1. Stack Molds and Mold Materials Chapter 15 and 16
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Stack Molds
Theory
Definitions and Nomenclature
Arrangement
Stroke and Support
Clamping Force
Rules for Stack Mold Design
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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
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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

5. 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

6. 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
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7. Stack Molds Support and Stroke
Some machines support the moving platen directly on the machine base and the slides on supporting ways.
Fig 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 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 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

8. 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.
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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.
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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
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11. Mold Materials Chapter 16
Material comparison
Average
Table 16.1
Material specifications for common mold materials
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12. 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
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Mold Materials
Heat treating
Stress relieving
Carburizing
Nitriding
Flame hardening, induction hardening
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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
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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
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16. 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
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Mold Materials
Mold finishing
Molding surface finishes
Symbols
Sand blasting and vapor honing
Polioshing and buffinh
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