Stack Molds and Mold Materials Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Stack Molds Theory Definitions and Nomenclature Arrangement Stroke and Support Clamping Force Rules for Stack Mold Design Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Stack Molds Theory Stack mold is usually a mold assembly consisting of only two single-face molds, mounted back-to-back Example, injection mold of home plate for baseball Plastic materials are thick parts and run vertically. Get extra compression to reduce flash. Used for rubber materials for thermosets up to 10 stacks Clamp force equals sum of the reaction forces R in tie bars For more than one set of cavities and core plates, Fig 15.1, The sum of forces p in the left core block equals F, The sum of the forces, p, in the right core block equals the sum of the reaction forces R The forces p within the cavity block to the left and to the right balance each other. The effective molding area is doubled, and the rated machine clamp force is available for each level (stack) of cavities and cores Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Stack Molds Theory Arrangement One of the two core sections is mounted on the moving platen (single face mold) The other is mounted on the stationary platen (Fig 15.2) Note: Core section mean the portions of mold opposite gate Cavities (gates) are mounted back to back on the center section or floating section of the mold, sandwiched between two core sections. Floating section is supported on the lower tie bars. Plastic enters the mold from the machine nozzle through an extended sprue bushing, (sprue bar) which is mounted in the hot runner manifold. Heated sprue bar passes through core section and is in contact with the machine nozzle only when mold is closed. Dr. Joseph Greene Copyright 2002 All Rights Reserved

Stack Molds Support and Stroke Mold opening and closing stroke of the center section is exactly one-half that of the moving platen. Synchronization of the travel is achieved by rack and pinion arrangement (Fig 15.3) As the mold opens, the lower rack moves to the left at the speed of the clamp. The upper rack is held without moving on the stationary platen. This causes the gear to turn and makes the center section move to the left at half the speed of the clamp motion. Two sets of rack are required; one in the front and one in the rear of the mold. Position of the racks in the rear are inverted to that of the front so that the left rack in the rear is at the top, and the right rack at the stationary side is at the bottom. Dr. Joseph Greene Copyright 2002 All Rights Reserved

Stack Molds Support and Stroke Adequate support of the center section (mass of 2,000 Kg or 2 metric tons) is very important for proper alignment Fig 15.4 Tie Bars supported on the base at the clamp housing and at the stationary platen Any deflection of the supports may seriously affect The life of the mold alignment features (leader pins, bushings, taper locks). Damage the cavities and cores. Deflections From the weight of the tie bars Equation 15.1 Here, w is weight of metal tie bar per in3, L is tie bar length, d is diameter of bar, E is modulus. Weight of the moving platen plus mold half Equation 15.2 Here, W is the weight of the mold, L is tie bar length, d is diameter of bar, E is modulus. Example, a machine that has tie bars of 72 inches and one with 96 inches Table 15.1 Calculated values for tie bar diameters 15.1 15.2 Dr. Joseph Greene Copyright 2002 All Rights Reserved

Stack Molds Support and Stroke Some machines support the moving platen directly on the machine base and the slides on supporting ways. Fig 15.5. Note: Both the upper and lower tie bars are supported indirectly by moving platen. Some machines support the lower tie bars by a number of supports. Fig 15.6. Supports for lower tie bars enable moving platen to slide on them as supporting ways. Moving platen slides on top of the lower tie bars, rather than passing through the platen. Supporting the lower tie bars, eliminates the deflection. Fig 15.7. Supported lower tie bars act as slide supports for the moving platen and center mold section. Note: Stack molds are more susceptible to misalignment caused by tie bar deflection because the distance between stationary and moving platens is greater than for comparable single level molds. Dr. Joseph Greene Copyright 2002 All Rights Reserved

