Thermosetting Plastic

THERMOSETTING PLASTICS:

          Thermoset, or thermosetting, plastics are synthetic materials that strengthen during being heated, but cannot be successfully remolded or reheated after their initial heat-forming. This is in contrast to thermoplastics, which soften when heated and harden and strengthen after cooling. Thermoplastics can be heated, shaped and cooled as often as necessary without causing a chemical change, while thermosetting plastics will burn when heated after the initial molding. Additionally, thermoplastics tend to be easier to mold than thermosetting plastics, which also take a longer time to produce (due to the time it takes to cure the heated material).
          Thermosetting plastics, however, have a number of advantages. Unlike thermoplastics, they retain their strength and shape even when heated. This makes thermosetting plastics well-suited to the production of permanent components and large, solid shapes. Additionally, these components have excellent strength attributes (although they are brittle), and will not become weaker when the temperature increases.
          Thermoset plastic products are typically produced by heating liquid or powder within a mold, allowing the material to cure into its hardened form. These products can be removed from the mold even without allowing it to cool. The reaction used to produce thermosetting plastic products is not always the result of heating, and is sometimes performed by chemical interaction between specialized materials. Typical types of thermosetting plastics are epoxies, polyesters, silicones and phenolics. Vulcanized rubber is also an excellent example of a thermosetting plastic; anyone who has ever driven an automobile can attest to the properties of a superheated tire—it burns but does not mold into a new shape.
          Each type of thermosetting plastic has a unique set of properties. Epoxies, for example, exhibit elasticity and exceptional chemical resistance, and are relatively easy to cure. Phenolics, while fairly simple to mold, are brittle, strong and hard. Because of their wide range of characteristics, thermosetting plastics find use in an extensive variety of applications, from electrical insulators to car bodies.

Injection Moulding

INJECTION MOULDING:

          This is the most common method of producing parts made of plastic. The process includes the injection or forcing of heated molten plastic into a mold which is in the form of the part to be made. Upon cooling and solidification, the part is ejected and the process continues. The injection molding process is capable of producing an infinite variety ofm part designs containing an equally infinite variety of details such as threads, springs, and hinges, and all in a single molding operation. 

          A plastic is defined as any natural or synthetic polymer that has a high molecular weight. There are two types of plastics, thermoplastics and thermosets. Thermosets will undergo a chemical reaction when heated and once formed cannot be resoftened. The thermoplastics, once cooled, can be ground up and reheated repeatedly. Thus, the thermoplastics are used primarily in injection molding.

There are four major elements that influence the process. They are:
• the molder
• the material
• the injection machine
• the mold

          Of these four, the injection machine and the mold are the most varied and mechanically diverse. Most injection machines have three platens. Newer models use just two platens and may be electrically operated as opposed to the traditional hydraulic models. They can range in size from table top models to some the size of a small house. Most function horizontally, but there are vertical models in use. All injection machines are built around an injection system and a clamping system.
          The injection system mechanism may be of the reciprocating screw type or, less frequently, the two-stage screw type. Also included is a hopper, a heated injection barrel encasing the screw, a hydraulic motor, and an injection cylinder. The system’s function is to heat the thermoplastic to the proper viscosity and inject it into the mold. As the resin enters the injection barrel, it is moved forward by the rotation of the screw. As this movement occurs, the resin is melted by frictional heat and supplementary heating of the barrel encasing the screw. The screw has three distinct zones which further processes the resin prior to actual injection.

          Injection is accomplished through an arrangement of valves and a nozzle, all acted upon by the screw and the hydraulic pump that pushes the resin into the mold. This so-called “packing action” occurs at pressures from 20,000 to 30,000 psi and higher. The temperature of the resin at this time is between 320o and 600o F. The clamping system’s function is to keep the plastic from leaking out or “flashing” at the mold’s parting line. The clamping system consists of a main hydraulic pressure acting on the mold platens and a secondary toggle action to maximize the total clamping pressure.
          The platens are heavy steel blocks that actually hold the mold tightly closed during the injection phase. Most injection machines have three platens. The “stationary” platen has a center hole that receives the injection nozzle and holds the cavity half of the mold. This platen also anchors the machine’s four horizontal tie bars. The “movable” platen holds the core half of the mold. This platen moves back and forth on the tie bars and as the mold opens, the mold’s ejection system of pins and posts expel the finished part. The “rear stationary” platen holds the opposite ends of the tie bars and anchors the whole clamping system. 

          All injection machines have some sort of safety interlock system that prevent access to the molds during the clamping and injection phases when the machine is operating semi-automatically. The operator removes the finished part, closes the door or gate, which sets in motion the next molding cycle. In full automatic operation, finished parts fall into a container, conveyor, or are removed by robot mechanisms.

ADVANTAGES:

1. Injection molding allows for high production output rates.

2. When producing your product you may use inserts within the mold. You may also use fillers for added strength.

3. Close tolerances on small intricate parts is possible with Injection Molding.

4. More than one material may be used at the same time when utilizing co-Injection Molding.

5. There is typically very little post production work required because the parts usually have a very finished look upon ejection.

6. All scrap may be reground to be reused, therefor there is very little waste.

7. Full automation is possible with Injection Molding.

DISADVANTAGES:

1. High set up costs - Moulds etc.

2. Complicated process.

3. can only be used for large quantities due to costs.

