Mechinopedia For Mechanical Engineers

The molding process involved in making dry sand molds are similar to those employed in green-sand molding except that a different sand mixture is used and all parts of the mold are dried in an oven before being reassembled for casting.  The green sand mold depends upon the moisture and the natural clay binder in the sand to retain its shape. But the sand used for dry sand molds depends upon added binding material such as flour, resin, molasses, or clay. The materials are thoroughly mixed and tempered with a thin clay water. The amount of binder is determined by the size of  the casting being made.  Metal flasks must be used for dry sand molds to withstand the heat in the oven. Before drying, the inside surfaces of the dry sand mold are coated with wet blacking - a mixture of carbon black and water with a small addition of a gum. This gives a smoother surface to the casting. These molds can be held for any length of  time before pouring, provided they are kept dry. Advantages of dry sa

This method is very much used and most suitable for split patterns as well as for solid patterns . One half of the pattern is placed with its flat side on molding board, and a drag is rammed and rolled over. It is now  possible to place the other half of the pattern and a cope box in proper position. After ramming, the cope is lifted off and the two halves of the pattern are rapped and drawn separately. The cope is next replaced on the drag to assemble the mold.

 If the upper surface of the casting is not flat or  must be smoother than the rough surfaces produced by open-sand molds, a solid pattern can also be molded by using a technique known as " bedded In ", in which a sand cover or cope is necessary. In bedded-In method , the pattern is pressed or hammered down to bed it into the sand of the foundry floor or in a drag partially filled with sand to form the mold cavity. To ensure that the sand is properly compacted, careful ramming of the sand close to the pattern is necessary. As a check , the pattern can be drawn and the mold cavity surface tested for soft spots. All soft spots should be filled in with extra sand and the pattern again pressed downward until properly rammed mold cavity is obtained.  After the joint has been smoothed , and parting sand spread , a cope is placed over the pattern. The cope is rammed up, runners and risers cut , and the cope box lifted, leaving the solid pattern in the floor or in the drag as the ca

  This is the simplest form of green sand molding , particularly suitable for solid patterns. The entire mold is made in foundry floor or in bed of sand above floor level for inconvenience in working and pouring. No molding box is necessary and the upper surface of the mold is open to air. The sand in the foundry floor is made loose and perfectly leveled to obtain uniform thickness of the casting. As there is no head of metal the sand may be rammed lightly, just hard enough to support the weight of metal only. After proper leveling, the pattern is pressed in the sand bed for making mold. The pouring basin is built up at one end of the mold, and the overflow channel is cut at the sides of the cavity at the exact height from the bottom face of the mold to give the desired thickness

Green-sand molds are prepared with natural molding sands or with mixtures of silica sand, bonding clay, and water. These materials are thoroughly mixed in proportions which will give the desired properties for the class for work being done. To make the green-sand mold the sand must be properly tempered before it can be used. If the sand is too dry, additional water is added if too wet, dry sand is added until it has the proper temper. To check the sand for proper temper, a handful is grasped in the first. The pressure is released, and the sand is broken in two sections. The sections of sand should retain their shape and the edges of the break should be sharp and firm.  The surface of the mold  which comes in contact with the molten metal forms the most important part in green-sand molds. In order to give the casting a clean and bright surface and to prevent the sand from the burning on the face of the mold , a layer of the facing sand is given surrounding the pattern. Facing-sand mixtu

Molding Processes in common use may be classified according to different forms. They may broadly be classified as :  (1) Hand Molding, and (2) Machine Molding .  In piece and, a small - lot production foundry practice, sand molds are made by hand ; molding machines are employed in large-lot and mass production. Molding processes are often classified according to  (1) The type of material of which the mold is made or ' (2) machine molding. In piece, and, small-lot production foundry practice, sand moulds are made by hand ; molding machines are employed in large -lot and mass production. Moulding processes are often classified according to  (1) The type of material of which the mold is made or  (2) Dry sand molds . (3) Skin-dried molds . (4) Loam Molds . Molds classified as to the methods commonly used as :  (1) bench Molding, (2) floor Molding , (3) Pit Molding , (4) Sweep Molding , (5) Plate Molding . 

