Hold-pressure control and clamping in stacked multi-parting molding system having desynchronized injection periods

A system and method of controlling hold pressure applied to molding cavities of a stacked multi-parting injection molding system having desynchronized injection periods while cyclic injection molding a plastic material. A combination of the center molding block and an injection system together provide an adjustable feed system. The feed system is adjusted to enable plastic material to be fed from the injection system through a left branch to the left molding cavity. The injection system is pressurized to inject the plastic material through the left branch to fill the left molding cavity. The injected plastic material is confined within the left branch and the left molding cavity; and the confined plastic material is hold pressurized by a movable packing element. A like sequence of steps is followed to hold pressurize the right molding cavity. When the molding cavities have desynchronous opening periods, the closed molding cavity is locked to maintain it in a closed position while the other molding cavity is open; and axial clamping force is applied to the locked molding cavity by expanding the thickness of at least one of the molding blocks that defines the locked molding cavity.

BACKGROUND OF THE INVENTION 
The present invention generally pertains to molding systems and methods and 
is particularly directed to control of hold pressure and clamping in a 
stacked multi-parting molding system having desynchronized injection 
periods. 
A stacked multi-parting molding system includes at least three molding 
blocks disposed for movement along a common axis with respect to each 
other. Molding cavities are defined between adjacent molding blocks. In a 
desynchronized stacked multi-parting molding system that includes left, 
center, and right molding blocks for defining a left molding cavity 
between the left and center molding blocks and a right molding cavity 
between the center and right molding blocks, positioning means move the 
left, center and right molding blocks along the common axis with respect 
to each other in such a sequence that the opening and closing of the left 
and right molding cavities may be desynchronized. Desynchronized stacked 
multi-parting molding systems are described in U.S. Pat. Nos. 4,400,431, 
4,464,327 and 4,539,171, all to Jens Ole Sorensen and 4,146,600 to Elly et 
al. Desynchronized injection of plastic material into the molding cavities 
may take place when both mold cavities are closed, as well as when one 
molding cavity is open and the other mold cavity is closed. 
Hold pressure is the pressure applied to plastic material injected into a 
molding cavity of an injection molding system after the molding cavity is 
initially filled with plastic by an injection system. Hold pressure is 
applied to force additional plastic material into the molding cavity as 
the initially injected material shrinks in the molding cavity upon 
hardening in order to assure that the plastic material completely fills 
the cavity. Otherwise the molded product may be crimped. 
In a prior art stacked multi-parting molding system, the injection system 
is used to apply hold pressure in the molding cavities. However, the use 
of the injection system to apply hold pressure may increase the cycle time 
because less time will be available during the molding cycle for 
plasticization of the molding material prior to injection. Also, with 
typical desynchronized stacked molding systems, when at least one of the 
molding cavities is opened, the injection system is separated from the 
remainder of the feed system that carries the plastic material to the 
molding cavities, thereby reducing the time available for the injection 
system to apply hold pressure. 
While hold pressure is being applied to the molding cavities, the molding 
cavities must be tightly clamped. While both molding cavities are closed, 
the means that position the molding blocks apply an axial clamping force 
to the molding cavities along their common axis to thereby tightly clamp 
the molding cavities. However, whenever one molding cavity is opened, this 
axial clamping force is no longer applied by the positioning means. 
Although Elly et al describe the use of wedge-shaped locking means for 
maintaining one molding cavity in a closed position while the other 
molding cavity is in an open position, the axial clamping force that is 
applied by the wedge-shaped locking means to the molding cavity that 
remains closed is significantly less than that applied by the positioning 
means when both molding cavities are closed, and typically is insufficient 
to enable adequate hold pressure to be applied to the closed molding 
cavity. 
SUMMARY OF THE INVENTION 
The present invention provides an improved system and method of controlling 
hold pressure applied to molding cavities of a stacked multi-parting 
injection molding system having desynchronized injection periods while 
cyclic injection molding a plastic material. The molding system includes 
left, center and right molding blocks disposed for movement with respect 
to each other along a common axis and defining a left molding cavity 
between the left and center molding blocks and a right molding cavity 
between the center and right molding blocks. A combination of the center 
molding block and an injection system together provide an adjustable feed 
system, which within the center molding block has a left branch for 
feeding the left molding cavity, a right branch for feeding the right 
molding cavity, and means for confining injected plastic material in the 
respective branches and molding cavities. This system accomplishes the 
method of the present invention by sequentially 
(a) adjusting the feed system to enable plastic material to be fed from the 
injection system through the left branch to the left molding cavity; 
(b) pressurizing the injection system to inject the plastic material 
through the left branch to fill the left molding cavity; 
(c) confining the plastic material injected into the left branch and the 
left molding cavity; and 
(d) hold pressurizing the plastic material confined in the left branch and 
the left molding cavity and by sequentially 
(e) adjusting the feed system to enable plastic material to be fed from the 
injection system through the right branch to the right molding cavity; 
(f) pressurizing the injection system to inject the plastic material 
through the right branch to fill the right molding cavity; 
(g) confining the plastic material injected into the right branch and the 
right molding cavity; and 
(h) hold pressurizing the plastic material confined in the right branch and 
the right molding cavity. 