Thermoplastic

THERMOPLASTIC:

           A thermoplastic polymer is a type of plastic that changes properties when heated and cooled. Thermoplastics become soft when heat is applied and have a smooth, hard finish when cooled. There are a wide range of available thermoplastic formulas that have been created for many different applications.

          A thermoplastic polymer is made up of long, unlinked polymer molecules, generally with a high molecular weight. Because the molecular chains are unlinked, they rely on other interactions, such as dipole-dipole interactions, aromatic ring stacking, or Van der Waals forces. Thermoplastics generally form a crystalline structure when cooled below a certain temperature, resulting in a smooth surface finish and significant structural strength. Above this temperature, thermoplastics are elastic. As the temperature increases, thermoplastics gradually soften, eventually melting.

          The material properties of a thermoplastic polymer can be adjusted to meet the needs of a specific application by blending the thermoplastic resin with other components. For example, shape memory polymer can be mixed with thermoplastic polymer to create a material that has shape memory characteristics, but retains the basic properties of the thermoplastic. Plasticizers can be added to a thermoplastic polymer to keep the material flexible at lower temperatures. This mixture is often used in plastic automobile body parts to prevent them from cracking during periods of cold temperatures.

        Some of the most commonly found thermoplastic polymers include polyethylene, polypropylene, polyvinyl chloride (PVC), polystyrene, polytetrafluoroethylene (PTFE, commonly known as Teflon), Acrylonitrile butadiene styrene (ABS plastic), and polyamide (commonly known as nylon).

          Because thermoplastics can be melted and reused without any change in material properties, these polymers can be actively recycled. Beverage bottles and household containers with resin identification codes are generally thermoplastic polymers. These containers are ground into chips, melted, refined to remove impurities, and reused as reclaimed material.

Thermit Welding

THERMIT WELDING:

          Thermit welding is an effective, highly mobile, method of joining heavy section steel structures such as rails. Essentially a casting process, the high heat input and metallurgical properties of the Thermit steel make the process ideal for welding high strength, high hardness steels such as those used for modern rails.

          Thermit Welding is a skilled welding process and must not be undertaken by anyone who has not been trained and certificated to use it.
          Detailed operating instructions are provided for each of our processes, but the welding methods all comprise of 6 main elements:

1. A carefully prepared gap must be produced between the two rails, which must then be accurately aligned by means of straightedges to ensure the finished joint is perfectly straight and flat.

2. Pre-formed refractory moulds which are manufactured to accurately fit around the specific rail profile are clamped around the rail gap, and then sealed in position. Equipment for locating the preheating burner and the Thermitt container is then assembled.

3. The weld cavity formed inside the mould is preheated using an oxy fuel gas burner with accurately set gas pressures for a prescribed time. The quality of the finished weld will depend upon the precision of this preheating process.

4. The Thermit® Portion is manufactured to produce steel with metallurgy compatible with the specific type of rail to be welded. On completion of the preheating, the container is fitted to the top of the moulds, the portion is ignited and the subsequent exothermic reaction produces the molten Thermitt Steel. The container incorporates an automatic tapping system enabling the liquid steel - which is at a temperature in excess of 2,500°C - to discharge directly into the weld cavity.

5. The welded joint is allowed to cool for a predetermined time before the excess steel and the mould material is removed from around the top of the rail with the aid of a hydraulic trimming device.

6. When cold the joint is cleaned of all debris, and the rail running surfaces are precision ground the profile. The finished weld must then be inspected before it is passed as ready for service.

 

ADVANTAGES:

1.The heat necessary for welding is obtained  from a chemical reaction and thus no costly power supply is required. Therefore broken parts (rails etc.) can be welded on the site itself.

2.For welding large fractured crankshafts. 

3.For welding broken frames of machines.

4.For building up worn wobblers.

5.For welding sections of castings where size prevents there being cast in one piece.

6.For replacing broken teeth on large gears.

7.Forgings and flame cut sections may be welded  together to make huge parts.

8.For welding new necks to rolling mill rolls and pinions.

9.For welding cables for electrical conductors.

10.For end welding of reinforcing bars to be used in concrete (building) construction. 

LIMITATIONS:

1.Thermit welding is applicable only to ferrous metal parts of heavy sections, i.e., mill housings and heavy rail sections.

2.The process is uneconomical if used to weld cheap metals or light parts.

Loam Moulding

LOAM MOULDING:

          Loam is one type of clay which is made with sand mixed with water to form a thin plastic mixture from which moulds are made. Loam sand also contains ganisters or fire clay. The loam must be sufficiently adhesive so that it can cling to the vertical surface. It always requires special provision to secure adequate ventilation. The object is opened out pores in the otherwise compact, closely knit mass, by artificial means. There are various kinds of organic matter such as chopped straw, and particularly horse manure, is mixed up with the sand, a typical loam sand mixture is given below :

1.	Silica Sand	22  vol.
2.	Clay	5    vol.
3.	Coke	10  percent
4.	Moisture	18-20 vol.

          This applied as plaster to the rough structure of the mould usually made of brick work and the exact shape is given by a rotating sweep around a central spindle. Cast iron plates and bars are used to reinforce the brick work which retains the moulding material. Loam moulds also be prepared by the use of skeleton pattern made of wood. The surfaces of loams are blackened and are dried before being assembled.

