12Mar 2013

Electrical Discharge Machining

The main components in EDM are the electric power supply, the dielectric medium, the work piece, the tool, and a servo control unit. The work piece and the tool are electrically connected to a DC electric power supply. The current density in the discharge of the channel is of the order of 10000 A / Square centimeter, and power density is nearly 500 MW / Square centimeter. The work piece is connected to the positive terminal of the electric source, so that it becomes the anode and the tool is the cathode.

A gap, known as SPARK GAP in the range, from 0.005 mm to 0.05 mm is maintained between the work piece and the tool, and suitable dielectric slurry, which is non conductor of electricity, is forced through this gap at a pressure of 2 kgf/square centimeter or lesser. When a suitable voltage in the range of 50 to 450 V is applied, the dielectric breaks down and electrons are emitted from the cathode and the gap is ionized. When more electrons collect in the gap, the resistance drops causing electric spark to jump between the work piece surface and the tool. Each electric discharge or spark causes a focused stream of electrons to move with a very high velocity and acceleration from the cathode towards the anode, and ultimately creates compression shock waves on both the electrode surface, particularly at high spots on the work piece surface, which are closest to the tool. The generation of compression shock waves develops a local rise in temperature.

The whole sequence of operation occurs within a few microseconds. The temperature of the spot is hit by the electrons is of the order of 10000 degree centigrade. This temperature is sufficient to melt a part of the metals. The forces of electric and magnetic fields caused by the spark produce a tensile force and tear off particles of molten and softened metal from this spot in the work piece. Thus, the metal is removed in this way from the work piece. The volume of the material removed per spark discharge is typically in the range of (1/1000000) to (1/10000) cubic millimeter. Tolerance value of + or – 0.05 mm could be easily achieved by EDM in normal production. The best surface finish that can be economically achieved on steel is 0.40 micron.

A variation of EDM is wire EDM. Wire EDM is otherwise called as Electrical Discharge Wire Cutting. In this process, a slowly moving wire travels along a prescribed path, cutting the work piece. This process is used to cut plates as thick as 300 mm and to make punches, tools, and dies from hard metals. It can also cut intricate components for the electronics industry. A schematic sketch of wire cut EDM is shown below in Figure-2.

28Jan 2013

Technology in Microgravity Solidification

Microgravity research in casting metallurgical engineering has grown in importance. Materials processing in space have been studied both theoretically and experimentally for over a quarter of a century. In the beginning, we naively spoke of zero gravity, elimination of convection, growth of perfect crystals, and eventual manufacturing in space. Currently focus has shifted to the use of a microgravity environment, where buoyancy driven convection and sedimentation need to be eliminated to validate physical models, as compared to space manufacturing. One of the research areas that have emerged as the forerunner in such studies is solidification science and technology. This is an obvious choice since solidification processing is an important step in most of the fabrication techniques where the starting material stock has to be cast as ingots. Solidification related transformations are significantly affected by the gravity level. This article provides the basic principles and helps us to understand the influence of microgravity on solidification processing of metals, and alloys.

The dreams of manufacturing perfect crystals using zero gravity began in the 1960’s during the Apollo era. Talk of manufacturing other substances continued through the 1970’s and much of the 1980’s. The space environment was not magic and the materials were not sufficiently better to warrant the costs. The value added did not exceed the additional cost. On the other hand, an immense amount was learned about gravitational effects on materials process, both through the results of space experiments and through related ground-based research. In fact, many results were anticipated and some await full explanation.

This knowledge has proven to be extremely useful in improvement and innovation of materials processing on earth.The study of the different states of matter and their interactions in microgravity is an exciting opportunity to expand the frontiers of science, engineering, and technology. Gravitational attraction is a fundamental and basic property of matter that exists throughout the known universe. A microgravity environment is one in which the apparent weight of a system is small compared to its actual weight due to gravity. In practice, the microgravity environments used by the researchers range from about 1% of the earth’s gravitational acceleration to better than one part in a million. The acceleration experienced by an object in a microgravity environment would be one millionth of that experienced at the earth’s surface.

The principal objective of microgravity materials science research is to gain a better understanding of how gravity driven phenomena affect the solidification and crystal growth of materials. Buoyancy driven convection, sedimentation, and hydrostatic pressure can create defects and irregularities in the internal structure of materials, which in turn alter their properties. Materials science and engineering research in microgravity leads to a better understanding of how materials are formed and how the properties of materials are influenced by their formation. Researchers are particularly interested in increasing their fundamental knowledge of the physics and chemistry of phase changes. This knowledge is applied to designing better process control strategies and production facilities in the laboratories on earth. In addition, microgravity experimentation will eventually enable the production of limited quantities of high quality materials and of materials that exhibit unique properties for use as benchmarks.

12Sep 2012

Advantages and Applications of Powder-Coating tech

The history of powder coating begins in the late 1940s and early 1950s, at a time in which organic polymers were still being spray coated in a powder form on to metallic bases. Dr. Erwin Gemmer, a German scientist, developed in those days the fluidized-bed process for the processing of thermosetting powder coatings, and registered an appropriate process patent in May 1953. Between 1958 and 1965, literally all powder coatings, generally only functional applications with a film thickness of 150 µm to 500 µm, were processed by means of fluidized-bed application. Electric insulation, corrosion and abrasion resistance were in the foreground. The coating materials in those days comprised nylon 11, CAB, polyethylene, plasticized PVC, polyester and chlorinated polyether, among others. It was the firm of Bosch that developed the basic type of expoxy resin powder when searching for a suitable electric insulation material.

The high film thicknesses for numerous applications, and the technology of electrostatic processing of powder coating, which was developed shortly after in the U.S.A., and was used commercially between 1962 and 1964 in the U.S.A. With the electrostatic spray-guns made by the firm of Sams for electrostatic application and which gave rise to the term “Samesizing”, this hurdle was also overcome. Between 1966 and 1973 the four basic types of thermosetting resins, which are still defining today, were developed and commercially marketed: epoxy, epoxy polyester hybrid, polyurethane and polyester. The number of powder-coating plants in Germany alone rose from four in 1966 to 51 in 1970.

From the early 1970s, powder coating then began its march of triumph worldwide, even though the growth of the powder coating market was until 1980 initially slight. The plants up to that time were expensive, the film thicknesses too high for commercial use, color-change problems and high curing temperatures greatly limited the color tone, effect and substrate diversity. From the early 1980s, powder coatings have developed worldwide through continuous growth, which, driven forward by continuous innovations in the raw materials available, improved formulation and advances in application technology.

Powder coating is a dry finishing process. Before coating, the parts to be coated are first pretreated similarly to conventional liquid coated parts. The pretreatment process is normally conducted in series with the coating and curing operations. Powder coating is a method by which electrically charged powder coating material is spray-applied to a grounded work piece. Electrostatic attraction holds the powder to the part to be coated until heat is added to flow the powder together and cure it. Since powder may adhere to the part for several hours, heat curing can be done at the user’s convenience. Should the uncured powder coat become damaged or blemished during handling, the powder can be simply blown off with air or vacuumed, and a new coat applied.