These types are called "newer" since these are not in use for many many years and have been developed very recently. The types are : 1.) Electron - Beam Welding  2.) Laser - Beam Welding.  LASER BEAM WELDING AND ITS PROCESS :  Lasers are devices which are capable of generating a very intense beam of optical radiation. The word "laser" is an acronym of Light Amplification by the stimulated Emission of Radiation .  As even more concentrated beam is produced, but a lower overall efficiency , with the laser beam. A CO2 laser pumped with 500 W emits far-infared light (10.6 Um wavelength) and develops a peak energy density of 80kW/mm2, yet the heat affected zone is only 0.005 to 0.1 mm wide. Oxygen blown on the surface of the metals reduces the heat reflection and increases material removal rates by oxidation ; inert gas increases heat transfer for non metals.  The laser has the advantage that vacuum is not necessary and it is finding limited but growing application,

  These types are called "newer" since these are not in use for many many years and have been developed very recently. The types are : 1.) Electron - Beam Welding  2.) Laser - Beam Welding.  Electron - Beam Welding And Its Process : Electron beam welding utilizes the energy from fast moving beam of electrons focused on the work piece. The electrons strike the metal surface , which gives up kinetic energy almost completely into heat. The beam is created in a high vacuum (10^-3 to 10 ^-5 mm Hg). If the work is done is such vacuum, no electrodes, gases, or filler metals can contaminate it, and pure welds can be made. Moreover, High vacuum is necessary around the filament so that it will not burn up and will also produce and focus a stable beam.  In all types of electron beam machines, a tungsten filament which serves as a cathode emits a mass of electron that are accelerated and focused to a 0.25 - 1mm diameter beam of high energy density up to 0.5 to 10 kW / mm2. The heat gener

The oxygen hydrogen process was once used extensively to weld low temperature metals such as aluminum, lead and magnesium ; but it is not as popular today because more versatile and faster welding process such as ( TIG ) Tungsten-Inert-gas and ( MIG ) Metal-Inert-Gas have replaced the oxygen-hydrogen flame. The process is similar to oxygen-acetylene system, with the only difference being a special regulator used in metering the hydrogen gas .   

 This process uses torch similar to Bunsen burner and operates on the Bunsen Burner principle. The air is drawn into the torch as required and mixed with fuel flame. The gas is then ejected and ignited, producing an air-fuel flame. The common fuels used in the air-fuel welding are acetylene , natural gas , propane and butane . This type of welding has limited use since the temperature is lower than that attained by other gas processes. The air-fuel welding processes are used successfully in lead welding and many low-melting-temperature metals and alloys such as in brazing and soldering processes. 

  Oxy - acetylene gas welding is accomplished by the melting edges or surface to be joined by gas flame and allowing the molten metal to flow together, thus forming a solid continuous joint upon cooling. This process is particularly suitable for joining metal sheets and plates having thickness of  2 to 50 mm. With material thicker than 15 mm, additional metal called filler metal is added to the weld in the form of welding rod. The composition of the filler rod is usually the same or nearly the same as that of the part being welded. To remove the impurities and oxides present on the surfaces of the metal to be joined and to obtain a satisfactory bond a flux is always employed during the welding except mild steel which has more manganese and silicon that act as a de-oxidizing agents. Various gas combinations can be used for producing a hot flame for welding metals. Common mixture of  gases is oxygen and acetylene , oxygen and hydrogen , oxygen and other fuel gas , and air and acetyl

 In explosive welding , strong metallurgical bonds can be produced between metal combinations which cannot be welded by other methods or processes. For example , tantalum can be explosively welded to steel although the welding point of tantalum is higher than the vaporization temperature of steel. Explosive welding is carried out by bringing together properly paired metal surfaces with high relative velocity at a high pressure and a proper orientation to each other so that a large amount of plastic interaction occurs between the surfaces. The work piece, held fixed is called the target plate and the other called flyer plate. While a variety of procedures have been successfully employed , the major techniques of the explosive welding can be divided into the contact techniques and Impact techniques.  In critical space and nuclear application , explosive welding permits fabrication of structures that cannot be made by any other means ; and in some commercial applications, explosive joini

The frictional energy generated when two bodies slide on each other is transformed into heat ; when the rate of movement is high and the heat is contained in a narrow zone, welding occurs. In the practical form of friction welding, one part is firmly held while the other (usually cylindrical) is rotated under simultaneous application of axial pressure. The temperature rises, partially formed welded spots are sheared , surface films are disrupted , and the rotation is suddenly arrested when the entire surface is welded. Some of the softened metal is squeezed out into a flash, but it is not fully clear whether melting takes place.  The heated zone being very thin, dissimilar metals are easily joined, for example, mild steel shanks can be fastened to high-speed-steel tool ends. 

