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Friday, June 27, 2014

Squeegees

Squeegees with either rubber or plastic blades are useful for scraping off excess resin from reinforcing cloth and woven roving 'When doing laminating or lay-up work. This is important for. Keeping the reinforcing-material-to-'resin ratio high for greater strength in the cured laminate. Squeegees can also be used for spreading resin quickly and removing air bubbles.

The blades on some squeegees are stiff. Others have some flexibility such as those used for cleaning window glass). It will take some experimenting with both types to determine which you prefer for fiberglassing work. Squeegees are available in various widths or in strips that you can cut to desired lengths. Those with blades from about 3 to 6 inches wide are about right for most fiberglassing repair work. Some squeegees are gripped by the side of the blade, others have handle. The choice is largely a matter of individual preference. Those with handle can be used to get at hard to reach areas.
Plastic Squeegees

Monday, June 23, 2014

Glass Fiber-Reinforced Polymer (GFRP) Composites

Fiberglass is simply a composite consisting of glass fibers, either continuous or discontinuous, contained within a polymer matrix; this type of composite is produced in the largest quantities. The composition of the glass that is most commonly drawn into fibers (sometimes referred to as E-glass) . fiber diameters normally range between 3 and 20 _m. Glass is popular as a fiber reinforcement material for several reasons:

1. It is easily drawn into high-strength fibers from the molten state.
2. It is readily available and may be fabricated into a glass-reinforced plastic economically using a wide variety of composite-manufacturing techniques.
3. As a fiber, it is relatively strong, and when embedded in a plastic matrix, it produces a composite having a very high specific strength.
4. When coupled with the various plastics, it possesses a chemical inertness that renders the composite useful in a variety of corrosive environments.

The surface characteristics of glass fibers are extremely important because even minute surface flaws can deleteriously affect the tensile properties. Surface flaws are easily introduced by rubbing or abrading the surface with another hard material. Also, glass surfaces that have been exposed to the normal atmosphere for even short time periods generally have a weakened surface layer that interferes with bonding to the matrix. Newly drawn fibers are normally coated during drawing with a ‘‘size,’’ a thin layer of a substance that protects the fiber surface from damage and undesirable environmental interactions. This size is ordinarily removed prior to composite fabrication and replaced with a ‘‘coupling agent’’ or finish that promotes a better bond between the fiber and matrix.

There are several limitations to this group of materials. In spite of having high strengths, they are not very stiff and do not display the rigidity that is necessary for some applications (e.g., as structural members for airplanes and bridges).

Most fiberglass materials are limited to service temperatures below 200C (400F); at higher temperatures, most polymers begin to flow or to deteriorate. Service temperatures may be extended to approximately 300C (575F) by using high-purity fused silica for the fibers and high-temperature polymers such as the polyimide resins. Many fiberglass applications are familiar: automotive and marine bodies, plastic pipes, storage containers, and industrial floorings. The transportation industries are utilizing increasing amounts of glass fiber-reinforced plastics in an effort to decrease

Wednesday, March 19, 2014

Flexural Testing for Fibre Reinforced Plastic


The stress-strain behavior of polymers in flexure is of interest to a designer as well as a polymer manufacturer. Flexural strength is the ability of the material to withstand bending forces applied perpendicular to its longitudinal axis. The stresses induced by the flexural load are a combination of compressive and tensile stresses. This effect is illustrated in Figure xx. Flexural properties are reported and calculated in terms of the maximum stress and strain that occur at the outside surface of the test 

Many polymers do not break under flexure even after a large deflection that makes determination of the ultimate flexural strength impractical for many polymers. In such cases, the common practice is to report flexural yield strength when the maximum strain in the outer fibre of the specimen has reached 5 percent.

For polymeric materials that break easily under flexural load, the specimen is deflected until a rupture occurs in the outer fibres. There are several advantages of flexural strength tests over tensile tests. If a material is used in the form of a beam and if the service failure occurs in bending, then a flexural test is more relevant for design or specification purposes than a tensile test, which may give a strength value very different from the calculated strength of the outer fibre in the bent beam

The flexural specimen is comparatively easy to prepare without residual strain. The specimen alignment is also more difficult in tensile tests. Also, the tight clamping of the test specimens creates stress concentration points. One other advantage of the flexural test is that at small strains, the actual deformations are sufficiently large to be measured accurately. There are two basic methods that cover the determination of flexural properties of plastics. Method 1 is a three-point loading system utilizing centre loading on a simple supported beam. A bar of rectangular cross section rests on two supports and is loaded by means of a loading nose midway between the supports. The maximum axial fibre stresses occur on a line under the loading nose.





Wednesday, March 12, 2014

Premix for Press Moulding

BULK MOLDING COMPOUND (BMC) / DOUGH MOULDING COMPOUND (DMC)
BMC/DMC has been defined as ‘a fiber reinforced thermoset molding compound not requiring advancement of cure, drying of volatile, or other processing after mixing to make it ready for use at the molding press. BMC can be molded without reaction by products under only enough pressure to flow and compact the material. BMC is usually manufactured by combining all the ingredients in an intensive mixing process.

Recent advances in BMC technology dictate that both the dry ingredients and wet ingredients be batch mixed separately and then combined together in an intensive mixer. The BMC is usually in a fibrous putty form when it comes out of the mixer and resembles ’sauerkraut’. It is usually compacted and extruded into bars or ’logs’ of simple cross section. 

The earliest BMC’s were probably made by employing a process of impregnating roving strands with blend of resin, filler, etc. and chopping them to a length in the wet stage. Since wetting glass fibers with a resin containing much filler is difficult and slow, these premixes had a high glass content. The first high volume commercial BMC was made with sisal fibers and used in molding automobile heater housings. Improvement in the binder chemistry of glass fibers, development of a chemical thickening system and thermoplastic low profile additives help BMC to attain strength, chemical resistance and to overcome surface irregularities. 

