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UNIT 15: Plastics Manufacturing Engineering Unit 15 Copyright © 2012 MDIS. All rights reserved. 1
Structure of Polymers if a basic group of elements is repeated in the formation of a molecule, what we get is a polymer. polymers have a very high molecular weight. In case of polymers, we can safely say that higher this molecular weight, greater the length of the polymer molecule and hence the repetitive units. The basic unit of the organic polymers is the H-C-H backbone. if two of the opposing Hydrogen atoms are replaced by the H-C-H, then we have the possibility of a long polymeric chain based on this H-C-H backbone. 5-2Unit 15 Copyright © 2012 MDIS. All rights reserved.
Basic Hydro-Carbons - metane, ethane, propane, butane, pentane, hexane, heptane etc. they all are based on the CnH2n+2 chemistry As the molecule increases in size, so does its characteristics like strength, flow ability, density and crystalline. Then again there are a group of organic compounds that are ring shaped in nature called the ring structures, examples are benzene, styrene and derivatives of benzene They are the Aromatic compounds 5-3Unit 15 Copyright © 2012 MDIS. All rights reserved.
Types of polymers Linear- when the entire chain of the polymer is straight it is called a liner polymer. Branched- Some polymers have outgrowth from the back bone structures at various points. Cross-linked – When several linear polymers get interlinked at various positions in the chain, they are called cross linked polymers. Network polymers- a complex interconnected group of polymers is called a network polymer. Co-polymers- If the repeating units of the polymer chain are all same, it becomes a homo polymer. Co-polymers contain two or more groups of such polymers. 5-4Unit 15 Copyright © 2012 MDIS. All rights reserved.
Thermosetting and Thermoplastic polymers Thermoplastic polymers are those grades which can be repeatedly heated and reshaped time and again. They are good for recycling and with each recycle operation, their molecular weight drops as the chain lengths reduce. So does their strength and flexibility. Examples of thermoplastic are: Polypropylene, Polythene, Acrylic, Nylons, PVC etc. These plastics are used in applications from domestic products like buckets and containers to some interesting machine parts like gears, and complex sub assembly structural parts. 5-5Unit 15 Copyright © 2012 MDIS. All rights reserved.
Thermosets – are those polymers which once heated cannot be reshaped and then get set in this shape permanently. If reheated again, they do not soften, and instead get charred and blacken. Examples of common thermosets are: Phenol- formaldehyde, urea formaldehyde, urethanes, epoxies, Silicon etc. 5-6Unit 15 Copyright © 2012 MDIS. All rights reserved.
Additives used in plastics extra elements introduced to polymers for enhanced properties. Plasticizers- they provide the flexibility necessary for the processing of polymers. UV blocking agents- All polymers are affected to some extent by exposure to Ultraviolet ranges of light. It causes the polymer chains to break, reducing their service life and strength drastically to the extent of failure by cracking, discoloration etc. These chemicals are introduced into the plastic chains to absorb the UV lights energy and prevent damage to the polymer chains themselves. Carbon black is the most commob uv stabilizer. 5-7Unit 15 Copyright © 2012 MDIS. All rights reserved.
Fillers- In order to improve the bulk of the polymer materials like talcum are added. They do not adversely affect its own property, but lend to an increase in the materials bulk and slight improvement in some areas of performance. Colorants- Polymers on their own have a very tasteless color range, and need to be provided with colorants to increase the range and vibrancy of the products manufactured. Colorants are basically pigments and dyes. Flame retardants- Polymers being made from highly inflamable raw amterials are themselves also highly flamable. To reduce their ability to catch fire and sustain the flames, we add flame retardants to the polymer. 5-8Unit 15 Copyright © 2012 MDIS. All rights reserved.
Typical flame retardands are chlorine compounds and similar chemicals from the same group. Lubricants- Lubricants are materials added to reduce the wear of the processing machinery used in their manufacture. They include simple compounds like wax, camphor etc. 5-9Unit 15 Copyright © 2012 MDIS. All rights reserved.
Lost foam casting (Also called evaporative pattern casting) pattern is made from an embedded pattern material that will eventually evaporate leaving no residue. Raw expandable styrene (EPS) beads are placed in a preheated die to take the shape of the die. As the beads fuse together, the die is cooled and the pattern removed, Complex patterns may be made in this way. This pattern is coated with a refractory slurry, and then placed in a flask with dry fine sand. The sand is compacted and then molten metal poured in to the cavity. As molten high temperature materials enters the cavity, the EPS vaporizes and leaves the cavity in its place into which the metal flows, thus creating a replica of the pattern. The process is simple, and there are no parting lines giving design flexibility. The pattern although lost is itself inexpensive. The casting requires minimal finishing and cleaning. And the process can be automated. 5-10Unit 15 Copyright © 2012 MDIS. All rights reserved.
The process is simple, and there are no parting lines giving design flexibility. The pattern although lost is itself inexpensive. The casting requires minimal finishing and cleaning Investment casting pattern made of a lasting material is created by a lost wax method. This pattern is then used in the process of casting and is thus called the investment casting method. Investment here refers to the surrounding of the pattern being surrounded with the refractory materials. 5-11Unit 15 Copyright © 2012 MDIS. All rights reserved.
The mold made around the wax is then held at elevated temperatures for few hours and inverted. This causes the wax to melt out. The hollow cast then is used to pour metal for the final casting process. Vacuum Casting This is a counter-gravity process. Molten metal is sucked by the vacuum pressures in the mold cavity to fill up against gravity. Resulting parts can be intricate and complex, with little chance of cavities. 5-12Unit 15 Copyright © 2012 MDIS. All rights reserved.
Mixture of fine sand ahd urethane is molded over metal dies and cured with amine vapor. It is then immersed into molten metal in an induction furnace. Gas turbine parts and thin walled components of 0.5mm are made using this process. Slush Casting As molten metal comes in contact with lower temperature form a thin skin like structure. molten metal is poured into a metallic mold. desired skin thickness is achieved, the mold is inverted to remove the excess metal, still in molten state. Ornaments, toys and low melting point metals can be used in this process. 5-13Unit 15 Copyright © 2012 MDIS. All rights reserved.
Control of Media Text - Tutorial Pressure Casting As in the vacuum casting, using pressure to pack molten metal into a cavity results in the same ability of intricate and complex mold Molten metal is force upwards by gas pressure into a graphite or metal mold. This method is used in precision and high-quality parts. Die Casting A die similar to the injection molding die in plastics is created and molten metal is forced into the die by hydralulic pressure on an injecting plunger. 5-14Unit 15 Copyright © 2012 MDIS. All rights reserved.
The arrangement can be horrizontal or vertical. High melting point alloys are used in this process. In the hot chamber process, the plunger pushes molten metal retained in a hot chamber such as a furnace through a goose neck path into the cavity. In the cold chamber process the metal is poured into the injecting chamber which is not heated hence this name. Centrifugal Casting the parts to be cast are normally revolved sections like pipes, bearing housings etc. Molten metal is poured into a rotating cavity with a horizontal axis of rotation. Vertical axis is not normal but can be used for short castings. Unit 15 Copyright © 2012 MDIS. All rights reserved. 5-15
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