FRP Composites Moulding Techniques PDF

Fiber Reinforced Plastics (FRP) Composites Moulding Rotational Molding Can form large hollow parts by this method. Two piece mold is designed such that it can be rotated about 2 perpendicular axes. Pre-measured amount of powder plastic is placed inside warm mold. Mold is h...

Fiber Reinforced Plastics (FRP) Composites Moulding Rotational Molding Can form large hollow parts by this method. Two piece mold is designed such that it can be rotated about 2 perpendicular axes. Pre-measured amount of powder plastic is placed inside warm mold. Mold is heated further and rotated about the two axes. Powder tumbles against mold walls, causing it to fuse without melting. Additional cross-linking occurs via a chemical agent. Typical products: trash cans, buckets, toys, boat hulls. https://www.youtube.com/watch?v=LPTb0jm8klA  Fibers as the Reinforcing Phase Common fiber materials: glass, carbon, and Kevlar (a polymer) In some fabrication processes, the filaments are continuous, while in others, they are chopped into short lengths In continuous form, individual filaments are usually available as rovings - collections of untwisted continuous strands, convenient form for handling By contrast, a yarn is a twisted collection of filaments  Fibers as the Reinforcing Phase The most familiar form of continuous fiber is a cloth ‑ a fabric of woven yarns Similar to a cloth is a woven roving, a fabric consisting of untwisted filaments rather than yarns Woven rovings can be produced with unequal numbers of strands in the two directions so that they possess greater strength in one direction Such unidirectional woven rovings are often preferred in laminated FRP composites  Fibers as the Reinforcing Phase Fibers can also be in a mat form ‑ It consisting of randomly oriented short fibers held loosely together with a binder Mats are commercially available as blankets of various weights, thicknesses, and widths Mats can be cut and shaped for use as preforms in some of the closed mold processes During molding, the resin impregnates the preform and then cures, thus yielding a fiber‑reinforced molding  Combining Matrix and Reinforcement 1. The starting materials arrive at the fabrication operation as separate entities and are combined into the composite during shaping Filament winding and pultrusion, in which reinforcing phase = continuous fibers 2. The two component materials are combined into some starting form that is convenient for use in the shaping process Molding compounds Prepregs  Molding Compounds FRP composite molding compounds consist of the resin matrix with short randomly dispersed fibers, similar to those used in plastic molding Most molding compounds for composite processing are thermosetting polymers Since they are designed for molding, they must be capable of flowing Accordingly, they have not been cured prior to shape processing Curing is done during and/or after final shaping  Prepregs Fibers impregnated with partially cured TS resins to facilitate shape processing Available as tapes or cross‑plied sheets or fabrics Curing is completed during and/or after shaping Advantage: prepregs are fabricated with continuous filaments rather than chopped random fibers, thus increasing strength and modulus  Open Mold Processes Family of FRP shaping processes that use a single positive or negative mold surface to produce laminated FRP structures The starting materials (resins, fibers, mats, and woven rovings) are applied to the mold in layers, building up to the desired thickness This is followed by curing and part removal Common resins are unsaturated polyesters and epoxies, using fiberglass as the reinforcement  Open Mold FRP Processes 1. Hand lay‑up 2. Spray‑up 3. Automated tape‑laying machines The differences are in the methods of applying the laminations to the mold, alternative curing techniques, and other differences  Hand Lay‑Up Method Open mold shaping method in which successive layers of resin and reinforcement are manually applied to an open mold to build the laminated FRP composite structure Labor‑intensive Finished molding must usually be trimmed with a power saw to size outside edges Oldest open mold method for FRP laminates, dating to the 1940s when it was first used for boat hulls  Hand Lay-Up Method Figure: Hand lay‑up : (1) mold is treated with mold release agent; (2) thin gel coat (resin) is applied, to the outside surface of molding; (3) when gel coat has partially set, layers of resin and fiber are applied, the fiber is in the form of mat or cloth; each layer is rolled to impregnate the fiber with resin and remove air; (4) part is cured; (5) fully hardened part is removed from mold.  Products Made by Hand Lay‑Up Generally large in size but low in production quantity - not economical for high production Applications: Boat hulls Swimming pools Large container tanks Movie and stage props Other formed sheets The largest molding ever made was ship hulls for the British Royal Navy: 85 m (280 ft) long  Spray‑up Method Spray-up is an open-moulding composites fabrication process where resin and reinforcements are sprayed onto a reusable mould. The resin and glass fibers may be applied separately or simultaneously "chopped" in a combined stream from a chopper gun. he processing steps are very similar to those in hand lay-up. In this process, the release agent is first applied to the mould and then a layer of gelcoat is applied. The gelcoat is left for sometime, until it hardens. Once the gelcoat hardens, a spray gun is used to deposit the fibre resin mixture onto the surface of the mould. The spray gun chops the incoming continuous rovings (one or more rovings) to a predetermined length and impels it through the resin/catalyst mixture.  Resin/catalyst mixing can take place inside the gun (gun mixing) or just in front of the gun. Spray guns are becoming popular because they provide more controlled spray patterns and reduced emission of volatiles. Once the material is sprayed on the mould, brushes or rollers are used to remove entrapped air as well as to ensure good fiber wetting. Fabric layers or continuous strand mats are added into the laminate, depending on performance requirements. The curing of the resin is done at room temperature. The curing of resin can take two to four hours, depending on the resin formulation. After curing, the part is removed from the mould and tested for finishing and structural requirements. Spray‑up Method  Automated Tape‑Laying Automated tape‑layingMachines machines operate by dispensing a prepreg tape onto an open mold following a programmed path Typical machine consists of overhead gantry to which the dispensing head is attached The gantry permits x‑y‑z travel of the head, for positioning and following a defined continuous path Curing is required of all thermosetting resins used in FRP laminated composites Curing cross‑links the polymer, transforming it from its liquid or highly plastic condition into a hardened product Three principal process parameters in curing: Time, Temperature and Pressure  Filament Resin‑impregnated Winding continuous fibers are wrapped around a rotating mandrel that has the internal shape of the desired FRP product; the resin is then cured and the mandrel removed The fiber rovings are pulled through a resin bath immediately before being wound (twisted) in a helical pattern onto the mandrel The operation is repeated to form additional layers, each having a criss-cross pattern with the previous, until the desired part thickness has been obtained Filament Winding Filament Winding  Pultrusion Processes Similar to extrusion (hence the name similarity) but work piece is pulled through die (so prefix "pul‑" in place of "ex‑") Like extrusion, pultrusion produces continuous straight sections of constant cross section Developed in early 1950s for making fishing rods of glass fiber reinforced polymer (GFRP) A related process, called pulforming, is used to make parts that are curved and which may have variations in cross section throughout their lengths  Pultrusion Processes Continuous fiber rovings are dipped into a resin bath and pulled through a shaping die where the impregnated resin cures The sections produced are reinforced throughout their length by continuous fibers Like extrusion, the pieces have a constant cross section, whose profile is determined by the shape of the die opening The cured product is cut into long straight sections  Pultrusion Processes solid rods, tubing, long flat sheets, structural sections (such as channels, angled and flanged beams), tool handles for high voltage work, and third rail covers for subways. Pultrusion Processes  Pulforming Pultrusion with additional steps (post processing) to form the length into a semicircular contour and alter the cross section at one or more locations along the length Pultrusion is limited to straight sections of constant cross section There is also a need for long parts with continuous fiber reinforcement that are curved rather than straight and whose cross sections may vary throughout length Pulforming is suited to these less regular shapes Pulforming