Mechinopedia For Mechanical Engineers

Metal Injection Molding (MIM) is a mass production technique to produce intricately shaped metallic components. A variety of metal co-injection molded components has been produced using several model material systems. This means that one type of material can provide the required surface characteristics , such as corrosion or wear resistance , with a cheaper material forming the core of the components. (eg. Iron as core and stainless steel as skin).  These are :  1. Feed Stock Mixing :    Metal powder is blended with a binder consisting of wax and polymer at about 160*C and   subsequently cooled and granulated to pellets. 2. Molding :   At about 160*C, the pellets are rammed into the mold at a pressure of 120 Mpa  to for the green molding. 3. Debinding : Hot air is passed over the components to remove binding materials , resulting in  brown  metal moldings. 4. Sintering :    The porous components are heated to 1600*C (for ferrous metal/alloys) at  very  low   pressure (10^-3 Mpa).

  Isostatic pressing is a relatively new development for powder metallurgy. In isostatic pressing, prepared metal powder is placed inside a flexible mold. A vacuum is created in the mold and it is then sealed. The mold along with the powder is then placed in a pressurized chamber into which a gas or oil is pumped through the chamber to generate pressure 3000 kg-f / cm2 or more. The pressure compresses the powder from all direction to give it the required shape.  There are two types of Isopressing ;  (1) Cold Isostatic pressing (CIP) and  (2) Hot Isostatic Pressing (HIP). In CIP, the metal powder is placed in a elastomeric material (an elastomer posses rubbery qualities such as high resilience and extensibility) and high pressure is applied at room temperature inside the pressure chamber. Water or oil is the pressure medium. The parts are removed and sintered. In HIP, the pressure is generated by an inert gas like helium or argon. The gas is reclaimed after every charge. However ,

PRESINTERING : Presintering is the process of heating the green compact to a temperature below the sintering temperature. This is done to remove the lubricants and binders added during blending and to increase the strength of the compact. All metals do not require presintering. But some metals like tungsten carbide are easily machined after presintering. After sintering they become so hard that they cannot be machined.  SINTERING :  After being compacted into a briquette having the shape of the finished workpiece, the cold-welded aggregate of metal particles is heated in a furnace to a temperature close to the melting point of the basic metal which goes into the mixture. This is carried out in controlled atmosphere furnaces. It may also be carried out under protective gas normally hydrogen or in a vacuum if material tends to react with the protective gas. The heating causes the metal particles to sinter, that is, a proportion of them partly melt and by so doing cement the remaining par

Compacting or briquetting is the process of converting loose powder into a " green compact " as it is called, of accurately defined size and shape. The briquette is considered fairly fragile, but it can be handled.  The compacting stage is carried out at room temperature in a die set-up on press. The die consists of a cavity , the shape of the desired part, but from two or ten times deeper, according to the material to be handled. Metal powder is poured into the cavity , and leveled off flush with the top of the die. The punches usually work from the top and the bottom of the die. Owing to interparticle friction , pressure applied from one direction will not be distributed uniformly throughout the part. This necessitates the use of  the both top and bottom die. The dies are forced together under pressure into the die cavity and the powder is compressed to the desired shape to approximately one-third of its original volume. In addition to two punches , a core rod extending up

The First step in the forming of powder metal parts is the mixing or blending of the powders. The blend of the powders determines many different properties that can be obtained. Many combinations of metals and of metals with ceramics or other materials that can be used as melted alloys are possible. These often give characteristics of heat resistance, frictional properties, heavy weight and hardness that are not obtainable by other methods.  The mixing may be done either wet or dry and an efficient mixer is used to produce a homogenous mixture. The type of mechanical mixer used will depend on the amount of powder handled and a type of powder.

