Hydraulic drive for the die closing unit of an injection molding machine

A hydraulic drive for the die closing unit of an injection molding machine in which at least one power cylinder and power piston with a coaxially attached auxiliary cylinder opens and closes the molding die by executing the opening and closing travel with a differential piston action in the power cylinder and/or with the auxiliary cylinder, while valve-closable bypass channels in the power piston are kept open. The closing of these channels switches the drive from an accelerated travel mode to a high-pressure die clamping mode. An annular valve plunger cooperates with a valve seat on the power piston to open and close the bypass channels, several alternatives of guiding and hydraulically moving the valve plunger being contemplated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to injection molding machines, and in 
particular to hydraulic drive means for the opening and closing of the two 
halves of a molding die into which plastic or metallic raw material is 
injected. More specifically, the present invention relates to hydraulic 
cylinder operated die closing units of injection molding machines in which 
the cylinder rods extend in the direction of the opening and closing 
motion of the movable die plate, which latter may be directly attached to 
the cylinder rods. 
2. Description of the Prior Art 
A prior art hydraulic drive for a die closing unit of the type mentioned 
above is disclosed in my U.S. Pat. No. 3,663,140. This drive consists of 
at least two parallel hydraulic power cylinders to which two auxiliary 
cylinders are coaxially connected. While a comparatively moderate force is 
normally sufficient to separate and approach the die halves, a much larger 
force is required to keep the die closed during the injection process. The 
known device takes advantage of this difference, by using the smaller 
auxiliary cylinders to produce the opening and closing travel of the unit, 
and using the power cylinders only to create the closing pressure. Under 
certain special circumstances, the power cylinder is also used to assist 
in the initial portion of the opening motion, when the molded part offers 
a resistance against die opening. 
In the prior art device, this is accomplished by arranging the pistons of 
the power cylinders so as to operate both as pistons and as bypass valves 
which, when open, permit the power piston to execute the opening and 
closing travel, without removing all the fluid from one side of the power 
cylinder and simultaneously taking in a comparable amount of fluid on the 
other side of the cylinder. Instead, the hydraulic fluid simply flows 
through bypass channels arranged inside the power piston from one side of 
the piston to the other, as a result of the valve action of the piston. At 
the end of the closing travel, for example, the power piston valve is 
closed, whereupon the effective area of the power piston, subjected to the 
full fluid pressure, exerts its maximum force against the die halves as a 
closing pressure. 
In the above-mentioned prior art arrangement, the power piston itself is 
the moving part of this internal bypass valve. The valve seat is provided 
in the form of an upstanding radial shoulder on the piston rod, the piston 
being seated against this shoulder in the closed valve position. This 
design has certain shortcomings, among them the material and machining 
costs of the piston rods with their radial shoulders, the manufacturing 
and assembly costs of the axially movable power pistons with their 
internal flow chamber and valve closing face. Another disadvantage relates 
to the considerable weight of the piston which, being the moving valve 
body, causes the latter to respond comparatively slowly to a reversal of 
movement direction, and which also creates a rather hard impact, when the 
power piston moves against its valve seat. Lastly, the entire closing 
pressure is transmitted from the power piston to the piston rod through 
the valve seat, meaning that the valve seat has to be comparatively large 
in area, with the result that the flow through the open valve seat is more 
restrained and not as straight and even as would be desirable. 
SUMMARY OF THE INVENTION 
Underlying the present invention is the primary objective of improving upon 
the above-described prior art die closing unit drive, by eliminating or at 
least reducing the mentioned shortcomings to the extent that savings are 
realized in the manufacture and assembly of the parts, while the 
possibility for operation at considerably higher speeds is afforded 
through the elimination of the power piston as a moving valve body and the 
rearrangement of the bypass channels inside the power piston for a fluid 
flow with a minimum of throttling action and with the least possible 
change in flow direction. 
The present invention proposes to attain the above objective by suggesting 
a hydraulic drive for the die closing unit of an injection molding machine 
in which the valve for controlling the bypass flow through the piston of 
the hydraulic power cylinder is constituted by an hollow valve plunger 
which is arranged to move axially on the pressure side of the power piston 
in cooperation with a cylinder surface which concentrically surrounds the 
piston rod, whereby the power piston itself is fixedly attached to the 
piston rod and has a valve seat for cooperation with the valve plunger 
which, when engaged against the seat, closes the bypass channels in the 
power piston. 
