Polymers Classifications of Polymers Polymer low density good

Polymers Classifications of Polymers Polymer – low density, good thermal & electrical insulation, high resistance to most chemicals and ability to take colours and opacities. But unreinforced bulk polymer are mechanically weaker, lower elastic moduli & high thermal expansion coefficients. Improvement Reinforced variety of fibrous materials Composites (PMC).

Plastics are synthetic chemical compounds Which are polymers and which are at some stage plastic and could be shaped by heat, with or without pressure into useful products.

Polymerization is an important reaction in plastic technology. A basic molecule is called a monomer. The combining of many monomers are to make macromolecules or polymers is known as polymerization.

Advantages : ease of manufacturing & versatility. Can manufacture into complicated shapes in one step with little need for further processing or surface treatment. Versatility : ability to produce accurate component, with excellent surface finish and attractive color, at low cost and high speed Application: automotive, electrical & electronic products, household appliance, toys, container, packaging, textiles Basic manufacturing processes for polymer parts are extrusion, molding, casting and forming of sheet.

Thermoset & thermoplastic Differ in the degree of their inter-molecular bonding Thermoplastic-litle cross bonding between polymer, soften when heated & harden when cooled Thermoset-strong intermolecular bonding which prevents fully cured materials from softening when heated Rubber are similar to plastic in structure and the difference is largely based on the degree of extensibility or stretching.

Properties of plastics Light in weight Resistant to most chemicals, corrosion resistant Electrical insulators Easy to shape into finished parts Adequately strong for many applications Sufficiently hard and in some cases flexible

Disadvantages Costs of materials are comparatively higher Most plastics cannot withstand even moderately high temperatures Thermosetting plastics will have practically no elastic or plastic deformation before rupturing, and thus are somewhat brittle.

Applications Plastics make cheap bearings that can easily replace wood, steel and zinc bearings. Plastic as bearing material is favoured in chemically aggressive environments, without the use of any lubricant and needs no maintenance. Plastics suitable for bearing applications are phenolic, nylon, flurocarbon, polycarbonatem acetal, and ultrahigh molecular weight ployethylene.

Gears in plastic-A plastic gear is made by moulding process. Fasteners in plastic- Screw assemblies, snap-and press-fits, inserts, ties and twists, and shrink wraps.

Rubber parts Rubbers can be classified into hard rubber and thermoplastic rubber. Rubber parts are formed to the desired shape mainly by moulding and extruding. Popular rubber products

Popular rubber products Rubber is combined with cord or fabric to form composites which possess both flexibility and high strength. Ex. , V belts, Hose and tyres Rubber products prepared by latex dipping are hollow and have extremely thin wall. Latex-dipped products are in the shape of bellows and similar designs.

Gaskets are frequently prepared by die-cutting rubber sheets, while rubber tubes may be cut on mandrel in a lathe to form washers. Rubber washers lend shock absorbing property to fasteners. ‘O’ rings and metal insert seals with lip contact are moulded from a variety of rubber compounds to provide resistance to leakage of fluids. Polyurethane parts manufactured by RIM (reaction injection moulding) have a dense skin and a porous core and are usually much stiffer than conventional rubber parts.

Some of the important applications of rubber in Mechanical design Water lubricated rubber bush bearings-these busings in rubber are press fitted in metallic housing. They have grooves parallel to their axis for retaining water which acts as lubricant. These bearings can tolerate and lubricate shafts when water contains sand particles and other materials.

Flexible mechanical couplings for transmission of power from one shaft to another with a slight permissible angle of twist utilise a rubber cross for connecting driving jaw with a driving jaw. Rubber pads for absorbing high frequency vibrations are widely used in foundations.

Design consideration for polymer Structural part/When the parts is to carry load Should remember the strength and stiffness of plastics vary with temperature. Troom data cannot be used in design calculation if the part will be used at other temp. Long term properties cannot be predicted from short term prop. Eg. Creep behavior Engineering plastics are britle (notched impact strength < 5. 4 J/cm) Avoid stress raiser

Design for ceramics Classification of Ceramic Materials Ceramics – inorganic compounds of one or more metals with a nonmetallic element. Eg Al 2 O 3, Si. C, Si 2 N 3. Crystal structure of ceramic are complex They accommodate more than one element of widely different atomic size. The interatomic forces generally alternate between ionic & covalent which leave few free electrons usually heat & electrical insulators. Strong ionic & covalent bonds give high hardness, stiffness & stability (thermal & hostile env. ).

Characteristics: Hard & brittleness, low mechanical & thermal shock High melting points Thermal conductivities between metal & polymer

Design consideration for ceramics Britle, low mechanical & thermal shock-need special consideration Ratio between tensile strength, modulus of rupture & compressive strength ~ 1: 2: 10. In design, load ceramic parts in compression & avoid tensile loading Sensitive to stress concentration Avoid stress raiser during design. Dimensional change take place during drying and firing, should be consider Large flat surface can cause wrapping Large changes in thickness of product can lead to nonuniform drying and cracking. Dimensional tolerances should be generous to avoid machining