It makes use of heat softening characteristics of thermoplastic materials. These materials soften when heated and reharden when cooled. No chemical change takes place within the material is heated or cooled, the change is entirely physical. For this reason in Injection Moulding, the softening, and rehardening cycle be repeated many number of times.
The granular material is loaded into a hopper from where it is metered out in a heating cylinder by a feeding device. The exact amount of material is delivered to a cylinder which is required to fill the mould completely. The injection ram pushes the material into the heating cylinder and in doing so pushes a small amount of heated material out of the other end of the cylinder through the nozzle and screw bushing and into the cavities of the closed mould.
The material is cooled to a rigid state in the mould. The mould is then opened and the piece is ejected out. The temperature to which the material is raised in the heating cylinder is usually between 180-280°C. The higher the temperature the lower the viscosity and more readily it can be pushed into the die. Every type of material has a characteristic moulding temperature, the softer formulations require lower temperature, and the harder formulations require a higher temperature. Intricate pieces, large pieces, several cavities in the die and long runners all tend to increase the temperature requirements.
When the plastic material is pushed from nozzle end of the cylinder, it enter through channels into the closed mould. In the majority of cases, the mould is kept cold, in order to cool the moulded articles ready to the point at which the mould can be opened and pieces ejected without distortion. This is done by circulating water through the mould frame. Sometimes it is necessary to use a warm mould, and mould temperature as high as 150°C is used for very special jobs. However, material sets faster in a cold die and the cycles are shorter. The cooling of plastics under pressure is desirable to avoid “shrink” marks on the surface. Automatic devices are commercially available to maintain mould temperature at the required level. For more related details on Mechanical engineering than click on it.
Electro forming is a process of producing precision metal parts, that is usually thin in section, by electro-deposition on to a form (variously called mandrel mold, matrices, or die), which is shaped exactly to the interior form of the product and which is subsequently removed. In the process, a slab or plate of the material of the product is immersed into the electrolyte (an aqueous solution of a salt of the same metal) and is connected to the positive terminal of a low voltage, high current d.c. power. So, it becomes an anode.
A correctly prepared master mandrel or pattern of correct shape and size is immersed at some distance from the anode and is connected to the negative terminal (cathode). The mandrels are made from a variety of materials, both metallic or non – metallic. If the material is non-conducting, a conductive coating must first be applied in order to perform electroplating. The mandrel should possess a mirror-like finish.
When the circuit is closed, metal ions are removed from the anode, transported through the electrolyte towards the cathode (master), and deposited there. After the deposition, the master is removed or destroyed. A metal shell is left, which conforms exactly to the contours of the master. It may take hours or days to get a deposit of sufficient thickness. The thickness of electro – forms ranges from 0 25 to 25 mm. The process is very much similar to electroplating, with the difference that whereas in electroplating, the deposit stays in place (on the cathode), in electroforming, it is stripped from the form. The electro-formed products are typically made from Nickel, Iron, Copper, or Silver, and more recently from copper-tin, nickel – cobalt and nickel – manganese alloys.
Low plant cost, cheap tooling, and the absence of heavy equipment.
Low labor operating costs.
The process can be designed to operate continuously throughout day and night.
Electrodeposition can produce good dense deposits, and compared with castings electroforming offers high purity, freedom from porosity with a homogeneous structure. These important qualities are seldom obtained to such a degree in machined parts, stampings, or forgings.
There is no restriction on the internal complexity of electro- forms, and this advantage eliminates in many instances, the costly joining processes.
The process has no equal for the reproduction of fine or complex details.
The use of inserts has widened the application of the process. Metal inserts are attached to or are embedded in wax or fusible alloy master, and, when the master is melted, the inserts remain attached to the electroformed.
A high-quality surface finish is obtained on both the internal and external surfaces of the electro-forms. Accuracies as close as 0.005 min with surface finishes up to 0.125 p.m can be produced.
Complex thin-walled parts can be produced with improved electrical properties.
Shell-like parts can be produced quickly and economically.
