The product made through forging has more strength and is made more precise, compared to products made by other manufacturing processes. Unfortunately even forged products can have some defects if proper care is not taken during the forging process. This article describes forging defects that normally occur on a forged part. Improper Grain Flow: This is caused by improper design of the die. The improper die design makes the flow of the metal in the wrong direction. Cold Shut: This appears as small cracks in the forged part’s corners. This happens mainly due to the improper design of the die uses. Scale Pits: Scale Pits are irregular lining on the surface of the forged part. This is mainly caused due to improper cleaning of the stock used. Die Shift: This is primarily caused by the wrong alignment of the die halve, making the two halves of the part to be improperly shaped. Flakes: Flakes are internal ruptures, caused by the improper cooling of the forged part. Rapid cooling to the part caused the exterior to cool too fast, causing internal fractures. Unfilled Section: Some section of the die cavity is not complete filled by the flowing metal. This is caused by the improper design of the forging die, or wrong use of forging technique.
Forging and its types Forging is the operation where the metal is heated and then a force is applied to manipulates the metals in such a way that the required final shape is obtained. Forging is generally a hot working process through cold forging is used us ed sometimes.
1. 2. 3. 4.
Type of forging smith forging drop forging press forging machine forging
1: Smith forging This is the traditional forging operation done openly or in-open dies by the village black smith or modern shop floor by manual hammering or by the power hammer. The process involves heating the stock in the black smith hearth and then beating it over the anvil. To get the desire shape the operator has to manipulate the component in between the blows. The types of operation available are fullering, flattening, bending , upsetting and swaging.
2: Drop forging Basic definition: This is the operation done in closed impression dies by means drop hammer here the force for shaping the component is applied in a series of blows. Drop forging utilizes a closed impression die to obtain the desire shape of the component , the shaping is done by the repeated hammering given to the material in the die cavity. The equipment use for delivering for blows are called drop hammers. The drop forging die consists of two halves. The lower halve of the die is fixed to the anvil of the machine , while the upper halve is fixed to ram. The heated stock is kept in the lower die, while the ram delivers 4-5 blows on the metal spreads and completely fills in the die cavity. When the two die of halves closed the complete is formed. The typical products obtained in drop forging are cranks, crank shaft, connecting rods, wrench, crane hooks etc. The types of operations are fullering, edging, bending, blocking , finishing and trimming etc.
3: Press forging Similar to the drop forging , the press forging is also done in closed impression dies with the expectation that the force is continuous squeezing type applied by the hydraulic press. Press forging dies are similar to drop forging dies as also the process in press forging, the metal is shaped not by means of a series of blows as in drop forging , but by means of a single continuous squeezing action. This squeezing is obtained by means of hydraulic presses. Because of the continuous action of by hydraulic presses, the material gets uniformly deform through out its entire depth ,the press forging dies with the various impression , such as fuller, bender and finisher impression properly arranged .
4: Machine forging:
Unlike the press or drop forging where the material is drawn out , in machine forging the material is only upset to get the desire shape. As it involves the upsetting operation some time it is simply called as upset forging. Originally this was develop for making bolts head in a continuous fashion, but now there are fairly large number of diverse. Uses of this process: Because of the beneficial grain flow obtain from upsetting. It is used for making gears, blanks, shafts, excels, and similar parts. Upsetting machine called up setter are generally horizontal acting. The die set consists of die and corresponding punch or a heading tool. The die consists of two parts, one called the stationary gripper die which is fixed to the machine frame and the other movable gripper die which moves along with the die slide of the up setter. The stock is held then between these two gripper dies. The upset forging cycle start with the, movable die sliding against the stationary die to grip the stock. The two dies when in closed position from the necessary die cavity then the heading tool advance against the stock and upset it to completely filled to the die cavity. Having completed the upsetting the heading tool moves back to its back position. Then the movable gripper die releases the stock by sliding backward. Similar to drop forging it is not possible to get the final shape in a single pass in machine forging also. Therefore the operation is carried out in number of stages. The die cavities is required for the various operations are all arrange vertically on the gripper dies. The stock is the move from stage one to another in proper sequence till the final forging is ready. A heading tool each for every upsetting stage is arranged on the heading slide of the upsetting machine. A typical upsetting die and heading tool is shown:
Forging defects: Though forging process give generally prior quality product compared other manufacturing processes. There are some defects that are lightly to come a proper care is not taken in forging process design.
A brief description of such defects and their remedial method is given below.
(A): Unfilled Section: In this some section of the die cavity is not completely filled by the flowing metal. The causes of this defects are improper design of the forging die or using forging techniques.
(B): Cold Shut: This appears as a small cracks at the corners of the forging. This is caused mainly by the improper design of die. Where in the corner and the fillet radius are small as a result of which metal does not flow properly into the corner and the ends up as a cold shut.
(C): Scale Pits: This is seen as irregular depurations on the surface of the forging. This is primarily caused because of improper cleaning of the stock used for forging. The oxide and scale gets embedded into the finish forging surface. When the forging is cleaned by pickling, these are seen as depurations on the forging surface.
