The compression molding process is presently the most technologically developed and incorporate either continuous or random chopped fibers into a structural composite. The process is more rapid and complex than the labor intensive hand lay up or liquid matched die molding methods that it often replaces, but it has a trade off with respect to fiber alignment. Depending on component shape and charge pattern, compression molding may involve regions of high resin flow that tend to orient fibers in the flow direction. Random orientation is desirable for chopped fiber sheet molding compound and long fiber thermoplastic materials. If directional fibers are desired, flow patterns can be developed.
The compression molding process is most commonly called the SMC process in reference to the precursor sheet molding compound material it uses. The primary application of this technology in the automotive industry has been for grille opening panels and, on selected vehicles, exterior panels. Tailgates, and hoods are two other examples of vehicular use of the SMC process. The entire cab of selected heavy trucks is also produced using this process.
A typical process cycle consists of placing sheets of SMC, which consist of 1 3 mm (0.5 in.) chopped glass fibers in chemically thickened thermoset resin that has a leathery consistency, into a heated mold (typically 150 °C, or 300 °F). The mold is closed under pressures of 7 to 14 MPa (1 to 2 ksi) for about 1 to 3 min to cure the material. Approximately 30 to 80% of the mold surface is covered by the SMC charge, and the material flows to fill the remaining cavity as the mold closes.
Fiber utilization in compression molding materials is very versatile. Continuous, cut length, or chopped glass fibers may be used, as well as random continuous glass mats. Carbon or aramid fibers may also be used. Sheet molding compounds can be made in various compositions and by various processes. Continuous, unidirectional molding compounds for structural components generally have 40 to 60 wt% glass fiber reinforcement. Normally, SMC for nonstructural automotive trim and body applications is 27 to 30 wt% glass fiber. Fillers are often used to minimize resin cost and lower thermal expansion of the product.
Resin chemistry has a major influence on the strength and reliability of the final composite component. Although the resin constitutes only 16 to 25 wt% of a typical SMC composite, it controls flow and moldability. Strength and corrosion resistance must be optimized by the resin selection. However, low viscosity high-acid resins are often desired for their good wetting and thickening characteristics; for example, magnesium oxide added to the resin system thickens it to a leathery consistency. The reaction takes place over a period of up to 5 days. Although epoxy resins are used in aerospace SMCs, vinyl ester and polyester resins are generally used in automotive applications because of their faster cure time and lower cost. Many SMC materials use a high reactivity, isophthalic polyester resin (for example, E980) with magnesium oxide for thickening. Ground limestone is generally used if a filler is required.
To form a component, sheets of SMC material are cut to a desired shape, the carrier film is removed, and the SMC sheets are stacked as a series in a charge pattern. For example in molding an automotive wheel nine sheets of SMC are stacked; the charge pattern is then placed between clean, preheated dies, and upon closing, the press spreads out to fill the die cavity. Normal die temperatures are 130 to 160 °C (270 to 320 °F). Typical cavity pressure varies from 4 to 21Mpa (2 to 3 ksi), depending on resin viscosity, glass fiber content, charge pattern, and mold complexity.
Cycle time ranges from 1 to 4 min, depening on component complexity, thckness, and the die clean up required. Cure time is very critical: If the resin cure exotherm is not properly controlled, cracking, blistering, or warping can occur. After all the other steps in SMC process are automated, the exotherm may well be the rate-controlling factor for thick parts of more than 10 mm (0.4 in.).
Advanced compression mold designs and molding systems are presently being developed to reduce SMC cycle time. Current minimum cycle time is about 1 min, button to button, die closed time. Tooling must be of high quality to maintain these fast molding rates and must be hardened in critical wear areas. It is also necessary in some cases to use compression molding heat transfer analysis techniques to maximize the heat transfer characterstics of the tool. Extra sets of tooling are expensive and introduce variance in part tolerance when numbers of parts must be assembled from moldings of several tools. It is therefore advantageous from several perspectives to produce parts as rapidly as possible using a minimum number of tools.
