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Level 21

13 B - PLASTIC SHAPING PROCESSES


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Coating processes
COAT SHEETS AND FILMS BETWEEN ROLLS, SPRAY AND DEEP TO COAT OBJECTS. Processes to coat wires, sheets, or objects with a layer of plastic. [1] Planar coating - flat fabric, paper, carboard, metal foil go through two rolls (roll method) or over a roll with a knife holding the polymer (doctor blade method), recieving a cover of polyethelene, polypropylene, nylon, PVC, or polyester. [2] Contour coating, the coting of three dimensional objects can be done by dipping the object into the plastic or spraying the polymer on the object. COAT SHEETS AND FILMS BETWEEN ROLLS, SPRAY AND DEEP TO COAT OBJECTS
Roll method
Roll method to combine polymer and substrate - roll press polymer and substrate together
Doctor blade method
Sharp knife controls the amount of polymer the substrate receives
Contour coating
Object receives a polymer coating by dipping into a polymer bath or by spraying.
Injection molding
Most widely used process for thermoplastics, due it's speed and versatility.
Injection molding machine
Consists of two parts: [1] Plastic injection unit - a heated extruder like screw capable of also pushing, a reciprocating screw with a non return valve at the end, melts and pushes the plastic into the mold , [2] Mold clamping unit - a hydraulically operated press holds the mold closed during injection, opening them up at the end.
Injection molding cycle
[1] The mold is closed and clamped; [2] Reciprocating screw injects a shot of melt into mold cavity, while the clamping unit maintains the pressure; [3] Screw rotates and retracts, allowing polymer to feel the forward portion of the barrel; [4] Clamping unit opens the mold and ejects the part from the mold. >>> CIRE: CLAMP, INJECT, RETRACT, EJECT
Injection molding mold
Specially designed and fabricated for a specific part, new types of parts require a new injection molding mold.
Two plate mold
RECEIVES MELT, EVACUATES AIR, COOLS MELT, EJECTS PART. A conventional, two plate mold for plastic ejection, each part mounted on a different platen of the injection unit. Consists of: (1) one or more cavities that determine part geometry, (2) distribution channels through which the polymer melt flows to the cavities - sprues, gates, runners: control the flow and the viscosity of polymer via the phenomena of pseudoplasticity, (3) an ejection system for part removal - ejector pins push part out, (4) a cooling system - water circulates through mold, and (5) vents to permit evacuation of air from the cavities - narrow channels allow air in and out, but keep the polymer in. >>> MELT FILLS CAVITIES THROUGH CHANNELS, AIR ESCAPES THROUGH THE VENTS, WATER COOLS THE MOLD, EJECTOR PINS REMOVE THE MOLD.
Three plate mold
SEPARATES THE GATING SYSTEM FROM THE PART UPON THE END OF MOLDING CYCLE, since the gaiting system is located in another plate, a mold better suited for automation.
Hot runner mold
ELIMINATES SOLIDIFICATION OF RUNNER AND SPRUE BY HEATING THEM UP, a mold used for products requiring virgin plastic, plastic that have not be recycled.
Injection molding machines
Injection molding machines differ in injection unit and clamping unit design.
Reciprocating screw
SCREW PLASTICIZES AND INJECTS. A screw compacts and melts by frictional action the heated polymer, injecting the melt into the mold by its reciprocating action, THE MOST COMMON INJECTION UNIT DESIGN.
Screw preplasticizer
SCREW MELTS, PLUNGER INJECTS. The screw melts the polymer, polymer enters a separate injection chamber only to be injected into the mold by a plunger.
Plunger type injection
Heaters melt the polymer and the plunger injects it.
Toggle clamp
CLAMPED BY A PISTON MOVING MECHANICAL LINKS, thus giving a fast speed at the start, high mechanical advantage at the end. Usually used on low tonnage machines.
Hydraulic clamp
HYDRAULIC CYLINDER CLAMPS THE UNITS WITH GREAT FORCE, a design suited for higher tonnage applications - 150[ton] to 1000[ton]
Hydromechanical clamps
MOVE AND CLAMP THE MOLD WITH A COMBINATION OF HYDRAULIC AND MECHANICAL MEANS. Move the mold with a hydraulic cylinder, clamp it in place by mechanical means, build tonnage by high pressure hydraulic cylinders.
Shrinkage
PLASTICS COOL WHEN SHRINK and have a high thermal expansion coefficient. $$D_{Cavity} = D_{part}+D_{part} \cdot S+D_{Part} \cdot S^{2}$$ SHRINKAGE DEPENDS ON MANY FACTORS - high temperature and pressure reduce shrinkage, compaction time, part thickness (thinker part - higher shrinkage).
Injection molding defects
Injection molding defects include the following phenomena: [1] Flashing - melt penetrates into clearances, [2] Short shots - mold doesn't fill fully, [3] Sinks and voids - sinks and voids on and below surface part, [4] Weld lines - flows of melt join poorly and form a 'weld line'. >>> FuS SeeVe WeLd: FLASHING, SHORT SHOTS,,, SINKS, VOIDS,,, WELD LINES
Short shots
MATERIAL SOLIDIFIES BEFORE COMPLETELY FILLING THE CAVITY - correct by increasing temperature or/and pressure, can be caused by a low shot capacity, the inability of the machine to inject enough material.
Flashing
MELT PENETRATES INTO PARTING LINE, VENT HOLES, EJECTION PINS - [1] Vents and clearances are too big, [2] Injection pressure too high compared with clamping force [3] Melt is too hot, [4] Shot size too big
Sink marks and voids
DEPRESSIONS OR HOLLOW VOIDS IN SURFACE PART, caused by contraction of the material, solved by increasing packing pressure and decreasing section thicknesses.
Weld lines
A WEAK POLYMER INTERFACE CONNECTION - melt flows around a core, reunites with itself, in some cases the two contacting interfaces forming a weak connection, a problem solved by higher melt temperatures, higher injection pressures, different gating locations, and better venting.
Thermoplastic foam injection molding
CREATES A PART WITH A DENSE OUTER SKIN AND A LIGHT FOAM CORE, injecting gas or using materials that do so in the melt, thus creating a part with high stiffness to weight ratio, quite often producing surface defects such as voids and sinks, thus requiring the part's surface to be processed afterward
Multi injection molding processes
INJECTS DIFFERENT PLASTICS INTO ONE MOLD, creating parts of different plastics and colors, like automotive tail lights
Thermoset injection molding
INJECTION MOLDS A THERMOSET WITH A FILLER. A shorter screw, to prevent premature curing, plasticizes thermoset pellets, injecting it into a heated mold where the thermoset sets, the process quite often combines filler materials enhance the thermoset properties, such as glass fibers, clay, wood fibers, carbon black, in effect creating a injection molded composite material part. STRONG FILLERS ENHANCE BRITTLE THERMOSETS.
Reaction injection molding
MIXES TWO REACTING CHEMICALS AND INJECT THEM INTO A MOLD - combining urethanes, epoxies, urea-formaldehydes before they go into the mold - a cheaper, low energy process than the injection molding, since it doesn't require high clamping pressures, and is more efficient at lager part sizes.