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Questions and Answers
What is a key characteristic of molding compounds used in composite processing?
What is the primary advantage of prepregs compared to molding compounds?
In open mold processes, what is done after applying the starting materials to the mold?
Which of the following is NOT a characteristic of open mold FRP processes?
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What type of resins are commonly used in open mold FRP processes?
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What is the primary process involved in rotational molding?
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Which fiber material is NOT commonly used for reinforcement in FRP composites?
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What distinguishes a yarn from a roving in fiber reinforcement?
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Which of the following is a characteristic of woven rovings?
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What is a distinguishing feature of mats used in fiber reinforcement?
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In which fabrication processes do continuous fibers serve as the reinforcing phase?
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What happens during the molding process involving mats as preforms?
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What is the advantage of using unidirectional woven rovings in FRP composites?
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What is the main disadvantage of the hand lay-up method?
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How does the spray-up method differ from the hand lay-up method?
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Which of the following is NOT a typical application of the hand lay-up method?
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What is the first step in the spray-up method?
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What is a characteristic of products made using the hand lay-up method?
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What unique characteristic do spray guns provide in the spray-up method?
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What curing technique is primarily associated with both the hand lay-up and spray-up methods?
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Which of the following statements about the history of hand lay-up is accurate?
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What is the primary purpose of using brushes or rollers after spraying material on the mould?
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What are the main process parameters that influence the curing of thermosetting resins?
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What characterizes the filament winding process when creating a reinforced polymer product?
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What is the primary function of a curing process for thermosetting resins?
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What is the main difference between pultrusion and extrusion processes?
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What type of products can be produced using the pultrusion process?
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In automated tape-laying, what allows the dispensing head to follow a programmed path?
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What is the main purpose of allowing resin-impregnated fiber rovings to be twisted in the filament winding process?
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What is the typical curing time range for the resin after application?
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Study Notes
Rotational Molding
- Forms large hollow parts by rotating a two-piece mold around two perpendicular axes.
- Powdered plastic is placed in the warm mold and heated further.
- Powder fuses without melting due to tumbling against mold walls.
- Cross-linking occurs through a chemical agent.
- Typical products include trash cans, buckets, toys, and boat hulls.
Fibers as the Reinforcing Phase
- Common fiber materials: glass, carbon, and Kevlar (a polymer).
- Filaments can be continuous or chopped into short lengths.
- Continuous filaments are often available as rovings, which are collections of untwisted strands.
- Yarns are twisted collections of filaments.
Fibers as the Reinforcing Phase
- The most common form of continuous fiber is cloth, a fabric of woven yarns.
- Woven rovings are fabrics consisting of untwisted filaments.
- Woven rovings can have unequal numbers of strands in different directions, creating greater strength in one direction.
- Unidirectional woven rovings are commonly used in laminated FRP composites.
Fibers as the Reinforcing Phase
- Mats consist of randomly oriented short fibers held together by a binder.
- Mats are available as blankets of varying weights, thicknesses, and widths.
- Mats can be used as preforms in closed mold processes.
- During molding, resin impregnates the preform and cures, creating a fiber-reinforced molding.
Combining Matrix and Reinforcement
- Starting materials are combined into a composite during shaping.
- Filament winding and pultrusion use continuous fibers as the reinforcing phase.
- Molding compounds and prepregs are starting forms used in shaping.
Molding Compounds
- FRP composite molding compounds consist of a resin matrix with randomly dispersed short fibers, similar to those used in plastic molding.
- Most molding compounds for composites are thermosetting polymers.
- They are designed to flow and are not cured prior to shaping, curing occurs during or after final shaping.
Prepregs
- Fibers impregnated with partially cured thermosetting (TS) resins to facilitate shaping.
- Available as tapes, cross-plied sheets, or fabrics.
- Curing is completed during or after shaping.
- Prepregs are fabricated with continuous filaments, increasing strength and modulus compared to chopped fibers.
Open Mold Processes
- A family of FRP shaping processes that use a single positive or negative mold surface to produce laminated FRP structures.
- Starting materials are applied to the mold in layers, building up to the desired thickness.
- Common resins are unsaturated polyesters and epoxies, using fiberglass as the reinforcement.
Open Mold FRP Processes
- Hand lay-up, spray-up, and automated tape-laying machines are used for applying laminations to molds.
- Each method varies in application methods, curing techniques, and other factors.
Hand Lay-Up Method
- Open mold shaping method where layers of resin and reinforcement are manually applied to a mold to build the FRP structure.
- Labor-intensive process.
- Finished moldings need to be trimmed to size.
- The oldest open mold method for FRP laminates, dating back to the 1940s.
Hand Lay-Up Method
- Mold is treated with a mold release agent.
- Thin gel coat (resin) is applied to the outside surface of molding.
- Layers of resin and fibers are applied after the gel coat partially sets.
- Part is cured and then removed from the mold.
Products Made by Hand Lay-Up
- Generally large in size but low production quantity.
- Applications include boat hulls, swimming pools, large container tanks, movie and stage props, and formed sheets.
- The largest molding ever made was ship hulls for the British Royal Navy: 85 m (280 ft) long.
Spray-Up Method
- An open-moulding composites fabrication process where resin and reinforcements are sprayed onto a reusable mould.
- Resin and glass fibers are applied separately or simultaneously "chopped" from a chopper gun.
- Process steps are similar to hand lay-up, with gel coat applied first.
- A spray gun chops continuous rovings and mixes them with resin/catalyst, spraying the mixture onto the mold.
- Once material is sprayed, brushes or rollers remove air and ensure good fiber wetting.
- Fabric layers or continuous strand mats can be added.
- Curing is done at room temperature, taking 2-4 hours depending on the resin.
- After curing, the part is removed from the mould and tested for finishing and structural requirements.
Automated Tape-Laying Machines
- Dispenses prepreg tape onto an open mold following a programmed path.
- Typical machine consists of an overhead gantry with a dispensing head attached.
- Gantry allows for x-y-z travel of the head, enabling positioning and following a defined continuous path.
- Curing is required for all thermosetting resins used in FRP laminated composites.
- Curing crosslinks the polymer, hardening it from a liquid or highly plastic state.
- Three key curing parameters: time, temperature, and pressure.
Filament Winding
- Resin-impregnated continuous fibers are wrapped around a rotating mandrel shaped like the desired FRP product.
- Fiber rovings are pulled through a resin bath and then wound onto the mandrel in a helical pattern.
- Multiple layers are created with criss-cross patterns, building up to the desired thickness.
- The resin is then cured and the mandrel removed.
Pultrusion Processes
- Similar to extrusion, but the work piece is pulled through a die.
- Produces continuous straight sections of constant cross-section.
- Developed in the early 1950s for making fishing rods from GFRP.
- A related process called pulforming makes curved parts with variations in cross-section.
Pultrusion Processes
- Continuous fiber rovings are dipped in a resin bath and pulled through a shaping die, where the impregnated resin cures.
- Continuous fibers reinforce the entire length of the product.
- Products have a constant cross-section determined by the die opening.
- The cured product is cut into long straight sections.
Pultrusion Processes
- Examples of pultruded products include solid rods, tubing, long flat sheets, structural sections, tool handles, and third rail covers for subways.
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Description
Explore the techniques of rotational molding and the role of fibers as reinforcing phases in materials science. This quiz covers the processes, common materials, and applications of both methods. Perfect for students studying materials engineering or manufacturing.