(D): Die Shift: This is caused by the mis- alignment of the die halve, making the two halve of the forging to be improper shape.
(E): Flakes: These are basically internal ruptures caused by the improper cooling of the large forging. Rapid cooling causes the exterior to cool quickly causing internal fractures. This can be remedied by following proper cooling practices.
(F): Improper Grain Flow: This is caused by the improper design of the die, which makes the flow of the metal not flowing the final interred direction.
Types of Plastics Plastics can be divided into two major categories: 1. Thermoset or thermosetting plastics. Once cooled and hardened, these plastics retain their shapes and cannot return to their original form. They are hard and durable. Thermosets can be used for auto parts, aircraft parts and tires. Examples include polyurethanes, polyesters, epoxy resins and phenolic resins.
2. Thermoplastics. Less rigid than thermosets, thermoplastics can soften upon heating and return to their original form. They are easily molded and extruded into films, fibers and packaging. Examples include polyethylene (PE), polypropylene (PP) and polyvinyl chloride (PVC). Let's look at some common plastics. Polyethylene terephthalate (PET or PETE): John Rex Whinfield invented a new polymer in 1941 when he condensed ethylene glycol with terephthalic acid. The condensate was polyethylene terephthalate (PET or PETE). PET is a thermoplastic that can be drawn into fibers (like Dacron) and films (like Mylar). It's the main plastic in ziplock food storage bags. Polystyrene (Styrofoam): Polystyrene is formed by styrene molecules. The double bond between the CH2 and CH parts of the molecule rearranges to form a bond with adjacent styrene molecules, thereby producing polystyrene. It can form a hard impact-resistant plastic for furniture, cabinets (for computer monitors and TVs), glasses and utensils. When polystyrene is heated and air blown through the mixture, it forms Styrofoam. Styrofoam is lightweight, moldable and an excellent insulator. Polyvinyl Chloride (PVC): PVC is a thermoplastic that is formed when vinyl chloride (CH2=CH-Cl) polymerizes. When made, it's brittle, so manufacturers add a plasticizer liquid to make it soft and moldable. PVC is commonly used for pipes and plumbing because it's durable, can't be corroded and is cheaper than metal pipes. Over long periods of time, however, the plasticizer may leach out of it, rendering it brittle and breakable. Polytetrafluoroethylene (Teflon): Teflon was made in 1938 by DuPont. It's created by polymerization of tetrafluoroethylene molecules (CF2=CF2). The polymer is stable, heatresistant, strong, resistant to many chemicals and has a nearly frictionless surface. Teflon is used in plumbing tape, cookware, tubing, waterproof coatings, films and bearings. Polyvinylidine Chloride (Saran): Dow makes Saran resins, which are synthesized by polymerization of vinylidine chloride molecules (CH2=CCl2). The polymer can be drawn into films and wraps that are impermeable to food odors. Saran wrap is a popular plastic for packaging foods. Polyethylene, LDPE and HDPE: The most common polymer in plastics is polyethylene, which is made from ethylene monomers (CH2=CH2). The first polyethylene was made in 1934. Today, we call it low-density polyethylene (LDPE) because it will float in a mixture of alcohol and water. In LDPE, the polymer strands are entangled and loosely organized, so it's soft and flexible. It was first used to insulate electrical wires, but today it's used in films, wraps, bottles, disposable gloves and garbage bags.
In the 1950s, Karl Ziegler polymerized ethylene in the presence of various metals. The resulting polyethylene polymer was composed of mostly linear polymers. This linear form produced tighter, denser, more organized structures and is now called high-density polyethylene (HDPE).
HDPE is a harder plastic with a higher melting point than LDPE, and it sinks in an alcohol-water mixture. HDPE was first introduced in the hula hoop, but today it's mostly used in containers. Polypropylene (PP): In 1953, Karl Ziegler and Giulio Natta, working independently, prepared polypropylene from propylene monomers (CH2=CHCH3) and received the Nobel Prize in Chemistry in 1963. The various forms of polypropylene have different melting points and hardnesses. Polypropylene is used in car trim, battery cases, bottles, tubes, filaments and bags.
