For certain polymers, primary bonding occurs between branches and other molecules at certain connection points to form Cross-linked Polymers.Cross-linking occurs because a certain proporiton of the monomers used to form the polymer are capable of bonding to adjacent monomers on more than two sides, thus allowing branches from other molecules to attach.Lightly crossilinked structures are characteristic of elastomers.When the polymer is highly cross-linked we refer to it as having a network structure in effect , the entire mass is one gigantic macromolucule.Thermosetting plastics take this structure after cutting.
Thermosetting plastics and elastomers are cross-linked polmers.Cross-linking causes the polymer to become chemically set; the reaction cannot be reserved.The effect is to permanently change the strucrure of the polymer ; upon heating , it degrades or burns rather than melts.Thermosets possess a high degree of cross-linking , while elastomers possess a low degree of cross-linking.Thermosets are hard and brittle , while elastomers are elastic and resilient.
(Mikell P.Groover, Fundamentals of Modern Manufacturing , materials,processes, and systems third edition page 151-152)
Explosive forming 19 mart 18:22
Explosive forming involves the use of an explosive charge to form sheet metal into a die cavity.One method of implementing the process is illustrated in figure 20,45.The workpart is clamped and sealed over the die , and vacuum is created in the cavity beneath.The apparatus is then placed in a large vessel of water.An explosive charge is placed in the water at certain distance above the work.Detonation of the charge results in shock wave whose energy is transmitted by the water to cause rapid forming of the part into the cavity.The size of the explosive charge and the distance at which it is placed above the part are largely a matter of art and experience.Explosive forming is reserved for large parts, typical of the aerospace industry.
(Mikell P.Groover, Fundamentals of Modern Manufacturing , materials,processes, and systems third edition page 473)
Bending Test 19 mart 18:37
Bending operations are used to form metal plates and sheets.The process of bending a rectangular cross section subjects the material to tensile stresses(and strains) in the outer half of bent section and compressive stresses in the inner half.If the material does nat fracture , it becomes permanently(plastically) bent.
Hard, brittle materials , which posses elasticity but little or no plasticity, are often ested by a method that subjects the specimen to a bending load.These metarials do not respond well to traditional tensile testing because of problems in preparing the test specimens and possible misalignment of the press jaws that hold the specimen.The bending test(also known as the flexure test) is used to test the strength of these materials, using a setup illustrated in the first diagram.In this procedure , a specimen of rectengular cross section is positioned between two supports, and a load is applied at its center.In this configuration, the test is called a three-point bending test.A four point configuration is also sometimes used.
(Mikell P.Groover, Fundamentals of Modern Manufacturing , materials,processes, and systems third edition page 49)
Wire EDM 19 mart 23:35
Wire cut EDM is an indispensable machine in tool rooms. This is used to make dies for blanking and piercing. The special feature of wire EDM is that the electrode is in the form of a thin wire of about 0.2 mm in dia. A small hole is drilled into the workpiece and the wire electrode is threaded through the workpiece onto a take-up roll. The movements of the work table through computer numerical control in X and Y directions enable cutting the component to the required shapes. In some cases where a relief is required for the die or the punch being machined, a 3-axis NC is used to tilt the wire axis relative to workpiece surface in the required direction. The direction of tilt varies with the contour and is controlled by the third axis of the CNC. Some of the latest wire EDM machines have the automatic drilling and self threading facility.
(P.Radhakrishnan,CAD/CAM/CIM,third edition page 369)
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