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13 C - PLASTIC SHAPING PROCESSES


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Compression molding
A HEATED MOLD COMPRESSES AND MELTS AN EXACT AMOUNT OF PREHEATED THERMOSET or ELASTOMER CHARGE. Makes simple parts in simple and cheaper molds. Takes more time than injection molding. Used to make electric plugs, sockets, pot handles.
Transfer molding
PLUNGER TRANSFERS PREHEATED THERMOSET CHARGE INTO CAVITY, leaving some behind: the sprue and the cull, parts that are scrap, since thermoplastics can't be recycled. $$1. Pot.Transfer.molding$$ Pot transfer molding - charge is injected from a pot through a vertical sprue. $$2.Plunger.Transfer.Molding$$ Plunger transfer molding: charge injected by plunger from side channels.
Blow molding
AIR INFLATES SOFT THERMOPLASTIC IN CAVITY. First a starting tube of plastic is manufactured, a parison, then it's either blown up inside the mold (Extrusion blow moding), or transfered into a different mold be blown up (injection and stretch blow molding).
Rotational molding
USES GRAVITY IN ROTATIONAL MOLD TO CREATE A HOLLOW FORM. Plastic is loaded in a heated and slowly rotating form, spreading across it's walls under the influence of gravity, creating complex parts such as small swimming pools, garbage cans, fashion mannequins. USEFUL GRAVITY SPREADS THIN A GARBAGE POOL
Thermoforming
HEATING A THERMOPLASTIC SHEET AND SHAPING IT INTO A DESIRED SHAPE, such as a packaging for consumer goods, door liners, bathtubs. Basic categories: [1] Vacuum thermoforming, [2] Pressure thermoforming, [3] Mechanical thermoforming. Can be used only with thermoplastic sheets - thermoset plastic sheets have already crosslinked and can't be reheated to a malleable state >>> ViPe Me: VACUUM, PRESSURE, MECHANICAL
Vacuum thermoforming
Small vacuum holes pull a heated thermoplastic sheet into a desired shape.
Pressure thermoforming
Air pressures the thermoplastic sheet from above, vacuum draws the sheet from below, pressing it much harder over the shape
Negative and positive mold differences
MOLD SURFACE IMPRINTS PATTERN ON SHEET. A positive mold has a convex shape; a negative mold has a concave cavity, and the sheet side touching the mold receives its contour - you can press a desired pattern on a finished product, the school of Athens or the Mona Lisa for example.
Sheet thinning
When stretched over a mold, the plastic sheet deforms and gets thin in different places - THINNING MINIMIZED BY PRESTRECHING THE SHEET
Mechanical thermoforming
POSITIVE MOLD MECHANICALLY STRETCHES THERMOPLASTIC SHEET INTO A NEGATIVE MOLD, a more expensice method, since it requires better two molds, yet it has a better dimensional control and the ability to detail both sides of the sheet.
Casting plastic
CASTING THE PLASTIC IN A MOLD, a slow process with low output, yet less-residual stresses than in die casting. Used to make acrillyc sheets, empty statutes with slush casting, and plastic covered electronics by encapsulation.
Polymer foam
LOW DENSITY, HIGH STRENGTH POLYMER FOAM with good elastic properties that can be made to be: [1] Elastomeric if made from rubbers, [2] Flexible if made from a soft thermoplastics, [3] Rigid if made from rigid thermoplastics, such as PVC.
Polymer foam forming
ADDING AIR, GAS, FOAMING AGENTS. There are several foaming processes: (1) mechanical agitation - mixing a liquid resin with air, then hardening the polymer by means of heat or chemical reaction; (2) mixing a physical blowing agent with the polymer - a gas such as nitrogen (N2 ) or pentane (C5H12 ) which can be dissolved in the polymer melt under pressure, so that the gas comes out of solution and expands when the pressure is subsequently reduced; and (3) mixing the polymer with chemical compounds, called chemical blowing agents, that decompose at elevated temperatures to liberate gases such as CO2 or N2 within the melt
Foam structures
CLOSED CELL AND OPEN CELL - the closed sell structure insulates is bubble of air with polymer, open cell structure has all the air bubbles connected to each other - A LIFE JACKET OF OPEN CELL FOAM WILL DROWN YOU.
Plastic product design considerations
(1) Plastics are not as strong or stiff as metals and should not be used in applications where high stresses will be encountered. (2) Impact resistance of plastics is general good, better than many ceramics. (3) Service temperatures of plastics are limited relative to engineering metals and ceramics. (4) Thermal expansion is greater for plastics than metals; so dimensional changes due to temperature variations are much more significant than for metals. (5) Many types of plastics degrade from sunlight and certain other forms of radiation. >>> WeaSeL TUb: WEAK, SOFT, LOW TEMPERATURES, HIGH THERMAL EXPANSION, UV INTOLERANT.