Composite Manufacturing Processes: A Technical Overview
Wet Lay-up / Hand Lay-up
Description: Resins are manually impregnated into fibers, which are in the form of woven, knitted, stitched, or bonded fabrics. This is typically accomplished using rollers or brushes. Increasingly, nip-roller type impregnators are used to force resin into the fabrics with rotating rollers and a resin bath. Laminates are cured under standard atmospheric conditions.
Material Options
- Resins: Any (e.g., epoxy, polyester, vinylester, phenolic).
- Fibers: Any, although heavy aramid fabrics can be difficult to wet-out by hand.
Advantages
- Widely used for many years.
- Simple principles, easy to teach.
- Low-cost tooling, if room-temperature cure resins are used.
- Wide choice of suppliers and material types.
- Higher fiber contents and longer fibers are possible compared to spray lay-up.
Disadvantages
- Resin mixing, laminate resin contents, and laminate quality are highly dependent on the laminator’s skill. Low resin content laminates are difficult to achieve without incorporating excessive voids.
- Health and safety considerations regarding resins. The lower molecular weights of hand lay-up resins generally mean they have the potential to be more harmful than higher molecular weight products. The lower viscosity also means they can more easily penetrate clothing.
- Limiting airborne styrene concentrations to legislated levels from polyesters and vinylesters is becoming increasingly difficult without expensive extraction systems.
- Resins need to be low in viscosity for hand workability. This generally compromises their mechanical/thermal properties due to the need for high diluent/styrene levels.
Typical Applications
Standard wind-turbine blades, production boats, architectural moldings.
Filament Winding
Description: This process is primarily used for hollow, generally circular or oval-sectioned components, such as pipes and tanks. Fiber tows are passed through a resin bath before being wound onto a mandrel in various orientations. The fiber feeding mechanism and the mandrel’s rotation rate control the winding pattern.
Material Options
- Resins: Any resin.
- Fibers: Any. Fibers are used directly from a creel and are not woven or stitched into a fabric form.
Advantages
- Can be a very fast and, therefore, economical method of laying down material.
- Resin content can be controlled by metering the resin onto each fiber tow through nips or dies.
- Fiber cost is minimized since there is no secondary process to convert fiber into fabric before use.
- Structural properties of laminates can be very good, as straight fibers can be laid in a complex pattern to match the applied loads.
Disadvantages
- Limited to convex-shaped components.
- Fiber cannot easily be laid exactly along the length of a component.
- Mandrel costs for large components can be high.
- The external surface of the component is unmolded and, therefore, cosmetically unattractive.
- Low-viscosity resins usually need to be used, with their attendant lower mechanical and health and safety properties.
Typical Applications
Chemical storage tanks and pipelines, gas cylinders, firefighters’ breathing tanks.
Pultrusion
Description: Fibers are pulled from a creel, through a resin bath, and then through a heated die. The die completes the fiber impregnation, controls the resin content, and cures the material into its final shape. The cured profile is then automatically cut to length. Fabrics may also be introduced into the die to provide fiber direction other than 0º. Pultrusion is a continuous process, producing a profile of constant cross-section.
Material Options
- Resins: Generally epoxy, polyester, vinylester, and phenolic.
- Fibers: Any.
Advantages
- Can be a very fast and, therefore, economical way of impregnating and curing materials.
- Resin content can be accurately controlled.
- Fiber cost is minimized since the majority is taken from a creel.
- Structural properties of laminates can be very good, as the profiles have very straight fibers, and high fiber volume fractions can be obtained.
- Resin impregnation area can be enclosed, thus limiting volatile emissions.
Disadvantages
- Limited to constant or near-constant cross-section components.
- Heated die costs can be high.
Typical Applications
Beams and girders used in roof structures, bridges, ladders, and frameworks.
Resin Transfer Molding (RTM)
Description: Fabrics are laid up as a dry stack of materials. These fabrics are sometimes pre-pressed to the mold shape and held together by a binder. These “preforms” are then more easily laid into the mold tool. A second mold tool is clamped over the first, and resin is injected into the cavity. Vacuum can also be applied to the mold cavity to assist resin in being drawn into the fabrics (VARTM – Vacuum Assisted Resin Transfer Molding). Once all the fabric is wet out, the resin inlets are closed, and the laminate is allowed to cure. Both injection and cure can take place at either ambient or elevated temperatures.
Material Options
- Resins: Generally epoxy, polyester, vinylester, and phenolic, although high-temperature resins such as bismaleimides can be used at elevated process temperatures.
- Fibers: Any. Stitched materials work well in this process since the gaps allow rapid resin transport. Some specially developed fabrics can assist with resin flow.
Advantages
- High fiber volume laminates can be obtained with very low void contents.
- Good health and safety, and environmental control due to enclosure of resin.
- Possible labor reductions.