These two sequences (a)-(d) and (e)-(h) may be overlapping. 
The molding system of the present invention may further include movable 
packing means for hold pressurizing the plastic material confined in the 
respective branches and molding cavities. Accordingly, step (d) further 
comprises moving a packing means to further hold pressurize the plastic 
material confined in the left branch and the left molding cavity; and step 
(h) further comprises moving a packing means to further hold pressurize 
the plastic material confined in the right branch and the right molding 
cavity. 
When the molding cavities have desynchronous opening periods, the method 
includes the further steps of 
(i) locking one of the molding cavities to maintain it in a closed position 
while the other molding cavity is open; and 
(j) applying axial clamping force to the locked molding cavity by expanding 
the thickness of at least one of the molding blocks that defines the 
locked molding cavity. 
The present invention thus also provides a system and method for providing 
additional axial clamping force to one molding cavity of a desynchronized 
stacked multi-parting molding system while one molding cavity is open and 
the other molding cavity is maintained in a closed position by a locking 
means, with said additional axial clamping force being provided by 
expanding the thickness of at least one of the molding blocks that defines 
the one molding cavity. 
Additional features of the invention are described with reference to the 
description of the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a preferred embodiment of a stacked multi-parting 
injection molding system in which hold pressure is controlled according to 
the present invention includes a left molding block 10, a center molding 
block 11 and a right molding block 12. The center molding block 11 and the 
left molding block 10 join at a first parting surface 13 and define a left 
molding cavity 14. The center molding block 11 and the right molding block 
12 join at a second parting surface 15 and define a right molding cavity 
16. The center molding block 11 may include a center platen of a molding 
machine. An injection system 18 and the center molding block 11 define an 
adjustable feed system, which may have a molten core. Within the center 
molding block 11, the feed system includes a runner stem 20, which is 
connected to a left branch 21 and a right branch 22. The left branch 21 
terminates in a gate 24 to the left molding cavity 14; and the right 
branch 22 terminates in a gate 25 to the right molding cavity 16. A first 
valve 27 in the runner stem 20 controls the flow of plastic material 
through the runner stem 20. The injection system 18 is disposed to inject 
plastic material into the runner stem 20. The injection system 18 may be 
separated from the remainder of the feed system when not being pressurized 
to inject the plastic material into the runner stem 20. The molding system 
has desynchronized injection periods. A piston system 29 including a 
packing element 30 is connected to the feed system for controlling the 
hold pressure in the left and right branches 21, 22. A controller 31 
controls hold pressure in the molding system of FIG. 1 by controlling the 
operations of the injection system 18, the valve 27 and the piston system 
29. 
The method of controlling hold pressure by utilizing the embodiment of the 
invention illustrated in FIG. 1 includes the following steps: 
(a) adjusting the feed system by opening the first valve 27 to enable 
plastic material 32 to be fed from the injection system 18 through the 
left branch 21 to the left molding cavity 14; 
(b) pressurizing the injection system 18 to inject the plastic material 32 
through the left branch 21 to fill the left molding cavity 14; 
(c) shutting the first valve 27 to confine the plastic material 32 injected 
into the left branch 21 and the left molding cavity 14; 
(d) hold pressurizing the plastic material 32 confined in the left branch 
21 and the left molding cavity 14 by the first valve 27; 
(e) adjusting the feed system by opening the first valve 27 to enable 
plastic material 32 to be fed from the injection system 18 through the 
right branch 22 to the right molding cavity 16; 
(f) pressurizing the injection system 18 to inject the plastic material 32 
through the right branch 22 to fill the right molding cavity 16; 
(g) shutting the first valve 27 to confine the plastic material 32 injected 
into the right branch 22 and the right molding cavity 16; and 
(h) hold pressurizing the plastic material 32 confined in the right branch 
22 and the right molding cavity 16 by the first valve 27. Steps (a) 
through (d) are performed in sequence; and steps (e) through (h) are 
performed in sequence. These two sequences (a)-(d) and (e)-(h) may be 
overlapping. Ejection of the product molded in the right molding cavity 16 
may conveniently take place in the period after step (b) and before step 
(f); and ejection of the product molded in the left molding cavity 14 may 
conveniently take place in the period after step (f) and before step (b). 
Alternatively, ejection of both molded products may take place during the 
period after step (f) and before step (b). 
Plasticization may commence immediately after step (b) and immediately 
after step (f) in order decrease the cycle time. 
