the reinforcement either through a resin bath or directly over a roller that contains a metered volume of resin controlled by a doctor blade. This is a low-cost system that is widely used in commercial applications with polyester resins. The resin content is affected by several parameters: resin viscosity, interface pressure at the mandrel surface, winding tension, the number of layers per inch, and the mandrel diameter.
Mandrel Preparation The tool around which the impregnated roving is wrapped is the mandrel. The principal types of mandrel in common usage in the filament winding industry are water soluble sand mandrels; spider/plaster mandrels for low volume products; segmented, collapsible mandrels for continuous production of pipes; tube mandrels; and unremovable liners, such as load sharing metal liners for pressure vessels. Water soluble sand mandrels are used mainly for rocket motor cases, and the insulator is t almost always reassembled with the mandrel. Wind axis, polar fittings, and other tooling are preassembled, and a water soluble sand solution is cast into the mold around the tooling. Following cure of the sand, the two halves of the mandrel are assembled and bonded. The two insulator halves are spliced using uncured rubber, which co cures with the case. The rough sanded) and cleaned with a solvent. Resin gel coat is applied to serve as an adhesive between the insulator and composite overwrap. The use of a film adhesive in place of the resin gel coat is becoming more common. This provides a controlled adhesive thickness with repeatable properties, but the cost can be prohibitive. Spider/plaster mandrels provide another approach to a high tolerance mandrel surface through use of a plaster sweep over removable or collapsible tooling. The plaster is cured, then overwrapped with Teflon tape or some other separator film. Following cure, the tooling is removed, the plaster is chipped out, and the release tape is removed, leaving the desired inside contour. In some instances, the rubber insulation is laid up and cured directly Waster contour. The process is completed by machining the insulation to the desired contours.
Segmented, collapsible mandrels are specialized and expensive, but the cost is justified for high production applications because of their reusability and the continuous winding process. Surface preparation before winding consists of an application of a mold release and then an ample gel coat to provide a continuous inside surface. The gel coat in this application is designed to provide a flexible barrier to prevent leakage at low strain levels. Tube mandrels are used in many applications involving cylindrical metal mandrels in which the cured composite is pushed (or pulled) off the surface after cure; this requires high quality tooling for trouble free usage. Chrome plating or hardened and polished surfaces assist in easy mandrel removal. A slight taper along the mandrel length is also beneficial.
Unremovable liners are used for metal lined pressure vessels, combining the high strength density advantage of composites with a thin, impermeable metal liner. Using this concept, high pressure low molecular weight gas such as helium or hydrogen can be effectively contained without leakage. The metal liner can be designed to carry a large or small portion of the, internal pressure, but in all cases the liner (which initially serves as the winding mandrel) become vital part of the pressure vessel. The mandrel preparation can vary from an adhesive system, where bonding to the composite is desired, to a released system, where independent movement between the liner and structural composite is desired.
Winding Preparation The manufacturing processes selected for the component are a function of product geometry, weight, and the availability of winding equipment. Most filament winding is still performed using the mechanical gear driven machines which evolved during the late 1950s. However, many of the winding machines now in use are numerically controlled (NC), providing the latitude to wind non optimum shapes where special considerations are required in order to wind the fibers on nongeodesic paths. Analysis techniques have been developed that derive a "slip coefficient" required to prevent fiber slippage from such paths. Because most reinforcements are packaged on rolls, tension can be introduced at the roll. Tensioning devices include magnetic or friction brakes, electronic rewind, and rotating scissor bars. Because the latter two techniques have the capability to rewind, they allow winding of low angle patterns around end domes, since the overtravel past the domes can be taken up. The tensioners are often mounted on reels, either remote to the winder or as a part of the carriage that actually travels with the delivery system. The tensioning devices should have variable but controlled tension levels, easy adjustment of tensions, rewind capability to prevent fiber slacking, and uniform tension regardless of roll size.
Component Winding For rocket motor cases, as for most high quality filament wound components, the manufacturing operation is controlled by detailed documentation. The operator follows this documentations carefully completing patterns, often changing from longitudinal to hoop winding, and verifying quality control. In the prototyping stage of development, the designer's calculations are checked using pi-tape measurements and thickness measurements at the polar bosses. In production, many quality control verifications can be eliminated, particularly in NC winding operations. Key elements in motor case construction are the skirts at the tangent zones where the domes and cylinders meet. these are attached by various bonding/winding or riveting methods to convey loads through the motor case assembly. The composite portion of the skirt usually consists of hoops and longitudinal fibers interspersed at approximately a 50:50 ratio. These are wound or laid up using temporary skirt tooling, which is removed following cure. The joint between the skirt and the motor case must transfer the combined loading through shear. Often a shear web consisting of rubber or film adhesive is laid up, overwound, and then co cured with the rocket motor case body.
Stage/Cure In the first step in the cure cycle, called B staging, resin viscosity is advanced by means of external heat (lamps or ovens) to the point where cross linking of the epoxide groups is initiated. At this point the resin is still soft to the touch and still exhibits some tack, but will not reflow upon the application of further heat. This operation is performed to allow removal of excess resin before proceeding with cure. This is often done with plastic paddles, wiping off the resin runs periodically until the resin has advanced into the B stage state. In many rocket motor case applications, the domes are B staged first, while the cylinder is temporarily covered with insulation to prevent resin advancement. This allows cocuring of the skirt to the cylinder, and guarantees a "run free" surface for the dome under the skirt tooling.
Fabrication of the composite portion of the assembly is completed by the curing operation. The types of equipment most commonly used are ovens (gas fired or electric), autoclaves, and microwave ovens. Most epoxy resin systems can be cured easily in gas fired ovens without supplemental pressure, using either air or inert gas environments. Recently, vacuum bags and bleeder cloths have been used by some manufacturers to produce more compact, void free laminates. Autoclaves are commonly used with more exotic resins such as bismaleimides and polyimides, where special considerations are required for proper handling of high volatile contents. Autoclaves are state of the art equipment for nonwound components such as skins and panels for aircraft components where the interface pressure between thc continuous fibers and thermally expanding mandrel does not develop during cure. Microwave cure requires a high initial investment but significantly reduces energy costs and cure times. However, special heating supplements such as induction heaters are required at the composite/metal interfaces.
Mandrel Removal The water soluble sand mandrels are the most easily removed; water is added through the wind axis tooling. The sand is washed out and the tooling assemblies removed. Removal is more difficult for mandrels where the tooling is segmented or collapsible. These may require that plaster be chipped out by hand. This operation is laborious and has a high potential for damage to the component.