FUNDAMENTALS OF INJECTION MOLDING Injection Moulding Machine Injection Molding & Mold Design Injection Molding • Injection molding is the most important process used to manufacture plastic products. • More than one third of all thermoplastic materials are injection molded. • It is ideally suited to manufacture massproduced parts of complex shapes that require precise dimensions. Injection Molding Process Mould Close & IU Forward INJECTION AND HOLDON DOSING SUCK BACK INJECTION UNIT RETRACT MOULD OPEN EJECTOR FORWARD EJECTOR BACK INJECTION UNIT Function: Rotation of screw for feeding and plasticising the material Axial movement of the screw for injection Pressure for injection, holding and back pressure Provide electrical heating to melt the material Movement on its support to bring the nozzle into contact with the sprue of the mould or to retract it. Keep pressure between the nozzle and sprue bush Mould close slow - fast - slow mould sensing locking Pause Time Nozzle Advance Injection Hold-on Pressure Metering Screw Suck back Ejector Operation Mould Open unlocking - slow 1 - fast - slow2 Nozzle Return Setting up the Injection Unit Barrel Temperature Feed throat temperature Metering Stroke Screw speed Back pressure Back Pressure Profiling IU retract stroke Setting up the Injection Unit…. Injection Speed Injection Pressure Holding Pressure Holding Time Cooling time Various stages in mold filling 1 2 3 4 Dosing stroke - stroke settings 1End point of injection stroke 2 Melt cushion 1 3 2 4 3 Switchover point 4 Dosing stop Factors affecting the flow behaviour of melt inside the cavity • • • • • • Viscosity of the melt Flow front speed Mould temperature Melt temperature Mould geometry Material characteristics Viscosity Viscosity of a polymer melt is a measure of its resistance to flow Therefore Higher the viscosity of the melt, Greater will be the pressure requirements during flow Typical viscosity data V I S C O S I T Y 450 400 350 300 250 200 150 100 50 0 PC PPO 300,360 280,320 ABS 240,260 Viscosity.... Newtonion and Non Newtonion flow Newtonion flow Viscosity Non Newtonion flow Flow rate INJECTION PRESSURE 360 deg C INJECTION SPEED 300 deg C Viscosity... Therefore the two major factors affecting viscosity are Temperature Flow rate The inherent viscosity of the material is dependent on its Molecular structure Fillers present A conceptual Model of melt flow inside the cavity Cavity Injection Unit 3 2 1 1 2 3 Heating and cooling effects during Mould Filling Heating Heat carried by the melt Heat generated during flow due to shear Cooling Heat losses due to conduction Heat loss due to expansion Molecular Orientation Gate Direction of flow O R I E N T A T I O N Frozen in Orientation Minimum orientation at core WALL THICKNESS Orientation... Factors affecting orientation Increasing melt Temp. Increasing mould Temp. Increasing inj. Speed Increasing Holdon Pressure Shrinkage Shrinkage is the reduction in dimensions of the component after cooling Cavity dimensions Part dimensions Expressed as mm/mm or inch/inch Factors affecting Shrinkage Factor Increasing Crystallinity Increase in part thickness Higher melt temp. Higher mold temp. Increased holdon time Larger gates Effect on Shrinkage Remarks Increase More closer packing of molecules Slow cooling of interior portion Material at higher temp. shrinks more Slow cooling Increase Increase Increase Decrease Decrease More time for packing Longer gate freeze off time WARPAGE Warpage is the shape distortion of a plastic object after demolding Major causes of warpage Differential cooling Cavity pressure differences Radial gating Material considerations RESIDUAL STRESSES Residual stresses are mechanical stresses that are present in the molding in the absence of external loading Outer Skin Layer Compressive stresses Sectional view of the part along flow direction Inner Core Tensile Stresses Residual Stresses... Influence of process parameters • Mold Over packing • Mold wall temperature • Cooling time Residual Stress • What Causes Residual Stress? • Unbalanced Residual Stress What Causes Residual Stress? Free Contraction Heat Molten Polymer Heat Frozen Layers Early Cooling Stage What Causes Residual Stress? Constrained Contraction Cooling Polymer Later Cooling Stage What Causes Residual Stress? Thermal Induced Residual Stresses Tensile (+) (-) Compressive Post-Molding Stage Unbalanced Residual Stress High Cooling Rate Heat Heat Low Cooling Rate Cooling Channel Early Cooling Stage: Uneven Cooling Unbalanced Residual Stress Tensile(+) (-)Compressive Post-Molding Stage: Asymmetrical Thermal-Induced Residual Stress Unbalanced