The system of FIG. 1 may further include two optional left and right valves 
34 and 35. The operations of the valves 34 and 35 are controlled by the 
controller 31. The feed system may be further adjusted during step (a) by 
opening the left valve 34 and closing the right valve 35 to thereby enable 
the plastic material to flow into only the left molding cavity 14 during 
step (b). During step (e) the left valve 34 is closed and the right valve 
35 is opened to thereby enable the plastic material to flow into only the 
right molding cavity during step (f). The optional valves 34 and 35 are 
not required in this embodiment when both steps (b) and (f) take place 
with both molding cavities 14, 16 closed, and the plastic material 
previously injected into one molding cavity provides resistance to further 
flow into such molding cavity and its adjacent branch while the plastic 
material is being injected into the other molding cavity. During 
initialization of the system, a previously molded product is placed in the 
one molding cavity to provide such resistance. The inclusion of the 
optional left and right valves 34, 35 allows hold pressure to be applied 
for a longer period because hold pressure can be continued to be applied 
to one molding cavity when the other molding cavity is opened. 
Step (d) may further include the step of (i) further hold pressurizing the 
plastic material confined in the left molding cavity 14 by operating the 
piston system 29 to protract the packing element 30; and step (h) may 
further include the step of (j) further hold pressurizing the plastic 
material confined in the right molding cavity 16 by operating the piston 
system 29 to protract the packing element 30. 
This method further includes the step of (k) depressurizing the feed system 
between the first valve 27 and the molding cavities 14, 16 by operating 
the piston system 27 to retract the packing element 30. 
Referring to FIG. 2, an alternative preferred embodiment of a stacked 
multi-parting molding system in which hold pressure is controlled 
according to the present invention includes a left molding block 40, a 
center molding block 41 and a right molding block 42. The center molding 
block 41 and the left molding block 40 join at a first parting surface 43 
and define a left molding cavity 44. The center molding block 41 and the 
right molding block 42 join at a second parting surface 45 and define a 
right molding cavity 46. The center molding block 41 may include a center 
platen of a molding machine. An injection system 48 and the center molding 
block 41 define an adjustable feed system, which may have a molten core. 
Within the center molding block 41, the feed system includes a runner stem 
50, which is connected to a left branch 51 and a right branch 52. The left 
branch 51 terminates in a gate 54 to the left molding cavity 44; and the 
right branch 52 terminates in a gate 55 to the right molding cavity 46. A 
first valve 57 in the left branch 51 controls the flow of plastic material 
through the left branch 51. A second valve 58 in the right branch 52 
controls the flow of plastic material through the right branch 52. The 
injection system 48 is disposed to inject plastic material into the runner 
stem 50. The injection system 48 may be separated from the remainder of 
the feed system when not being pressurized to inject the plastic material 
into the runner stem 50. The molding system has desynchronized injection 
periods. A left piston system 59 including a left packing element 60 is 
connected to the left branch 51 for controlling the hold pressure in the 
left branch 51 and the left molding cavity 44. A right piston system 61 
including a right packing element 62 is connected to the right branch 52 
for controlling the hold pressure in the right branch and the right 
molding cavity 46. A controller 63 controls hold pressure in the molding 
system of FIG. 2 by controlling the operations of the injection system 48, 
the valves 57, 58 and the piston system 59, 61. 
The method of controlling hold pressure by utilizing the embodiment of the 
invention illustrated in FIG. 2 includes the following steps: 
(a) adjusting the feed system by opening the first valve 57 to enable 
plastic material 64 to be fed from the injection system 48 through the 
runner stem 50 and the left branch 51 to the left molding cavity 44; 
(b) pressurizing the injection system 48 to inject the plastic material 64 
through the left branch 51 to fill the left molding cavity 44; 
(c) shutting the first valve 57 to confine the plastic material 64 injected 
into the left branch 51 and the left molding cavity 44; 
(d) hold pressurizing the plastic material 64 confined in the left branch 
51 and the left molding cavity 44 by the first valve 57; 
(e) adjusting the feed system by opening the second valve 58 to enable 
plastic material 65 to be fed from the injection system 18 through the 
runner stem 50 and the right branch 52 to the right molding cavity 46; 
(f) pressurizing the injection system 48 to inject the plastic material 65 
through the right branch 52 to fill the right molding cavity 46; 
(g) shutting the second valve 58 to confine the plastic material 65 
injected into the right branch 52 and the right molding cavity 46; and 
(h) hold pressurizing the plastic material 65 confined in the right branch 
52 and the right molding cavity 46 by the first valve 57. Steps (a) 
through (d) are performed in sequence; and steps (e) through (f) are 
performed in sequences. These two sequences (a)-(d) and (e)-(h) may be 
overlapping. 
Step (d) may further include the step of (i) further hold pressurizing the 
plastic material 64 confined in the left molding cavity 44 by operating 
the left piston system 59 to protract the left packing element 60; and 
step (h) may further include the step of (j) further hold pressurizing the 
plastic material 65 confined in the right molding cavity 46 by operating 
the right piston system 61 to protract the right packing element 62. 
