BackgroundThe injection moulding has seen steady growth since its beginnings in the late1800’s. Thetechnique has evolved from the production of the simple things like combs andbuttons to major consumer, industrial, medical, and aerospace products.The invention of an injection molding machine was achieved by John Wesley whoinjected hot celluloid into a mold which resulted in billiard balls which wereused as a replacement for ivory which was based on the pressure die castingtechnique for metals.
The industry progressed slowly over the years, producingproducts such as collar stays, buttons, and hair combs. The industry expandedrapidly in the 1940s because World War II created a huge demand for inexpensive,mass-produced products. In 1946, American inventor James Watson Hendry built thefirst screw injection molding machine, which allowed much more precise controlover the speed of injection and the quality of articles produced. This machinealso allowed material to be mixed before injection, so that colored or recycledplastic could be added to virgin material and mixed thoroughly before beinginjected.The main concept of plastic molding is placing a polymer in a molten state intothe mold cavity, so that the polymer can take the required shape with the helpof varying temperature and pressure.
There are different ways of molding aplastic some of them are blow molding, Injection molding, rotational moldingand compression molding. Each technique has their own advantages in themanufacturing of specific item.1.2 Injection MoldingMachineInjection moldingmachines are normally general-purpose machines. An Injection molding machine is a machine formanufacturing plastic products by the injection molding process.Injection molding is a method of forming a plastic product from thermoplasticsbyfeeding the material through the machine component called the hopper to aheated chamberin order to make it soft and force the material into the mold by the use of thescrew. In thiswhole process pressure should be constant till the material is hardened and isready to beremoved from the mold. This is the most common and preferable way of producinga plasticproducts with any complexity and size.
Injection molding permits mass production net shape manufacturing of highprecision, threedimensional of plastic parts. It consists of two main parts,an injection unit and a clamping unit. 1.
2.1 Injection / Plasticizing unit: Theplasticizing unit melts, homogenizes, conveys, meters and injects the polymerinto the mold. Thus the injection unit has two main tasks: On one hand itplasticizes the polymer and on the other hand it injects the melt into themold. The injection unit consists of hopper, barrel, screw/plungerand the heating section. Barrel, screw and nozzle areexchangeable. In doing so the injection unit is adaptable to the type ofpolymer and the shot volume.Types ofinjection units-Plunger type injection: Thedesign of this unit was based on a method used to mould rubber. Material ismetered by a dosage device and transported through the heated plasticisingcylinder until it is in front of the plunger at the correct temperature.
Thematerial residence time in the cylinder is very long, making this methodunsuitable for heat sensitive material such as rigid PVC and thermosets. Thistype of machine is not widely used as it was replaced by the reciprocatingscrew piston injection unit.Screw type injection:Standard machines have typically reciprocating screws, which means the screwacts also as an injection piston. Nowadays there are also approaches existingwhich separate plasticizing and injection unit. Common screws for injectionmolding consist of three zones: Feed section, Compression section, Meteringsection the screw rotates in a heated barrel and a feed hopper deliversgranulate or powder into the barrel.
Usually the injection unit is mountedflexibly on the machine bed.1.2.2 Clamping unit: The clamping unitresembles to a horizontal press. The moving platen slides on four bars and thehalves of the mold are mounted on both platens.
Types of clampingunits- ManualClamping: Manualclamping in only seen in the case of manual or Hand Injection Moulding Machineor some time in pneumatic Injection Moulding Machine. The proper clamping isdepend on the skill of the operator. The Clamping force is direct and notmeasured. The position of the sprue with respect to nozzle axis is critical.The ejection of the part from the mould is difficult. ToggleClamping: A toggleis mechanically device to amplify force. In a moulding machine, which consistsof two bars joined, together end to end with a pivot .
The end of one bar isattached to a stationary platen, and the other end of a second bar is attachedto the movable platen. When the mould is open, the toggle is in the shape of aV. When pressure is applied to the pivot, the two bars form a straight line.HydraulicClamping: A clampingunit actuated by hydraulic cylinder, which is directly connected to the moving,closed the mould.
In this case ram of hydraulic system is attached to movingplaten. There are two halves in hydraulic cylinder, which is actually inlet andoutlet of oil. When oilgoes to the cylinder with pressure oil pushes the ram to forward direction bywhich moving platen moves and mould closed and when oil comes from the cylinderthe ram come back and mould is open. Hydro-Mechanical ThisClamping System is combination of Toggle & Hydraulic Clamping System. Tomove the toggle a hydraulic cylinder is operated.
TIE-BARLESS CLAMPINGTie-Barless clamping system is basically Hydraulic clamping system without any tiebar. The platen is moved on a rail system. The mainadvantage of this system there is no limitation of mould platen size. As thereis no tie bar so the mould dimension is not so important. Also mounting of themould is easy and it is very useful when products eject from the mould ismanual.
1.3 Injection Molding ProcessThe procedure ofinjection molding is depicted in the figure It is clear that allprocedure steps of injection molding take place one after another except forthe important cooling stage, which overlaps with the holding pressure, thebackward movement of the plasticizing unit, metering and plasticizing stage.The machine control coordinates each process step and repeats it everyinjection molding cycle. To achieve a high output and hence a high processefficiency the cycle time should be kept as short as possible.