          Loam moulds are employed chiefly in the making of large casting for which it would be expensive to use full pattern and ordinary flasks equipment. Objects such as large cylinders, chemical pans, large gears, round bottoms, kettles and other machining parts are produces in the loam moulding.

Skin Dried Moulding

Skin Dried Moulding:

          Skin-dried or air-dried molds are sometimes preferred to green sand molds where assurance is desired that the surface moisture and other gas-forming materials are lowered. By skin drying the face of the mold after special bonding materials have been added to the sand molding mixture, a firm mold face is produced similar to that obtained in dry sand practice. Shakeout of the mold is almost as good as that obtained with green sand molding. Skin-dried molds are commonly employed in making medium-heavy and heavy castings.

          Generally, the surface of the mold is washed or sprayed with a refractory mold coating. The most common method of drying the refractory mold coating uses hot air, gas or oil flame. Skin drying of the mold can be accomplished with the aid of torches, a bank of radiant heating lamps or electrical heating elements directed at the mold surface.

Advantages :

• This process reduces surface moisture and other gas-forming materials from mold. It can commonly be used in the production of medium-heavy to heavy castings.

Disadvantages :

• These molds are more expensive to produce. Mold sections must be completely dry and cool prior to assembly.

Transfer Moulding

Transfer Moulding :

          Transfer molding is similar to compression molding in that a carefully calculated, pre-measured amount of uncured molding compound is used for the molding process. The difference is, instead of loading the polymer into an open mold, the plastic material is pre-heated and loaded into a holding champer called the pot. The material is then forced/transferred into the pre-heated mold cavity by a hydraulic plunger through a channel called sprue. The mold remains closed until the material inside is cured. 

          Transfer molded parts inherently have less flash (excess material that runs along the parting line of the mold) than their compression molded counterparts because the mold remains closed when the plastic enters the mold cavity. However, transfer molding still produces more waste material than compression molding because of the sprue, the air holes and the overflow grooves that are often needed to allow air to escape and material to overflow. 

           One of the key advantages of transfer molding over compression molding is that different inserts, such as metal prongs, semiconductor chips, dry composite fibers, ceramics, etc., can be placed/positioned in the mold cavity before the polymer is injected/drawn into the cavity. This ability makes transform molding the leading manufacturing process for integrated circuit packaging and electronic components with molded terminals, pins, studs, connectors, and so on.

          In the composite industry, fiber-reinforced composites are often manufactured by a processed called Resin Transform Molding (RTM). Layers of textile preforms (long fibers woven or knitted in patterns) are pre-arranged in the mold. The resin is then injected to impregnate the performs. Vacuum is often used to avoid air bubbles and help draw the resin into the cavity. In addition, the resin used has to be relatively low in viscosity.

ADVANTAGES:

• Loading a preform into the pot takesless time than loading preforms intoeach mold cavity.
• Tool maintenance is generally low, although gates and runners aresusceptible to normal wear.
• Longer core pins can be used and canbe supported on both ends, allowingsmaller diameters.
• Because the mold is closed before theprocess begins, delicate inserts andsections can be molded.
• Higher tensile and flexural strengths areeasier to obtain with transfer molding.
• Automatic de-gating of the mold's tunnelgates provides cosmetic advantages.

DISADVANTAGES:

• Molded parts may contain knit lines in back of pins and inserts.
• The cull and runner system of transfer molding leaves waste material, but this scrap can be greatly reduced by injection molding with live sprues and Runnerless Injection Compression (RIC).
• Fiber degradation of orientation occurring in the gate and runner system reduces the molded part's impact strength.
• Compared to compression molding, high molding pressures are required for the transfer process, so fewer cavities can be put into a press of the same tonnage.

Compression molding

Compression molding :   

          Compression molding is a method of molding in which the molding material, generally preheated, is first placed in an open, heated mold cavity. The mold is closed with a top force or plug member, pressure is applied to force the material into contact with all mold areas, while heat and pressure are maintained until the molding material has cured. The process employs thermosetting resins in a partially cured stage, either in the form of granules, putty-like masses, or preforms. Compression molding is a high-volume, high-pressure method suitable for molding complex, high-strength fiberglass reinforcements. Advanced composite thermoplastics can also be compression molded with unidirectional tapes, woven fabrics, randomly oriented fiber mat or chopped strand. The advantage of compression molding is its ability to mold large, fairly intricate parts. Also, it is one of the lowest cost molding methods compared with other methods such as transfer molding and injection molding; moreover it wastes relatively little material, giving it an advantage when working with expensive compounds. However, compression molding often provides poor product consistency and difficulty in controlling flashing, and it is not suitable for some types of parts. Fewer knit lines are produced and a smaller amount of fiber-length degradation is noticeable when compared to injection molding. Compression-molding is also suitable for ultra-large basic shape production in sizes beyond the capacity of extrusion techniques. Materials that are typically manufactured through compression molding include: Polyester fiberglass resin systems (SMC/BMC), Torlon, Vespel, Poly(p-phenylene sulfide) (PPS), and many grades of PEEK.

