Plastics – a large group of synthetic materials which are made from a number of common substances such as coal, salt, oil. Natural gas, cotton, wood and water. From these, relatively simple chemicals known as monomers, which are capable of reacting with one another are produced. These are then built up into chainlike molecules of high molecular weight called polymers.
Two General Classification:
1. Thermoplastics – become soft when heated and hard when cooled, regardless of the number of times the process is repeated. Include in the thermoplastics are acrylic cellulosis, polyethylene, polyvinyl chloride (PVC) polysterene, polyallomers polycarbonates, polyimide, polypropylene, polysulfone, phenylene oxide, nylons, methyl pertenes, ionomer, fluoroplastics, acetal and acryonitrile butadieniene styrene (ABS).
2. Thermosetting Plastics – set into a permanent shape when heat and pressure are applied to them during the forming stage. Thermosetting group includes phenolics, aminos (urea and melamine) epoxies, polyesters, polyurethane, alkyd silicones and diallyl phthalate (DAP).
Production: Plastics products are formed by a number of methods which include:
1. Injection Molding Process – measured amount of powder or granules is heated and when flowing forced through the nozzle of the barrel into a shaped cavity, where it cools of solidities.
2. Blow-Molding Process – an extruder extrudes a hollow tube which is captured between the two halves of a hollow. As the mold closes, air is blown into the tube and expand it to fit inside surface of the mold.
3. Rotational Molding – used to form hollow units with complex shapes and heavy walls, a premeasured amount of powder or liquid resin is placed at the bottom half of a cold mold which is then closed.
4. Expandable Bead Molding – a process used to produce light weight products of polysterene foam, small granules of polysterene with a small amount of an expanding agent are placed in a rolling drum and steam heated.
5. Compression Molding – a measured quantity of powder in a heated mold, which is then closed. Heat and pressure are applied to the powder which melts and flows to all parts of the mold.
6. Transfer Molding – similar to compression molding except that the powder is heated and liquefied outside the mold and injected into the mold under heat and pressure, where the forming and setting takes place.
7. Foamed Plastics – are made by expanding agent with either granules or powder and then heating. Heat melts the plastic and causes the formation of a gas which expands the molten material into foamed structure.
8. Thermoset Foam – made by mixing the appropriate resin with a curing agent and an expanding agent and then heating them in a mold.
9. Extrusion Forming – used for mass-produced materials which have a constant cross section, and it is done in two ways by forcing of semi-liquid plastic through a die of the proper size and shape in a manner similar to that used for forming brick by extrusion.
10. Thermoforming Process – sheet plastic is heated until soft and then forced by air pressure against a cold and hardens in shape.
11. Laminating Process – consists of impregnating sheets of paper, glass fiber, or cloth with a thermosetting liquid resin and then applying heat and pressure to a number of sheets to form a laminated product.
12. Casting – a simple process in which liquid plastics, with their appropriate curing agent, are poured into molds and set, with or without heat.
13. Calendaring Process – plastic is fed to revolving rollers which turn out a thin sheet or film the thickness of the product is determined by the roller spacing, and the surface of the sheet may be smooth or matted, depending on the roller surface.
Plastic Laminates – consists of three or more layers of material bonded or laminated together with plastic adhesive under high pressure.
ADHESIVES AND SEALANTS
Cohesiveness – the ability of particles of a material to cling tightly to one another.
Adhesiveness – the ability of a material to fix itself and cling to an entirely different material.
Sealers – products which are used to seal the surface of various materials against the penetration of water or other liquids or in some cases to prevent the escape of water through the surface.