 The powders of almost all metals and of a large quantity of alloys are used at the present times. The powders most commonly used are copper-base and iron-base materials. But stainless steel , titanium, nickel , chromium , metal powders are also used. Amongst powder properties, composition, size, form and structure of particle, specific surface, porosity and volume characteristics, fluidity , strength , hardness , permeability , regarding liquids and gases , electric conductivity , compressibility and sinterability are of great interest in powder metallurgy . The particle size of powders fall into a range of 0.1u to several millimeters (1u = 10^-6 mm) . In the majority of powders , the size of the particles varies from the several microns to 0.5 mm . There are various methods of producing powders of those size. The most commonly used methods are mechanical , atomization , reduction , electrolysis , and shotting . Mechanical, In this method metals are disintegrated to produce the

For making metallic pipes, a continuous casting method, known as Osprey process is utilized. This method is cheaper to produce metallic pipes in comparison to traditional pipe manufacturing techniques. In this process the molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipes. The Osprey process is essentially a rapid solidification technique for the direct conversion of  liquid metal into shaped preforms by means of  an integrated gas-atomizing /spray-depositing operation . In the osprey process, a controlled stream of molten metal is poured into a gas-atomizing device where it is impacted by high-velocity jets of gas , usually nitrogen or argon. The resulting spray of  metal particles is directed onto a "collector" where the hot particles re-coalesce to form a highly dense preform. 

 In Powder Metallurgy the articles are produced by pressure and heat. Usually the pressure and heating stages are separate and are termed compacting and sintering stages respectively. The compacting is also called briquetting and compressing. The manufacture of parts by powder metallurgy process usually involves a series of steps as follows : * The manufacture of powders  * Blending * Compacting * Presintering. * Sintering. A number of secondary operations such as  sizing,  coining,  machining,  impregnation,  infiltration,  plating and  heat treatment. The essence of powder metallurgy is that a mixture, composed of  specially selected and prepared powders, is compressed in dies under pressures of 10.2 to 102 Kgf / mm2 (100 to 1000 MN / m2) . The half finished object obtained ( the pressing) has a strength which, although insufficient for the articles to be used, permits transportation to the next technological operation. The final mechanical strength of the materials is achieved o

The stripping plate 4 is arranged between the flask 2 and pattern plate 3. The stripping plate has a recess whose contour equal those of the pattern 1. When the mold is ready the pattern is withdrawn from the mold downwards through the stripping plate, which supports the mold when the pattern is removed. 

 In the straight-draw molding machine, the pattern l is fixed on the pattern plate 3 on the table-5 , and the flask or molding box 2 is placed over it and filled with sand. It is then roughly rammed round the edges of the box. The squeeze head is next swung over in the position and it squeezes the mold. The flask is then lifted from the pattern by stripping pins 6.

  In the slinging operation, the consolidation and ramming are obtained by the impact of sand which falls at a very high velocity. The overhead impeller head consists of  the housing 1 in which the blade 2 rotates at a very high speed. The sand is delivered to the impeller through the opening 3 by means of conveyor buckets. The impeller head by the rotation of the blade throws the sand through the outlet 4 down into the flask over the pattern at a rate ranging from 500 to 2000 kg per min. The density of the sand can be controlled by the speed of the blade. Mold produced by the method having adequate strength, since hardness is a function of sand velocity, which is controllable in a sand slinger . These machines are most often used for ramming medium-size to large molds. 

  In order to overcome the drawbacks of  both the squeeze and jolt principles of ramming the sand, a combination of  squeeze and jolt action is often employed. A jolting action is used to consolidate the sand on the face of the pattern and it is followed by a squeezing action to impart the desired density throughout the mass of the sand.  The Jolt-Squeeze machine is so constructed that both squeeze and jolt actions can be obtained one after the other. A high pressure jolt-squeeze machine is capable of producing molds of maximum hardness, rammed uniformly throughout the flask.

 In the jolting method, the flask is first filled with the molding sand and then the table supporting the flask is mechanically raised and dropped in succession. Due to the sudden change in inertia at the end of each fall, the sand gets packed and rammed. The action of raising and sudden dropping the table is called " jolting ". The principle of  jolt molding machine in which the table 1, with the platen and flask 3 , filled with molding sand , is raised to 30 to 80 mm at short intervals by plunger 8 when the compressed air is admitted through the hose 9 and the channel 10. The air is next released through the opening 11 and the table drops down suddenly and strikes the guiding cylinder 12 at its bottom. This sudden action causes the sand to pack evenly around the pattern. Springs 13 are used to cushion the table blows and thus reduce noise and prevent destruction of the mechanism and the foundation.  The draw back in this method is that sand is rammed hardest at the parting