Consequently, BMC was accepted for use in the electrical, chemical and appliance industries. Today, BMCs are accepted as high performance engineering thermoset molding compounds and used extensively in the electrical, automotive and consumer goods industries. BMC is increasingly injection molded to take advantage of the automation and reproducibility afforded by the process, although it is also both transfer molded and compression molded. 

Wednesday, March 5, 2014

Press Moulding in Fibre Reinforced Plastic



Press moulding is one of the primary manufacturing process used for automotive composite applications today. The process is also used to manufacture parts and components for other industrial and consumer applications.

A typical manufacturing chain involves the conversion of composite constituent materials, often using a semifinished product (or preform), into an end-use application. Fully formed parts are molded in matched metal compression molds that give the final part shape; usually these undergo secondary operations such as deburring, hole punching, insert assembly, and, in some cases, painting, and adhesion priming or friction welding for tertiary assembly operations with other parts and components. This type of composite component manufacture is based on either thermoplastic or thermosetting matrix materials reinforced, in the overwhelming majority of instances, by glass fibers. Emerging new materials also use combinations with natural fibers and polymeric fibers.

The three main groups of materials that are compression molded are:
Glass-fiber-mat-reinforced thermoplastics (GMT)
Long-fiber-reinforced thermoplastics (LFT)
Sheet molding compounds (SMC) (thermosets)

Wednesday, February 26, 2014

Advantages and Disadvantages of Filament Winding Process

The most important advantage of filament-winding is its low cost, which is less than the prepreg cost for most composites. The reduced costs are possible in filament-winding because a relatively expensive fiber can be combined with an inexpensive resin to yield a relatively inexpensive composite. Also, cost reductions accrue because of the high speed of fiber lay-down. Other advantages of filament-winding compared to other compacting and curing processes are:

Highly repetitive and accurate fiber placement (from part to part and from layer to layer). The accuracy can be superior to that of fiber placement and automated tape-laying machines.
The capacity to use continuous fibers over the whole component area (without joints) and to orient fibers easily in the load direction. This simplifies the fabrication of structures such as aircraft fuselages and reduces numbers of joints for increased reliability and lower costs.
Elimination of the capital expense (and size restrictions) of an autoclave and the recurring expense for inert gas. Thick-walled structures can be built that are larger than any autoclave can accommodate.
Ability to manufacture a composite with high fiber volume
Mandrel costs can be lower than other tooling costs because there is usually only one tool, the male mandrel, that sets the inside diameter and the inner surface finish.
Lower cost for large numbers of components because there can be less labor than many other processes. It is possible to filament wind multiple small components, such as up to 20 golf shafts at once (Fig. 8), leading to sharply reduced costs compared to flag rolling. Costs are eliminated for bagging and disassembly of the bagging materials, as well as the recurring costs of these materials.
Costs are relatively low for material since fiber and resin can be used in their lowest cost form rather than as prepreg.

Disadvantages

Need for mandrel, which can be complex or expensive
Necessity for a component shape that permits mandrel removal. Long, tubular mandrels generally do not have a taper. Unless nonuniform shapes are capable of mechanical disassembly, mandrels must be made from a dissolvable or frangible material. Different mandrel materials, because of differing thermal expansion and differing composite materials and laminate lay-up percentages of hoops versus helical plies, will demonstrate varying amounts of difficulty in removal of the part from the mandrel.
Difficulty in winding reverse curvature
Inability to change fiber path easily (in one lamina)
Poor external surface finish, which may hamper aerodynamics or aesthetics.

It is important to note that most of the disadvantages are application-specific and, in many cases, have been circumvented by innovative design and equipment modifications.

Wednesday, February 19, 2014

Type of Mandrel in Filament Winding Process

In mandrel design and material selection, the following criteria should be considered:
1. cost
2. mandrel reusability (durability)
3. production quantity
4. mandrel material thermal characteristics
5. mandrel strength/ability to resist deflection during winding and cure
6. final part tolerances required
7. dimensional stability

To ease part removal, mandrels may be constructed from water-soluble materials (sand), plaster, or an assemblage of metal shells that is collapsible or segmented. Tube mandrels constructed with a high-quality surface finish and a slight taper are often used for cylindrical parts.

The mandrel, which determines accurate internal geometry for the component, is generally the only major tool. Low-cost mandrel materials such as cardboard or wood can often be used when winding low-cost routine parts.
For critical parts requiring close tolerances, expensive mandrels designed for long-term use may be required. For high-temperature cure 315°C (600"F), graphite mandrels with low thermal expansion may be advantageous, however some attention should be paid to the potential difficulties of mandrel removal. Gas containment pressure vessels often require metal liners because composites are porous; these metal liners can also serve as mandrels.

Mandrels can be group to four types of mandrels. First is non-removable, which the mandrel remaining as a part of the wound structure. The other three are removable and classified according to the removal technique as:
-entirely removed (for example, tubular mandrels with or without taper and with release agent);
-collapsible (the mandrel is disassembled or removed piece by piece)
-Breakable or soluble (plaster, sand or salts).

The selection of a mandrel involves several trade-offs. These include part size and complexity, size of openings, resin system and its curing cycle and the number of components to be fabricated. The basic requirements for a mandrel, whether it will be removed from the part after winding or remain as a part of the structure, are:
-It must be stiff and strong enough to support its own weight and the weight of the applied composite while resisting the fiber tension pressure from winding and curing.
-It must be dimensionally stable and should have a thermal coefficient of expansion greater than the transverse coefficient of the composite structure.