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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

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)