 In the squeeze method, molding sand in the flask is squeezed between the machine table and the overhead squeeze board pneumatically or hydraulically until the mold attain the desired density. The principle of operation of a top squeezer machine. The pattern 2 is placed on a mold board which is clamped on the table 1. The flask 3 is then placed on the mold board  and the sand frame 4 on the flask. The flask and frame are filled with molding sand and leveled off. Next the table is raised by the table lift mechanism against the platen 5 on the stationary squeezer head 6. The platen enters the sand frame up to the dotted line and compacts the molding sand . After the squeeze , the table returns to its initial position. As before the pattern is placed on the mold board which is clamped to the table. The flask 3 is placed on the frame 7 and is filled with sand. Next the squeeze head is brought against the top of the flask and the table with the pattern is raised upon the dotted line. After

Molding processes may be classified as hand molding or machine molding according to whether the mold is prepared by hand tools or with the aid of molding machines. Hand molding is found to be economical when only a few castings are required. Hand molding is slow and it requires considerable skill to produce good castings . On the other hand, the use of  the molding machine is advisable when the large number of repetitive castings are to be produced since hand molding is more time consuming, laborious, and becomes expensive. The dimensions of machine cast castings are more accurate, in other words , it is possible to produce castings to close tolerances. As a consequence, the weight of  the castings is reduced  and material saved. The working time per mold is similar than that required for hand molding, this means that, related to same shop area , the output of castings is increased per unit of  time, In fine, machine molding offers higher production rates and better quality casting

 In This process , the pattern is divided into half across the parting and mounted in halves on to plates with parallel sides of the same shape as the parting. The use of the plates gives the following advantages : 1. The patterns can be handled easily and rapidly. 2. The task of making the joint between the two parts of the mold is relieved as the plate provides its own joint when the flask is rammed up. 3. The pattern can be drawn quickly, as the plate overlaps the side of the box and the pins which hold it in the position acts as guides during the drawing operation. 

Sweep moldings are employed for molding parts whose shape is that of a surface of revolution. In the preliminary process, a base 1 and spindle 2 is well placed in the foundry floor. The sand is filled in and rammed until the excavation forms approximately the shape and size of the required casting. A sweep holder 5 is then placed in the spindle land the sweep 6 is attached by bolts and nuts. The surface of the mold is produced by the profile of the sweep as it is rotated about the spindle. After sweeping, the spindle is removed and the mold patched at the center. The gate is then cut and the mold is ready for pouring. 

  Bench molding applies chiefly to molds small enough to be made on a work bench of height convenient to the molder. Very heavy castings or castings of a considerable depth or area may be molded in the sand of the foundry floor in much the same way as green-sand or dry-sand molding. In such cases, the floor itself acts as the drag, and this may be covered with a cope or the mold may be cast open. 

 Molds are large jobs are generally prepared in pit dug in the foundry floor which facilitates in lifting the pattern and casting the mold easily. Since a pit which functions as drag cannot be rolled over, the sand under the pattern may be rammed by bedded-in. The pattern may be suspended in correct location as the sand is rammed under it. In other cases, if the bottom surface of a pattern is flat, the pattern can be placed on a flat-level surface rammed up for it. A bed of coke is laid on the bottom of the pit, covered with straw and then a layer of sand , which is rammed and leveled . The coke bed is connected with atmosphere by vertical vent pipes in the corners of the pit to provide an outlet for the gases generated. If the floor is lightly damp, the inside surfaces of the pit are lined with tar-paper, bricks, or wooden planks. Generally, one box is required to complete the mold. Runners, pouring basins, feeders, are cut in it. 

Loam is clay and sand mixed with water to form a thin plastic mixture from which molds are made. Loam sand also contains fire clay or ganisters . The loam must be sufficiently adhesive so that it can cling to the vertical surfaces. Loam molds always require special provision to secure adequate ventilation. The object is to open out pores in the otherwise compact, closely knit mass, by artificial means. Thus various kinds of organic matter such as chopper straw, and particularly horse manure, is mixed up with the sand . A typical loam sand mixture is , 1. Silica - 22 %  2. Clay  - 5 % 3. Coke - 10 %   4. Moisture - 18-20 % This is applied as plaster to the rough structure of  the mold usually made of brickwork and the exact shape is given by a rotating sweep around a central spindle. Cast iron plates and bars are used to reinforce the brickwork which retains the molding material. Loam molds may also be prepared by use of a skeleton pattern made of wood. The surfaces of  loams are black