Contents: 1.Introduction to polymer 2.Historical Development 3.Polymer Casting and various processes 4.Brief introduction to : Thermoforming Compression molding Rotational Molding
Polymer Polymer: Large number of molecules joining together repeatedly in the form of long and flexible chains. Types of Polymers: 1.Thermoplastics : These can be melted by heating, solidified by cooling and re-melted repeatedly. 2.Thermosets: Hardened by application of heat and pressure. These cannot be softened by heating for reprocessing or cannot re-melted. Because due to cross-linking of structure, these create permanent 3-D network.
Historical Development In 1811, Henri Braconnot did pioneering work in derivative cellulose compounds which was the earliest important work in polymer science. In 1907, Leo Baekeland created the first synthetic polymer, Bakelite, by reacting phenol and formaldehyde. Bakelite was then publicly introduced in 1909.
Polymer Casting: Operations carried out on polymeric materials to increase their utility. The conversion of polymeric raw materials into useful finished products. Processes of polymer casting: 1. Thermoforming 2. Compression and transfer molding 3. Rotational molding and sintering 4. Extrusion 5. Extrusion-based processes 6. Injection molding 7. Blow molding 8. Plastic foam molding
Selection of a process depends upon: - Quantity and production rate. - Dimensional accuracy and surface finish. - Form and detail of the product. - Nature of material. - Size of final product.
Phases of polymer processing: 1. Heating - To soften or melt the plastic. 2. Shaping / Forming - Under constraint of some kind like application of specific pressure etc. 3. Cooling - So that it retains its shape.
Thermo plastics start as regular pellets or sheet and can be re-melted. Thermosets start as liquids often called "resins", or powders which need heat for the shaping phase. For shaping a chemical reaction is required, which means that the material does not soften on reheating.
Thermoforming Only thermoplastics sheet can be processed by this method. This sheet is heated so that it can soften. This softened sheet is placed in the mould and pressure is applied to form it into the shape of mould. Fig:
Applications: For food packaging industries. Other industries like toiletries, pharmaceuticals and electronics. Fig:
Compression and transfer molding Fig:
Place a pre-weighed or fixed amount of material in a matched metal mold and closing the mold. Heat and pressure cause the material to liquify and flow into the voids in the tool where it chemically reacts and hardens into the final shape.
Rotational molding : Fig:
Application: This process is used to produce hollow complex shapes. Charge of plastic powder is placed in one half of a metal mold. The mold halves are then clamped together and heated while the mold rotates. The powder particles melt and form a homogenous layer on the surface of the mold during rotation. The mold is then cooled to solidify.