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Chapter 8

Chapter 8. Design for Injection Molding. Outline. 8.7 MOLDING CYCLE TIME 8.7.1 Injection Time 8.7.2 Cooling Time 8.7.3 Mold Resetting 8.8 MOLD COST ESTIMATION 8.8.1 Mold Base Costs 8.8.2 Cavity and Core Manufacturing Costs Geometrical Complexity Counting Procedure

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Chapter 8

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  1. Chapter 8 Design for Injection Molding Dr. Mohammad Abuhaiba

  2. Outline 8.7 MOLDING CYCLE TIME 8.7.1 Injection Time 8.7.2 Cooling Time 8.7.3 Mold Resetting 8.8 MOLD COST ESTIMATION 8.8.1 Mold Base Costs 8.8.2 Cavity and Core Manufacturing Costs Geometrical Complexity Counting Procedure 8.9 MOLD COST POINT SYSTEM 8.10 ESTIMATION OF THE OPTIMUM NUMBER OF CAVITIES 8.11 DESIGN EXAMPLE 8.12 INSERT MOLDING 8.13 DESIGN GUIDELINES 8.14 ASSEMBLY TECHNIQUES 8.1 INTRODUCTION 8.2 INJECTION MOLDING MATERIALS 8.3 THE MOLDING CYCLE 8.3.1 Injection or Filling Stage 8.3.2 Cooling or Freezing Stage 8.3.3 Ejection and Resetting Stage 8.4 INJECTION MOLDING SYSTEMS 8.4.1 Injection Unit 8.4.2 Clamp Unit 8.5 INJECTION MOLDS 8.5.1 Mold Construction & Operation 8.5.2 Mold Types 8.5.3 Sprue, Runner, and Gates 8.6 MOLDING MACHINE SIZE Dr. Mohammad Abuhaiba

  3. 8.1 INTRODUCTION Injection molding (IM) technology consists of: • Heating thermoplastic material until it melts • Forcing this melted plastic into a steel mold, where it cools and solidifies. Dr. Mohammad Abuhaiba

  4. 8.2 INJECTION MOLDING MATERIALS • Polymers that are capable of being brought to a state of fluidity can be injection-molded. • Polymers can be divided into two categories: • thermoplastic • thermosetting Dr. Mohammad Abuhaiba

  5. 8.2 INJECTION MOLDING MATERIALS - Thermoplastic polymers (TP) • Capable of being softened by heat and of hardening on cooling. • This is because long-chain molecules always remain separate entities and do not form chemical bonds to one another • Most TP materials offer: • high impact strength • good corrosion resistance • easy processing with good flow characteristics for molding complex designs. Dr. Mohammad Abuhaiba

  6. 8.2 INJECTION MOLDING MATERIALS- Thermoplastic polymers (TP) • Thermoplastics are generally divided into two classes: • Crystalline (CP) • Amorphous (AP) • Crystalline polymers: • ordered molecular arrangement • sharp melting point • Because of the ordered arrangement of molecules, CP reflect most incident light and generally appear opaque. • High shrinkage or reduction in volume during solidification. • More resistant to organic solvents • have good fatigue and wear-resistance properties. • are denser and have better mechanical properties than amorphous polymers Dr. Mohammad Abuhaiba

  7. 8.2 INJECTION MOLDING MATERIALS- Service temperature Heat deflection temperature: • Temperature at which a thermoplastic can be operated under load. • This is the temperature at which a simply supported beam specimen of the material, with a centrally applied load, reaches a predefined deflection. Dr. Mohammad Abuhaiba

  8. 8.2 INJECTION MOLDING MATERIALS - Thermosetting • Chemical bonds are formed between the separate molecule chains during processing. • Referred to as cross-linking, is the hardening mechanism. Dr. Mohammad Abuhaiba

  9. 8.2 INJECTION MOLDING MATERIALS Dr. Mohammad Abuhaiba

  10. 8.3 THE MOLDING CYCLE • Stages of injection molding: • injection or filling • cooling • ejection and resetting Dr. Mohammad Abuhaiba

  11. 8.3 THE MOLDING CYCLE • During 1st stage, material in molten state is a highly nonlinear viscous fluid. • It flows through mold passages and is subject to rapid cooling from mold wall, on one hand, and internal shear heating, on the other. • Melt then undergoes solidification under high packing and holding pressure. • Mold is opened, part is ejected, and machine is reset for next cycle to begin. Dr. Mohammad Abuhaiba