This method further includes the steps of (k) depressurizing the left 
branch 51 between the first valve 57 and the left molding cavity 44 by 
operating the left piston system 59 to retract the left packing element 
60; and (l) depressurizing the right between the second valve 58 and the 
right molding cavity 46 by operating the right piston system 61 to retract 
the right packing element 62. 
Referring to FIGS. 3A and 3B, another alternative preferred embodiment of a 
stacked multi-parting injection molding system in which hold pressure is 
controlled according to the present invention includes a left molding 
block 67, a center molding block 68 and a right molding block 69. The 
center molding block 68 and the left molding block 67 join at a first 
parting surface 70 and define a left molding cavity 71. The center molding 
block 68 and the right molding block 69 join at a second parting surface 
72 and define a right molding cavity 73. The center molding block 68 may 
include a center platen of a molding machine. An injection system 74 and 
the center molding block 68 define an adjustable feed system, which may 
have a molten core. The center molding block 68 includes a snorkle unit 75 
and a piston system 76. The piston system 76 includes a packing element 
77. The packing element 77 includes a one-way valve that permits plastic 
material to flow only toward the molding cavities 71, 73. Within the 
center molding block 68, the feed system includes a horizontal runner stem 
78 and a vertical runner stem 79 connected together at an angle. The 
vertical runner stem 79 extends in both directions from its juncture with 
the horizontal runner stem 78, and includes a recess 79a. The vertical 
runner stem 79 is connected to a left branch 81 and a right branch 82. The 
packing element 77 of the piston system is positioned within the vertical 
runner stem 79. When the piston system 76 is retracted, the packing 
element 77 is withdrawn into the recess 79a. When the piston system 76 is 
protracted the packing element 77 is protracted to a position within the 
vertical runner stem 79 between the horizontal runner stem 78 and the left 
and right branches 81, 82. Within the center molding block 68, the left 
branch 81 terminates in a gate 83 to the left molding cavity 71; and the 
right branch 82 terminates in a gate 84 to the right molding cavity 73. An 
optional first valve 85 in the left branch 81 controls the flow of plastic 
material through the left branch 81. An optional second valve 86 in the 
right branch 82 controls the flow of plastic material through the right 
branch 82. The injection system 74 is disposed to inject plastic material 
into the inlet stem 78 of the snorkle unit 75. An opening third valve 87 
is disposed in the inlet stem 78 to prevent plastic material from leaking 
out of the horizontal runner stem 78. As shown in FIG. 3B, the injection 
system 74 may be separated from the remainder of the feed system when the 
right molding cavity 73 is opened to enable a product 88 to be ejected 
therefrom. The molding system of FIGS. 3A and 3B has desynchronized 
injection periods. A controller 89 controls the hold pressure control 
system of FIGS. 3A and 3B by controlling the operation of the injection 
system 74, the piston system 76, and the valves 85, 86, 87. In an 
alternative embodiment (not shown), the packing element 77 is a 
cylindrical piston instead of a one-way valve. 
The method of controlling hold pressure by utilizing the embodiment of the 
invention illustrated in FIGS. 3A and 3B includes the following steps: 
(a) adjusting the feed system by retracting the packing element 77 into the 
recess 79a and opening the first valve 85 and the third valve 87 to enable 
plastic material to be fed from the injection system 74 through the 
horizontal runner stem 78, the vertical runner stem 79 and the left branch 
81 to the left molding cavity 71; 
(b) pressurizing the injection system 74 to inject the plastic material 
through the horizontal runner stem 78, the vertical runner stem 79 and the 
left branch 81 to fill the left molding cavity 71; 
(c) protracting the packing element 77 into that portion of the vertical 
runner stem 79 between the horizontal runner stem 78 and the left branch 
81 to confine the plastic material injected into the left branch 81 and 
the left molding cavity 71; 
(d) maintaining the packing element 77 in its protracted position and/or 
further protracting the packing element 77 to hold pressurize the plastic 
material confined in the left branch 81 and the left molding cavity 71 by 
the packing element 77; 
(e) adjusting the feed system by retracting the packing element 77 into the 
recess 79a and opening the second valve 86 and the third valve 87 to 
enable plastic material to be fed from the injection system 74 through the 
horizontal runner stem 78, the vertical runner stem 79 and the right 
branch 82 to the right molding cavity 73; 
(f) pressurizing the injection system 74 to inject the plastic material 
through the horizontal runner stem 78, the vertical runner stem 79 and the 
right branch 82 to fill the right molding cavity 73; 
(g) protracting the packing element 77 into that portion of the vertical 
runner stem 79 between the horizontal runner stem 78 and the right branch 
82 to confine the plastic material injected into the right branch 82 and 
the right molding cavity 73; and 
(h) maintaining the packing element 77 in its protracted position and/or 
further protracting the packing element 77 to hold pressurize the plastic 
material confined in the right branch 82 and the right molding cavity 73 
by the packing element 77. 
Steps (c) and (d) may be performed while the right molding cavity 73 is 
opened as shown in FIG. 3B. Likewise, steps (g) and (h) may be performed 
while the left molding cavity 71 is opened. 
