ISE 2500 Chapter 14: Processing of Polymer Matrix Composites and Rubber
Curing methods based on heating
oven curing, infraded heating, curing in an autoclave
Synthetic rubbers
produced from petroleum by the same polymerization techniques used to synthesize other polymers
Tire production sequence
1.Preforming of components 2.Building the carcass and adding rubber strips to form the sidewalls and treads 3.Molding and curing the components into one integral piece
Duration of rubber vulcanization
15-20 minutes
Compounding
Additives to the rubber that can cause vulcanization, enhance the mechanical properties, or extend the rubber to reduce cost
Roller die process
Combination of extrusion and calendering that results in better quality product than either extrusion or calendering alone
Pneumatic tires
Support the weight of the vehicle, passengers, and cargo, transmit the motor torque to propel the vehicle, absorb road vibrations and shock to provide a comfortable ride
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T/F: Carbon black provides protection from ultraviolet radiation
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T/F: Cured FRPs are tough, abrasive, and difficult‑to‑cut
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T/F: Curing (vulcanizing) is accomplished in the mold in all three molding processes
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T/F: Curing is required of all thermosetting resins used in FRP laminated composites
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T/F: Curing normally occurs at room temp for the TS resins used in hand lay-up or spray-up procedures
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T/F: Pultrusion produces continuous straight sections of constant cross section
Coagulation
an acid such as formic acid is used to transform latex into soft solid slabs
autoclave
an enclosed chamber equipped to apply heat and/or pressure at controlled levels
Automated tape-laying machines
machines that operate by dispensing a prepreg tape onto an open mold following a programmed path
Building drum
Thing that carcass is assembled on. Its main element is a cylindrical arbor that rotates
Three principle process parameters in curing
Time, pressure, temperature
Carcass
internal structure of the tire, consists of multiple layers of plies (rubber-coated cords)
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
Closed mold processes
performed in molds consisting of two sections that open and close each molding cycle
Plies
strands of rubber-coated nylon, polyester, fiber glass, or steel, which provide inextensibility to reinforce the rubber in the carcass
Conventional Reaction injection molding
two reactive ingredients are mixed and injected into a mold cavity where curing and solidification occur
Yarn
A twisted collection of filaments
Advantages of closed molds
(1) good finish on all part surfaces, (2) higher production rates, (3) closer control over tolerances, and (4) more complex three‑dimensional shapes are possible
Steps of hand lay-up method
(1) mold is treated with mold release agent; (2) thin gel coat (resin) is applied to outside surface of mold; (3) when gel coat has partially set, layers of resin and fiber mat or cloth are applied, 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
Other PMC Shaping processes
Centrifugal casting, tube rolling, continuous laminating, cutting of FRPs, many of the traditional thermoplastic shaping processes
Compression molding
Charge is placed in lower mold section and mold sections are brought together under pressure, forcing charge to take the shape of the cavity
Injection molding
Charge of thermosetting resin with short fibers is placed in a pot, heated, and squeezed by ram action into one or more mold cavities
Rovings
Collections of untwisted continuous strands; convenient form for handling
Preforming
Components and others are produced by continuous processes and then pre-cut to size and shape for subsequent assembly
Reasons for inefficient shaping techniques for composites
Composites are more complex than other materials, one needs to orient the reinforcing phase in FRPs, composite processing technologies have not been the object of refinement over as many years as processes for other materials
Three categories of closed mold processes
Compression molding, transfer molding, injection molding
Curing
Cross-links the polymer, transforming it from its liquid or highly plastic state into a solid product
Two-stage mixing
Done to avoid premature vulcanization Stage 1: carbon black and other non‑vulcanizing additives are combined with the raw rubber. Step 2: After cooling time has been allowed, stage 2 mixing is carried out in which vulcanizing agents are added
Cloth
Fabric of woven yarns
Advantages of prepregs
Fabricated with continuous filaments rather than chopped random fibers, thus increasing strength and Youngs modulus
Open mold processes
Family of FRP shaping processes that use a single positive or negative mold structure to produce laminated FRP structures
Mat
Felt consisting of randomly oriented short fibers held loosely together with a binder
Prepregs
Fibers impregnated with partially cured TS resins to facilitate shape processing
3 primary methods for open mold FRP processes
Hand lay-up, spray-up, automated tape-laying machines
Coating
Important industrial process for producing tires, conveyor belts, inflatable rafts, and waterproof cloth
Properties of hand lay-up products
Large in size but low in production quantity (not economical for high production)
Spray-up method
Liquid resin and chopped fibers are sprayed onto an open mold to build successive FRP lamination. It is an attempt to mechanize the application of resin-fiber layers and reduce lay-up time (alternative to step 3 in the hand lay-up procedure)
Processing of thermoplastic elastomers
Most common shaping processes are injection molding and extrusion. Generally more economical and faster than the traditional processes for rubbers that must be vulcanized
Pulforming
Pultrusion with additional steps to form the length into a semicircular contour and alter the cross section at one or more locations along the length
Filament winding
Resin‑impregnated continuous fibers are wrapped around a rotating mandrel that has the internal shape of the desired FRP product
Calendering
Rubber stock is passed through a series of gaps of decreasing size by a stand of rotating rolls
Woven roving
Similar to a cloth, but it consists of untwisted filaments rather than yarns
Pultrusion process
Similar to extrusion (hence the name similarity) but workpiece is pulled through die. Continuous fiber rovings are dipped into a resin bath and pulled through a shaping die where the impregnated resin cures
Grades of natural rubber
Smoke-house --> air dry --> pale crepe
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T/F: Cutting of FRP laminated composites is required in both uncured and cured states (trim excess material, cut holes and outlines, and so on)
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T/F: Many products require filament reinforcement to reduce extensibility but retain the other desirable properties
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T/F: Rubber uses same molding processes as FRPs
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T/F: The additives must be thoroughly mixed with the base rubber to achieve uniform dispersion of ingredients
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T/F: Thermosetting polymers are the most common matrix materials
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T/F: Tires are about 75% of total rubber tonnage
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T/F: Tires can be made from TPEs (thermoplastic elastomers)
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T/F: Unidirectional woven rovings are often preferred in laminated FRP composites
Carbon black
The single most important reinforcing filler in rubber, it is a colloidal form of carbon obtained by thermal decomposition of hydrocarbons
Vulcanization
Treatment that accomplishes cross‑linking of elastomer molecules. Makes the rubber stiffer and stronger but retain extensibility
Properties of injection molding
Used for both TP and TS type FRPS, virtually all TPs can be reinforced with fibers, chopped fibers must be used, during injection into the mold cavity, fibers tend to become aligned as they pass through the nozzle
Polymer matrix composite
composite material consisting of a polymer imbedded with a reinforcing phase such as fibers or powders
Molding compounds
consists of a resin matrix with short randomly dispersed fibers, similar to those used in plastic molding
Four basic categories of shaping processes
extrusion, calendering, coating, molding/casting
Pale crepe rubber
high grade of rubber that involves two coagulation steps followed by warm air drying
Thermoplastic elastomers
processed like thermoplastics, but their applications are those of an elastomer. The most common shaping processes are injection molding and extrusion
Extrusion
screw extruders generally used, the L/D ratio of the extruder barrel is less than thermoplastic, die swell occurs in rubber extrudates, rubber has not yet been vulcanized
Advantages of reinforced reaction injection molding
similar to RIM but includes reinforcing fibers, typically glass fibers, in the mixture
Reinforced reaction injection molding
similar to RIM but includes reinforcing fibers, typically glass fibers, in the mixture
Latex
starting ingredient of rubber naturally grown in tropical climates