  12. 8.3 THE MOLDING CYCLEInjection or Filling Stage • Forward stroke of plunger to facilitate flow of molten material from the heating cylinder through nozzle and into mold. • Gradual increase in pressure. • As soon as cavity is filled, pressure increases rapidly, and packing occurs. • During packing part, flow of material continues, at a slower rate, to account for any loss in volume of material due to partial solidification and shrinkage. • After packing, injection plunger is withdrawn and pressure in mold cavity begins to drop. • At this stage, next charge of material is fed into the heating cylinder in preparation for next shot. Dr. Mohammad Abuhaiba

  13. 8.3 THE MOLDING CYCLECooling Stage • Cooling starts from 1st rapid filling of cavity and continues during packing and then following withdrawal of the plunger, with the resulting removal of pressure from the mold and nozzle area. • Upon pressure removal, gate of mold may still be relatively fluid. • Because of pressure drop, there is a chance for reverse flow of material from mold until material adjacent to the gate solidifies and the sealing point is reached. • Reverse flow is minimized by proper design of gates such that quicker sealing action takes place upon plunger withdrawal. • Following the sealing point, there is a continuous drop in pressure as material in cavity continues to cool and solidifies in readiness for ejection. • Length of sealed cooling stage depends on: • wall thickness of part • material used • mold temperature • Because of low thermal conductivity of polymers, cooling time is usually the longest period in the molding cycle. Dr. Mohammad Abuhaiba

  14. 8.3 THE MOLDING CYCLEEjection and Resetting Stage • During this stage: • mold is opened • part is ejected, • mold is then closed again in readiness for next cycle to begin. • rapid movements may cause: • undue strain on the equipment • damage the edges of the cavities. • Adequate time must be allowed for mold ejection. • This time depends on part dimensions • For parts to be molded with metal inserts, resetting involves reloading of inserts into mold. After resetting, mold is closed and locked, thus completing one cycle. Dr. Mohammad Abuhaiba

  15. 8.4 INJECTION MOLDING SYSTEMS Components of injection molding system: • injection unit • clamp unit • mold Dr. Mohammad Abuhaiba

  16. 8.4 INJECTION MOLDING SYSTEMSInjection Unit • The injection unit has two functions: • to melt pellets or powder • to inject the melt into a mold. • Most widely used types of injection units: • conventional units: consists of a cylinder and a plunger • reciprocating screw units: a barrel and a screw that rotates to melt & pump the plastic mix from hopper to end of screw and then moves forward to push the melt into mold. Dr. Mohammad Abuhaiba

  17. 8.4 INJECTION MOLDING SYSTEMSInjection Unit • Injection units are usually rated with two numbers: • First rating No.: Shot capacity • Second rating number: plasticating rate • Shot capacity: max volume of polymer that can be displaced by one forward stroke of injection plunger or screw. • recommended shot sizes: 20 to 80% of rated capacity. Dr. Mohammad Abuhaiba

  18. 8.4 INJECTION MOLDING SYSTEMSInjection Unit • Plasticating rate: amount of material that can be softened into a molten form by heating in the cylinder of machine in a given time. • It is usually expressed as No. of pounds of polystyrene material that the equipment can heat to molding temperature in one hour Dr. Mohammad Abuhaiba

  19. 8.4 INJECTION MOLDING SYSTEMSClamp Unit • Clamp unit has three functions: • open and close mold halves • eject the part • hold mold closed with sufficient force to resist melt pressure inside mold as it is filled • Required holding force: 30 to70 MN/m2 of projected area of part Dr. Mohammad Abuhaiba

  20. 8.4 INJECTION MOLDING SYSTEMSClamp Unit • Magnitude of initial opening force required depends on: • packing pressure • Material • part geometry (depth and draft) • is approximately equal to 10 to 20% of nominal clamp force. Dr. Mohammad Abuhaiba

  21. 8.4 INJECTION MOLDING SYSTEMSClamp Unit Two common types of clamp designs: • Linkage or toggle clamp: • very fast closing and opening actions • lower in cost than alternative systems • clamp force is not precisely controlled • Hydraulic clamp units: • long term reliability • precise control of clamp force • relatively slow and expensive compared to toggle clamp systems. Dr. Mohammad Abuhaiba

  22. 8.4 INJECTION MOLDING SYSTEMSClamp Unit • Force required to eject the part depends on: • Material • part geometry • packing pressure • less than 1% of nominal clamp force Dr. Mohammad Abuhaiba

  23. 8.5 INJECTION MOLDS • Functions of a mold: • impart the desired shape to the plasticized polymer • cool the molded part • A mold is made up of: • the cavities and cores • the base in which the cavities and cores are mounted Dr. Mohammad Abuhaiba