The primary advantages resulting from this improvement are: a 
simplification of the constituent parts which reduces the cost of 
manufacture, especially in large quantities; a considerable increase in 
operating speed, due to the greatly reduced weight of the valve plunger, 
with the additional benefit of greatly reducing the impact forces on the 
valve seat and hence the contact area on the latter; greater longevity; 
and improved adaptability of the design to varying design requirements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of the invention is illustrated in FIG. 1, where a 
movable die plate 12 carries one half of the injection molding die 13, 
while the other half is attached to a block-shaped cylinder mount 10 which 
also serves as a base for the die closing unit. In this cylinder mount are 
arranged two parallelly spaced cylinder bores defining cylinder pressure 
spaces 75 and 76 on the front and back of two identical power pistons 24. 
It should be understood, that the proposed die closing unit may also be 
equipped with four power cylinders, for example, or, in the case of the 
push-type unit to be described further below, with only one such power 
cylinder. The power pistons 24 are attached to piston rods 16 extending 
axially from the cylinder mount 10 in both directions, the forward 
extremities of the piston rods 16 carrying the movable die plate 12 with 
the movable half of the molding die 13. The rearwardly extending piston 
rod portions 16' extend into two coaxially aligned auxiliary cylinders 11, 
thereby forming annular cylinder chambers 80 and 81. The longitudinal 
guidance of the piston rods is obtained by means of guide bores in a pair 
of guide bushings 17 and 18 arranged in the forward wall of the cylinder 
mount and by similar guide bores in a pair of suitable connecting flanges 
of the auxiliary cylinders 11. To the rear extremity of one of the piston 
rod portions 16' is attached a piston 14. 
The cylinder space 75 which is located ahead of the power piston 24, 
hereinafter referred to as the high pressure space, is connected to the 
hydraulic system of the injection molding machine via an inlet 19 at the 
front end of the power cylinder, while a similar inlet 20 at its rear end 
leads to the rear cylinder space 76, hereinafter referred to as the low 
pressure space. Hydraulic lines lead from these inlets to the hydraulic 
controls of the injection molding machine (not shown), which selectively 
connect one or the other, or both inlets, to a source of pressure fluid, 
while one or the other inlet may be connected to the fluid tank of the 
machine. Similar inlets 21 and 22 lead to the cylinder spaces 80 and 81 of 
the two auxiliary cylinders 11. From FIG. 1 it can readily be seen that a 
pressurization of the cylinder space 80 of the right-hand auxiliary 
cylinder - as seen from behind the unit - produces a closing travel of the 
unit, while pressurization of the cylinder space 81 of the left-hand 
auxiliary cylinder produces an opening travel of the unit. Both auxiliary 
cylinders are thus single-acting cylinders, in contrast to the 
double-acting power cylinders; one auxiliary cylinder serves exclusively 
as an opening cylinder, while the other serves as a closing cylinder. A 
bore 23 allows air to enter and exit from the outer extremity of the 
right-hand auxiliary cylinder. 
A central flow channel 31 leads from the cylinder space 81 of the opening 
auxiliary cylinder to its associated power piston and through a cross 
connection in the movable die plate 12 from the left-hand piston rod into 
the right-hand piston rod and to the right-hand power piston. As can 
better be seen in FIGS. 2 and 3, the central channel 31 opens into an 
annular valve plunger pressure space 29, located axially between the power 
piston 24 and a hollow valve plunger 25, which latter is arranged for 
axial sliding motion on the piston rod 16. While the inner diameter of the 
valve plunger 25 engages the piston rod 16, a concentric outer diameter 
thereof engages a matching recessed bore in the power piston 24 so that 
the valve plunger is not only responsive to the pressure created in the 
plunger pressure space 29, but also to the pressure to which the high 
pressure space 75 of the power cylinder is subjected. Suitable gaskets 36 
and 37 in the valve plunger 25 and in the piston rod 16, respectively, 
provide a seal for the inner and outer cylinder surfaces of valve plunger 
25. 
