into performing fixtures where the section is partially shaped and excess resin and/or air are removed. Then it goes into a heat die where the section is cured continuously.
Preformed molding compounds
A large number of reinforced thermosetting resin products are made by matched die mold-ing processes such as hot-press compression moldmold-ing, injection moldmold-ing and transfer moldmold-ing.
Matched die molding can be a wet process, but it is most convenient to use a preformed molding compound or premix to which all necessary ingredients have been added. This enables faster production rates to be achieved. Molding compounds can be divided into three broad categories:
• bulk or dough molding compounds (BMC, DMC),
• sheet molding compound (SMC),
• pre-preg.
3.3.2 Fabrication of thermoplastic resin matrix composites
The principal method used for the production of parts with short fiber reinforced thermo-plastics is injection molding. The choice of compounding technique depends on the requirements of fiber length, volume fraction, and degree of dispersion of fibers throughout the matrix. The two most common compounding methods are extruder compounding and strand coating.
3.4 Woven fabrics reinforcements
Weaves are generally referred and defined by a notation such as: 2x2, 4x4, and 3x1 for example. The first number in this set, for example the 3 in 3x1, refers to how many strands are crossed “over” before going “under” the perpendicular strands (in a 90◦ weave). The second number refers to how many strands are crossed “under” before going back “over” the perpendicular strands (again in a 90◦ weave). That is, a 3x1 weave would run: over, over, over, under, over, over, over, under, over, over, over, etc. A 1x1 weave would run: over, under, over, under, over, under, etc. [29].
3.4.1 Plain weave
A plain weave is defined as a 1x1 weave. In this most simple weave pattern, warp and fill yarns are interlaced over and under each other in alternating fashion. The plain weave provides good stability, porosity and the least yarn slippage for a given yarn count. For high strength panel and linear angle part fabrication plain pattern (see Figure 3.10(a)) is used when high strength parts are desired. It is ideal for construction, composites reinforcement, mold making, aircraft and auto parts tooling, marine and other composite lightweight applications.
3.4. WOVEN FABRICS REINFORCEMENTS
(a) Plain weave structure (b) Twill weave structure
Figure 3.10: Plain and twill weave structures
3.4.2 Twill weave
Twill weave is more pliable than the plain weave and has better drapability while maintaining more fabric stability than a four or eight harness satin weave. Twill weave also wets out better than plain weave. The weave pattern is characterized by a diagonal rib created by one warp yarn floating over at least two filling yarns. This enables a greater number of yarns per unit area than a plain weave, while losing little fabric stability. This type of fabric looks different on one side than on the other. A twill weave has that 45◦, or 3D, “look” to it, which is so often desired. It is much easier to bend around complex curves than a plain weave, because its weave is looser.
Therefore, a 4x4 twill will bend around curves better than a 2x2 twill weave. The looser the fabric, the more likely the fabric will fray at the ends and create spaces in the fabric when bent around complex curves. But a loose fabric will fit around complex curves much better than a tighter weave fabric [29].
(a) 4 harness satin weave (b) 8 harness satin weave
Figure 3.11: Satin weave structures
3.4. WOVEN FABRICS REINFORCEMENTS
3.4.3 Satin Weave
This style fabric is one of the easiest fabrics to use and it is ideal for laying up cowls, fuselages, ducts and other contoured surfaces with minimal distortions. The fabric is more pliable and can comply with complex contours and spherical shapes. Because of its tight weave style, satin weaves are typically used as the surface ply for heavier and courser weaves. This technique helps reduce fabric print through and requires less gel coat to create a smoother surface. A harness-satin, sometimes referred to as: harness, satin or crowfoot, is defined as any number larger than 1, followed by x, and a larger number. That is, a 3x1 harness-satin is referred to as a harness-satin 4, H4, or 4HS. A 4x1 harness-satin is referred to as a harness-satin 5 (5HS or H5) and a 7x1 harness-satin is referred to as a harness-satin 8, H8, or 8HS. In the H4 weave picture below the weave runs: over, over, over, under, over, over, over, under, etc.
Four harness satin (crowfoot)
The four harness satin weave is more pliable than the plain weave and is easier to conform to curved surfaces typical in reinforced plastics. In this weave pattern there is a three by one
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Figure 3.12: Schematic diagram of a weaving loom
3.4. WOVEN FABRICS REINFORCEMENTS
interfacing where a filling yarn floats over three warp yarns and under one (see Figure 3.11(a)).
Eight harness satin
The eight harness satin (see Figure 3.11(b)) is similar to the four harness satin except that one filling yarn floats over seven warp yarns and under one. This is a very pliable weave and is used for forming over curved surfaces.
Which one for what?
A harness-satin bends over complex curves better than either a plain or twill weave. A harness-satin almost always has more weaves per inch than a plain or twill weave (defined as
“pics”). So a 17pic x 17pic has more weaves per inch than a 12pic x 12pic (the difference between the weave definition, such as 2x2, with a pic definition, such as 12pic x 12pic should not be confused!). So, because a harness-satin has a higher pic count, it will hold together a bit better than a twill weave, when handled carefully.
The best 90◦ woven fabric for going around complex curves is a harness-satin 8 (8HS). A fabric that easy pivots, or can easily change fiber angle will also fit very complex curves. To allow a fabric to change fiber angle easily, it may be necessary to cut or remove the selvage. Most flat fabric weaves, such as plain weave, twills, harness-satins, etc. are woven on a machine called loom (see Figure 3.12). This machine weave at a 90◦ angle. That is, fiber going lengthwise and widthwise [29].
In general, if there are no complex curves and we do not care about aesthetics, a plain weave is the best option. If aesthetics are very important, generally a twill weave is selected, but for a
Figure 3.13: Hierarchical structure of the NCF composites