Material cost for compression molded SMC was estimated to be approximately 3.5 times the cost of steel by weight. To compete polyester composites must integrate several parts into one, to save assembly, floor space and storage costs. If suitable part integration could be accomplished, polyester SMC would compete with steel for up to 227000 units per year. To compete beyond this volume, the SMC cycle time must be reduced. Halving the cycle time would double the level at which composite components would compete with steel. Research must be done to speed up the process; eliminate SMC storage by making it on-site; and immediately recycle rejected, uncured SMC to bulk molding compound(BMC).
Many supply companies are conducting research to obtain more reproducible SMC products as well as faster cycle times. The rate of pressure application and parallelism of the molding die are being closely controlled by a microcomputer controlled flow system. Many SMC applications are for visible components requiring high quality surface finishes. Using low-profile resins and highly polished dies improves the as molded surface. Control of material flow and tooling surface (parallelism) also reduces waviness and improves appearance. A technique for obtaining better surface finish that is currently being used in many applications is called in mold coating.
To apply an in mold coating, a thermoset resin is injected into the mold after the SMC component is partially cured. To provide a space for the coating, the press is opened slightly. This opening can be conveniently achieved by using a counteracting force system or, as in case of the recently developed high pressure in mold coating systems, by injecting the coating with a slight yet pressure to compress the partially cured SMC. The thermoset resin is injected into the die and the die is reclamped. The press forces the coating to surround and impregnate any surface voids in the SMC in a uniform manner. Urethane is a common in-mold coat. The in-mold coating can eliminate some paint priming operations and significantly reduce the hand finishing required for SMC. Class A surfaces can be obtained with a minimum number of paint pops after baking. Research is currently being done to apply in mold coating to deep sections, such as fender extensions. Present technology extends to shapes such as Corvette hood outer panels and similar large, rather simple geometries. This process does not address problems that exist in areas where subsequent machining will expose new, uncoated surfaces, such as trimmed edges.
The other system in use today to improve SMC surface quality relies on a vacuum to decrease the amount of trapped air and gasses in the molded component. As the mold is closed on the SMC charge, a seal closes around the entire mold and the mold area is evacuated. As the material flows to fill the die, the vacuum enhances the natural expulsion of air and styrene vapor from the SMC material, resulting in higher surface quality and less tendency for the occurrence of subsequent defects during painting.
Process Advantages The SMC method has several advantages over methods such as hand lay up or spray up. Having the liquid resin, catalyst, and glass fiber precombined into a unit can allow better quality control over chemistry, mix, and distribution prior to forming. Using matched metal dies is a elosed mold process that gives better dimensional control and stability than open mold processes. Higher pressures require more expensive tooling than is necessary for hand lay up or spray up but less expensive tooling than is needed for stamping or injection molding.
Higher pressures during compression molding may reduce the blistering, splitting, and paint popping often encountered during paintbaking cycles in moldings made at lower pressure. The short flow lengths that are possible in compression molding tend to minimize fiber movement, and reduce stresses and tool wear, in contrast to injection molding. The process does not constrain the mold designer with sprue and runner lay outs. Absence of runners and sprues reduces resin degradation due to shear heating and eliminates reinforcement length reduction, which, in the case of injection molding, severely limits the length of reinforcement fibers. Venting tends to be uniform by force, and the dimensional stability of the formed components is better than it is for components formed by higher flow processes, such as injection molding, or by processes such as hand lay up.
Process Limitations There are disadvantages associated with both the SMC material and the compression molding process that must be recognized. A high capital investment is required for sheet forming and molding equipment. The cost of SMC material is also high because of the post finishing labor and equipment required after molding. The SMC material must be stored properly to prevent thermal and moisture degradation. Cracking and warpage of the final part may result from using at degraded SMC sheet. Also, the SMC sheet-forming line must be properly protected to prevent toxic vapor emissions. During the compression molding cycle, most problems are flow related. Finite element numerical techniques are showing promise in predicting flow of filled polymer melts of the type encountered in composite fabrication; however, these are not expected to be operational for several years. The problem areas that exist presently are residual stresses, warpage, weld lines, flow orientation of fibers, fiber kinks and folds, delamination, and breakdown of the resin paste and glass interface. While all these problems can be minimized by varying the charge pattern, cycle parameters, or SMC chemistry, the existence of this large number of variables adds complexity and cost to the process.
