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13 A - PLASTIC SHAPING PROCESSES


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Why plastics
PLASTICS ARE EASY TO PROCESS, requiring less energy and lower temperatures. Plastics hold a number of advantages over metals and ceramics:(1) many of the processes are net shape processes; (2) in general, less energy is employed than in metalworking processes; (3) lower temperatures are required to process plastics than metals or ceramics; there is great flexibility in geometry; and (5) painting and other finishing processes are generally not required >>> MOLDABLE PLASTICS MAKE CHEAPER BOTTLES.
Plastic shaping processes
Classification of plastic shaping processes is according to resultant product geometry: [1] Continuous, non-flat, constant cross section; [2] Continuous sheet and films; [3] continuous filaments (fibers); [4] Mostly solid molded parts; [5] hollow molded parts with thin walls; [6] Discrete parts made of formed sheets and films; [7] Castings; [8] Foamed products. >>> TUBES, FILMS, FILAMENTS, MOLDED PARTS, PLASTIC SHEET PARTS, CASTINGS, FOAMED PRODUCTS
Properties of polymer melts
LIQUID PLASTIC HAS DISTINCT PROPERTIES. In order to shape a thermoplastic polymer it needs to be softened to a consistency of a liquid, to a polymer melt, a state in which the thermoplastic has certain physical properties such as VISCOSITY AND VISCOELASTICI
Polymer melt
Melted thermoplastic with a liquid like consistency.
Polymer melt viscosity
Viscosity of a polymer melt depends on (1) temperature and (2) shear rate (flow rate). Also, (3) the molecular weight of the polymer affects viscosity. >>> FaT Mee >>> FLOW RATE, TEMPERATURE, MOLECULAR WEIGHT
Pseudoplasticity
DECREASE OF VISCOSITY AT HIGH SHEAR RATES, that is at high flow rates the polymer melt becomes thinner.
Viscoelasticity
MATERIAL BEHAVES BOTH LIKE A VISCOUS LIQUID AND ELASTIC SOLID - stretch it slow and will flow; tug it fast and it will snap like a mast, $$---$$ A combination of viscous and elastic properties which cause the melt to exhibit memory - the tendency to return to its previous shape, as exhibited by die swell in extrusion.
Die swell
SWELLING OF EXTRUDED PLASTIC AFTER EXITING THE DIE - the viscoelastic material strives to retain it's original shape, after being compressed by the die, a phenomena minimized by making a longer die channel. >>> LONG CHANNEL PASSIFIES SWELLED SILICONE
Swell ratio
Ratio between the diameter of the die channel and the diameter of the extruded plastic: $$r_s=\frac{D_x}{D_d}$$
Extrusion
FORCING MATERIAL THROUGH A DIE TO GIVE IT THE DIE'S CROSS-SECTIONAL SHAPE. a polymer melt is compressed to flow through a die orifice and thus the continuous length of the plastic assumes a cross-sectional shape that is approximately the same as that of the orifice. >>> HARD DIE SHAPES DESIRED CROSS-SECTION
Extrusion process and equipment
EXTRUDER HEATS, SCREWS, AND FORCES POLYMER PELLETS THOUGH A DIE. The extruder heats and melts the plastic pellets fed to it through the hopper, before forcing them through the breaker plate supported screen pack while heating them by the means of compression and added electric heaters, a set of small holes that remove the polymer's memory of the rotation, and finally forcing the polymer through the die.
Plastic extruder
A TIGHTLY FIT SCREW INSIDE A BARREL. Can be divided into three sections: [1] The feed section, in which the feed stock is fed from the hopper and heated; [2] The compression section, in which the polymer changes to a viscous fluid due to friction and generated heat; and [3] The metering section, in which pressure is developed to pump the plastic through the die orifice. >>> FEEDS AND HEATS, COMPRESSES AND MELTS, METERS AND EXTRUDES
Screen pack and breaker plate
A FLOW STRAIGHTENING FILTER. The polymer melt passes through the screen pack - a filter of fine wire mesh, a filter supported by the breaker plate - a plate with a set of small, axial holes. their functions are: (1) filter dirt and lumps, (2) build pressure, (3) straighten the flow and remove memory of the polymer melt. A FILTER TO REMOVE DIRT, BUILD PRESSURE, AND STRAIGHTEN THE FLOW.
Analysis of extrusion
The flow rate of material as determined by the screw is the drag flow the screw create minus the back pressure flow due to a constriction at the die, yet the more pressure you apply to the die, the more material comes out. $Q_{Out} = Q_{DragFlow} - Q_{BackPressure}$
Extruder and die characteristics
CURVES RELATING THE RELATIONSHIP BETWEEN PRESSURE AND MELT FLOW, the intersection of the curves is the extruder operating point - the more pressure you apply to the die, the more material it will poop out; the more pressure the screw feels, the less material it can propel forth. SCREW EXPELS MAXIMUM AMOUNT OF MATERIAL AT THE OPERATING POINT.