The first valve 85 may be closed during steps (e) and (f); and the second 
valve 86 may be closed during steps (a) and (b). 
Steps (a) through (d) are performed in sequence; and steps (e) through (h) 
are performed in sequence. These two sequences may be overlapping. 
This method further includes the steps of (i) depressurizing the left 
branch 81 between the packing element 77 and the left molding cavity 71 by 
operating the piston system 76 to retract the packing element 77; and (j) 
depressurizing the right branch 82 between the packing element 77 and the 
right molding cavity 73 by operating the piston system 76 to retract the 
packing element 77. 
Referring to FIG. 4, still another alternative preferred embodiment of the 
hold pressure control system of the present invention is a variation of 
the system described above with reference to FIG. 2. Common reference 
numerals are used to describe the common elements of these two systems. 
This embodiment is a stacked multi-parting molding system including a left 
molding block 40, a center molding block 41 and a right molding block 42. 
The center molding block 41 and the left molding block 40 join at a first 
parting surface 43 and define a left molding cavity 44. The center molding 
block 41 and the right molding block 42 join at a second parting surface 
45 and define a right molding cavity 46. The center molding block 41 may 
be either a center platen of a molding machine. An injection system 48 and 
the center molding block 41 define an adjustable feed system, which may 
have a molten core. Within the center molding block 41, the feed system 
includes a runner stem 50, which is connected to a left branch and a right 
branch. The left branch includes a vertical portion 91 and a horizontal 
portion 92. The vertical portion 91 of the left branch extends in both 
directions from its juncture with the horizontal portion 92 of the left 
branch. The right branch includes a vertical portion 93 and a horizontal 
portion 94. The vertical portion 93 of the right branch extends in both 
directions from its juncture with the horizontal portion 94 of the right 
branch. 
A left piston system 59 including a left packing element 60 is connected to 
the vertical portion 91 of the left branch for controlling the hold 
pressure in the left branch and the left molding cavity 44. A right piston 
system 61 including a right packing element 62 is connected to the 
vertical portion 93 of the right branch for controlling the hold pressure 
in the right branch and the right molding cavity 46. The packing elements 
60, 62 are cylindrical pistons and do not include any valves. The packing 
element 60 of the piston system 59 is positioned for movement within the 
vertical portion 91 of the left branch. When the piston system 59 is 
retracted the packing element 60 is withdrawn to open the juncture between 
the vertical 91 and horizontal 92 portions of left branch. When the piston 
system 59 is protracted the packing element 60 blocks the flow of plastic 
material through the vertical portion 91 of the left branch from the 
horizontal portion 92 of the left branch. The packing element 62 of the 
piston system 61 is positioned for movement within the vertical portion 93 
of the right branch. When the piston system 61 is retracted the packing 
element 62 is withdrawn to open the juncture between the vertical 93 and 
horizontal 94 portions of the right branch. When the piston system 61 is 
protracted the packing element 62 blocks the flow of plastic material 
through the vertical portion 93 of the right branch from the horizontal 
portion 94 of the right branch. The left branch terminates in a gate 54 to 
the left molding cavity 44; and the right branch terminates in a gate 55 
to the right molding cavity 46. The injection system 48 is disposed to 
inject plastic material into the runner stem 50. The injection system 48 
may be separated from the remainder of the feed system when not being 
pressurized to inject the plastic material into the runner stem 50. The 
molding system has desynchronized injection periods. A controller 95 
controls the hold pressure control system of FIG. 4 by controlling the 
operation of the injection system 48, the left piston system 59 and the 
right piston system 61. The hold pressure control system of FIG. 4 does 
not require valves in the feed system. 
The method of controlling hold pressure by utilizing the embodiment of the 
invention illustrated in FIG. 4 includes the following steps: 
(a) adjusting the feed system by retracting the left packing element 60 to 
enable plastic material to be feed from the injection system 48 through 
the runner stem 50, and the horizontal 92 and vertical 91 portions of the 
left branch to the left molding cavity 44; 
(b) pressurizing the injection system 48 to inject the plastic material 
through the runner stem 50, and the horizontal 92 and vertical 91 portions 
of the left branch to fill the left molding cavity 44; 
(c) protracting the left packing element 60 into the vertical portion 91 of 
the left branch between the horizontal portion 92 of the left branch and 
the left molding cavity 44 to confine the plastic material injected into 
the vertical portion of the left branch and the left molding cavity 44; 
(d) maintaining the left packing element 60 in its protracted position 
and/or further protracting the left packing element 60 to hold pressurize 
the plastic material confined in the vertical portion 91 of left branch 
and the left molding cavity 44 by the packing element 60; 
(e) adjusting the feed system by retracting the right packing element 62 to 
enable plastic material to be fed from the injection system 48 through the 
runner stem 50, and the horizontal 94 and vertical 93 portions of the 
right branch to the right molding cavity 46; 
(f) pressurizing the injection system 48 to inject the plastic material 
through the runner stem 50, and the horizontal 94 and vertical 93 portions 
of the right branch to fill the right molding cavity 46; 
(g) protracting the right packing element 62 into the vertical portion 93 
of the right branch between the horizontal portion 94 of the right branch 
and the right molding cavity 46 to confine the plastic material injected 
into the right branch and the right molding cavity 46; and 
(h) maintaining the right packing element 62 in its protracted position 
and/or further protracting the right packing element 62 to hold pressurize 
the plastic material confined in the right branch and the right molding 
cavity 44 of the right packing element 62. 