There is a wide range of applications of electroforming process:
Molds and dies feature high on the list. Molds for the production of artificial teeth, rubber and glass products, and high -strength thermosetting plastics are now commonplace. The molds can be made with undulating parting lines which have made a considerable impact upon the production of thermoplastic toys and novelties.
Radar and electronic industry Radar waveguides, probes, complicated grids, screens, and meshes can be produced much more easily, to find accuracies and at a lesser cost.
Spline, thread, and other types of form gauges.
Cathodes for ECM arid electrodes for. EDM.
Electro-formed core boxes with inbuilt heating elements. Electro-formed molds for the wax patterns.
Electro-formed precision tubing, parallel and tapered, formed to different shapes to eliminate the need for bending which distorts the bore.
Electrotypes floats, bellows, venturi tubes, fountain pen caps, reflectors, heat exchanger parts, honeycomb sandwich, parts for gas appliances and musical instruments, radio parts, spraying masks and stencils, seamless screen cylinders for textile printing, filters and dies for stamping of high-fidelity records.
Electro-forming is particularly useful for:
Low production quantities
Quantity of identical parts, for example, a multi- impression mold.
The possibility of using a single master for the production of a number of electro forms.
Whereas intricate female impression is required, so that is would be much easier to produce a male form, that is, the master.
The process of linking together of monomers, that is, of obtaining macromolecules is called “‘polymerization”. It can be achieved by one of the two processing techniques. (a) Addition Polymerization: In addition or chain polymerization under suitable conditions of temperature and pressure and in the presence of a catalyst called an initiator, the polymer is produced by adding a second monomer to the first, then a third monomer to this dimer, and a fourth to the trimer, and so on until the long polymer chain is terminated. Polyethylene is produced by the addition polymerization of ethylene monomers. This linear polymer can also be converted to a branched polymer by removing a side group and replacing it with a chain. If many such branches are formed, a network structure results.
“Co-polymerization” is the addition polymerization of two or more different monomers Many monomers will not polymerize with themselves. but will copolymerize with other compounds
(b) Condensation Polymerizations:
In this process, two or more reacting compounds may be involved and there is a repetitive elimination of smaller molecules, to form a by-product. For example, in the case of phenol-formaldehyde (bakelite), the compounds are formaldehyde and phenol. Meta cresol acts as a catalyst and the by-product is water. The structure of the ‘Mer’ is more complex. Also, there is a growth perpendicular to the direction of the chain. This is called cross-linking’.
Size of a Polymer:
The polymer chemist can control the average length of the molecules by terminating the reaction. Thus, the molecular weight (the weight of the average molecule, in grams, of 6.02x molecules) or degree of polymerization, D.P., (the number of members in the average molecule) can be controlled. For example, the length of molecules may range from some 700 repeat units In low-density polyethylene to 1,70,000 repeat units in ultrahigh-molecular-weight polyethylene.
These plastics undergo a number of chemical changes on heating and cure to infusible and practically insoluble articles. The chemical change is not reversible Thermosetting plastics do not soften on reheating and can not be reworked. They rather become harder due to the completion of any left-over polymerization reaction. Eventually, at high temperatures, the useful properties of the plastics get destroyed. This is called degradation. The commonest thermosetting plastics are alkyds, epoxies, melamines, polyesters, phenolics, and ureas. for a more related topic click on it Mechanical engineering.
These have superior mechanical properties and yet are lightweight. The reinforcing fibers are usually glass, graphite, boron, etc. Epoxies and polyester commonly serve as a matrix material. Reinforced plastics are being developed rapidly. New developments concern metal-matrix and ceramic-matrix composites and honeycomb structure (Honeycomb structure consists of a core of honeycomb or other corrugated shapes bonded to two thin outer skins. Ceramic-matrix cutting tools are being developed, made of silicon carbide reinforced alumina, with greatly improved tool life. A composite material, as stated above, contains more than one component. The compound materials are incorporated into the composite to take advantage of their attributes, thus obtaining improved material. They become cohesive structures made by physically combining two or more compatible materials. Fiber-reinforced composites are heterogeneous materials prepared by associating and bonding in a single structure of materials possessing different properties. Due to complementary nature, the composite material possesses additional and superior properties. These thus become ideal materials for structural applications requiring high strength-to-weight and stiffness-to-weight ratios. Fiber-reinforced materials exhibit anisotropic properties. Glass fibers are strong but if notched they fracture readily. By encapsulating them in a polyester resin matrix, they can be from damage. Fibers of graphite and boron are also used in composites. Commonly used fibers for composite materials are-glass, silica, and boron for amorphous structure, ceramic and metallic for single crystals as well as polycrystals, carbon, and boron (amorphous) materials for multiphase structure, and organic material for macromolecular structure. For two-dimensional structural applications such as in plates, walls, shells, cylinders, pipes, etc. a planar reinforcement is much more advantageous as compared to the linear reinforcement.
