Bond between adjacent long chain molecules (secondary bond) Covalent bond between mers (primary bond)•The overall strength of plastic depend on the secondary bond•Increased temperature (Tm and Tg) weaken secondary bond (thermal vibration oflong molecules) and the chain are easily move by external shaping.
Also known as thermosoftening plastic Can be remelted, recycled and remoulded. More elastic and flexible Example: vinyl ,acrylic, polycarbonate, Polyamide, polyethylene and polypropylene. Application : lense, bubble top, windshield, zipper.
Cannot be melted and re-shaped after it is cured Cannot be recycled Better suited to high-temperature applications Generally stronger than thermoplastic materials due to this 3-D network of bonds Example : Polyester, Bakelite, Vulcanized rubber, epoxy, melamine, polyamides Application : Electrical insulators and plastic ware, car parts, body parts of modern airplanes and printed circuit boards
Raw material (pellet, granules, powder) are placed into hopper and fed into the barrel of screw extruder. Helical screw blend the pellet and convey them down the barrel toward the die Barrel heater and the screw internal friction heat the pellet and liquidizes them. Molten plastic was then forced through the die. Extruded part was then cooled by exposing it to blowing air or pass through water-filled chanel
Parts- Solid rod, sections, channel, sheet, tubing, pipe, architectural parts. Defects- surfaces fractures (uneven die shapes), bambooing and sharkskin effect ( friction at die-polymer interface and elastic recovery and non-uniform deformation of product outer layer.
Control parameters- extruder screw rotational speed, barrel wall temperature, die design, rate of cooling, drawing speed. The flowrate- control by the shape, pitch and flight angle of helical screw Cooling rate and uniformity – control by cooling medium such as air and water Design of die and extruder Pressure and heat rate Continuous supply of raw material
Has high production rates (fast cycle time) Good dimensional control Complex shapes with good dimensional accuracy Parts generally weigh 100g-600g (small parts) or heavier ( automobile components) High tooling cost (die)
Product - Cup, container, housing, knob, toy, electrical and telecommunication devices Examples of product
Design capabilities Proper control of temperature and pressure Mold design modifications Good control of flowrate in the die cavitiesDefect Weld lines- molten plastic flows in from 2 opposite runners and meet in the middle of cavities Void/Porosity-runner section too small, premature solidification, preventing full filling of mold cavity Flash- die do not close completely Sink marks/ Pull in- thick molded section
Extrusion blow moulding Injection blow moulding Multilayer blow moulding Modified/ combined extrusion and injection moulding process. Product : hollow container, water bottle, drinking cup, pharmaceutical and cosmetic containers.
A tube or pre form (parison) is first extruded. It is clamped into the mold with cavity much larger than the parison diameter It is then blown upward to fill the mould cavity The pieces was then cooled and ejected from mold
Short tubular pieces (parison) is injection molded in cool dies. The dies then open and the parison is transferred by an indexing mechanism. Hot air is injected from the blow pin into the parison, expanding to the wall of mold cavity The pieces was then cooled and ejected from mold after the blow pin is removed
Parts produces are hollow tubing and containers Sufficient blowing pressure Strong and rigid die ( typical material : steel, aluminum, beryllium copper) Improper filled molds The blow pressure of hot blasting ranging from 350 to 700 kPa depending on the material types Blow ratio can be up to 7:1. Drum with volume up to 2000 liters can be made through this process
Process of forming thermoplastic sheet/film over a mold through the application of heat and pressure. Sheet is clamp and heated to sag point ( above Tg) by radiant heating. It was then force using vacuum or pressurized air against the mold surfaces.
Typical parts- tray, house appliances, panel for shower stall, etc Cannot made part with holes/opening due to unable to maintain pressure during forming Material had to exhibit high and uniform elongation. The molds is made from aluminum due to low strength and low cost requirement The mold has small holes (<0.5mm) to help vacuum forming
Tearing of sheet during forming Non-uniform wall thickness Improper filled molds Poor part definition Lack of surfaces details
Made through roll method or doctor blade method. In roll method, the polymer coating material is squeeze against the substrate (fabric, paper, cardboard, metal foil), by mean of opposing rolls. This cause the polymer to uniformly cover the base material and the take-up reel will roll and gather the coated material In doctor blade method, a sharp knife edge control the amount of polymer melt that is coasted in the substrate. Similarly, the coated material will be collected through a take up reel.