Residual Stress Warped Part Warpage Warpage is the deformation of the part after ejection due to high internal stresses Warpage Unbalanced cooling Cavity pressure differences High ejection temperatures Low Holding time/pressure Shrinkage and Warpage • Why Does One Plastic Shrink Differently Than Another? • What Influences Shrinkage? • What Causes Part Warpage? • How to Minimize Warpage Problems Shrinkage What Influences Shrinkage? Melt Temperature Mold Temperature Injection Rate Shrinkage What Influences Shrinkage? Holding Pressure Holding Time Part Thickness What Causes Part Warpage? Hot Part Cold Part Warps Toward Hot Surface Hot Cold Part Warps Toward Hot Surface Unbalanced Cooling What Causes Part Warpage? High Shrinkage High Cooling Rate Low Crystallization Level Low Cooling Rate High Crystallization Level Warped Part Non-Uniform Wall Thickness What Causes Part Warpage? Unfilled Materials High Gate Pressure Low Pressure Flow Direction Low High Shrinkage Shrinkage Differential Shrinkage Fiber-Filled Materials Molecular and Fiber Orientation Fiber Orientation How to Minimize Warpage Problems • Avoid Large Pressure Variations – Wall Thickness Variation – Long Flow Lengths – Uneven Filling Pattern • Avoid Large Temperature Variations – Wall Thickness Variation – Unbalanced Cooling Cavity Pressure profile during mold filling process CAVITY PRESSURE CAVITY PRESSURE PROFILE 1400 1200 1000 800 600 400 200 0 1 2 3 4 5 TIME 6 7 8 CAVITY PRESSURE... Reasons for pressure rise during injection phase • Increase in melt viscosity • Increasing flow lengths • Narrowing melt flow channel Switchover Point Type of switchover How triggered Time dependent Stroke dependent Hydraulic Pressure Cavity pressure After a preset inj. time After a preset inj.stroke After a preset Hyd. Pr After a preset cavity pr. Remarks Highly inflexible Rarely used Most commonly used Only one pr. transducer needed Each mould needs a transducer INJECTION SPEED PROFILING Need for injection speed profiling to avoid Jetting to prevent diesel effect to maintain a constant flow front speed to have a smooth switchover INJECTION SPEED PROFILING... CAVITY GATE INJECTION SPEED PROFILE RUNNER HOLDING PRESSURE PROFILING HOLDING PRESSURE TIME Pressure Pressure Requirements P1 P2 Flow Length What Influences Injection Pressure? • • • • Part Design Mold Design Process Conditions Material Properties Part Design Impact • Wall Thickness Thin Part Thick Part Higher Pressure Lower Pressure Part Design Impact • Surface Area More Wall Cooling & Drag Force Less Wall Cooling & Drag Force Higher Pressure Lower Pressure Process Impact • Fill Time Too Short Too Long Higher Pressure Optimal Fill Time Lower Pressure Slow vs. Fast Fill Time Thick Frozen Layer Melt Long Fill Time Low Injection Speed Thin Frozen Layer Melt Short Fill Time High Injection Speed Fill Time vs. Injection Pressure Maximum Injection Pressure (MPa) 80 Optimal Fill 60 Time Range 40 20 0 4 8 12 Fill Time (sec) 16 20 Process Impact • Melt Temperature Colder Melt Hotter Melt Higher Pressure Lower Pressure Melt Temperature Affects Resin Viscosity Viscosity (lb ft-s/ft^2) Viscosity for Polycarbonate 100 590 F 640 F 10 1 1 10 100 1000 Shear Rate (1/s) 10000 Process Impact • Mold Temperature Colder Coolant Temperature Hotter Coolant Temperature Higher Pressure Lower Pressure So What’s a Process Window? GE Lexan 101 @ 624 (F) Melt Temperature (F) 650 640 630 620 610 600 590 5000 10000 15000 Injection Pressure (psi) 20000 Process window (MAD) Flow Length = 5” Center Gate 650 Melt Temperature (F) Melt Temperature (F) Thickness Change Affects the Process Window 640 630 620 610 600 590 5000 10000 15000 20000 Injection Pressure (psi) 650 Flow Length = 10” End Gate 640 630 620 610 600 590 5000 10000 15000 20000 Injection Pressure (psi) MVD(Molding volume diagram) Flash Short shots Dosing stroke Injection speed Injection pressure Mould temperature Melt temperature Venting Holding time/pressure Material Impact • Resin Flow Properties Low MFR Material W Higher Pressure High MFR Material W Lower Pressure Mold Design Impact • Flow Length (Gate Location) Long Flow Length Short Flow Length Higher Pressure Lower Pressure Mold Design Impact • Gate Size Restrictive Gate Generous Gate Higher Pressure Lower Pressure Sink marks and voids Poor design Sink marks Void Better design Weld lines Weld lines are formed where the the flow fronts merge JETTING Slow melt front speed at the gate crossing point establishes laminar flow High melt front speeds at the gate crossing point causes ‘jetting’ After jetting the laminar flow front will be established. But the jetted portion will