SIU IT 208 - Chapter 18 – Manufacturing with Plastics

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Chapter 18 – Manufacturing with PlasticsPolymers – based on a carbon backbone and are thus organic materials.Plastics – Polymers compounded with various additives.- Additives – Agents that enter the molecular structure and are designed to change its properties (Antioxidants, flame retardants, lubricants, etc.)- Fillers – Incorporated to improve mechanical properties, often called reinforcing agents. (Increase bulk, stiffness, etc.)CHEMISTRY OF POLYMERIZATION The carbon atom must always have four covalent bonds. Polymer - many repeating parts “Poly” = many “meros” = part (called mers in long chains)Units are bonded togetherH H H H H HC C ------- C C ------- C C PolyEthylene H H H H H HEthylene Ethylene EthyleneWhy use polymers?-low density-high corrosion resistance-electrical insulation-ease of manufactured into complex shapesWhere do polymers come from?Early 1900's + + +Basic Building Block1CokeMethaneAcetyleneEthylene Hydrogen3 Classes of Plastics1. Thermoplastic (linear) - Soften repeatedly when heated (straight chains) -c-c-c-c- PET - Polyethylene Terphalate (Beverage Containers) HDPE - High Density Polyethylene (Milk bottles, detergent bottles) PVC - Polyvinyl Chloride (Food wrap, vegetable oil bottles) LDPE - Low Density Polyethelene (Shrink wrap, plastic bottles) PP - Polypropylene (Margerine and yogurt containers) PS - Polystyrene (Egg cartons, fast food trays)OTHER - Multi resin containers (All other categories) Cannot mix types and reuseCan be reused and recycled simply by melting and remolding them. Repeated heating/cooling of thermoplastics causes degradation and thermal aging.2. Thermosetting (cross linked) Thermosets -When hardened cannot melt again-toaster parts, automotive engine parts, electrical outlet parts, handles on pots and pans, ash traysEarliest (human made) synthetic polymer was a phenol-formaldehyde, thermoset developed in 1906 called Bakelite.3. Elastomers - are polymers that can be stretched elastically and then return to their original shape and size. One-third of plastic production is in disposable products sector (bottles, packaging, garbage bags) -starch based bio-plastics (starch granules) -lactic based bio-plastics (polymerized lactic acid used) -bio-plastic from fermentation of sugar  Biodegradable - designed to degrade completely when exposed to micro-organisms in soil or water with no toxic byproducts2MANUFACTURING WITH PLASTICS Classification by process rather than product or material is more relevant, because most processes are suitable for making products from a large variety of plasticsCasting - filling a mold by gravity- Liquid resins (epoxy)- Hot melted plastic poured into a casting (nylon)- Slush casting for thin walled products (snow boots, gloves, toys)- Wet spinning -fibers formed by spinning the solution through multi-hole dies- Rotational molding - variation of slush casting, mold is heated as it is rotated. Works exceptionally well for larger parts (IDOT barrels, chemical containers)Molding - melt processing-polymers are deformed with the aid of applied pressure- Results in a finished part- Need three things to be a molding operation (Time, temperature, and pressure)Extrusion – Pellets, granules or powder is placed into a hopper and fed into the extruder barrel. The screw in the barrel that turns it blends and moves the material down the barrel. The material then forced through a die that is in desired shape. It is cooled by water or air and cut to length. Largest production volume of plastics (bar, tube, sheet, film) Rated by the length to diameter ratio of the screw 3Injection molding – Similar to die casting metal. Pellets are fed into heated cylinder where they are melted. The screw rotates much like extrusion molder (it moves back as material in front of it are melted) then it rams forward pushing the melted material into the die. most wide spread technique for making 3-D configurations  uses either reciprocating screw or reciprocating plunger (RAM) rated by clamping pressure of die 4- Insert molding - special type of plastic injection molding process where a metal orother plastic object is placed into a mold and the plastic is injected around it (electrical plug, bi-colored tail lights)- Molds in injection molding are usually expensive and complexOther molding techniques:- Reaction Injection Molding (RIM) – Two reactant liquids are heated and brought together under high pressure5- Compression molding – Pre-measured amount of polymer introduced into the heated mold then the top half comes down and applies pressure. Usually uses thermosetting plastics and produces products like dishes, container caps, etc.Blow Molding - Extruded tube or preform is expanded by internal pressure (most bottles)- Extrusion blow molding - continuous tube is extruded, pinched off, placed in a mold and blown into shape- Injection blow molding - part is injected into a mold then transferred and blown into shape Thermoforming - Heating a thermoplastic sheet and using a vacuum to pull the sheet overperforated mold.  Fast food containers, advertising signs, panels for shower stalls Hot air pressure and vacuum forming - drive sheet into female die


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SIU IT 208 - Chapter 18 – Manufacturing with Plastics

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