Die configurations
Plastic extrusion dies can extrude the following types of profiles: [1] solid profiles; [2] Hollow profiles - tubes and shit; [3] Wire and cable coating; [4] Sheet and film; [4] Filaments.
Solid profile
DIE PROFILE TO EXTRUDE A SOLID CROSS-SECTION. For non round profiles the die profile differs from the extrudite profile due to the viscoelasticity of the melt, thus for a square cross-section you need an almost star shaped die. DIE SHAPE COMPENSATES FOR VISCOELASTICITY.
Extrudite cooling
After exiting the die, the extrudite is often cooled by air flow, water spray, or water bath.
Hollow profile
A HOLLOW AND AIR BLOWING MANDREL CREATES A HOLLOW PROFILE, supported by a few spider legs within the die, blowing air into the extrudite in order to keep it hollow, creating tubes, pipes, and hoses. MANDREL BLOWS UP TUBE.
Wire and cable coating
POLYMER COATS WIRE IN VACUUM, the wire introduced into the extrudite through a mandrel, assisted in adhearing by the application of partial vacuum, and cooled as it exits the die. VACUUM HELPS TO COAT WIRES WITH POLYMER.
Defects in extrusion
A turbulent flow and high flow velocities can result extrusion defects such as melt fractures, shark skin, and bambooing.
Melt fracture
NARROW DIE FRACTURES SURFACE. Sudden reduction of cross-section causes turbulence and subjects melt to high stress, resulting in a highly irregular extrudite surface.
Sharkskin
ROUGH EXTRUDITE SURFACE. Friction with die walls creates a problematic velocity profile, resulting in a rough extrudite surface. VELOCITY PROFILE MANGLES SOFT SURFACE.
Bambooing
Extreme velocity gradient in the die creates a bamboo-like surface of the extruded polymer.
Sheet and film production
Methods: [1] Slit-die extrusion; [2] Blown film extrusion process; [3] Calendering. SCOOBY: SLIT-DIE; CALENDERING; BLOWN EXTRUSION.
Sheet
0.5[mm] to 12.5[mm] thick stock
Film
Stock below 0.5[mm] in thickness
Slit die extrusion
ALSO KNOWN AS CHILL-ROLL EXTRUSION. A narrow slitted manifold extrudes a thin and wide film or sheet through it's tight, narrow, well lubricated, tightly fitted slot, passing on the film onto chilling rolls with speeds of about 5[m/s]. the problem in this method is with the uniform thickness of the film - it's hard to evenly distribute the material, and the sides of the film are usually cut off. THIN FILM LOOSES UNEVEN EDGES.
Blown film extrusion
MAKES A TUBE OF HOMOGENEOUS FILM. process for making thin polyethylene film for packaging. It combines extrusion and blowing to produce a tube of thin film. The process begins with the extrusion of a tube that is immediately drawn upward while still molten and simultaneously expanded in size by air inflated into it through the die mandrel. EXTRUDES AND BLOWS UP TUBE OF FILM
Calendering
MAKES FILM AND SHEET OUT OF RUBBER OR RUBBERY THERMOPLASTICS. The process passes material through a set of rolls, pressing the plastic into a final thickness with speeds up to 2.5[m/s] and good thickness control, rolling the plasticized PVC ad other plastics to create PVC floor coverings, shower curtains, inflatable boats. ROLLS RUBBERY PLASTICS INTO SLEEK SHEETS.
Plastic filament production
EXTRUDES FILAMENTS, COLLECTS THEM ON SPOOL. Melt spinning, wet spinning, and dry spinning operate on the same principle - extrude a lot of long plastic filaments, cooling them with air and conditioning them with steam, before collecting them on a spool
Melt spinning
MELTS PLASTIC AND EXTRUDES PLASTIC FILAMENTS. The process uses a pump to force polymer melt into the spinneret, a die with a multitude of holes - about 50, 0.25[mm], counter sinked holes, cooling and conditioning the polymer before spooling it on the collection bobbin. PROCESS USED FOR MOST IMPORTANT SYNTHETIC FIBERS - POLYESTER AND NYLONS.
Wet and dry spinning
SIMILAR TO MELT SPINNING - POLYMER KEPT IN SOLVENT. Same as melt spinning, only the polymer is kept in a solution and separated from it upon extrusion.
Difference between fiber and filament
FIBER - A LONG STRAND; FILAMENT - CONTINUOUS FIBER. Fiber is a long, thin strand of material whose length is at least 100 times its diameter; a filament is a fiber of continuous length