Injection Moulding Slides

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Injection Moulding 1 Polymer Engineering Polymer Engineering (MM3POE) (MM3POE) http://www.nottingham.ac.uk/~eazacl/MM3POE INJECTION MOULDING INJECTION MOULDING
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Injection Moulding 1Polymer Engineering Polymer Engineering(MM3POE) (MM3POE)http://www.nottingham.ac.uk/~eazacl/MM3POEINJECTION MOULDING INJECTION MOULDINGInjection Moulding 2ContentsContents Principles of injection moulding Reciprocating screw machineMoulding sequenceMachine features Injection mould design Mould filling calculationsFilling pressuresClamping forcesFilling times Component design for injection mouldingInjection Moulding 31. Introduction 1. IntroductionPrinciples of Injection Moulding:Melting : Thermoplastic material (granules/pellets)heated to melt polymerMelt Transport & Shaping : Polymer melt is forcedthrough a nozzle into a closed mouldStabilisation : Component cools in relatively coldmould prior to ejectionMain Advantages: Automation & high production rates Manufacture of parts with close tolerances Versatility in moulding wide range of productseg. appliance housings,washing up bowls, gearwheels,fascia panels, crash helmets, air intake manifoldsInjection Moulding 41. Introduction 1. IntroductionPlunger Type MachinesFig. 4.30 Plunger type injection moulding machineR J Crawford (1998) Plastics Engineering, Butterworth-Heinemann.Injection Moulding 51. Introduction 1. IntroductionPlunger Type MachinesFig. 7.3 Injection moulding machine with screw preplasticator unitN G McCrum et al (1997) Principles of Polymer Engineering, Oxford Science Publications.Injection Moulding 62. Reciprocating Screw Machine 2. Reciprocating Screw MachineMachine Features:Feed hopperScrew/plungerDRIVEHeater bandsCLAMPINGUNITNozzle Back flowcheck valveInjection Moulding 72. Reciprocating Screw Machine 2. Reciprocating Screw Machine(a) Mould closes & screw (not rotating) injectsmelt into mould.Injection Moulding 82. Reciprocating Screw Machine 2. Reciprocating Screw Machine(b) Screw maintains pressure until materialsets at the gate.Injection Moulding 92. Reciprocating Screw Machine 2. Reciprocating Screw Machine(c) Screw (rotating) draws material fromhopper & plasticises it. Back pressureforces screw back until shot volume reached.Injection Moulding 102. Reciprocating Screw Machine 2. Reciprocating Screw Machine(d) When moulding has set, mould opens & partis ejected.Animation from:www.bpf.co.ukInjection Moulding 112. Reciprocating Screw Machine 2. Reciprocating Screw MachineInjection moulder with nanocomposite tensile specimens (inset)HopperToolScrew & ramInjection moulder with nanocomposite tensile specimens (inset)HopperToolScrew & ramInjection Moulding 122. Reciprocating Screw Machine 2. Reciprocating Screw MachineScrews Screws: Similar to extruder screws Length/Diameter ratios 15 15- -25 25 Compression ratios 2.5 2.5- -4.0 : 1 4.0 : 1 Injection pressure up to 200 200 MPa MPaInjection Moulding 133. Injection Mould Design 3. Injection Mould Designbacking plates backing platesguide pin guide pinlocating locatingring ringsprue bush sprue bushInjection Moulding 143. Injection Mould Design 3. Injection Mould Designejector ejectorpin pinshoulder shoulderscrew screwInjection Moulding 153. Injection Mould Design 3. Injection Mould Designcavity cavityrunner runnersprue spruecooling coolingchannels channelsvent ventchannel channelInjection Moulding 163. Injection Mould Design 3. Injection Mould DesignFig. 7.31 Feed system of multi-impression mouldN G McCrum et al (1997) Principles of Polymer Engineering, Oxford Science Publications.Injection Moulding 173. Injection Mould Design 3. Injection Mould DesignAalborg Universitet, Denmarkhttp://www.aaue.dk/bm/mfg/Injection Moulding 183. Injection Mould Design 3. Injection Mould DesignGate Gate: Narrow constriction at entrance to cavity cavity (impression).Incorrect gating can lead to problems during flow:Injection Moulding 193. Injection Mould Design 3. Injection Mould DesignGate Gate: Narrow constriction at entrance to cavity cavity (impression).Incorrect gating can lead to problems during flow:Injection Moulding 203. Injection Mould Design 3. Injection Mould DesignTypical process conditions: Typical process conditions: Process is non-isothermal as mould & barrel are atdifferent temperatures:Table 1: Injection moulding conditions for thermoplastics (after Elias)Polymer TG/oC TM/oC Tpoly/oC Tmould/oCAmorphous polymersPC 150 - 280 - 320 85 - 120SAN 120 - 200 - 260 30 - 85ABS 100 - 200 - 280 40 - 80PS 100 - 170 - 280 5 - 70PMMA 105 - 150 - 200 50 - 90uPVC 82 - 180 - 210 20 - 60Semi-crystalline polymersPET 70 265 270 - 280 120 - 140PTFE 40 220 220 - 280 80 - 130PA 6 50 215 230 - 290 40 - 60POM -82 181 180 - 230 60 - 120PP -15 176 200 - 300 20 - 60HDPE -80 135 240 - 300 20 - 60LDPE -80 115 180 - 260 20 - 60Injection Moulding 215. Mould Filling Calculations 5. Mould Filling Calculations Require expressions to calculate maximuminjection pressure injection pressure to fill a partTo design/select injection systemTo determine clamping force Can obtain reasonable approximation from:Isothermal analysisMould cavity onlyConstant flow rate In practice moulding operation can be complex:Non-isothermal & non-Newtonian - hence = f (T, )Flow within sprue, runners, gate & mould cavityInjection sequence can be relatively complexInjection Moulding 225.1 Filling Pressures 5.1 Filling PressuresInjection pressure (Pmin) for given flow rate (Q) can bedetermined from non-Newtonian flow expressions(see Melt Rheology & Processing Melt Rheology & Processing notes).Rectangular cavity (depth h):TLGate0 Pmin