          Compression molding was first developed to manufacture composite parts for metal replacement applications, compression molding is typically used to make larger flat or moderately curved parts. This method of molding is greatly used in manufacturing automotive parts such as hoods, fenders, scoops, spoilers, as well as smaller more intricate parts. The material to be molded is positioned in the mold cavity and the heated platens are closed by a hydraulic ram. Bulk molding compound (BMC) or sheet molding compound (SMC), are conformed to the mold form by the applied pressure and heated until the curing reaction occurs. SMC feed material usually is cut to conform to the surface area of the mold. The mold is then cooled and the part removed. Materials may be loaded into the mold either in the form of pellets or sheet, or the mold may be loaded from a plasticating extruder. Materials are heated above their melting points, formed and cooled. The more evenly the feed material is distributed over the mold surface, the less flow orientation occurs during the compression stage.
          In compression molding there are six important considerations that an engineer should bear in mind :

• Determining the proper amount of material.
• Determining the minimum amount of energy required to heat the material.
• Determining the minimum time required to heat the material.
• Determining the appropriate heating technique.
• Predicting the required force, to ensure that shot attains the proper shape.
• Designing the mold for rapid cooling after the material has been compressed into the mold.

 ADVANTAGES:

• Lowest cost molds
• Little "throw away" material provides advantage on expensive compounds
• Often better for large parts
• Lower labor costs
• Minimum amount of wasted material & Improved material efficiency
• Internal stress and warping are minimized.
• Dimensional accuracy & stability is excellent.
• Shrinkage is minimized and closely reprodcible.
• Thick sections and large parts are practical.
• Lower molding pressures allow molding of large parts on presses of lower tonnage.

DISADVANTAGES:

• Offers least product consistency
• Not suitable for fragile mold features, or small holds
• Uneven parting lines present a mold design problem
• High impact composites make flash control & removal difficult.
• The depth of the molded holds is limited to 2 or 3 times their diameter
• Shot weight must be tightly controlled
• Dimension across the parting line may be difficult to hold but good accuracy may be obtained through tight process control.

Metal Inert Gas ( MIG )

Metal Inert Gas ( MIG ):

           Gas metal arc welding (GMAW), sometimes referred to by its subtypes metal inert gas (MIG) welding or metal active gas (MAG) welding, is a semi-automatic or automatic arc welding process in which a continuous and consumable wire electrode and a shielding gas are fed through a welding gun. A constant voltage, direct current power source is most commonly used with GMAW, but constant current systems, as well as alternating current, can be used. There are four primary methods of metal transfer in GMAW, called globular, short-circuiting, spray, and pulsed-spray, each of which has distinct properties and corresponding advantages and limitations.

          Originally developed for welding aluminum and other non-ferrous materials in the 1940s, GMAW was soon applied to steels because it allowed for lower welding time compared to other welding processes. The cost of inert gas limited its use in steels until several years later, when the use of semi-inert gases such as carbon dioxide became common. Further developments during the 1950s and 1960s gave the process more versatility and as a result, it became a highly used industrial process. Today, GMAW is the most common industrial welding process, preferred for its versatility, speed and the relative ease of adapting the process to robotic automation. The automobile industry in particular uses GMAW welding almost exclusively. Unlike welding processes that do not employ a shielding gas, such as shielded metal arc welding, it is rarely used outdoors or in other areas of air volatility. A related process, flux cored arc welding, often does not utilize a shielding gas, instead employing a hollow electrode wire that is filled with flux on the inside.

           For most of its applications gas metal arc welding is a fairly simple welding process to learn requiring no more than a week or two to master basic welding technique. Even when welding is performed by well-trained operators weld quality can fluctuate since it depends on a number of external factors. All GMAW is dangerous, though perhaps less so than some other welding methods, such as shielded metal arc welding.

Technique :

          The basic technique for GMAW is quite simple, since the electrode is fed automatically through the torch. By contrast, in gas tungsten arc welding, the welder must handle a welding torch in one hand and a separate filler wire in the other, and in shielded metal arc welding, the operator must frequently chip off slag and change welding electrodes. GMAW requires only that the operator guide the welding gun with proper position and orientation along the area being welded. Keeping a consistent contact tip-to-work distance (the stick out distance) is important, because a long stick-out distance can cause the electrode to overheat and will also waste shielding gas. Stick-out distance varies for different GMAW weld processes and applications.

            For short-circuit transfer, the stick-out is generally 1/4 inch to 1/2 inch, for spray transfer the stickout is generally 1/2 inch. The position of the end of the contact tip to the gas nozzle are related to the stickout distance and also varies with transfer type and application. The orientation of the gun is also important—it should be held so as to bisect the angle between the workpieces; that is, at 45 degrees for a fillet weld and 90 degrees for welding a flat surface. The travel angle, or lead angle, is the angle of the torch with respect to the direction of travel, and it should generally remain approximately vertical. However, the desirable angle changes somewhat depending on the type of shielding gas used—with pure inert gases, the bottom of the torch is often slightly in front of the upper section, while the opposite is true when the welding atmosphere is carbon dioxide.

Quality :

          Two of the most prevalent quality problems in GMAW are dross and porosity. If not controlled, they can lead to weaker, less ductile welds. Dross is an especially common problem in aluminum GMAW welds, normally coming from particles of aluminum oxide or aluminum nitride present in the electrode or base materials. Electrodes and workpieces must be brushed with a wire brush or chemically treated to remove oxides on the surface. Any oxygen in contact with the weld pool, whether from the atmosphere or the shielding gas, causes dross as well. As a result, sufficient flow of inert shielding gases is necessary, and welding in volatile air should be avoided.