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Tooling used in Composite Fabrication | ||
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Woven materials in laminate form, are currently displacing more traditional structural forms primarily because of the availability of fibers (such as glass, carbon and aramid) whose enhanced mechanical properties in composite form surpass the property values of corresponding hardware in aluminum or steel on a strength-to-weight basis.
Autoclave curing of composites is of prime importance to manufacturing high-quality aerospace laminates. Curing is achieved through a combination of pressure, temperature and heat under inert conditions in an enclosed vessel. Processing materials must be added to a composite ply lay-up before autoclave curing. These materials control the resin content of the cured part and ensure proper application of autoclave pressure to the lay-up. The materials usually used in preparing a lay-up for autoclave curing are peel ply, separator, bleeder, barrier, breather, dam and vacuum bag. The materials are compatible with the maximum cure temperature and pressures required for the matrix system being cured.
The peel ply if used is placed immediately on top of or under the composite laminate. It is ultimately removed just before bonding or painting operations so that a clean, bondable surface is available. It is usually a woven fabric and may be either nylon, polyester, or fiberglass. The fabric is treated with a release agent that must not transfer to the laminate; otherwise, subsequent bonding or painting operations may not be satisfactory. Nylon, polyester, or fiberglass peel ply can be used with most matrix system.
A separator (release material) is placed on top of or under the laminate and peel ply. It allows volatile and air to escape from the laminate and excess resin to be bled from the laminate into the bleeder plies during cure. It also gives the cured part a smooth surface, except for porous Teflon, which gives a slightly textured surface.
The purpose of the bleeder is to absorb excess resin from the lay-up during cure, thereby producing the desired fiber volume. Fiberglass fabric or other absorbent materials or fabrics are used for this purpose. The amount of bleeder used is a function of its absorbency, the fiber volume desired in the part, and the resin content of the prepreg material used in the lay-up. In advanced composites essentially all excess resin is bled from the surface of the laminate, with edge bleeding being minimized by properly damming the lay-up edges.
To determine the correct number of plies to use:
The barrier is commonly placed between the bleeder plies, and breather plies. In the case of epoxy resin, it is frequently an unperforated film, so resin removal from the part, can be controlled. For resins that produce volatile by-products during cure, a film with small perforations and large spacing is used to prevent the breather materials from becoming clogged with resin and unable to perform its function.
The breather is a material placed on top of the barrier film to allow uniform application of vacuum pressure over the lay-up and removal of entrapped air or volatile during cure. It may be drapable, loosely woven fabric, or felt.
The dam is sometimes located peripherally to minimize edge bleeding. It may be an integral part of the tool or built-in position using materials such a rubber neoprene cork pressure-sensitive tape, silicone rubber or Teflon or metal bars.
The vacuum bag is used to contain any vacuum pressure applied to the lay-up before and during cure and to transmit external autoclave pressure to the part. It prevents any gaseous pressurizing medium used in the autoclave (air or inert gas) from permeating the part and causing porosity and poor or unacceptable part quality.
An autoclave system allows a complex chemical reaction to occur inside a pressure vessel according to a specified schedule in order to process a variety of materials. The evolution of materials and processes has taken autoclave operating conditions from 120 degree C and 40 psi pressure to well over 760 degree C and 10,000 psi. The materials processed in autoclaves range from metal bonding adhesives, reinforced epoxy laminates, thermoplastic laminates, metal, ceramic and carbon matrix materials, to may other aerospace and electronic components. The major elements of an autoclave system are: a vessel to contain pressure, sources to heat the gas stream and circulate it uniformly within the vessel, a subsystem to pressurize the gas stream, a subsystem to apply vacuum to parts covered by a vacuum bag, a subsystem to control operating parameters, and a subsystem to load the molds into the autoclave.
In industry autoclave curing of composites is used to improve cured product quality and reduce fabrication costs by providing:
Process Optimization
Reduced process inconsistencies and product rejections
Accurate, real-time quality assurance with rapid error detection and correction
Verification of process reaction behavior kinetics
Nondestructive verification of cured properties
Accurate, permanent process documentation
Flexibility in adapting to new or modified processes
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Pultrusion is an automated process for manufacturing composite materials into continuous constant cross-sectional profiles. It is one of the most versatile composite, but it is still one of the least understood.
