Composite Materials 61Valery Marinov, Manufacturing Technology

COMPOSITE MATERIALS

Introduction

Composite materials are polymers that have been reinforced with organic or in-organic fiber materials (Fiber-reinforced polymers, FRPs). The principal advantage of these materials is the very high strength-to-weight ratio, which makes them attractive in aircrafts, spacecrafts, cars, boats, and sport equipment.

Gates-Piaggio aircraft is an example of the use of composite

materials in aircraft industry

Starting materials

Fiber-reinforced polymers consist of two components, polymer matrix and reinforced phase. They are produced separately before being combined to make the composite part.

Polymermatrix

Reinforcedphase

Structure of a fiber-reinforced composite material

Thermoplastics, thermosets or elastomers are used for the polymer matrix. Thermosetting polymers are the most common matrix materials, especially epoxies.

Reinforcing components are fibers, cloth (fabrics), and mat of glass, boron, carbon (graphite), polymers.

Thread

Fiber

Cloth Mat

Materials used as reinforcing phase in fiber-reinforced composite materials

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Fibers are used in some fabrication processes in a continuous form, known as roving, which is a collection of untwisted continuous threads. Each thread consists of 1000~30 000 single fibers.

Cloth is a fabric of woven yearns. Some special cloths can consists of different fibers, for example carbon + glass fibers:

Mat is a material that consists of randomly oriented short fibers.

In some applications, raw materials are combined prior to the shaping operations (the resulting material is known as a prepreg). The typical thickness of the prepreg is 0.125 mm with 34% polymer resin. The prepreg is available as strip with width 300, 500, or 1000 mm and length 50 or 100 m.

Glass fibers

Carbon fibers

Carbon-glass fibers cloth

PVC film

Polymermatrix

Fibers, cloth or mat

The structure of prepreg

Shaping processes

Open mold process

This process uses a single positive or negative mold surface. The starting materials are applied to the mold in layers, building up to the desired thickness. Next follows curing and after that part is removed from the mold. Different open-mold processes were developed depending on the way the raw material is applied to the mold. The oldest one is a hand Lay-up Process:

Composite Materials 63Valery Marinov, Manufacturing Technology

The entire lay-up process is both time-consuming and labor-intensive.

(Left) Adding successive layers of resin and glass cloth in a hand lay-up process for producing a boat hull (Right) Finishing the fiberglass reinforced boat hull

Other open mold processes include automated tape-laying machines, spray-up process, etc.

Closed mold processes

In these processes, molds consist of two sections that open and close during each molding cycle. In compression molding processes, starting materials are placed in the lower mold section, and the mold is closed under pressure. Next, the mold is heated to cure the polymer matrix. After curing, the mold is opened and the part is removed. Different compression molding processes were developed for different starting materials.

Compression molding of a thermoplastic sheet Pick-and-place machine used in resin-transfer molding to automatically cut to shape and stack in position the carbon

fabric plies

In Resin Transfer Molding (RTM), the resin is injected into fiber preforms enclosed in heated mold cavities. In advanced RTM instead of cloth or math continuous fiber reinforcement is used. Material is sintered before with powdered binder material and send to the pick-and-place machine where it is cut to shape and stacked in position. The preforms are then combined with tooling, heated, and the resin is injected. After cooling, parts are cleaned and trimmed.

Composite Materials64 Valery Marinov, Manufacturing Technology

Injection molding process can also be adapted to FRPs to produce low-cost parts in large quantities.

Filament winding

In this process, resin-impregnated fibers are wrapped around a rotating mandrel that has the internal shape of the desired FRP part:

Wet filament winding

Fiber can be impregnated just prior to winding pulling them through the liquid resin (wet winding). In prepreg winding, the filament band is used instead of fibers. It is possible also the filament to be impregnated after the winding by brushing or other techniques (postimpregnation).

Filament winding machine

Pultrusion process

The process is similar to extrusion, but it involves pulling of the workpiece. Pultrusion process is used to produce simple shapes of uniform cross section, such as round, rectangular, tubular, and structural products. The bundles of fibers are drawn through a bath of polymer resin and then gathered to produce a desired cross-sectional product. The material is then drawn through one or more heated dies for further shaping and curing. After cooling, material is cut to length.

Resin bathForming

die

Heateddie orcuringoven

Reinforced fibers

Schematic diagram of the pultrusion process