          In GMAW the primary cause of porosity is gas entrapment in the weld pool, which occurs when the metal solidifies before the gas escapes. The gas can come from impurities in the shielding gas or on the workpiece, as well as from an excessively long or violent arc. Generally, the amount of gas entrapped is directly related to the cooling rate of the weld pool. Because of its higher thermal conductivity, aluminum welds are especially susceptible to greater cooling rates and thus additional porosity. To reduce it, the workpiece and electrode should be clean, the welding speed diminished and the current set high enough to provide sufficient heat input and stable metal transfer but low enough that the arc remains steady. Preheating can also help reduce the cooling rate in some cases by reducing the temperature gradient between the weld area and the base material.

ADVANTAGES:

1) Higher welding speeds.
2) Greater deposition rates.
3) Less post welding cleaning (e.g. no slag to chip off weld).
4) Better weld pool visibility.
5) No stub end losses or wasted man hours caused by changing electrodes.
6) Low skill factor required to operate M.I.G / M.A.G.S welding torch.
7) Positional welding offers no problems when compared to other processes. (Use dip or pulsed mode of transfer).
8) The process is easily automated.
9) No fluxes required in most cases.
10) Ultra low hydrogen process.

DISADVANTAGES:

1) Higher initial setup cost
2) Atmosphere surrounding the welding process has to be stable (hence the shielding gasses), therefore this process is limited to draught free conditions
3) Higher maintenance costs due to extra electronic components
4) The setting of plant variables requires a high skill level
5) Less efficient where high duty cycle requirements are necessary
6) Radiation effects are more severe

Tungsten Inert Gas (TIG)

Tungsten Inert Gas (TIG):

          Gas tungsten arc welding (GTAW), also known as tungsten inert gas (TIG) welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld. The weld area is protected from atmospheric contamination by a shielding gas (usually an inert gas such as argon), and a filler metal is normally used, though some welds, known as autogenous welds, do not require it. A constant-current welding power supply produces energy which is conducted across the arc through a column of highly ionized gas and metal vapors known as a plasma.

          GTAW is most commonly used to weld thin sections of stainless steel and non-ferrous metals such as aluminum, magnesium, and copper alloys. The process grants the operator greater control over the weld than competing processes such as shielded metal arc welding and gas metal arc welding, allowing for stronger, higher quality welds. However, GTAW is comparatively more complex and difficult to master, and furthermore, it is significantly slower than most other welding techniques. A related process, plasma arc welding, uses a slightly different welding torch to create a more focused welding arc and as a result is often automated.

OPERATION :

          Manual gas tungsten arc welding is often considered the most difficult of all the welding processes commonly used in industry. Because the welder must maintain a short arc length, great care and skill are required to prevent contact between the electrode and the workpiece. Similar to torch welding, GTAW normally requires two hands, since most applications require that the welder manually feed a filler metal into the weld area with one hand while manipulating the welding torch in the other. However, some welds combining thin materials (known as autogenous or fusion welds) can be accomplished without filler metal; most notably edge, corner, and butt joints.

          To strike the welding arc, a high frequency generator (similar to a Tesla coil) provides an electric spark; this spark is a conductive path for the welding current through the shielding gas and allows the arc to be initiated while the electrode and the workpiece are separated, typically about 1.5–3 mm (0.06–0.12 in) apart. This high voltage, high frequency burst can be damaging to some vehicle electrical systems and electronics, because induced voltages on vehicle wiring can also cause small conductive sparks in the vehicle wiring or within semiconductor packaging. Vehicle 12V power may conduct across these ionized paths, driven by the high-current 12V vehicle battery. These currents can be sufficiently destructive as to disable the vehicle; thus the warning to disconnect the vehicle battery power from both +12 and ground before using welding equipment on vehicles.

          An alternate way to initiate the arc is the "scratch start". Scratching the electrode against the work with the power on also serve to strike an arc, in the same way as SMAW ("stick") arc welding. However, scratch starting can cause contamination of the weld and electrode. Some GTAW equipment is capable of a mode called "touch start" or "lift arc"; here the equipment reduces the voltage on the electrode to only a few volts, with a current limit of one or two amps (well below the limit that causes metal to transfer and contamination of the weld or electrode). When the GTAW equipment detects that the electrode has left the surface and a spark is present, it immediately (within microseconds) increases power, converting the spark to a full arc.

Once the arc is struck, the welder moves the torch in a small circle to create a welding pool, the size of which depends on the size of the electrode and the amount of current. While maintaining a constant separation between the electrode and the workpiece, the operator then moves the torch back slightly and tilts it backward about 10–15 degrees from vertical. Filler metal is added manually to the front end of the weld pool as it is needed^.

          Welders often develop a technique of rapidly alternating between moving the torch forward (to advance the weld pool) and adding filler metal. The filler rod is withdrawn from the weld pool each time the electrode advances, but it is never removed from the gas shield to prevent oxidation of its surface and contamination of the weld. Filler rods composed of metals with low melting temperature, such as aluminum, require that the operator maintain some distance from the arc while staying inside the gas shield. If held too close to the arc, the filler rod can melt before it makes contact with the weld puddle. As the weld nears completion, the arc current is often gradually reduced to allow the weld crater to solidify and prevent the formation of crater cracks at the end of the weld.