The term pultrusion refers to the final product and to the process. Most simply, it refers to a non homogeneous compilation of materials; pulled through a die. In virtually every case, a reinforcing fiber is integral to the finished product. The matrix used is typically a thermosetting resin, which chemically reacts when heat is introduced to create an exothermic reaction. The resulting profile is shaped to the point at which it cannot be reshaped or otherwise altered within its operating temperature range, unlike thermoplastics. In contrast, the extrusion of aluminum materials involves homogenous materials that are heated and pushed through a die, then allowed to cool into the final shape. Because the material is initially heated and then cooled, it can be heated again re-formed into another shape.
The pultrusion process has developed relatively slowly compared to other composite processes. There is a significant amount of art in combining the continuous reinforcements and resins in a continuous operation, and developing the science from the art has taken time. During the 1980s, there has been a dramatic increase in market acceptance, technology development, and pultrusion industry sophistication. Today, the number of technically competent personnel in pultrusion is sufficient to provide the base from which dramatic changes can occur. This, coupled with continual increase in cost-competitive advantages, will enable pultruded composites to become a traditional material alongside steel, wood, and aluminum before the end of the 20th century.
The process begins when reinforcing fibers are pulled from a series of creels. The fibers proceed through a bath, where they are impregnated with formulated resin. The resin impregnated fibers are preformed to the shape of the profile to be produced. This composite material is placed in a heated steel die that has been precision machined to the final shape of the part to be manufactured. Heat initiates an exothermic reaction in the thermosetting resin matrix. The profile is continuously pulled and exits the mold as a hot, constant cross sectional profile. The profile cools in ambient or forced air, or assisted by water as it is continuously pulled by a mechanism that simultaneously clamps and pulls. The product emerges from the puller mechanism and is cut to the desired length by an automatic, flying cutoff saw.
There are two categories of pultrusion products. The first category consists of solid rod and bar stock produced from axial fiberglass reinforcements and polyester resins; these are used to make fishing rods and electrical insulator rods, which require high axial tensile strength. The second category is structural profiles, which use a combination of axial fibers and multidirectional fiber mats to create a set of properties that meet the requirements of the application in the transverse and longitudinal directions.
More than 90% of all pultruded products are fiberglass reinforced polyester. When better corrosion resistance is required, vinyl ester resins are used. When a combination of superior mechanical and electrical properties is required, epoxy resin is used. Higher temperature resistance and superior mechanical properties generally dictate the use of epoxy resins reinforced with aramid or carbon fibers.
Another advantage is that complex, thin wall shapes, such as those extruded in aluminum, or polyvinyl chloride (PVC), are now possible because of recent process technology advances. Hollow sections can be produced by using cantilevered steel mandrels. A third advantage is that wire, wood, or foam inserts can be encapsulated on a continuous basis in pultruded products.
In addition to symmetrical walls, which are always easier to pultrude, variable wall thicknesses in a constant cross section can be pultruded. A fourth advantage of the process, which is less obvious, is its ability to use a wide variety of reinforcement types, forms, and styles with many thermosetting resins and fillers. Virtually no other composite process offers as much versatility as pultrusion. Reinforcements can be placed precisely where they are needed for mechanical strength and can be consistently repeated.
Finally, pultruded shapes can be made as large as required because equipment can be built in any size. A corollary advantage of larger equipment is its ability to produce multiple cavities of the same or different profiles, which enables pultrusion to compete with traditional materials because of a relatively low labor cost. The cost of dies for pultruded shapes is also low compared to other composite processes.
There are several markets in which pultruded profiles have made a significant penetration that is not tied to just price competitiveness. Pultruded products are successful because they provide corollary advantages that are not available with traditional competitive materials.
The highest volume application of pultrusion is the fabrication of nonconductive ladder rails for in plant and communication utility use. Corrosion resistant fiberglass sucker rods have replaced steel in the extraction of oil. Semiflexible highway delineator posts that deflect without permanent deformation are used instead of rigid cold rolled steel posts with plastic reflectors.
In highly corrosive environments, pultruded grating systems have become the standard because of their durability, replacing steel, aluminum, and even stainless steel systems. They are also used in elevated walkways and on steps where the supports are structural profiles, such as l beams, channels, angles, and tubular shapes, that are made to the same dimensions as steel or aluminum supports. Cable trays of steel are being replaced by pultruded composite cable trays because of their superior corrosion resistance and better electrical insulation values.
