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