Pressurization of the cylinder space 75 thus causes the valve plunger 25 to 
move to the right until an enlarged flange portion 25' on the valve 
plunger 25 comes to axially abut against a matching valve seat 30 of the 
power piston 24. The latter is mounted on the same diameter of the piston 
rod 16, being axially restrained against the effect of the hydraulic 
pressure in the high pressure space 75 by a split abutment ring 27 seated 
in a groove of the piston rod. If the line 31 and the pressure space 29 
are pressurized higher than the pressure which exists inside the cylinder 
space 75, then the valve plunger 25 moves to the left, away from the valve 
seat 30, until it is stopped by a retaining ring 26. In this retracted 
position, the valve plunger 25 opens an axial recess 28' in the power 
piston 24. The latter in turn is in communication with a series of axially 
oriented bypass channels 28, which thus connect the low pressure side of 
the power piston to its high pressure side, when the valve plunger is in 
the open position (FIG. 2). For a smooth flow path of the hydraulic fluid 
through the bypass channels 28 and through the valve itself, the valve 
seat 30 of the power piston and the mating face of the flange portion 25' 
of the valve plunger are preferably inclined at an angle of approximately 
45.degree. against the cylinder axes a--a. The outer diameter of the 
flange portion 25', which is preferably also the outer diameter of the 
valve seat 30, is approximately identical to the center circle on which 
the axes b--b of the bypass channel 28 are located. 
The operative sequence of this die closing unit drive, starting from a 
closed die position and assuming that the molded part is ready for 
ejection, is as follows: 
In order to open the die halves 13, pressurized hydraulic fluid is pumped 
into the cylinder space 81 of the opening auxiliary cylinder 11, via its 
intake 22. This pressurization causes the piston rod portion 16' to act 
itself as a piston, pushing both piston rods, which are rigidly 
interconnected at the movable die plate 12 to form a moving assembly 
therewith, to the left in FIG. 1. However, since the valve plunger 
pressure spaces 29 are hydraulically connected to the cylinder space 81, 
the pressurization of the latter also causes both valve plungers 25 to 
move to the left, thereby opening the bypass valves of the two power 
pistons 24. With the power piston valves thus opened (FIG. 2), the pistons 
are free to move to the left inside their power cylinders, as the 
hydraulic fluid simply moves from the high pressure side of the power 
pistons to their low pressure side, without leaving the cylinder. This 
produces a rapid opening motion of the die closing unit, while a 
comparatively small volume of pressure fluid is used, as determined by the 
diameter of the piston rod portion 16' and the required opening stroke of 
the molding die 13. 
It may be desirable to remove and renew at least a portion of the hydraulic 
fluid in the power cylinders during each cycle. This is accomplished 
either through appropriate controls in the hydraulic lines connected to 
the intake openings 19 and 20, or it may be a built-in feature, if the 
piston rods 16 have rear piston rod portions 16' which are larger or 
smaller in diameter than the remainder of the piston rods. 
The closing travel is accomplished by means of the closing auxiliary 
cylinder on the right-hand side of the unit which, when the cylinder space 
80 is pressurized, cause the auxiliary piston 14, and with it the entire 
moving assembly, to move rearwardly, thereby approaching the mold halves 
13 against each other. In this case, the valves of the power pistons are 
again in their open positions (FIG. 2), the hydraulic fluid flowing freely 
from the low pressure side to the high pressure side of the power pistons 
24. In this case, the power piston valve is held open either as a result 
of an axially fixed connection between the power piston 24 and the piston 
rod 16, or as a result of a certain residual pressure inside the valve 
plunger pressure space 29, which latter may be obtained through a 
throttling action against the hydraulic fluid being expulsed from the 
cylinder space 81 of the opening auxiliary cylinder. As soon as the die 
halves 13 are closed against each other, the high pressure spaces 75 of 
the power cylinders are pressurized. This pressure causes the valve 
plungers 25 to move to the right, thereby shutting off the bypass flow 
through the power pistons 25. The latter now become pressurized over their 
entire area, and the resultant force is transmitted to the piston rods 16 
and to the die 13 via the split abutment rings 27 in the piston rods. 
In FIGS. 4 and 5 is illustrated a slightly modified embodiment of the power 
piston and valve assembly just described. Here, the annular space which 
previously served as a valve plunger pressure space is elongated in the 
axial direction, accommodating therein a compression spring 47 which gives 
the valve plunger 25 a bias toward its open position (FIG. 4). The central 
bore 60 serves in this case merely as a relief bore for the evacuation and 
supply of air from and to the annular space occupied by the spring 47. 