This method further includes the steps of (i) depressurizing the vertical 
portion 91 of the left branch between the left packing element 60 and the 
left molding cavity 44 by operating the left piston system 59 to retract 
the left packing element 60; and (l) depressurizing the vertical portion 
93 of right branch between the right packing element 62 and the right 
molding cavity 46 by operating the right piston system 61 to retract the 
right packing element 62. 
Steps (a) through (d) are performed in sequence; and steps (e) through (h) 
are performed in sequence. These two sequences may be overlapping. 
Referring to FIG. 5, a further alternative preferred embodiment of the hold 
pressure control system of the present invention is a variation of the 
system described above with reference to FIG. 4. Common reference numerals 
are used to describe the common elements of these two systems. This 
embodiment is a stacked multi-parting molding system including a left 
molding block 40, a center molding block 41 and a right molding block 42. 
The center molding block 41 and the left molding block 40 join at a first 
parting surface 43 and define a left molding cavity 44. The center molding 
block 41 and the right molding block 42 join at a second parting surface 
45 and define a right molding cavity 46. The center molding block 41 may 
be either a center platen of a molding machine. An injection system 
including a left injection unit 96 and a right injection unit 97 and the 
center molding block 41 define an adjustable feed system, which may have a 
molten core. Within the center molding block 41, the feed system includes 
a left runner stem 98, which is connected to a left branch, and a right 
runner stem 99, which is connected to a right branch. The left branch 
includes a vertical portion 91 and a horizontal portion 92. The vertical 
portion 91 of the left branch extends in both directions from its juncture 
with the horizontal portion 92 of the left branch. The right branch 
includes a vertical portion 93 and a horizontal portion 94. The vertical 
portion 93 of the right branch extends in both directions from its 
juncture with the horizontal portion 94 of the right branch. 
The left runner stem 98 includes an orifice 100 for receiving plastic 
material from the left injection unit 96; and the right runner stem 99 
includes an orifice 101 for receiving plastic material from the right 
injection unit 97. Alternatively, the system includes only a single 
injection unit 96, which is moved back and forth between the left and 
right injection orifices 100, 102 for injecting plastic material into the 
respective left and right runner stems 98, 99. In another alternative 
embodiment, as shown in FIG. 5A, the injection system includes a single 
injection unit 102 having two injection nozzles 103, 104 for injecting 
plastic material into the respective left and right injection orifices 
100, 101. A first valve 105 is located in the left runner stem 98 and a 
second valve 106 is located in the left branch 91. A third valve 107 is 
located in the right runner stem 99 and fourth valve 108 is located in the 
right branch 93. 
A left piston system 59 including a left packing element 60 is connected to 
the vertical portion 91 of the left branch for controlling the hold 
pressure in the left branch and the left molding cavity 44. A right piston 
system 61 including a right packing element 62 is connected to the 
vertical portion 93 of the right branch for controlling the hold pressure 
in the right branch and the right molding cavity 46. The packing elements 
60, 62 are cylindrical pistons and do not include any valves. The packing 
element 60 of the piston system 59 is positioned for movement within the 
vertical portion 91 of the left branch. When the piston system 59 is 
retracted the packing element 60 is withdrawn to open the juncture between 
the vertical 91 and horizontal 92 portions of left branch. When the piston 
system 59 is protracted the packing element 60 blocks the flow of plastic 
material through the vertical portion 91 of the left branch from the 
horizontal portion 92 of the left branch. The packing element 62 of the 
piston system 61 is positioned for movement within the vertical portion 93 
of the right branch. When the piston system 61 is retracted the packing 
element 62 is withdrawn to open the juncture between the vertical 93 and 
horizontal 94 portions of the right branch. When the piston system 61 is 
protracted the packing element 62 blocks the flow of plastic material 
through the vertical portion 93 of the right branch from the horizontal 
portion 94 of the right branch. The left branch terminates in a gate 54 to 
the left molding cavity 44; and the right branch terminates in a gate 55 
to the right molding cavity 46. The molding system has desynchronized 
injection periods. A controller 109 controls the hold pressure control 
system of FIG. 5 by controlling the operation of the left and right 
injection units 96, 97, the left piston system 59, the right piston system 
61, and the valves 105, 106, 107 and 108. 