Duplex Composite Components:
Components subjected to severe wear and high contact stresses can be made of duplex composite, the composite layer is located on the outer or inner. Surface depending on the requirement. Aluminum composite alloys reinforced by ceramic have been developed and these have a relatively high strength to weight ratio, high modulus of elasticity, and good wear characteristics.
Silicon carbide particles are incorporated into the surface of aluminum alloy heated to its mushy state and pressure is applied to get a good wetting between the aluminum alloy and the silicon carbide particles. Experiments can be carried out to determine the semi-solid forming conditions. Specimens surrounded by SiC particles are heated up to this temperature for about 45 minutes in order to homogenize the temperature through the specimen. A hydraulic press is used to apply the necessary low pressure for the semi-solid forming process. There is an optimum combination of temperature and pressure values to obtain optimum mechanical properties. In this way, a composite layer of about 2.5 mm width can be formed with uniformly distributed particles having a good bond with the aluminum matrix, with no separation or porosity at the composite layer/matrix interface. The surface composite layer has the hardness and wears resistance about 1.75 and 10 times those of as-received aluminum matrix alloy. for more related topic to Mechanical engineering click on it.
In almost every type of production tooling, the most desirable feature to have is a very hard surface layer on a low strength but the tough body. Toughness is needed to survive mechanical shocks, that is, impact loading in interrupted cuts. Shocks occur in even continuous chip formation processes when the tool encounters a localized hard spot. The examples of such tooling include metal cutting tools, rock drill, cutting blades, forging screws for extrusion of plastic and food products and sawmills and so on. Other applications, including parts for earth moving machinery, valves and valve seats for diesel engines, and many such parts involving high heat applications and in general, applications requiring wear resistance. Surface Coatings for Tooling
The various techniques employed for this purpose are discussed below:
1. Hard Facing This is a welding technique and has already been discussed in Chapter on “Welding Process”, under Art. 2. Nitriding Case Hardening : Discussed in Chapter. 3. Hard Chrome Plating : Hard chrome plating is done by the Electrolytic electroplating technique (See Art.). It is the most common process for wear resistance. 4. Flame Plating Flame plating is a process developed to prolong the life of certain types of cutting tools and for severe wear applications. By this process, a carefully controlled coating of tungsten carbide, chromium carbide (Cr, C2) or aluminum oxide is applied to a wide range of base metals. The more common materials which have been successfully flame – plated include aluminum, brass, bronze. Cast iron, ceramics, copper. glass, H.S.S., magnesium, molybdenum, nickle, steel and titanium and their alloys. The process uses a specially designed gun into which is admitted metered amounts of oxygen and acetylene. A change, of fine particles of the selected plating mixtures, is injected into the mixture of oxygen and acetylene. Immediately after this, a valve opens to admit a stream of nitrogen to protect the valves during the subsequent detonation. The mixture is now ignited and an explosion takes place which plasticizes the particles and hurls them from the gun barrel at 750 m/s. The particles get embedded in the surface of the component and a microscopic welding action takes place, which produces a highly tenacious bond.