  24. 8.5 INJECTION MOLDSMold Constructionand Operation Dr. Mohammad Abuhaiba

  25. 8.5 INJECTION MOLDSMold Constructionand Operation • Fixed Clamping Plate • Runner Stripper Plate • Cavity plate • Movable Cavity Plate or Cavity plate • Back up Plate • Spacer Block • Ejector retainer plate • Ejector Plate • Movable Clamping Plate Dr. Mohammad Abuhaiba

  26. 8.5 INJECTION MOLDSMold Construction and Operation • Mold basically consists of two parts: • a stationary half (cavity plate) • a moving half (core plate) • Parting line: separating line between the two mold halves • The injected material is transferred through a central feed channel, called the sprue. • In multi-cavity molds, sprue feeds polymer melt to a runner system. Dr. Mohammad Abuhaiba

  27. 8.5 INJECTION MOLDSMold Construction and Operation • Core plate holds the main core. • Purpose of main core is to establish the inside configuration of the part. • The core plate has a backup plate. • Backup plate in turn is supported by pillars against the U shaped structure known as the ejector housing, which consists of the rear clamping plate and spacer blocks. • The U-shaped structure, which is bolted to core plate, provides the space for the ejection stroke. Dr. Mohammad Abuhaiba

  28. 8.5 INJECTION MOLDSMold Construction and Operation • During solidification part shrinks around main core so that when mold opens, part and sprue are carried along with moving mold half • Both mold halves are provided with cooling channels • Mold cavities incorporate fine vents (0.02 to 0.08mm by 5mm) Dr. Mohammad Abuhaiba

  29. 8.5 INJECTION MOLDSMold Types Most common types of molds: • Two-plate molds • Three-plate molds • Side-action molds • Unscrewing molds Dr. Mohammad Abuhaiba

  30. 8.5 INJECTION MOLDSMold Types - A two-plate mold • consists of two active plates (Fig. 8.3) (cavity and core plates) into which cavity and core inserts are mounted, as shown in Fig. 8.4. • Runner system, sprue, runners, and gates solidify with part being molded and are ejected as a single connected item. Dr. Mohammad Abuhaiba

  31. 8.5 INJECTION MOLDSMold Types - A two-plate mold Dr. Mohammad Abuhaiba

  32. 8.5 INJECTION MOLDSMold Types - three-plate mold • Consists of: • Stationary or runner plate, which contains sprue and half of runner • Middle or cavity plate, which contains other half of runner, gates, and cavities and is allowed to float when mold is open • Movable or core plate, which contains cores and ejector system. • Facilitates separation of runner system and part when mold opens Dr. Mohammad Abuhaiba

  33. 8.5 INJECTION MOLDSMold Types - Hot runner system • Three main plates • Runner is contained completely in the fixed plate, which is heated and insulated from the rest of the cooled mold. • Runner section of the mold is not opened during molding cycle. • There are no side products (gates, runner, or sprues) to be disposed of or reused • There is no need for separation of gate from part. Dr. Mohammad Abuhaiba

  34. 8.5 INJECTION MOLDSMold Types - Side-acting molds • are used in molding components with external depressions or holes parallel to the parting plane. • Undercuts prevent molded parts from being removed from cavity in axial direction. • The usual way of providing the side action needed to release the part is with side cores mounted on slides. • These are activated by angle pins, or by air or hydraulic cylinders that pull the side cores outward during opening of the mold. Dr. Mohammad Abuhaiba

  35. 8.5 INJECTION MOLDSMold Types - Side-acting molds • The slide, which carries the secondary side core pin, is moved by the angle pin mounted in the stationary half of the mold. • As the two halves of the mold move apart during mold opening, the slide, which is mounted on the moving plate, is forced to move sideways by the angle of the pin. Dr. Mohammad Abuhaiba

  36. 8.5 INJECTION MOLDSMold Types - Side-acting molds Dr. Mohammad Abuhaiba

  37. 8.5 INJECTION MOLDSMold Types - unscrewing molds Dr. Mohammad Abuhaiba

  38. 8.5 INJECTION MOLDSSprue, Runner, and Gates • Fig. 8.4 • Sprue: an inlet channel for molten material from the heating chamber into the mold or runner system. • The gate: a constriction between feed system and mold cavity, serves several purposes: • It freezes rapidly and prevents material from flowing out of cavity when injection pressure is removed. • It provides an easy way of separating moldings from runner system. Dr. Mohammad Abuhaiba