Ultrasonic welding will join similar or dissimilar  metals by the Introduction of  high-frequency vibratory energy (frequency being 20000 to 60000 Hz) into overlapping metals into the area to be joined. No flux or filler metals are used, no electrical current passes through the weld metal, and usually no heat is applied.  The parts to be joined are clamped together between a welding tip and a supporting member under low-static pressure. High-Frequency vibratory energy is then transmitted into the weld area for a brief interval. This process produces a sound bond without an arc or melting weld metal and in the absence of  filler metal or fluxes. The ultrasonic welding process can be utilized in spot welding , continuous seam welding , etc. The maximum thickness by these processes ultrasonically may vary from 0.38 to 2.5mm depending upon the metal.  

Diffusion welding is a process that does not necessarily need heat to produce a fusion weld. Rather it needs two kinds of surfaces that can come into intimate contact under pressure. This pressure is applied for a period of  hours. In this process, although heating is not essential , if the temperature is raised, the diffusion rate will be cut sufficiently. It might take many hours to perform a certain bonding, but with heat the time element can be cut to a matter of hours of minutes. The goldsmith has for centuries made filled gold by placing a weight on top of  a sandwich composed of silver or copper core with gold face sheets. When this is held in a furnace for a prolonged time, a permanent bond is obtained. That is what is done in diffusion bonding in principle.  This process makes it possible to join metal to metal , metal to ceramic, and metal to metal with intermediate bonding materials, Temperatures that approach is approximately 900*C. This extreme temperature limits diffusio

  Interatomic bonds   may be established by bringing atoms of two surfaces in close enough proximity to assure adhesion. Relative moment of the surfaces under pressure and controlled roughness are helpful in breaking through surface films. While theoretically no pressure would be required for bonding, in practice a certain normal pressure is necessary to assure conformity with the contacting surfaces. In principle, however, any material can be bonded and solid state bonding is often applied when other technique fails.  SOLID STATE WELDING METHODS  Solid State Welding Includes :  (1) Friction welding (2) Ultrasonic welding and  (3) Diffusion Welding. (4) Explosive Welding 

Thermit Welding is primarily a fusion-welding process in which the weld is effected by pouring superheated liquid Thermit steel around the parts to be united. In the case of thermit pressure welding, only the heat of the "thermit" reaction is utilized to bring the surface of the metal to be welded in a plastic state and mechanical pressure is then applied to complete the weld.  The thermit process for welding metal is based on the chemical reaction between finely divided aluminum and iron oxide.  During the reaction, the oxygen leaves the iron oxide and combines with aluminum, producing aluminum oxide , or slag, and superheated thermit steel. The thermit is a mixture of finely divided aluminum and iron oxide, the ratio by weight being approximately three parts of iron oxide to one part of aluminum. The mixture, placed in a refractory-lined crucible, is ignited with the aid of highly inflammable powder composed largely of barium peroxide. The temperature produced by the therm

 The operation is performed with one part held in stationary holder and the other in a clamp mounted on a slide which is backed up against pressure from a heavy spring. In the welding operation, the movable clamp is released rapidly carrying the part forward. When the two parts are approximately 1.5 mm apart , a sudden discharge of electrical energy is released, causing an intense arc between the two surfaces. Too complete, it takes about 0.1 second. No upset or flash occurs at the weld. This method of welding is limited to small areas of  144 mm 2 maximum. Percussion welding is a fast method and it can handle dissimilar metals. This is highly suitable for welding small wires to electrical components.  

Projection welding is a modification of spot welding . The current and pressure are localized at the weld section by the use of the embossed, machined or coined projections on one or both pieces of the work. The flattening out of these projections under pressure results in good welds at all points of contact.  Projection welding applies to nearly all the metal combinations that can be spot welded, but the design  must be strong enough to support the projection. Annular, or ring projections are often used on screw machine parts such as bosses and studs which are to be welded to sheets up to approximately 3 mm thick. For thicker sheets a dome type of projection seems to work out better.  Only clean, scale free surfaces should be used in projection welding. A dirty substance will cause much variation in the resistance between the parts being joined, with resulting variation in current flow and weld strength. Line projection welds are recommended over a point welds when sections are subje

Seam welding is a method of making a continuous joint between two overlapping pieces of sheet metal. The normal procedure for making a seam weld is to place the work between the wheels which serve as conductors for producing continuous welds. As pressure is applied, the drive is started and the welding current switched on. Then at the same time, the over-lapping surfaces of the metal are forced together as fast as they are heated. A coolant is applied to conserve the electrodes and cool the work rapidly to speed the operation.  The materials that may be seam welded include most of those that may be spot welded. Steel plates 10 mm thick have been seam welded to hold about 200 kg/cm2 (20000 KN/m2) pressure. Seam welding is used on many types of  pressure tight or leak proof tanks for various purposes and numerous other products.