Thus, the valve is automatically maintained open during the closing and 
opening travels of the guide closing unit, and it is closed only, when the 
high pressure cylinders space 75 is pressurized, while a certain counter 
pressure is created in the low pressure space 76. This pressure creates a 
closing force on the hollow valve plunger 25. 
Both the first-described embodiment (FIGS. 1-3) and its modification (FIGS. 
4 and 5) have essentially the same hollow valve plunger 25 which is seated 
for sliding axial motion on the outer diameter of the piston rod 16 and 
which has an outer cylindrical surface that sealingly engages a recessed 
bore of the power piston 24. This arrangement renders the valve plunger 25 
responsive to the pressure in the high pressure space 75 of the power 
cylinder, which pressure forces the valve plunger into its closed 
position. The valve plunger returns to its open position, as soon as the 
space 75 is de-pressurized, moving either under the action of the spring 
47 (FIGS. 4 and 5), or in response to a hydraulic opening pressure 
reaching the pressure space 29 through line 31 (FIGS.2 and 3). 
A different operative response is provided in all the other embodiments of 
the invention, as exemplified by FIGS. 8-12, for example. Here, the 
annular valve plunger 61 is made responsive to a valve plunger pressure 
space 77 which is arranged on the opposite axial end of the valve plunger, 
between the latter and a cylinder sleeve 71. Thus, the valve plunger is 
urged into its open position (FIG. 10), whenever a pressure exists in 
either the high pressure space 75 or the low pressure space 76, as long as 
the valve plunger pressure space 77 and the supply line 31 are not 
pressurized. As can be seen in FIG. 9, for example, the hollow valve 
plunger 61 has again a tapered contact face engaging a matching valve seat 
62 on the periphery of an axial recess 73' of the power piston 72. 
However, in this case the valve diameter is slighty smaller than the 
hydraulically effective outer diameter of the valve plunger 61. Thus, the 
closing motion of the power piston valve can be controlled and timed 
independently of the opening and closing travel motions, making it 
possible to slow down the closing motion just before the die halves 13 
meet, by closing the valve slightly before the end of closing travel. 
Similarly, the full power piston force could be used to initiate the 
opening travel, by holding the power piston valve closed during an initial 
pressurization of the low pressure space 76. 
A modified embodiment is again illustrated in FIGS. 11 and 12, where a 
compression spring 47 is arranged between the cylinder sleeve 75 and the 
valve plunger 61. In this case, the spring provides a closing bias on the 
valve plunger 61, which bias adds itself to the closing pressure from the 
line 31, but which can be overcome, in order to open the valve, when both 
power cylinder pressure spaces 75 and 76 are under pressure during the 
opening and closing travels. 
It will be noted that the die closing unit of FIG. 8 does not have two 
different auxiliary cylinders 11 as is the case in the unit if FIG. 1. 
Here, both auxiliary cylinders operate simultaneously to provide the 
opening travel only. The closing travel, on the other hand, is obtained 
through a pressurization of both power cylinder spaces 75 and 76 which, 
due to different diameters of the piston rod 16 on the front and rear of 
the power piston 72, produces a differential pressure on the power piston 
72 in the sense of a closing motion. As mentioned, this pressure also 
automatically opens the power piston valve. 
A similar, but somewhat modified embodiment is shown in FIGS. 6 and 7, 
where the cylinder sleeve 71 of the previously described embodiment is 
replaced by an integral cylinder extension 45' of the power piston 45, the 
open end of the cylinder extension 45' being closed off by means of an end 
cover 63, thereby confining the valve plunger 61 inside the power piston 
45. A plurality of radial bores 28'" complete the flow path of the fluid 
through the power piston 45 from the bypass channel 28 and the axial 
recess 28', when the valve plunger 61 is in its open position. The central 
channel 31, leading to the valve plunger pressure space 64 inside the 
cover 63, supplies the pressure which closes the valve, while the pressure 
inside the power cylinder spaces 75 and 76 causes the valve to open. The 
operative characteristics of this assembly are thus similar to those of 
the embodiment described in connection with FIGS. 8-12. 
A still further embodiment of the invention is illustrated in FIGS. 13-17. 