Stack Molds Clamping Force Force required to prevent flashing for each level in stack mold is the same as the force for a comparable single face mold. Since both levels are back to back the projected surface area for two is the same as one. The compressive forces within the center section balance each other Total clamping force is about the same as that for a single level mold with the same projected molding area. Injection forces is usually up to 10 tonnes for small machines and up to 20 tonnes for larger ones. Rule of thumb. Clamping force required for a stack mold is 10% greater than clamp force for single-face mold. Differential cavity space at the bottom of the product can be used to reduce required tonnage by increasing the bottom space of the cavities in the face near clamp side to ensure the same product thickness Fig 15.8. Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Stack Molds Rules Rules Shot volume- twice that required for single face cavities for the same product since you have two stack molds. Injection rate- twice that of single-face cavities to fill the cavities the same time as single face cavity. Cavity layout Shape of product is horseshoe 2, 3, 4, 8, and 12 cavity arrangement is OK. 6 is not Length of sprue bar Sprue bar must not be too long to ensure the machine nozzle does not project too far toward the injection unit. Sprue bar must not be too short to ensure the seat of the antidrool bushing is properly seated. Heating of sprue bar Requires very little heat for maintaining temperature during operation, but requires heat during start-up. Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Stack Molds Rules Rules Clamp stroke Required clamp stroke is twice that is required for single-face mold. Stroke in both levels of the mold must be the same, even if one on face is 40 mm high and the second face is 20 mm high then the stroke must be 40 mm. Ejection mechanism Air ejection is preferred For hydraulic ejection needs to be on both sides of the mold versus standard moving side actuators. Stack molds require for the cores mounted on stationary platen have to have ejection mechanisms added, including: Chains or pull rods Hydraulic or air cylinders to stationary platen Hydraulic or air cylinders to mold plate Ejection linked with mold movement Two-stage ejection Dr. Joseph Greene Copyright 2002 All Rights Reserved

Mold Materials Chapter 16 Material comparison Average Table 16.1 Material specifications for common mold materials Dr. Joseph Greene Copyright 2002 All Rights Reserved

Mold Materials Chapter 16 Table 16.2 Properties of mold materials rated from Table 16.1 Table 16.3 Materials selection guide Table 16.4 Specification comparison for mold materials Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Mold Materials Heat treating Stress relieving Carburizing Nitriding Flame hardening, induction hardening Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Tempering of Steels Rapid drops in temperature causes internal stresses in metals. Tempering is the process of re-heating the metal immediatley after hardening to a temperature below the transformation temperature [700F and 800F] for 1 hour per inch of thickness then cooled to increase the ductility and toughness of steel. Tempering is also called drawing because it “draws” the hardness from the metal Types of tempering martempering: part is quenched to a temperature just above the Ms line [between 500F and 600 F]for a few seconds to allow temperature throughout the part to stablilize. Then the part is quenched through the martensitic range to room temperature Provides more uniform grain structure as it enters martensitic range More stress free Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Tempering of Steels Types of tempering (continued) austempering: resembles martempering, except after leveling the temperature at 700F, it is held for a longer period of time while it passes through the Ps and Pf lines. Bainite is formed which is the region of transformation between the rapid cooling curves for martensite and slower cooling pearlite. Bainite has superior ductility and tougness but inferior hardness and strength versus martensite. Once Bainite is formed, the steel is quenched to room temperature isothermal quenching and tempering fits somewhere between martempering and austempering. Steel is harder and stronger than austempering, yet more ductile and stress free than martempering. Structure is combination of bainite and tempered martensite. Metal is heated to autenite range then quenched to about 50% transformation from austenite to martensite. Temperature of 300F is held for a few seconds while remaiing austenite transforms to bainite. Quenched to room temperature Dr. Joseph Greene Copyright 2002 All Rights Reserved

Case Hardening of Steels Case hardening involves four different methods carburizing: crowing considerable amounts of carbon into the outer surface of the steel. Placing low-carbon steel in high carbon atmosphere and heating to 1400F. Nitriding: same process as carburizing except that nitrogen is added to the outer shell of the part by plaicng the part in a nitrogen rich atmosphere. Cyaniding: supply carbon and nitrogen to the steel. Hot steel is immersed in sodium cyanide for several hours while C and N disperse in steel. Carbonitriding: same as Cyaniding Case hardening is used on parts for gear teeth, cutting wheels, and tools. Flame hardening Induction hardening Dr. Joseph Greene Copyright 2002 All Rights Reserved

Dr. Joseph Greene Copyright 2002 All Rights Reserved Mold Materials Mold finishing Molding surface finishes Symbols Sand blasting and vapor honing Polioshing and buffinh Dr. Joseph Greene Copyright 2002 All Rights Reserved