= AnTh1).2Q + (2nhL. C P drop pressure = P2n2minThLQ 123an~ (Newtonian, ie. n=1)Injection Moulding 235.1 Filling Pressures 5.1 Filling PressuresInjection pressure (Pmin) for given flow rate (Q) can bedetermined from non-Newtonian flow expressions(see Melt Rheology & Processing Melt Rheology & Processing notes).Rectangular cavity (depth h):TLGate0 Pmin

= AnTh1).2Q + (2nhL. C P drop pressure = P2n2minThLQ 123an~ (Newtonian, ie. n=1)Min. pressure usually not sufficientApply additional pressure to: Compact material Counteract shrinkageHold Hold- -on on pressure of up to 2 x PminInjection Moulding 245.2 Clamping Forces 5.2 Clamping ForcesTLGatePGPXxdxRectangular cavity:Force required to clamp element of mould dx:F = PxA = PxT dxTotal clamping force:dx TP= F xL0 PLx-P=P G x AInjection Moulding 255.2 Clamping Forces 5.2 Clamping ForcesTLGatePGPXxdxRectangular cavity:PLx-P=P G x AAssuming linear linear pressuredistribution:dx PLx-PT = F GL0

A

A2P L- LPT = G

A2P-PTL = GTherefore:Projected area Projected area Clamping force Clamping force = = x x Mean pressure Mean pressureInjection Moulding 265.3 Mould Filling Times 5.3 Mould Filling TimesThe time to fill a mould is simply:rate flow volumevolume total=tfeg. for a rectangular mould cavity:TLGate0 PminQTLh=tfInjection Moulding 275. Mould Filling Calculations 5. Mould Filling CalculationsWorked Example - Injection Mould FillingCalculate the minumum gate pressure required to fill a rectangular plaque cavity,150mm x 25mm x 3mm, with Acrylic resin in one second, assuming thefollowing conditions:(a) Newtonian flow with apparent viscosity a = 1000 Ns/m2or (b) Non-Newtonian flow using Acrylic flow data.If the gate pressure is 1.5 X this minimum, estimate the mould clamping forcerequired for a double impression mould.Page 11Injection Moulding 285. Mould Filling Calculations 5. Mould Filling CalculationsPage 13n = 0.254C = 5.395 x 104n = 0.566C = 9.333 x 1031 s-110 s-140 s-150 s-1102s-1103s-1n = 0.254C = 5.395 x 104n = 0.566C = 9.333 x 1031 s-110 s-140 s-150 s-1102s-1103s-1Injection Moulding 295. Mould Filling Calculations 5. Mould Filling CalculationsExercise sheet 4,Qu. 1 Fig. Q1