Metering the screw,which rotates in the plasticizing cylinder, conveys the melt from the hopper tothe end of the screw. This way the material is going to be densified andbrought into a molten stage. While the screw conveys the melt, the screw movesbackward because the delivered material conglomerates at the screw tip. Whilemoving backwards a definite pressure is applied to the screw, which is calledback pressure. In doing so the screw has to convey against this pressure andthe homogenization and the dissipation heating improves. This back pressure isadjustable.At a certain screwposition the metering stops, because the mass of melt for filling the wholecavities is reached. The position of the screw or the way the screw movesbackward respectively represents the metering volume and is called feedingstroke or metering stroke.
Both parameters have to be adjusted for each newmolding process. Injection During injection the screw movesforward without rotating and injects the metered material through the nozzleinto the mold. In doing so the screw acts like an injection piston. A residualmelt cushion remains in front of the screw for applying the following holdingpressure. The maximum injection pressure is a prefixed parameter and an upperlimit for the applied injection pressure.
Another prefixed parameter is theinjection speed. However it is variable during injection stage. It is alsopossible to apply an increasing or decreasing velocity profile during injectionand holding pressure stage.Melt flow in thecavities during injection the melt flow can be divided into three stages,depicted in Figure 8: A) Injection stage B) Compression stage C) Holdingpressure During injection stagethe mold is going to be filled volumetrically. Afterwards the velocity slowsdown and compression takes place. For densification more melt is conveyed intothe mold, while the pressure in the cavity rises rapidly and steeply. Whenreaching a certain pressure value the holding pressure stage begins. Whenswitching too late from compression MSci PolySci P104 – Injection molding 11 toholding pressure stage a high peak pressure arises in the melt that exhibitsstill a very low viscosity.
Thus overmolding becomes possible. While coolingdown the polymer shrinkage occurs. Hence more molten material has to beconveyed into the cavity during holding pressure stage in order to keep thevolume of the molding constant.
This stage is finished when the gate freezes.The switching point from compression to holding pressure is of high importance.When switching too early the density of the molding is too low and shrink marksare visible on the surface.Cooling stage Coolingbegins with injection and ends with demolding. Afterwards the molding has toexhibit a certain temperature and enough stability. Cooling channels in themold nearby the cavities support cooling. A cooling medium mostly oil or waterflows through the channels. Its temperature is set at a definite value andcontrolled by a cooling aggregate.
1.4 Material SelectionMaterialselection depends to a large extent on the functional constraints of the part.Both amorphous and crystalline thermoplastic resins are used in injectionmoulding. Short glass fibres are commonly used as reinforcements.Types of Plastic Materials:Thermoplastics:-Thermoplasticsaretheplasticsthatdonotundergochemicalchangeintheircompositionwhenheatedandcanbemoldedagainandagain.Theyareeasilymoldedandextrudedintofilms,fibersandpackaging.Examplesincludepolyethylene(PE),polypropylene(PP)andpolyvinylchloride(PVC).Thermosettingplastics:-Thermosettingplasticswhichareformedbyheatprocessbutarethenset(likeconcrete)andcannotchangeshapebyreheating.
Commonlyused thermoplastics in injection moulding include: ?Acrylonitrile butadiene styrene (ABS) ?Acetal ?Acrylic ?Polycarbonate (PC) ?Polyester ?Polyethylene ?Fluoroplastic ?Polyimide ?Nylon ?Polyphenylene oxide ?Polypropylene (PP) ?Polystyrene (PS) ?Polysulphone ?Polyvinyl chloride (PVC) The physical propertiesof the materials (density, thermal conductivity, melting temperature, etc.)must be considered in light of the required mechanical properties of thefinished part (strength, stiffness, hardness, etc.)This schematicillustrates the performance spectrum of a variety of plastic materials. Various additives may be added to injection moulding compounds to accomplish various purposes. This table summarizes some of them. Additive Function Examples Filler increase bulk density calcium carbonate, talc, limestone Plasticizer improve process ability, reduce product brittleness phthalate esters, phosphate esters Antioxidant prevent polymer oxidation phenols, aromatic amines Colorant provide desired part application colour oil-soluble dyes, organic pigments Flame retardant reduce polymer flammability antimony trioxide Stabilizer stabilize polymer against heat or UV light carbon black, hydroxybenzophenone Reinforcement improve strength E-glass, S-glass, carbon, Kevlar fibres Polymer physicalproperties dictate temperature processing window (Tg < Tproc < Tdeg) Theoperating temperature must lie in the range between the glass transitiontemperature and the degradation temperature of the polymer.
2.6.1 Resin data table Generic Name Melt Temperature (ºC) Mould Temperature (ºC) Min. Rec.
Max. Min. Rec. Max.
ABS 200 230 280 25 50 80 PA 12 230 255 300 30 80 110 PA 6 230 255 300 70 85 110 PA 66 260 280 320 70 80 110 PBT 220 250 280 15 60 80 PC 260 305 340 70 95 120 PC-ABS 230 265 300 50 75 100 PC-PBT 250 265 280 40 60 85 PE-HD 180 220 280 20 40 95 PE-LD 180 220 280 20 40 70 PEI 340 400 440 70 140 175 PET 265 270 290 80 100 120 PETG 220 255 290 10 15 30 PMMA 240 250 280 35 60 80 POM 180 225 235 50 70 105 PP 200 230 280 20 50 80 PPE-PPO 240 280 320 60 80 110 PS 180 230 280 20 50 70 PVC 160 190 220 20 40 70 SAN 200 230 270 40 60 80