Operation modes :

          GTAW can use a positive direct current, negative direct current or an alternating current, depending on the power supply set up. A negative direct current from the electrode causes a stream of electrons to collide with the surface, generating large amounts of heat at the weld region. This creates a deep, narrow weld. In the opposite process where the electrode is connected to the positive power supply terminal, electrons flow from the part being welded to the tip of the electrode instead, so the heating action of the electrons is mostly on the electrode. This mode also helps to remove oxide layers from the surface of the region to be welded, which is good for metals such as aluminum or magnesium. A shallow, wide weld is produced from this mode, with minimum heat input. Alternating current gives a combination of negative and positive modes, giving a cleaning effect and imparts a lot of heat as well.

ADVANTAGES:

1.No flux is used, hence there is no danger of flux entrapment when welding refrigerator and air conditioner components.
2.Because of clear visibility of the arc and the job, the operator can exercise a better control on the welding process.
3.This process can weld in all positions and produces smooth and sound welds with less spatter.
4.TIG welding is very much suitable for high quality welding of thin materials (as thin as 0.125 mm).
5.It is a very good process for welding nonferrous metals (aluminium etc.) and stainless steel. 

DISADVANTAGES:

1. Under similar applications, MIG welding is a much faster process as compared to TIG welding, since. TIG welding requires a separate filler rod.
2. Tungsten if it transfers to molten weld pool can contaminate the same. Tungsten inclusion is hard and brittle.
3. Filler rod end if it by chance comes out of the inert gas shield can cause weld metal contamination.
4. Equipment costs are higher than that for flux shielded metal arc welding.  

APPLICATIONS:

1. Welding aluminium, magnesium, copper, nickel and their alloys, carbon, alloy or stainless steels, inconel, high temperature and hard surfacing alloys like zirconium, titanium etc.
2. Welding sheet metal and thinner sections.
3. Welding of expansion bellows, transistor cases, instrument diaphragms, and can sealing joints.
4. Precision welding in atomic energy, aircraft, chemical and instrument industries.
5. Rocket motor chamber fabrications in launch vehicles.

Foundry Shop Hand Tools

FOUNDRY SHOP HAND TOOLS :

1.Showel: It consists of iron pan with a wooden handle. It can be used for mixing and conditioning the sand.

 

2. Trowels: These are used for finishing flat surfaces and comers inside a mould. Common shapes of trowels are shown as under. They are made of iron with a wooden handle.

 

3. Lifter: A lifter is a finishing tool used for repairing the mould and finishing the mould sand. Lifter is also used for removing loose sand from mould.

4. Hand riddle: It is used for ridding of sand to remove foreign material from it. It consists of a wooden frame fitted with a screen of standard wire mesh at the bottom.

5.Strike off bar: It is a flat bar, made of wood or iron to strike off the excess sand from the top of a box after ramming.

     Its one edge made beveled and the surface perfectly smooth and plane.

6. Vent wire: It is a thin steel rod or wire carrying a pointed edge at one end and a wooden handle or a bent loop at the other. After ramming and striking off the excess sand it is used to make small holes, called vents, in the sand mould to allow the exit of gases and steam during casting.

7. Rammers: Rammers are used for striking the sand mass in the moulding box to pack it closely around one pattern. Common types of rammers are shown as under.

8. Swab: It is a hemp fiber brush used for moistening the edges of sand mould, which are in contact with the pattern surface, before withdrawing the pattern. It is also used for coating the liquid blacking on the mould faces in dry sand moulds.

9. Sprue pin: It is a tapered rod of wood or iron, which is embedded in the sand and later withdrawn to produce a hole, called runner, through which the molten metal is poured into the mould.

10. Sprue cutter: It is also used for the same purpose as a sprue pin, but there is a marked difference between their use in that the cutter is used to produce the hole after ramming the mould. It is in the form of a tapered hollow tube, which is inserted in the sand to produce the hole.

Advantages and Disadvantages of die casting

Advantages of die casting are :

1) It requires less floor space as compared to other casting processes.

2) Rate of production is high. 75 to 150 casts per hour in cold chamber and 300 to 350 casts per hour in hot

chamber process.

3) Die casting dies retain their accuracy for a very long time.

4) Very thin sections can be cast and holes upto minimum of 1.6 mm diameter can be easily cored.

5) High surface finish is obtained and often no further finishing is required.

6) Cost per unit is minimum hence economical.

Disadvantages of die casting are :

1) All metals and alloys can not be cast.

2) The cost of machines, dies and other equipment used is high.

3) Not economical for small quantity production.

4) Heavy casting cannot be cast.

5) Special precautions are necessary for evacuation of air from die cavity, otherwise cause porosity.

Cupola Furnace zone

Cupola Furnace zone :

A number of chemical reaction take place in these zones which are explained below :

1. Well :

     It is the space between the bottom of the tuyeres and the sand bed. The metal, after melting, trickles down and collects in this space before it is tapped out.