The method of controlling hold pressure by utilizing the embodiment of the 
invention illustrated in FIG. 5 includes the following steps: 
(a) adjusting the feed system by retracting the left packing element 60 and 
opening the valves 105 and 106 to enable plastic material to be fed from 
the left injection unit 96 through the left runner stem 98, and the 
horizontal 92 and vertical 91 portions of the left branch to the left 
molding cavity 44; 
(b) pressurizing the left injection unit 96 to inject the plastic material 
through the left runner stem 98, and the horizontal 92 and vertical 91 
portions of the left branch to fill the left molding cavity 44; 
(c) protracting the left packing element 60 into the vertical portion 91 of 
the left branch between the horizontal portion 92 of the left branch and 
the left molding cavity 44 to confine the plastic material injected into 
the vertical portion 91 of the left branch and the left molding cavity 44; 
(d) maintaining the left packing element 60 in its protracted position 
and/or further protracting the left packing element 60 to hold pressurize 
the plastic material confined in the vertical portion 91 of the left 
branch and the left molding cavity 44 by the packing element 60; 
(e) adjusting the feed system by retracting the right packing element 62 
and opening the valves 107 and 108 to enable plastic material to be fed 
from the right injection unit 97 through the right runner stem 99, and the 
horizontal 94 and vertical 93 portions of the right branch to the right 
molding cavity 46; 
(f) pressurizing the right injection unit 97 to inject the plastic material 
through the right runner stem 99, and the horizontal 94 and vertical 93 
portions of the right branch to fill the right molding cavity 46; 
(g) protracting the right packing element 62 into the vertical portion 93 
of the right branch between the horizontal portion 94 of the right branch 
and the right molding cavity 46 to confine the plastic material injected 
into the right branch and the right molding cavity 46; and 
(h) maintaining the right packing element 62 in its protracted position 
and/or further protracting the right packing element 62 to hold pressurize 
the plastic material confined in the right branch and the right molding 
cavity 44 of the right packing element 62. 
This method further includes the steps of (i) depressurizing the vertical 
portion 91 of left branch between the left packing element 60 and the left 
molding cavity 44 by operating the left piston system 59 to retract the 
left packing element 60; and (l) depressurizing the vertical portion 93 of 
right branch between the right packing element 62 and the right molding 
cavity 46 by operating the right piston system 61 to retract the right 
packing element 62. 
The first and second valves 105, 106 may be closed during steps (e) and 
(f); and the third and fourth valves 107, 108 may be closed during steps 
(a) and (b). The valves 105, 106, 107 and 108 are optional since the left 
and right branches may be depressurized to prevent leakage as described 
above. 
Steps (a) through (d) are performed in sequence; and steps (e) through (h) 
are performed in sequence. These two sequences may be overlapping. 
It is usually desirable to provide a high injection pressure at the gate to 
a molding cavity immediately as the injection is started. But due to the 
feed system of a multi-parting injection molding system tending to be 
rather long in distance, and hot molten plastic material being quite 
compressible, such high injection pressure at the gate is difficult to 
obtain. It should be noted that the piston systems of the system and 
method of the present invention may be used to increase initial pressure 
at the gate at the beginning of injection. 
The piston systems in the embodiments described above may be hydraulic, as 
shown, or may be electrically or spring powered. The valves in the 
embodiments described above may be rotary or multidependent. Alternatively 
the valve function may be provided ultrasonicly or by heat impulse to the 
plastic material at the gate. Also the different types of confining 
systems described herein may be used interchangeably. 
Referring to FIGS. 6 and 7, a preferred embodiment of the multi-parting 
molding system of the present invention includes a left platen 110, a 
center platen 112, a right platen 114, a base block 116, positioning bars 
118, and a hydraulic positioning mechanism containing a cylinder 120, a 
rod 122, a plurality of locking mechanisms, each including a hydraulic 
cylinder 124, a rod 126, a wedge-shaped locking plate 128, a locking bar 
130 and a wedge-shaped nut 132; mold parts 134, 136, 138, 140, 142 and 
144; a plurality of springs 46 and a hydraulic cylinder 148. The 
combination of the center platen 112 and the mold parts 138, 140 form a 
center molding block in a stacked molding system and may be adapted to 
include the different embodiments of the hold pressure control system of 
the present invention described herein with reference to FIGS. 1 through 
5A. 
The right platen 114 and the base block 116 are stationary, and are 
connected by the positioning bars 118. The left and center platens 110, 
112 are supported by the bars 118 between the base block 116 and the right 
platen 114 for lateral movement in directions toward and away from the 
right platen 114 and the base block 116. The cylinder 120 of the hydraulic 
positioning mechanism is attached to the base block 116, and the rod 122 
of the hydraulic positioning mechanism is attached to the left platen 110. 
For each of the plurality of locking mechanisms, the cylinder 124 is 
attached to the left or right platen 110 or 114, the rod 126 moves in the 
cylinder 124 and is connected to the locking plate 128; one end of the bar 
130 is secured to the center platen 112; and the nut 132 is fastened to 
the other end of the bar 130. The other end of the bar 130 is threaded so 
that the position of the nut 132 may be adjusted. 