Here, the hollow valve plunger 61 defines a valve configuration with the 
power piston 72 which is similar to that of the embodiment of FIGS. 8-10 
but, instead of being surrounded by a cylinder sleeve 71, the valve 
plunger 61 encloses a fixed ring 71', in order to form the desired valve 
plunger pressure space 77. The piston rods 16 are again shown of a uniform 
diameter over their entire length, rather than with differential diameters 
as in FIG. 8, which means that the power piston 72 is positioned against 
the piston rod by means of a split ring 27. It further requires that one 
of the auxiliary cylinders serves again as a closing cylinder, while the 
other serves as an opening cylinder. 
It will be readily recognized that the operative characteristics of the 
embodiment of FIGS. 13-17 are quite similar to those of the embodiment of 
FIGS. 8-12. The major difference between these two embodiments is one of 
analogous inversion of certain portions of the hollow valve plunger 61 and 
its cooperating part, which together form the valve plunger pressure space 
77. This cooperating part - cylinder sleeve 71 in one case, and ring 71' 
in the other case - is axially positioned on the piston rod 16 be means of 
retaining rings 26 and 74. The outer diameter of the valve plunger 
pressure space 77 is in both cases the same and, because it is larger than 
the diameter of the valve seat 62, the valve opens under pressurization of 
the high pressure space 75, unless the valve plunger pressure space 77 is 
likewise pressurized, in which case the valve plunger closes against the 
power piston 72. With the exception of the piston rod shoulder 70 of the 
split ring 27 of this embodiment, the power piston 72 is essentially the 
same in both cases. 
For practical purposes, the radial width of the valve plunger pressure 
space 77, i.e. the radial step between the inner and outer effective 
diameters of the valve plunger 61, is preferably approximately between 
one-eighth and one-sixth of the diameter of the piston rod 16. 
In FIGS. 16 and 17 is again shown a modified version of the embodiment of 
FIGS. 14 and 15, the closing of the valve plunger 61 being assisted by a 
compression spring 47. In this case, however, the valve plunger has to be 
axially longer, in order to accommodate the spring inside an axially 
extended valve plunger pressure space 77. A small shoulder 95 limits the 
opening travel of the valve plunger 61 against the fixed ring 71'. 
In order to open the molding die 13, the cylinder space 81 of the opening 
auxiliary cylinder 11 is pressurized via the inlet 21, so that the piston 
rod portion 16' acts as a piston, pushing the movable assembly into the 
open position. During this opening travel, the power cylinder spaces 75 
and 76 are both pressurized just enough to cause the valve plunger 61 to 
move against the non-pressurized valve plunger pressure space 77, i.e. to 
open, so that the power piston 72 can move freely through the pressure 
fluid contained inside the power cylinder. As can readily be seen from the 
drawing, the flow path of the fluid to the bypass channels 73 of the power 
piston 72 has a minimum of directional change. 
The closing travel of the movable assembly is achieved by pressurizing the 
pressure space 80 of the closing auxiliary cylinder 11, via its inlet 22. 
The auxiliary piston 14, which is attached to the rear extremity of the 
piston rod portion 16, then causes the movable assembly to travel 
rearwardly, until the molding die 13 is closed. The valve plunger 61 is 
again kept open through a slight pressurization of the power cylinder 
spaces 75 and 76. Upon termination of the closing travel, the valve 
plunger pressure spaces 77 are pressurized via the fluid channels 31, so 
that the valve plunger 61 closes the bypass channels 73 of the power 
piston by contacting its valve seat 62. 
As stated earlier, the diameter of the valve plunger pressure space 77 is 
somewhat larger than the diameter of the valve seat 62 on the edge of the 
axial recess 73' on the high pressure side of the power piston 72. This 
difference in diameters signifies that the valve plunger 61 is not only 
urged into its open position (FIG. 15), as long as it is axially spaced 
from the valve seat 62, but also when it contacts the latter, because the 
hydraulically effective area on the valve plunger which is located 
radially outside the valve seat 62 is larger than the oppositely oriented 
effective area of the valve plunger which is located radially outside the 
fixed ring 71'. 
As can be seen especially in FIGS. 1, 8 and 13, the three major embodiments 
of the invention described so far are very similar in regard to their die 
mounting configurations. In each case the movable die plate is pulled 
closed, leading to the designation of this type of die closing unit as a 
"pull-type" die closing unit. This piston rods extend axially past the 
molding die 13, thereby pulling the movable die plate against the 
stationary die plate. 
