Processing Techniques

Polymer Processing Techniques Injection Molding Injection molding is one of the most important techniques for the forming of thermoplastic materialsA number of various items can be manufactured with great ease.Important TerminologiesMolding: To shape something into a stable (preform) structureInjection Line: The sequencing of an injection molding machine with all other particulars to produce a product.Shot size: The weight of material that can be processed in one cycleShot Cycle: The time required to process one shot size.Mold Cycle: The total time required to produce a stable product.Sprue Channel: The channel between barrel nozzle and the mold.Clamping unit: The mechanical assembly that is used to hold the moveable mold.Reciprocating Action: To and Fro motionRam Action: Piston actionMold definition: The internal smoothness and surface finish of the mold Multicavity Mold: A mold in which more than two cavities are present.Family Mold: A mold containing multiple cavities of different types.Injection LineIt consists of three major units:

Injection UnitClamping Unit + ControlMold

Injection Molding CycleInjection molding involves two basic steps:Melt generation by a rotating screwForward movement of the screw to fill the mold with melt and to maintain the injected melt under high pressure

Injection molding is a cyclic process:Injection: The polymer is injected into the mold cavity. Hold on time: Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. Cooling: The molding cools and solidifies. Screw-back: At the same time, the screw retracts and turns, feeding the next shot in towards the frontMold opening: Once the part is sufficiently cool, the mold opens and the part is ejectedThe mold closes and clamps in preparation for another cycle.6

Process & machine schematics

*Schematic of thermoplastic Injection molding machineInjection UnitPurpose: to liquefy the plastic materials and then inject the liquid into moldResin is introduced through hopperSome machines can have several hoppers (to fed filler, colorants, other additives)However, due to limited size of barrel, mixing capability is poor

Injection UnitBarrel made of heavy steel cylinder to withstand the pressure and temperature involved in melting the resin2 types of system used in injection molding;Reciprocating screw- similar to extruder screw but with unique reciprocating actionRam injector: plunger actionInjection Unit3 sectionsFeed section- to advance the resinCompression section- to melt the resinMetering section- to homogenize the resin and pump it forwardThe screw of injection molding machine is shorter than extruder, L/D ratios are 12:1 and 20:1Low L/D ratios suggest the mixing is less efficient in the injection molding machine

Reciprocating Screw Injection Molding MachineAdvantagesMore uniform meltingImproved mixing or additives and dispersion throughout the resinLower injection pressureFewer stresses in the partFaster total cycleRam Injection- Injection Molding MachineIn this type of injection molding, the resin is fed from a hopper into the barrel, and heated through thermal energy from the heatersThe molten resin is collect in a pool in a barrel celled injection chamberThe molten resin is then push forward by the action of plunger (ram or piston)

Mold Basics

Cavity PlateCavityMouldingCoreCore PlateRunnerCavityGateNozzleSprueMelt Delivery

PartCavityCoreStripper plateInjection Molding for a plastic cup

RunnerKnobNozzleComplete IM for a plastic cupRunnerPartCavityNozzlePartCavityKnobStripper plateRunnerPartCavityNozzleReaction Injection Molding

Liquid monomers are placed in the mold avoiding the need to use temperature to melt the polymer or pressure to inject it. The monomers polymerize in the mold forming the part. 19ExtrusionExtrusion a Greek word: To push outAn Extruder is a pump that supplies a continuous stream of material to a shaping tool or post shaping process.

ObjectiveOne of the most famous and easiest method of plastic processing.Use is not restricted to Plastics, some metals are also extruded (e,g. Al window frames).

Extruder : Mechanical assembly used to melt and push the materialExtrusion Line: A typical set up that shows the complete arrangement and sequencing of all the important auxiliaries involved along with extruder to produce any uniform product.Barrel: The upper portion of extruder that covers the screw.Pipes or tubings: The parts which are in hollow cylindrical shape.Profiles: Parts that are hollow but not cylindrical.Die: The shaping tool installed usually at the end of a typical processing equipment.Advantages and DisadvantagesUniform products Continuous ProcessHigh productionLow processing costFlexibility of Raw materialsCan act as compounder.No narrow and complex part Low extrusion pressuresUniform products onlyScrew is almost material specificExtrusion LineFigure 10.1

ExtrusionPellets of the polymer are mixed with coloring and additives. The material is heated to its proper plasticity. The material is forced through a die. under pressure resulting in an endless product of constant cross section The material is cooled.