  39. 8.6 MOLDING MACHINE SIZE • Determination of appropriate size of an injection molding machine is based primarily on required clamp force. • This in turn depends upon projected area of cavities in mold and max pressure in the mold during mold filling. • For a 15cm diameter plain disk, projected area is 176.7 cm2. • If the disk has a single 10cm diameter through hole in any position, projected area is 98.2 cm2 Dr. Mohammad Abuhaiba

  40. 8.6 MOLDING MACHINE SIZE • Size of runner system depends upon size of part. • As a first approximation, these figures will also be applied to give projected area of runner system as a percentage of projected area of part. Dr. Mohammad Abuhaiba

  41. 8.6 MOLDING MACHINE SIZE • 50% of pressure generated in machine injection unit is lost because of flow resistance in sprue, runner systems, and gates Dr. Mohammad Abuhaiba

  42. 8.6 MOLDING MACHINE SIZEExample A batch of 15 cm diadisks with a thickness of 4 mm is to be molded from ABS in a 6-cavity mold. Determine appropriate machine size. • Projected area of each part = 177cm2. • Table 8.2: % increase in area due to runner system = 15%. • Total projected shot area = 6x1.15x177= 1221.3 cm2 • Table 8.5: injection pressure for ABS =1000 bars • Max cavity pressure = 500 bars= 500x105 N/m2 • Max separating force F =(1221.3x10-4)x500x105N = 6106.5 kN Dr. Mohammad Abuhaiba

  43. 8.6 MOLDING MACHINE SIZEExample A batch of 15 cm diadisks with a thickness of 4 mm is to be molded from ABS in a 6-cavity mold. Determine appropriate machine size. • Table 8.4: appropriate machine would be the one with a max clamp force of 8500kN. • Required shot size = volume of six disks +volume of runner system = 6x 1.15 x (177x0.4) = 489 cm3, which is easily within max machine shot size of 3636 cm3. Dr. Mohammad Abuhaiba

  44. 8.6 MOLDING MACHINE SIZEExample A batch of 15 cm diadisks with a thickness of 4 mm is to be molded from ABS in a 6-cavity mold. Determine appropriate machine size. • For the 8500 kNmachine, machine clamp stroke = 85 cm • This stroke is sufficient to mold a hollow part up to a depth of approximately 40 cm. • For such a part, the 85 cm stroke would separate the molded part from both the cavity and the core with a clearance of approximately 5 cm for the part to fall between the end of the core and the cavity plate. • This stroke is excessive for molding of 4 mm thick flat disks. Dr. Mohammad Abuhaiba

  45. MOLDING CYCLE TIME Molding cycle: • injection or filling time • cooling time • mold-resetting time Dr. Mohammad Abuhaiba

  46. MOLDING CYCLE TIMEInjection Time • Initial flow rate gradually decrease as mold is filled: • flow resistance in mold channels • constriction of channels as polymer solidifies against the walls • Flow rate suffers a constant deceleration to reach a low value at the point at which mold is nominally filled. • Under these circumstances, the fill time would be estimated as • Pj = injection power, W • Pj = recommended injection pressure, N/m2 • Vs = required shot size, m3 Dr. Mohammad Abuhaiba

  47. MOLDING CYCLE TIMEInjection Time - Example • For the 15 cm dia disks molded in a six-cavity mold, required shot size is 489 cm3. • Recommended injection pressure for ABS is 1000 bars, or 100 MN/m2. • Available power at injection unit of the 8500kN machine is 90 kW. Thus estimated fill time is • tf = 2 x (489 x 10-6) x (100 x 106)/(90 x 103) = 1.09 s Dr. Mohammad Abuhaiba

  48. MOLDING CYCLE TIMECooling Time • Mold opening and ejection are assumed to be permissible when injected polymer has cooled to the point where the highest temperature in mold (at thickest wall center plane) equals Tx, recommended ejection temperature. Dr. Mohammad Abuhaiba

  49. MOLDING CYCLE TIMECooling Time • Cooling time is given by • hmax = max wall thickness, mm • Tx = recommended part ejection temperature, °C • Tm = recommended mold temperature, °C • Ti= polymer injection temperature, °C • a = thermal diffusivity coefficient, mm2/s Dr. Mohammad Abuhaiba

  50. MOLDING CYCLE TIMECooling Time • Eq. (8.5) tends to underestimate cooling time for very thin wall moldings. • For such parts thickness of runner system is often greater than parts themselves and greater delay is needed to ensure that runners can be ejected cleanly from the mold. • 3 s be taken as min cooling time even if Eq. (8.5) predicts a smaller value. Dr. Mohammad Abuhaiba

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