A different type of die closing unit is shown in FIGS. 18 and 19, where the 
movable die plate is pushed against the stationary die plate in a 
so-called "push-type" die closing unit. Unlike the pull-type die closing 
unit, where at least two power cylinders are necessary, the push-type die 
closing unit can operate with a single power cylinder, if appropriate 
guides are provided between the die halves 13 and/or the die plates of the 
die closing unit. FIG. 18 shows the power piston and bypass valve 
configuration to be essentially the same as in the last-described 
embodiment (FIGS. 13-17), except that the valve is arranged at a larger 
diameter, with the fixed ring 71' having a tubular axial extension 83 
reaching against the power piston 72. The fixed ring 71' thus can serve a 
dual purpose, by clamping the power piston 72 against a clamping shoulder 
84 of the piston rod 16 (FIG. 19) - or, alternatively, against a split 
ring while serving as a guide for the valve plunger 61 on two concentric 
cylindrical surfaces and thereby defining the valve plunger pressure space 
between these two parts. The fixed ring 71' and the corresponding length 
portion of the piston rod 92 are preferably provided with threads 97, so 
that the ring 71' can be axially tightened against the power piston 72. 
The fully tightened ring 71' is lastly secured in place by means of a 
retaining ring 86 seated in a groove of the piston rod 92. 
A very favorable bypass flow path is obtained in the configuration of FIG. 
19, where the smaller guide diameter of the fixed ring 71' is 
approximately aligned with the diameter of the piston rod 92 on the low 
pressure side of the power piston 72 and the axially oriented bypass 
channels 73 in the piston have their innermost wall portion likewise 
located in alignment with the aforementioned diameters of the ring 71' and 
rod 92. The flow path of the hydraulic fluid through the power piston 72 
thus has a virtually straight inner contour, until it reaches the valve 
plunger 61, where a 45.degree. outward deviation takes place. The center 
circle at which the axes b--b of the bypass channels 73 approximately 
coincides with the diameter of the axial recess 73' of the power piston 72 
and the valve seat 62 has approximately the same diameter. Lastly, the 
outer diameter of the fixed ring 71', which determines the outer guide 
surface between the valve plunger 61 and the ring 71', is likewise 
approximately the same as the diameter of the valve seat 62, or it may be 
somewhat larger, as will be explained further below. The power piston 72 
itself has a planar inner clamping face 83 for clamping contact with the 
extension 72' of ring 71'; axially offset outer planar faces 88 and 89, 
with a tapered intermediate face portion provide a smooth flow path for 
the hydraulic fluid into and out of the bypass channels 73. On the outer 
periphery of the power piston 72 are two annular grooves, one of them 
accommodating a thin guide sleeve 34, and the other holding a piston seal 
35. 
It will be noted that, when the diameter of the valve seat 62 and the outer 
diameter of the ring 71', defining a cylinder surface 96, are identical, a 
pressurization of the high pressure cylinder space 75 does not 
automatically retract the valve plunger 61 from its closed position on the 
valve seat 62. However, because the high pressure space 75 has a larger 
effective area than the low pressure space 76 behind the power piston 72, 
a pressurization of both spaces will move the power piston to the right in 
FIG. 19, i.e. in the direction of die closing. This also means, however, 
that the valve plunger 61 receives additional opening pressure from the 
low pressure side of the power piston, on its area which is located inside 
the valve seat 62. This additional pressure and the impingement of the 
fluid flow through the bypass channels 73 against the plunger 61 will 
maintain the latter in its open position, until the valve plunger pressure 
space 77 is pressurized via the channels 31 and 91. As soon as the valve 
is thus closed, the power cylinder space 75 can be fully pressurized to 
produce the closing pressure. 
In FIG. 18 is shown an auxiliary opening cylinder 96 which is coaxially 
attached to the power cylinder of the cylinder mount 10. This auxiliary 
opening cylinder is basically similar in design to the opening cylinders 
of the pull-type embodiment of FIGS. 1 and 13, having a piston 94 attached 
to the far extremity of the piston rod portion 92'. Pressurization of this 
cylinder causes the movable assembly to travel to the left in FIG. 18, 
until it reaches the fully open position shown in that figure. 