Each particle in the coating is elongated and flattened into a thin disc. The coating has a dense, fine – grain laminar structure with negligible porosity and an absence of voids or visible oxide layers. The layer of the plated material is about 0.006 mm, and this layer can be built up, by repeating the explosions, to thicknesses ranging from 0.05 to 0.75 mm, according to the requirements of any subsequent operations. The resultant coating is dense, hard and well bonded. Because of the hard dense structure of the coatings, flame – plating has provided the industry with a valuable tool for the solving of many abrasion, erosion and wear problems. For example, bushes for many applications, core pins for powder metallurgy, dies, gauges, journals, mandrels, and seals for high – duty pumps, have all been given much longer lives.
The process has influenced considerably certain types of cutting processes, especially in the glass, leather, paper, rubber, soap and textile industries and has proved to be of great advantage for components involving high heat applications such as “hot-end” of gas turbines. The coatings show an excellent resistance to galling and corrosion. Flame-plated coatings can be ground and lapped, if necessary. Resultant surface finish can be within the region of 0.025 Another advantage is that the components can be masked to enable the coatings to be placed precisely where required. The mixture of tungsten carbide coating material consists of cobalt ranging from 7 to l7% and the balance of tungsten carbide. Aluminum oxide plating mixture is almost of A103 (Above 99%). Chromium carbide plating mixture consists of about 75 to 85% of Cr3C, and balance of (Ni-Cr). for more related topics on mechanical engineering topics.
Elements of a Simple Gas Turbine Plants: A simple gas turbine plant is shown in Fig. It consists of compressor combustion and turbine. When the units run the atmospheric air is drawn into the compressor, raised to static pressure several times that of the atmosphere. The compressed air then flows to the combustion chamber, where the fuel is injected. The produces of combustion, comprising a mixture of gases at high temperature and pressure, are passed through the turbine where they expand and develop motive force for turning the turbine rotor. After expansion, the gases leave the turbine at atmospheric pressure.
The temperature of the products of combustion is nearly 1000° to 1500°F, The temperature of the exhaust gases is in the range of 900° to 1100°F. The compressor is mounted on the same shaft as that of the turbine. The major portion of the work developed in the turbine is used to drive the compressor and the remainder is available as net power output.
Turbine drives the compressor and the load. Both impulse and reaction turbines can be used in gas turbine plants. As compared to steam turbines gas turbines have tew stages because they operate on smaller pressure drops.
Axial flow type turbines are commonly used. The various requirements of turbines are as follows: (i) Light Weight (i) High Efficiency (iii) Reliability in operation (iv) Long working life.
Combustion Chamber :
In the combustion chamber, combustion of fuel takes place. The combustion process taking place inside the combution chamber is quite important because it is in this process that energy, which is later converted into work by the turbine, is supplied. Therefore, the combustion products and air so that complete combustion and uniform temperature distribution in the combustion gases may be achieved.
Combustion should take place at high efficiency, because losses incurred in the combustion process have a direct effect on the thermal efficiency of the gas turbine cycle. Further the pressure losses in the combustion chamber should provide sufficient volume and length for complete combustion of the fuel.
Initially the temperature developed in combustion chamber is too high. The difficulty is avoided by adding a satisfactory amount of air to maintain stable combustion conditions and then the products of combustion are cooled to a temperature suitable for use in gas secondary the combustion chamber. In combustion chamber used for aircraft engines a large quantity of air is used to keep the temperature of combustion chamber to about 650°C. The air fuel ratio may be of the order of 60 :1 in this case
The requirements of a combustion chamber are as follows:
(i) Low-pressure loss (ii) High combustion efficiency (iii) Good flame stability (iv) Low weight (v) Through mixing of cold air and hot products of combustion to generate uniform temperature. (vi) Reliability (vi) Low carbon deposit in the turbine, and combustion chamber.
The various compressors used are the reciprocating compressor, centrifugal compressors and axial flow compressor. The reciprocating compressors are not preferred due to the friction in sliding parts, more weight, less speed and inability to handle large volumes of air. For a gas turbine power plant of high output and efficiency generally pressure ratios of 10: 1 or more is used. It is observed that when a single compressor with a pressure ratio not more than 4:1 is required the centrifugal compressor is the most suitable.