not fuse with the surrounding material Record grooves (Ripple effect) (Resemble the grooves of gramaphone record ) Low injection speed Low injection pressure Mould too cold Low melt temperature Silver streaks (mica marks) Streaky silvery appearance of the moulding nearly always radiating from the gate area Moisture in the pellets Volatiles due to over heating Low back pressure CLAMPING UNIT Function : * Open and close the mould * Keep the mould locked when subjected to injection and follow up pressure * Eject the moulded part. *To accommodate different size of MOULDS Various Clamping Mechanisms Direct clamping by •Hydraulic Cylinders * Single Cylinder * Double Cylinder Clamping by Toggles actuated by small hydraulic cylinder * 4 point * 5 point twin toggle twin toggle Clamping by special Mechanisms * Swing plate * Slide plate Various Types of TOGGLE Clamping Mechanism Machine Setup Mould clamping Settin temperatures Speeds / Strokes / Tonnage Mould protection Ejector strokes / speeds Dosing stroke Screw speed / back pressure Injection speed / pressure Switchover point Holding pressure & time Cooling time Machine Setup Setting up the Clamping Unit Mounting the Mold Setting the Strokes / Speeds Setting the Tonnage Setting Mold Sensing Setting Ejector Strokes MOULD SETUP CHECKLIST The shut height (H) of the mould should be within the min. and maximum limits of the machine H Mould locating ring diameter should match the bore diameter in the fixed platen T W The width (W) of the mould should be lesser than the distance between tie-bars (T) I.e. W < T MOULD SETUP CHECKLIST The opening stroke required by the mould (S) should be within the maximum opening stroke (L) of the machine S L The weight of the mould should not exceed the recommended maximum mould weight The minimum mould diameter must be greater than the specified lower limit. MOULD SETUP CHECKLIST Nozzle nose cone radius (r) must be lesser than the sprue bush radius (R). Any mis-match (as shown in the figure left will lead to material drool and sprue getting stuck R r WRONG RIGHT Nozzle orifice must be lesser than the sprue bush orifice MOULD SETUP CHECKLIST Likely fouling points Ensure that the nozzle heater and thermocouple do not foul with the mould while the nozzle makes contact with the sprue bush. If the sprue bush is located deep inside the mould, use extended nozzles MOULD SETUP - Loading the mould 1 Close the mould fully till the toggles are straight. Take the end platen backwards by ‘Mould Height’ gear, till sufficient gap is created for lowering the mould. the X Change operating mode to “setting” during mould setting 2 Lower the mould from the top. Take care to see that the mould doesn't touch the tie-bars while being lowered 3 4 Clamp the mould halves to the platens and Remove the link piece (if any). Remove the crane chain. Locate the mould properly and bring the moving platen forward by ‘Mould Height’ adjustment till the moving platen touches the back plate of the mould MOULD SETTING - Mould Open the X Change operating mode to “setting” during mould setting V2 V1 V3 3 2 1 1. Start with low opening speeds. (V1) 2. Increase the speed after the mould opens slightly. (V2) 3. Decrease the speed again in the end to avoid jerk and override during stopping at end position.(V3) MOULD SETTING - Mould Close the X Change operating mode to “setting” during mould setting V2 V3 V1 1 2 3 1. Start with low closing speeds. (V1) 2. Increase the speed after the mould starts closing. (V2) 3. Decrease the speed again just before the two mould halves make contact. (V3) MOULD SETTING - Mould Close the X Change operating mode to “setting” during mould setting Set minimum mould sensing pressure which is just sufficient to move the mould in the sensing zone Set mould sensing time slightly higher than the time taken by the moving platen to clear the sensing zone Sensing zone Lock Set the ‘start mould sensing’ at a point just before the projecting surfaces of the moving half (such as guide pins, finger cams, core etc.) start entering into the fixed half of the mould. Set the ‘mould sensing stop’ at a nominal minimum distance from the fixed half of the mould. Normally this will about 2 to 3mm. Setting the strokes of the moving platen A A= Mold open stroke B= Mold shut height with toggles fully stretched B A Mold open speed Stroke Mold close speed Stroke Setting the tonnage LOW TONNAGE…. Leads to mold flashing VERY HIGH TONNAGE…. Leads to mold damage OPTIMUM TONNAGE SHOULD BE BASED ON THE ESTIMATED AVERAGE CAVITY PRESSURE Setting Ejector stroke E E = Maximum permissible stroke Basic Structure of an Injection Mold