- Both sides of the component have a molded surface.
Disadvantages
- Matched tooling is expensive and heavy in order to withstand pressures.
- Generally limited to smaller components.
Manufacturing of Prepregs
Description: Prepreg stands for “pre-impregnated” composite sheets. They are made of sheets of woven, unidirectional fibers or mats already impregnated with the matrix material. The thermoset matrix is slightly cured to provide consistency to the pre-impregnated sheet.
Main Advantages
- Controlled fiber content and consistent mechanical properties.
- Benefit from automatic tape laying technology.
- Easiness of laminate manufacturing: cutting, pile-up, and consolidation.
Main Disadvantages
- Prepregs are perishable goods. Their life is limited by the progressive curing of the matrix.
- They have to be stored at -18ºC, and their life at this temperature is approximately 6 months.
- Cost (as compared with dry fabrics, fiber tapes, or rovings).
Vacuum Bagging
Description: This is basically an extension of the wet lay-up process described above, where pressure is applied to the laminate once laid-up in order to improve its consolidation. This is achieved by sealing a plastic film over the wet laid-up laminate and onto the tool. The air under the bag is extracted by a vacuum pump, and thus up to one atmosphere of pressure can be applied to the laminate to consolidate it.
Material Options
- Resins: Primarily epoxy and phenolic. Polyesters and vinylesters may have problems due to excessive extraction of styrene from the resin by the vacuum pump.
- Fibers: Any, including heavy fabrics.
Advantages
- Higher fiber content laminates can usually be achieved than with standard wet lay-up techniques.
- Lower void contents are achieved than with wet lay-up.
- Better fiber wet-out due to pressure and resin flow throughout structural fibers, with excess into bagging materials.
- Health and safety: The vacuum bag reduces the amount of volatiles emitted during cure.
Disadvantages
- The extra process adds cost, both in labor and in disposable bagging materials.
- A higher level of skill is required by the operators.
- Mixing and control of resin content are still largely determined by operator skill.
Autoclave Consolidation of Prepregs
Description: Fabrics and fibers are pre-impregnated by the materials manufacturer, under heat and pressure or with solvent, with a pre-catalyzed resin. The catalyst is largely latent at ambient temperatures, giving the materials several weeks, or sometimes months, of useful life when defrosted. However, to prolong storage life, the materials are stored frozen. The resin is usually a near-solid at ambient temperatures, and so the pre-impregnated materials (prepregs) have a light, sticky feel to them, such as that of adhesive tape. Unidirectional materials take fiber directly from a creel and are held together by the resin alone. The prepregs are laid up by hand or machine onto a mold surface, vacuum bagged, and then heated to typically 120-180 ºC. This allows the resin to initially reflow and eventually to cure. Additional pressure for the molding is usually provided by an autoclave, which can apply up to 5 atmospheres to the laminate.
Advantages
- Resin/catalyst levels and the resin content in the fiber are accurately set by the materials manufacturer. High fiber contents can be safely achieved.
- The materials have excellent health and safety characteristics and are clean to work with.
- Resin chemistry can be optimized for mechanical and thermal performance, with the high-viscosity resins being impregnable due to the manufacturing process.
- The extended working times (of up to several months at room temperatures) means that structurally optimized, complex lay-ups can be readily achieved.
- Potential for automation and labor saving.
Disadvantages
- Materials cost is higher for pre-impregnated fabrics.
- Autoclaves are usually required to cure the component. These are expensive, slow to operate, and limited in size.
- Tooling needs to be able to withstand the process temperatures involved.
Injection Molding
Description: The most common method to manufacture short-fiber and particle-reinforced thermoplastics and is very similar to the process used for injection of thermoplastics. Polymer pellets and fiber/ particles are mixed in a hot Archimedes screw, where the shear forces homogenize the mixture. The mixture is finally injected into a mold.
Material Options
- Thermoplastics: PE, PP, PVC, nylon.
- Fibers: Chopped glass fibers and/or any kind of particles (including nanoparticles).
Advantages
- High production rates, highly automatized process.
Disadvantages
- Tooling needs to be able to withstand the process temperatures involved.
- Wear of parts is increased by the presence of the fibers.
- Homogeneous dispersion of nanoparticles/fibers is critical to achieve good properties.
Compression Molding
Description: Fiber preforms pre-impregnated with a thermoplastic (or laminates of thermoplastic film/fiber preforms) are laid up on a mold and formed by the application of heat and pressure. Preform can be form by fiber mats or fabrics. The material is heat above Tm.
Advantages
- Rapid forming process.
- Complex shapes and cross-sections are possible.
- Inserts and attachments can be accommodated.
- Different fillers (short fibers, particles, etc.) can be used.
- High dimensional control.
Disadvantages
- Tooling needs to be able to withstand the process temperatures involved.
- High cost of the precursor material.
- Part size is limited by press size.