2. Combustion zone :

     It is also known as oxidizing zone. It is located between the top of the tuyeres and a theoretical level above it. The total height of this zone is normally from 15 cm. To 30 cm. The actual combustion takes place in this zone, consuming all free oxygen from the air blast and producing a lot of heat, which is sufficient enough to meet the requirements of other zones of cupola. More heat is evolved due to oxidation of silicon and manganese. A temperature of about 15400C to 18700C is produced in this zone. The exothermic reactions taking place in this zone can be represented thus.

3. Reducing zone :

     It is also known as the protective zone. It is located between the top of the combustion zone and the top level of the coke bed. CO2 is reduced to CO in this zone through an endothermic reaction, as a result of which the temperature falls from combustion zone temperature to about 12000C at the top of this zone. The reaction is as follows : Nitrogen, the other main constituent of the upward moving hot gases does not participate in the reaction. This zone, on account of the reducing atmosphere in it, protects the charge against oxidation.

4. Melting zone :

     The first layer of metal charge above the coke bed constitutes this zone. The solid metal charge changes to molten state in this zone and trickles down through the coke to the well. The molten metal picks up sufficient carbon content in this zone as represented by the following reaction :

5. Preheating zone :

     It extends from above the melting zone to the bottom level of the charging door and contains a number of alternate layers of coke and metal charges. The function of this zone is to preheat the charges from atmospheric temperature to about 10930C before they settle downwards to enter the melting zone. This preheating takes place due to the upward advancing hot gases, from which the solid metal also picks up some sulfur content.

6. Stack zone :

     The empty portion of cupola above the preheating zone, which provides the passage to hot gases to go to atmosphere, is known as stack zone.

Pattern

Pattern :

     A pattern may be defined as a replica or facsimile model of the desired casting which, when packed or embedded in a suitable moulding material, produces a cavity called mould. This cavity, when filled with molten metal, produces the desired casting after solidification of the poured metal. Since it is a direct duplication, the

pattern very closely conforms to the shape and size of the desired casting, except for a few variations due to the

necessary allowances. The ways in which a pattern differs from an actual component are :

1. It carries an additional allowance to compensate for metal shrinkage.

2. It carries additional allowances over those portions, which are to be machined or finished otherwise.

Crucible furnaces

Crucible furnaces :

     These are the simplest of all the furnaces used in foundries. They are sparingly used in most of the small foundries where melting is not continuous and a large variety of metals is to be melted in small quantities. In these furnaces the entire melting of metal takes place inside a melting pot, called crucible, which is made of clay and graphite. The sizes of these crucibles vary from No.1 to No.400 each number representing a definite quantity  of metal that can be held conveniently by the crucible.

Pattern materials

Pattern materials :

The common materials of which the patterns are made are the following :
1) Wood :
     It is the most common material used for pattern making because of the following Advantages :
(i) It is cheap and available in abundance.
(ii) It can be easily shaped into different forms and intricate designs.
(iii) Its manipulation is easy because of lightness in weight.

(iv) Good surface finish can be easily obtained by only planning and sanding. 

(v) It can be preserved for a fairly long time by applying proper preservatives like shellac varnish.

On the other hand, it has certain disadvantages also as follows:
(i) It wears out quickly due to its low resistance to sand abrasion. As such, a wooden pattern cannot stand a long constant use.
(ii) It is very susceptible to moisture, which may lead to its warping or splitting. This needs its careful storing in a dry place and the application of preservatives.
(iii) Its life, owing to the above reasons, is short as compared to other pattern materials. This confines its use to such cases only when a small number of castings are required.

2) Metals :

     Metals are used with advantage, as pattern material, only when the number of castings to be made is very high and a closer dimensional accuracy is desired. They have a much longer life than wooden patterns and eliminate the inherent disadvantages of wood to a great extent.
But they also carry the following Disadvantages :
(i) They are costlier than wood and, therefore, cannot be used with advantage, where a smaller number of
castings is to be made.
(ii) For giving different shapes and fine surface finish they need machining. This again adds to their cost.
(iii) Most of them are very heavy and in case of large castings the weight of the pattern always poses a problem in its manipulation.
(iv) A large number of them have a tendency to get rusted.

3) Plaster :

     Plaster of Paris or gypsum cement is advantageously used as a pattern material since it can be easily casted into intricate shapes and can be easily worked also. Its expansion can be easily controlled and it carries a very high compression strength. Its specific use is in making small patterns and core boxes involving
intricate shapes and closer dimensional control. A marked feature of this cement is that contrary to the action of metals, it expands on being solidified. Thus, if a cement of proper coefficient of expansion is selected, the effect of shrinkage of casting can be automatically neutralized.

4) Plastics :

     Plastics are gradually gaining favor as pattern materials due to their following specific characteristics :
1. Lightness in weight.
2. High strength.
3. High resistance to wear.
4. High resistance to corrosion due to moisture.
5. Fine surface finish.
6. Low solid shrinkage.
7. Very reasonable cost.

     The plastics used as pattern materials are thermosetting resins. Phenolic resin plastic and foam plastic suit best for this purpose. For making the pattern, first the moulds are made, usually from plaster of Paris. The resin is then poured into these moulds and the two heated. At a specific temperature, the resin solidifies to give the plastic pattern.

5) Wax :

     Wax patterns are exclusively used in investment casting. For this a die or metal mould is made in two
halves into which the heated wax is poured. The die is kept cool by circulating water around it. As the wax
sets on cooling, the die parts are separated and the wax pattern taken out.