It is quite rugged in construction, can operate more efficiently over a wide range of mass rate of flow of air than a comparable axial flow compressor. Centrifugal compressor is mainly used in super chargers and in jet aircraft plants, where lower pressure ratios and small volumes of air is needed. For higher pressure ratios multi-stage centrifugal compressor does not prove to be as useful as an equipment axial flow compressor. Therefore, when high pressure ratios are needed, axial compressor is advantageous and is always used for industrial gas turbine installations. Further is desirable that more than one compressor should be used when the pressure ratio exceeds ratio 6:1. Although the axial flow compressor is heavier than the centrifugal compressor but it has higher efficiency than the centrifugal compressor. It is important that air entering the compressor should be free from best.
Fig. shows nuclear power plant using B.W.R. In this reactors enriched uranium (enriched uranium contains more fissionable isotope U235 then the naturally occurring percentage 0.7%) is used as nuclear fuel and water is used as coolant. Water enters the reactors at the bottom. It takes up the heat generated due to the fission of fuel and gets converted into steam. Steam leaves the reactor at the top and flows into the turbine. Water also serves as moderator. India’s first nuclear power plant at Tarapur has two reactors (each of 200 MW capacity) of boiling water reactor type.
Pressurized Water Reactors (P.W.R.)
A P.W.R. nuclear plant is shown in Fig. It uses enriched U as fuel. Water is used as coolant and moderator. Water passes through the reactor core and takes up the heat liberated due to nuclear fission of the fuel. In order that water may not boil (due to its low boiling point 212 F at atmospheric conditions) and remain in liquid state it is kept under a pressure of about 1200 р.s..g. by the pressurizer. This enables water to take up more heat from the reactors. Form the pressurizer water flows to the steam generator where it passes on its heat to the feed water which in turn gets converted into steam.
Fast Breeder Reactors (F.B.R.)
Fig. shows a fast breeder reactor system In this reactors, the core containing U235 is surrounded by a blanket (a layer of fertile material placed outside the core) or fertile material U238. In this reactors no moderator is used. The fast-moving neutrons liberated due to fission of U235 are absorbed by U238 which gets converted into fissionable material Pu239 which is capable of sustaining chain reaction. Thus this reactor is important because it breads fissionable material from fertile material U238 available in large quantities. Like sodium graphite nuclear reactors this reactor also uses two liquid metal circulated through the tubes of intermediate heat exchange transfers its heat to secondary coolant sodium-potassium alloy. The secondary coolant while flowing through the tubes of steam generator transfer its heat to feed water. Fast breeder reactors are better than conventional reactor both from the point of view of safety and thermal efficiency. For India which already is fast advancing towards self-reliance in the field of nuclear power technology, the fast breeder reactor becomes inescapable in view of the massive reserves of thorium and the finite limits of its uranium resources. The research and development efforts in the fast breeder reactors technology will have to be stepped up considerably if nuclear power generation is to make any impact on the country’s total energy needs in the not too distant future.
Coolants for Fast Breeder Reactors: The commonly used coolants for fast breeder reactors are as follows: (i) Liquid metal (Na or NaK) (ii) Helium (He) (iii) Carbon dioxide Sodium has the following advantages (i) It has very low absorption cross-sectional area. (ii) It possesses good heat transfer properties at high temperature and low pressure. (iii) It does not react on any of the structural materials used in primary circuits.
Degree of Freedom: A system is said to be n-degrees of freedom system if it needs n independent coordinates to specify compectely the configuration of the system at any instant. A mass supported by a spring and constrained to move in one direction without rotation is a single degree of freedom system. The same is true for a simple pendulum oscillating in one plane. A crank-slider mechanism is also a single degree freedom system since only the crank angle is sufficient to define the system completely. These vibratory systems are illustrated in Fig. (a).
On the other hand a spring-supported rigid mass which can move in the direction of spring and can also have angular motion in one plane has two degrees of freedom. A two-mass, two spring system constrain direction without rotation has also two degrees of freedom [see Fig. (b)] A body in space has six degrees of freedom, three transitional and three rotational. A flexible beam between two supports has a infinite number of degrees of freedom. These system are shown in fig.