24Blown Film ExtrusionA web under 0.254 mm thickness is called a film (Packaging industry uses 0.10mm)Materials over these dimensions are called sheets.Thermoplastics are usually blown into films.Die is fed from side.Tubular die is used after extruderThe inflated film is air cooled.The size is controlled through die gap and size rings around the blown tube, by the internal air pressure and speed of take-off rolls.Several guide rolls are used to align the blown tube.A collapsing device is used for to make passage of tube through nip rolls.

The trapped air that forms the continuous tube is directed through mandrel via die.Once the bubble is formed, the controlled air pressure required to make tube stable is kept constant.To speed up lines and improve output performances, internal bubble cooling (IBC) systems are extensively used. They direct cool air at low velocity to enter and exit the inside of the bubble.

Drag the extrudate up to the top nip

Pass it through idle rolls to the winderAdjust the blow up ratio by introducing the air by slowly turning the air valve.

After the desired size is obtained, close the valve Extrusion Blow MoldingResembles film blowing in some aspectsCylindrical extrudate is required so a mandrel is to be used.Air may be inserted through a pin in the mandrel or through the bottom of the mold into the parison.Extrusion is a continuous process where as blow molding is a discrete process.Continuous EBM (Rising Mold System, Parison Transfer System, Rotational Molding)Intermittent EBM (Reciprocating Screw, Accumulator System)

Bottle forming process

Compression MoldingPre-formed blanks, powders or pellets are placed in the bottom section of a heated mold or die. The other half of the mold is lowered and pressure is applied. The material softens under heat and pressure, flowing to fill the mold. Excess is squeezed from the mold. If the polymer is a thermoset, cross-linking occurs in the mold. The mold is opened and the part is removed.

1) Placement of Charge5) De-moulding3) Squeeze tofinal dimensions2) Mould Closure4) Resin CureThermoformingShaping of materials (sheets and films) by heating until the material softens and then forcing the material to adopt the shape of mold.

The forcing medium includes vacuum, air pressure, or mechanical actions.

Pros & ConsProsLow machine costLow temp. requirementsLow mold costLow pressure requirementsLarger parts can easily be formedFast mold cyclesConsHigh raw material costHigh scapLimited part shapePoor part finishInherent wall thickness variationCommon internal stressesTypes of Thermoforming processStraight vacuum formingPressure FormingPlug-Assist FormingReverse Draw FormingFree FormingDrape FormingSnap-back FormingMatched-Die FormingMechanical FormingStraight Vacuum FormingSimplest from all othersStraight sheet of thermoplastic is clamped above the moldMold has vents connected to vacuum pumpOut side pressure of air forces the sheet to press due to vacuum.Max. Pressure therefore will be 1 atm.Very poor control over wall thickness and corners

Pressure FormingPositive air pressure is used to force the material in heated form.Same mechanism as that was of straight vacuum forming except the vacuum.The process can be done at low temperatures and faster cycles.Good finish than vacuum forming is obtained.

Rotational MoldingProcess Description

MouldArmPlateMateriala) Charging the Mould

Final Partd) Demoulding A large hollow mould is charged with thermoplastic powder, and rotated.

Heatb) Heating the Mould The rotating mould is transferred into an oven, polymer sticking to the walls.

Spray Nozzles/Fanc) Cooling Once the powder consolidates, the mould is cooled, and the part removed.

Rotational MoldingProducts

Wide range of complex hollow products:

TP PultrusionProcess Description Prepreg is fed into a preheating section, bringing TP close to Tmelt. The heated die has a slight taper, gradually shaping to the final cross section. The product is cooled, taking on the required solid cross section.Prepreg Tape or SheetsHeated DieCooled DiePreheatingTP PultrusionEquipmentExperimental TP Pultrusion Line

Close-up of preheater and dies