Sand Grain

Sand Grain :

    The shape and size of the sand grains has a remarkable effect on the physical properties of the foundry sand. The sand grains may have smooth, conchoidal or rough surfaces. Out of these the first type i.e., smooth, is preferred for moulding for the reason that such a surface renders higher permeability, sinter point and plasticity to the sand mass, but the percentage of binder required is also equally high.

     Similarly the sand grains may have different shapes. The commonly formed shapes are rounded, sub-angular, angular and compound. The rounded grains do not bind together two well when rammed and, hence, render the sand mould highly permeable but the strength of the mould is also reduced.

     Sub-angular grains give a relatively stronger bond than above but the permeability is reduced. Angular or
sharp grains produce a much stronger bond and a low permeability when rammed. Thus they enable a mould of greater strength. Sand grains which are cemented together such that they do not separate when screened are called compound. They may consist of one, two or a combination of all the above three shapes. They are not much preferred.

     Like the shape the size of sand grains also effects the mould structure and its characteristics. Large, regular and uniform grains increase permeability. Smaller grains increase smoothness on mould surfaces.

Core prints

Core prints :

     When a casting is required to have a hole, through or blind, a core is used in the mould to produce the same.
This core has to be properly seated in the mould on formed impressions in the sand. To form these impressions, extra projections are added on the pattern surface at proper places. These projections are known as core prints.

Master Pattern

Master Pattern :

  Master patterns are used for preparing the mouldsfor metal castings which are later used as patterns for further moulding work, called metal patterns. The masterpatterns are accurately finished wooden patterns, whichcarry double shrinkage allowance and the required machiningallowance. For example, an alluminium pattern is to bemade which is to be used further for making moulds forbrass castings. The alluminium pattern should, obviously,be larger than the desired brass casting by an amount equalto shrinkage that will take place during solidification ofthis casting. For making this alluminium pattern a woodenpattern is to be used which should be larger than thealluminium pattern by an amount equal to the alluminiumshrinkage, added with proper machining allowance forfinishing the alluminium casting. Mathematically, it can be represented thus :
Let Sb represent the size of the desired casting in brass.

And  Let Sa represent the size of aluminum pattern.
And  Let Cb represent the contraction allowance for brass.
Then  Sa=Sb+Cb

Again, let S represent the size of the master pattern.

And let Ca represent the contraction allowance for aluminum.

Also let Am represent the machining allowance required
to finish the aluminum casting to the required size of
pattern and to give smooth surface finish.

Then S = Sa+Ca+Am = Sb+Cb+Ca+Am 

or
Size of master pattern = Size of the final casting to be Made + shrinkage allowance for the material of final casting + shrinkage allowance of the metal of which the pattern is to be made + Finishing allowance for the metal pattern.

Shell Moulding Process

Shell Moulding Process :

Introduction: 

     A process that can be fully automated, shell molding is the most rapidly used technique for mold and core making. Also known a croning process, this casting technique was invented and patented by J.Croning during World War II. Also know as the “C” process, shell molding technique is used for making thin sections and for acquiring surface finish and dimensional accuracy.

Process:

     In the first stage of shell molding, a metal pattern is made which is resistant to high temperature and can withstand abrasion due to contact with sand. The sand and resin mixture for the shell mold is brought in contact with the pattern. The mold is placed in an oven where the resin is cured. This process causes the formation of a thin shell around the pattern. The thickness of the mold can be 10-20mm as compared to the heavy mold made for sand castings. When fully cured the skin is removed from the pattern, which is the shell mold.

      For each shell molds there are two halves know as the cope and drag section. The two sections are joined by resin to form a complete shell mold. If an interior desing is required, the cores are placed inside the mold before sealing the two parts.

For heavy castings, shell mold are held together by metals or other materials. Now, the molten metal is poured into the mold, and once it solidifies, the shell is broken to remove the casting.This process is highly useful for near net shape castings. Another advantage is that shell molding can be automated.

Automated Shell Molding Machines :

     Shell molding machines like the cold shell molding machines helps in making castings with little molding material. In a cold shell molding machine the molds are made using cold binding materials. In it patterns made of wood, metal or plaster can be used. And the greatest benefit is that the mold can be kept horizontally or vertically.

Robotizing:

     Using robots for shell molding is a milestone for the old molding technology. Robots which are multifunctional and re programmable are used in some foundries. Robots are used for a number of activities like robotic gate and sprue removal, robotic cutting of “wedges” for gate valves, robotic core setting, etc. The robots are reliable, consistent, more productive, provides better surface finish, and less machining etc.

Advantage:

     A sizable amount of the casting in the steel industry are made by shell molding process, that ensures better profitability. Carbon steel, alloy steel, stainless steel, low alloys, aluminum alloys, copper, are all cast using shell molding process. Casting that require thin section and excellent dimensional accuracy are cast using this process. Body panes, truck hoods, small size boats, bath tubs, shells of drums, connecting rods, gear housings, lever arms, etc. are cast using croning process.

Advantage:

• Thin sections, complex parts and intricate designs can be cast
• Excellent surface finish and goal size tolerances
• Less machining required for the castings
• Near net shape castings, almost 'as cast' quality
• Simplified process that can be handled by semi skilled operators
• Full mechanized and automated casting process
• Less foundry space required.