A left molding block includes the left platen 110 and mold parts 134 and 
136. A center molding block includes the center platen 112 and mold parts 
138 and 140. A right molding block includes the right platen 114 and mold 
parts 142 and 144. A left molding cavity 150 is defined by the mold parts 
136 and 138; and a right molding cavity 152 is defined by the mold parts 
140 and 142. 
To close both the left molding cavity 150 and the right molding cavity 152, 
the rod 122 of the hydraulic positioning mechanism is protracted, as shown 
in FIG. 6. The amount of axial clamping force applied to the left and 
right mold cavities 150, 152 can be controlled by controlling the amount 
of protraction of the rod 122 of the hydraulic positioning mechanism. To 
open the molding cavities 150, 152, the hydraulic positioning mechanism is 
operated to retract the rod 122. When both molding cavities 150, 152 are 
closed the locking mechanisms are operated to lock both molding cavities 
150, 152 the closed position, as shown in FIG. 6. To lock a molding cavity 
(for example, the left molding cavity 150) in a closed position, the rod 
126 is protracted to wedge the locking plate 128 between the nut 132 and 
the left platen 110, as shown in FIGS. 7 and 9. To open the left molding 
cavity 150, the rod 126 is retracted to remove the locking plate 128 from 
between the nut 132 and the left platen 110, as shown in FIGS. 7 and 9. 
When one of the two molding cavities 150, 152 is opened at a time when it 
is desired to maintain the other molding cavity in a closed position, it 
is necessary to lock the other molding cavity in the closed position. 
Thus, when the left molding cavity 150 is opened, as shown in FIG. 7, the 
locking mechanisms attached to the left platen 110 are operated to unlock 
the left platen 110 from the bars 130. The locking mechanisms attached to 
the right platen 114 remain in their locked position. The hydraulic 
positioning mechanism is then operated to retract the rod 122 and thereby 
move the left platen 110 to the left, which results in the opening of the 
left molding cavity 150. Since the locking mechanisms attached to the 
right platen 114 remain in their locked position, the center platen 112 
remains attached to the right platen 114 and resists movement when the rod 
122 is retracted to move the left platen 110 and open the left molding 
cavity 150. Thus the right molding cavity 152 remains closed when the left 
molding cavity 150 is opened. It will be readily apparent that by a 
reverse operation of the locking mechanisms, the left molding cavity 150 
can be maintained in a locked position while the right molding cavity 152 
is opened. 
When the axial clamping force applied by the positioning mechanism 118, 
120, 122 is removed from the right molding cavity 152 as a result of the 
retraction of the rod 122, with the right molding cavity being maintained 
in a closed position, as shown in FIG. 7, axial clamping force in addition 
to that provided by the locking plates 128 is applied to the right molding 
cavity 152 by expanding the right molding block in response to hydraulic 
pressure applied within the hydraulic cylinder 148 within the right 
molding block. The amount of the clamping force applied within the right 
molding block can be actively controlled by controlling the hydraulic 
pressure within the hydraulic cylinder 148. 
When the clamping force applied by the positioning mechanism 118, 120, 122 
is removed from the left molding cavity 150 as a result of the retraction 
of the rod 122, with the left molding cavity being maintained in a closed 
position (not shown), axial clamping force in addition to those provided 
by the locking plates 128 is applied to the left molding cavity 150 by 
expanding the left molding block in response to spring pressure passively 
applied by the springs 146 within the left molding block. The springs 146 
preferably are parabolic steel washers, which are sometimes referred to as 
Belleville washers. The springs 146 are compressed by the clamping force 
applied by the positioning mechanism when the rod 22 is protracted, and 
expand within the left molding block when the rod 122 is retracted. 
In other preferred embodiments, both the left molding block and the right 
molding block may contain the same type of expanding means, whether 
passive expanding means, such as the springs 146, or controllable 
expanding means, such as the hydraulic cylinder 148. 
Other variations may be made in the preferred embodiment shown herein 
without departing from the present invention. For example, the locking 
plates 128 and the nuts 132 need not be wedge shaped; the clamping means, 
such as the springs 146 and/or the hydraulic cylinder 148, may be located 
apart from rather than within the molding blocks; and each of the left and 
right molding blocks may be of integral construction without necessarily 
including platens; and the positioning mechanism may be a tuggle system 
rather than the hydraulic mechanism 120, 122. 
In an embodiment (not shown) utilizing a tuggle system for the positioning 
mechanism and controllable expanding means, such as the hydraulic cylinder 
148, the total axial clamping force provided by the tuggle system and the 
hydraulic cylinder 148 is controlled by controlling the amount of 
hydraulic pressure applied within the hydraulic cylinder 148. 
The hold pressure control systems and clamping systems of the present 
invention are particularly compatible with the various desynchronized 
molding cycles described in the aforementioned U.S. Pat. Nos. 4,400,431, 
4,464,327 and 4,539,171.