Now that we have discussed the various types of plastics, let's look at how plastics are made
Plastics are everywhere. While you're reading this article, there are probably numerous plastic items within your reach (your computer, your pen, your phone). A plastic is any material that can be shaped or molded into any form -- some are naturally occurring, but most are man-made. Plastics are made from oil. Oil is a carbon-rich raw material, and plastics are large carboncontaining compounds. They're large molecules called polymers, which are composed of repeating units of shorter carbon-containing compounds called monomers. Chemists combine various types of monomers in many different arrangements to make an almost infinite variety of plastics with different chemical properties. Most plastic is chemically inert and will not react chemically with other substances -- you can store alcohol, soap, water, acid or gasoline in a plastic container without dissolving the container itself. Plastic can be molded into an almost infinite variety of shapes, so you can find it in toys, cups, bottles, utensils, wiring, cars, even in bubble gum. Plastics have revolutionized the world. Because plastic doesn't react chemically with most other substances, it doesn't decay. Therefore, plastic disposal poses a difficult and significant environmental problem. Plastic hangs around in the environment for centuries, so recycling is the best method of disposal. However, new technologies are being developed to make plastic from biological substances like corn oil. These types of plastics would be biodegradable and better for the environment. In this article, we'll examine the chemistry of plastic, how it's made, how it's used, and how it's disposed of and recycled. We'll also look at some new biologically based plastics and their role in the future of plastic
Condensation and Addition Reactions There are a few ways that monomers combine to form the polymers of plastics. One method is a type of chemical reaction called a condensation reaction. In a condensation reaction, two molecules combine with the loss of a smaller molecule, usually water, an alcohol or an acid. To understand condensation reactions, let's look at another hypothetical polymer reaction. Monomers 1 and 2 both have hydrogen (H) and hydroxyl groups (OH) attached to them. When they come together with an appropriate catalyst (an atom or a molecule that speeds up the chemical reaction without being used up in it), one monomer loses a hydrogen while the other loses a hydroxyl group. The hydrogen and hydroxyl groups combine to form water (H2O), and the remaining electrons form a covalent chemical bond between the monomers. The resulting compound is the basic subunit of copolymers 1 and 2. This reaction occurs over and over again until you get a long chain of copolymers 1 and 2. Another way that monomers can combine to form polymers is through addition reactions. Addition reactions involve rearranging electrons of the double bonds within a monomer to form single bonds with other molecules. Imagine that two people (each a monomer) stand close together and each person has his/her arms folded (double bond). Then they unfold their arms and hold hands (single bond). The two people now make a polymer, and the process can be repeated.
Various polymer chains can interact and cross-link by forming strong or weak bonds between monomers on different polymer chains. This interaction between polymer chains contributes to the properties of specific plastics (soft/hard, stretchy/rigid, clear/opaque, chemically inert).
Making Plastics To make plastics, chemists and chemical engineers must do the following on an industrial scale: 1. 2. 3. 4.
Prepare raw materials and monomers Carry out polymerization reactions Process the polymers into final polymer resins Produce finished products
First, they must start with various raw materials that make up the monomers. Ethylene and propylene, for example, come from crude oil, which contains the hydrocarbons that make up the monomers. The hydrocarbon raw materials are obtained from the "cracking process" used in refining oil and natural gas (see How Oil Refining Works). Once various hydrocarbons are obtained from cracking, they are chemically processed to make hydrocarbon monomers and other carbon monomers (like styrene, vinyl chloride, acrylonitrile) used in plastics.
Next, the monomers carry out polymerization reactions in large polymerization plants. The reactions produce polymer resins, which are collected and further processed. Processing can include the addition of plasticizers, dyes and flame-retardant chemicals. The final polymer resins are usually in the forms of pellets or beads. Finally, the polymer resins are processed into final plastic products. Generally, they are heated, molded and allowed to cool. There are several processes involved in this stage, depending upon the type of product. Extrusion: Pellets are heated and mechanically mixed in a long chamber, forced through a small opening and cooled with air or water. This method is used to make plastic films. Injection molding: The resin pellets are heated and mechanically mixed in a chamber and then forced under high pressure into a cooled mold. This process is used for containers like butter and yogurt tubs. (Custompart.net has a great lesson on injection molding.) Blow molding: This technique is used in conjunction with extrusion or injection molding. The resin pellets are heated and compressed into a liquid tube, like toothpaste. The resin goes into the chilled mold, and compressed air gets blown into the resin tube. The air expands the resin against the walls of the mold. This process is used to make plastic bottles. Rotational molding: The resin pellets are heated and cooled in a mold that can be rotated in three dimensions. The rotation evenly distributes the plastic along the walls of the mold. This technique is used to make large, hollow plastic items (toys, furniture, sporting equipment, septic tanks, garbage cans and kayaks).
On the next page we'll learn about new innovations in plastics and how they're recycled.
Chemistry of Plastics All plastics are polymers, but not all polymers are plastics. Some familiar nonplastic polymers include starches (polymers of sugars), proteins (polymers of amino acids) and DNA (polymers of nucleotides -- see How DNA Works). The simplified diagram below shows the relationship between monomers and polymers. Identical monomers can combine with each other to form homopolymers, which can be straight or branched chains. Different monomers may combine together to form copolymers, which also may be branched or straight. The chemical properties of a polymer depend on:
The type of monomer or monomers that make up the polymer. The chemical properties of homopolymer 1 are different from those of homopolymer 2 or the copolymers. The arrangement of monomers within the polymer. The chemical properties of the straight polymers are different from those of the branched polymers.
The monomers that are found in many plastics include organic compounds like ethylene, propylene, styrene, phenol, formaldehyde, ethylene glycol, vinyl chloride and acetonitrile (we'll examine many of these as we discuss various plastics). Because there are so many different monomers that can combine in many different ways, we can make many kinds of plastics.