The document discusses stack molds and mold materials. It provides information on: - The theory and design of stack molds, which consist of two single-face molds mounted back-to-back to double molding capacity. - Key aspects of stack mold design including arrangement, stroke, support, clamping force, and rules. - Different mold materials, their properties, specifications, heat treating processes, and applications. - Processes for case hardening steels like carburizing and nitriding to harden outer surfaces.

1. 1
Stack Molds and Mold Materials
Chapter 15 and 16

2. 2
Stack Molds
• Theory
• Definitions and Nomenclature
• Arrangement
• Stroke and Support
• Clamping Force
• Rules for Stack Mold Design

3. 3
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

4. 4
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.

5. 5
Stack Molds Support and Stroke
• 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.

6. 6
Stack Molds Support and Stroke
• Support
– 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
2
4
1 208.0
Ed
wL
f =
4
3
2
35.2 Ed
WL
f =
15.1
15.2

7. 7
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.

8. 8
Stack Molds Clamping Force
• 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.

9. 9
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.

10. 10
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
– Chains or pull rods
– Hydraulic or air cylinders to mold plate
– Ejection linked with mold movement
– Two-stage ejection

11. 11
Mold Materials Chapter 16
• Material comparison
– Average
• Table 16.1
– Material specifications for common mold materials
Type Designation Hardness
1 Prehardened 4140 35
2 P20 35
3 SS Prehardened 420SS 35
4 Carburizing Steels P5 61
5 P6 60
6 Oil Hardening O1 62
7 Air Hardening H13 51
8 A2 60
9 D2 58
10 Maraging 250 52
11 Maraging SS 455M 48
12 High Speed M2 62
13 Beryllium-Copper Be-Cu 32

12. 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

13. 13
Mold Materials
• Heat treating
– Stress relieving
– Carburizing
– Nitriding
– Flame hardening, induction hardening

14. 14
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

15. 15
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

16. 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

17. 17
Mold Materials
• Mold finishing
– Molding surface finishes
– Symbols
– Sand blasting and vapor honing
– Polioshing and buffinh