Pultruded solid rectangular and square bars are being used in transformers to separate the windings and to permit air circulation. Utility market applications include guy strain insulators, stand-off insulators, hot line maintenance tools, and the booms for electrical bucket trucks. Other electrical applications include tool handles, bus bar insulator supports, fuse tubes, and lighting poles.
Dunnage bars that separate and isolate loads in trucks and railcars have been made from pultruded lineals for many years. The back doors that roll into the roof of the truck are now also pultruded, as are the structural Z sections between the inner and outer walls of a refrigerated truck trailer. In many buses, the luggage rack is pultruded. Hollow sections within the rack allow air to be passed for heating or cooling. Because of its continuous nature, the process produces the rack in one piece to span the length of the bus. In other rapid transit applications, continuous lengths of coverboard are pultruded in one piece to cover the current carrying third rail on rapid transit systems. Because of the design flexibility of composite profiles, the shape is designed to snap over the rail and yet support a load dropped from above.
Some fiberglass rovings are available as centerpull twistless; that is, the natural twist as a consequence of winding has been offset by a built in reverse twist. Continuous fibers of glass, carbon, and organic polymers can also be supplied on packages with cardboard cores designed for outside pay out to avoid twist. This style of package dictates the use of a multiple spindle creel design in which the packages are oriented horizontally. Multiple bushings are again used to guide and protect fibers as they are delivered to the front of the creel. An additional consideration of package rotation and the resulting tension necessitates the use of a spindle bearing to provide uniform tension regardless of package size. Because packages in this configuration are usually smaller, a greater number of packages may be found on such a creel design.
The continuous fiber creels are usually the first station on a process line. Directly after the roving creels is a creel designed to accommodate rolls of mat, fabric, or veil. The roll materials are usually supplied in diameters between 305 and 610 mm (12 and 24 in.) with cores of 75 or 100 mm (3 or 4 in. ) in inside diameter. The creel must be able to accommodate both the size of the roll and the inside diameter of the core, along with appropriate spindle spacing and core bushings. The ability to lock the position of the roll in the desired location will ensure proper delivery of the material to the desired location. In some cases, it is also necessary to provide for the pay out of web material in a vertical, rather than a horizontal, format. This requires independent stands in a "lazy Susan" type of configuration.
As materials travel forward toward the impregnation area, it is necessary to control the alignment to prevent twisting, knotting, and damage to the reinforcements. This can be accomplished by using creel cards that have predefined specific locations for each material. In some cases, these cards can be used for only one profile. In other cases, a general format for roving and web locations can be easily for a variety of common profiles.
Forming is usually accomplished after impregnation, although some initial steps can be carried out during the impregnation process. Forming guides are usually attached to the pultrusion die to ensure positive alignment of the formed materials with the cavity. In the case of tubular pultruded products, a mandrel support is necessary to extend the mandrel in a cantilevered fashion through the pultrusion die while resisting the forward drag on the mandrel. Materials must form sequentially around the mandrel in an alternating fashion to prevent weak areas due to ply overlap joints. Sizing the forming guide slots, holes, and clearances must be done to prevent excess tension on the relatively weak and wet materials, but must allow sufficient resin removal to prevent too high of a hydrostatic force at the die entrance.
An alternative impregnating and forming method consists of injecting the resin directly into the forming guide or die after the dry materials have been formed. Although this technique minimizes the problems associated with with the wet-out bath systems, some limitations exist in the areas of wet out, air entrapment and maximum filler content. A combination of techniques may be the answer for a specific profile, depending on its complexity.
The materials commonly used for guides include Teflon, ultrahigh molecular weight polyethylene, chromium plated steel and various sheet steel alloys. The pultrusion processor who employs a craftsman capable of converting sheet metal and plastic stock into forming guides with precise control would be most successful in processing complex shapes
Another popular die station uses heated platens that have fixed zones of heating control with thermocouple feedback from within the platen. The advantage of this method is that all dies can be heated uniformly with reduced temperature cycling, because changes in temperature are detected early at the source of heat rather than at the load. In the same respect, however, a temperature offset will be common between the platen set point and the actual die temperature. With knowledge of the differential, an appropriate set point can be established. When provided with the means to separate the automatically, the advantage of quick setup and replacement of dies can lead to increased productivity through reduced down time. One machinery supplier also uses the bottom platen height adjustment feature to exactly align the die centerline with the pulling mechanism to eliminate any product distortion associated with misalignment.
