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INTRODUCTION
One method that’s frequently accustomed produce composite products is first to put layers of sheet-formed fabrics on a form having the required shape and so to impregnate the material with wet resin.
Each layer of material may be adjusted in its orientation to provide special properties of the finished article.
After the lay-up and resin impregnation are completed, the whole system is subjected to heat and pressure while a curing agent reacts with the bottom resin to supply cross-linking
that binds all of the weather into a three-dimensional, unified structure.
The polymer binds to the fibers and holds them in their preferred position and orientation during use.
An alternative method of fabricating composite products starts with a process of preimpregnating the fibers with the resin material to supply strands, tape, braids, or sheets.
The resulting form, called a prepreg, can then be stacked into layers or wound onto a form to provide the specified shape and thickness.
The final step is that the curing cycle as described for the wet process.
Injection molding and extrusion will be used for composites made of thermoplastics in conventional plastics processing equipment, provided the reinforcement fibers are short that the material can flow properly.
Glass fiber reinforcement is most typically used.
Sheet molding compounds (SMC) are employed in compression molding of panels and shells used for automotive body panels, industrial and business machine cabinets, watercraft, and electrical parts.
Complex three-dimensional products are often made in final form or near-net-shape using SMC materials.
Glass reinforcement fibers [typically in lengths from 1/2 in to 1.0 in (12 mm to 25 mm) and from 10% to 60% volume fraction] are randomly mixed with a polymer resin (often polyester), catalysts, colorants, thickeners, release agents, and inert fillers.
Plastic films of nylon or polyethylene cover the mixture top and bottom as a sandwich and the combination is rolled to the specified thickness.
At the time of use, the sheets are placed into a mold and shaped and cured simultaneously under heat and pressure.
Where higher mechanical properties are required, SMC sheets are often made of an epoxy matrix and carbon fiber reinforcements.
Pultrusion may be a process during which the fiber reinforcement is coated with resin because it is pulled through a heated die to provide never-ending form within the desired shape.
This process is employed to supply rod, tubing, structural shapes (I-beams, channels, angles, and so on), tees, and hat sections used as stiffeners in aircraft structures, truck
and trailer bodies, and similar large structures.
Filament winding is employed to create pipe, pressure vessels, rocket motor cases, instrument enclosures, and other cylindrical containers.
The continuous filament may be placed during a type of patterns, including helical,
axial, and circumferential, to supply desired strength and stiffness characteristics.
SELECTION OF COMPOSITE MATERIALS
Selection of which matrix, reinforcement, and processing method to use for a component made of composites is indeed a fancy process.
However, some generalizations will be made counting on two major criterion, cost and performance.
When low cost dominates the look criteria, fiberglass is that the predominant choice.
Common fiberglass made up of thermosetting polyester resin and optical fiber reinforcement is most frequently utilized in boats, piping, housings, canisters, and structural components.
For high-volume applications which will be made up of injection molding or extrusion, low-cost designs are often made using thermoplastics like polyamides and polycarbonates with short glass reinforcements.
Higher strength or stiffness properties may result when using PEEK, PPS, or PVC.
When high performance is required, carbon fiber in an epoxy matrix is that the predominant choice.
Typical applications include aircraft structural components; high-end equipment like tennis rackets, golf clubs, and fishing poles; and machine components.
ADVANTAGES OF COMPOSITES
Designers typically seek to supply products that are safe, strong, stiff, lightweight, and highly tolerant of the environment within which the merchandise will operate.
Composites often excel in meeting these objectives in comparison with alternative materials like metals, wood, and unreinforced plastics.
Two parameters that are wont to compare materials are specific strength and specific
modulus, defined as follows:
Specific strength is that the ratio of the strength of a cloth to its specific weight.
Specific modulus is that the ratio of the modulus of elasticity of a cloth to its specific weight.
Because the modulus of elasticity could be a measure of the stiffness of a fabric, the particular modulus is usually called specific stiffness.
Although obviously not a length, both of those quantities have the unit of length, derived from the ratio of the units for strength or modulus of elasticity and therefore the units for specific weight.
Thus, the unit for specific strength or specific modulus is inches.
In SI units, strength and modulus are expressed in N/m2 (pascals), whereas specific weight is in N/m3.
LIMITATIONS OF COMPOSITES
Designers must balance many properties of materials in their designs while simultaneously considering manufacturing operations, costs, safety, life, and repair of the merchandise. Listed next are a number of the foremost concerns when using composites.
- Material costs for composites are typically above for several alternative materials.
- Fabrication techniques are quite different from those used to shape metals. New manufacturing equipment could also be required, together with additional training for production operators.
- The performance of products made up of some composite production techniques is subject to a wider range of variability than the performance of products made up of most metal fabrication techniques.
- The operating temperature limits for composites having a polymeric matrix are approximately 500°F (260°C).