A source of cooling water or air is essential in the front of the die at start up and during temporary shutdown periods to prevent premature gelation of the resin at the tapered or radiused die entrance. This can be accomplished by using either a jacket or a self contained zone within the heating platen. Alternatively, the first section of the die can be unheated, and cooling can be accomplished through convection. The most critical pultrusion process control parameter is the die heating profile because it determines the rate of reaction, the position of reaction within the die, and the magnitude of the peak exotherm. Improperly cured materials will exhibit poor physical and mechanical properties, yet may appear identical to adequately cured products. Excess heat input may result in products with thermal or crazes, which destroy the electrical, corrosion resistance, and mechanical properties of the composite. Heat sinking zones at the end die or auxiliary cooling may be necessary to remove heat prior to the exit of the product from the die.
To increase process rates and to reduce temperature differentials that coontribute to thermal cracking in large mass products, it is desirable to deliver heat to the material before it enters the die. This is accomplished by radio frequency preheating, induction heating, or conventional conductive heating. Such heating devices are available as either integral units or stand alone devices, which can be positioned before the die entrance.
One supplier has developed a process optimization instrument that allows tracking in a convenient graphic format of external die temperature profiles and internal product temperatures as a function of die position during the curing process. The data collected at a specific process speed become essentially a photograph of steady state process conditions to be used for quality control, process engineering, and quality assurance documentation. Further process control developments of this nature will provide improved process capability and production efficiency.
The earliest pultrusion machines used a singular clamp, which was hydraulically operated to grip the part between contoured pads. A carriage containing this clamping unit was then pulled by a continuous chain, which was driven by a variable speed reversible drive train for a stroke of 3 to 4 m ( 10 to 12 ft). At the end of the stroke, the clamp released, and the clamping carriage returned to its starting point. During this return interval, the product remained stationary until the clamping and pulling cycle could be reinitiated. Because of this pull pause sequence, this style became known as an intermittent pull machine. Variations of this design are still found in the industry, including multiple clamping heads for multiple cavity production.
The continuous pull reciprocating clamp machine, which has become the most popular style, takes this concept one step farther. Its clamping, extension, and retraction cycles are synchronized between two pullers to provide a continuous pulling motion to the product. The value of using the intermittent pull cycle with slow cure materials or for purging die buildups is reflected in the fact that commercial reciprocating clamp machines now have intermittent-pull sequences. Subtle variations exist in the use of such drive methods as direct acting hydraulic cylinders, hydraulic motor chain drives, or recirculating ball screws. Methods of clamping can be hydraulic, pneumatic, or a mechanical wedge action. The basic prerequisite is that sufficient clamping pressure be available on a relatively short (< 460 mm, or 18 in.), contoured puller block that is held within the clamping envelope. In addition, sufficient thrust must be provided to the clamping unit to overcome the die resistance and to maintain a uniform pulling speed. An advantage of the reciprocating clamp system is its need for only two matched puller pads to attain a continuous pulling motion. These pads are easily changed and are generally of durable urethane coated steel for long life.
Continuous belt pullers have evolved from extrusion take off pullers, but they have been modified for higher loads. These pullers are unsuitable for single cavity or multiple cavity production when they are all of the same physical size. Even with this restriction, uneven belt wear can result in slippage of adjacent cavities. On a positive note, the contact area of the belted puller is generally longer than that found with the reciprocating clamp pullers, which allows lower unit pressures on the pultrusion. A more flexible version of the continuous belt machine is the cleated chain (or caterpillar) puller, which has many individually contoured puller pads attached to chain ears along the chain length. This modification allows the production of complex shapes and multiple cavities. Machine controls are used to ensure that even pressure is maintained between opposing chain pullers. The number of individually contoured puller pads can vary widely, depending on the complexity of the part. For the average part, the number of pads will vary between 12 and 60.