Certain types of molding dies and raw materials may require considerable 
forces for the initial separation of the die halves. In such a case, the 
valve plunger pressure space 77 is maintained under pressure during the 
initial portion of the opening stroke, while the low pressure side of the 
power cylinder is pressurized, in order to create an opening force on the 
power piston 72 itself. This force is then transmitted to the piston rod 
92 through the fixed ring 71' and the thread connection 97. Following the 
initial separation of the die halves, the pressure space 77 can be 
de-pressurized, whereupon the valve plunger 61 opens under the pressure 
exerted against it through the fluid in the recess 73' which is in 
communication with the low pressure space 76 of the power cylinder. The 
full force potential, e.g. 20 tons, which is available for the creation of 
a closing pressure on the die halves can thus also be utilized for the 
initial opening travel, the auxiliary opening cylinder producing the 
remainder of the opening travel in an accelerated motion. 
For the closing travel, the power piston 72 itself acts as a 
differential-diameter piston, because the low pressure space 76 is smaller 
in effective area than the high pressure space 75. This is due to the fact 
that the diameter of the piston rod 92 is larger than the diameter of the 
piston rod portion 92' reaching into the auxiliary opening cylinder. Thus, 
if both power cylinder spaces are pressurized evenly, with the bypass 
channels 73, open, an effective piston area equal to the difference 
between the two piston rod cross sections is obtained under this pressure, 
producing an accelerated closing travel motion. The diameter of the piston 
rod 92 is preferably equal to the inner diameter of the auxiliary 
cylinder, with the result that identical effective piston areas are 
available for both the opening and closing travel. 
In order to obtain an automatic opening motion on the valve plunger 61 
under the effect of the pressurization of the high pressure space 75, as 
soon as the pressure in the valve plunger pressure space 77 is relaxed, 
the outer diameter of the pressure space 77, i.e. the outer diameter of 
the fixed ring 71', may be chosen somewhat larger than the diameter of the 
valve seat 62, meaning that the valve plunger has a larger effective area 
subjected to the pressure of pressure space 75 in the direction of opening 
motion, viz. the area located outside the valve seat 62, than the area 
which is subjected to the same pressure in the direction of closing 
motion. Once the valve plunger is thus retracted into its open position, 
the high pressure space 75 is in communication with the low pressure space 
76. But even in this condition, a certain closing pressure can be 
maintained through the differential-piston effect of the power cylinder. 
Removal of the pressure from the power cylinder spaces 75 and 76 and 
pressurization of the auxiliary piston space 93 causes the assembly to 
execute the accelerated opening travel. It follows that, because this 
opening travel, as well as the closing travel, are produced with 
relatively small effective piston areas, they can be executed very 
rapidly, without the need for a large pumping capacity. 
As can readily be seen from FIGS. 18 and 19, the fixed ring 71', with its 
extension 71", and the hollow valve plunger 61 form a convenient 
sub-assembly which is readily removable from the piston rod. The valve 
plunger no longer needs to have a sliding fit on the piston rod itself. 
Also, because the piston rod diameter in the rod portion which carries the 
power piston 72 and the fixed ring 71' is greater by the height of the 
thread connection 97, these parts are easier to slide in place on the 
piston rod. A gasket 36 between the valve plunger 61 and the ring 
extension 71" serves as a seal for the pressure space 77, similar gaskets 
being provided in all other embodiments. The power piston itself is again 
equipped with a guide sleeve 34 and a piston seal 35. 
The construction of the power cylinder as a differential pressure cylinder 
has the additional advantage of providing unequal capacities in its high 
pressure and low pressure spaces 75 and 76, respectively. This means that, 
during each operating cycle, a portion of the hydraulic fluid contained in 
the power cylinder is evacuated during the opening stroke and fresh 
hydraulic fluid is re-introduced during the subsequent closing stroke. The 
hydraulic fluid thus removed is passed through the cooling circuit of the 
injection molding machine, while cooled fluid enters the power cylinder. 
This feature prevents an undesirable heat buildup in the power cylinders 
of the die closing unit. 
The embodiments of FIGS. 13-17 and of FIGS. 18 and 19 have the additional 
advantage of being more compact in the axial direction than the embodiment 
of FIGS. 8-12. This makes it possible to obtain a longer opening stroke 
with a given length of the power cylinder, or to correspondingly shorten 
the power cylinder for a given opening stroke. The fact that the power 
piston 72 is axially clamped against the shoulder 84 of the piston rod 92 
further makes it possible to considerably reduce the axial length of the 
power piston, thereby accordingly shortening the length of the bypass 
channels 73 and thus reducing the flow resistance therethrough during 
opening and closing travel. The reduction of this flow resistance, in 
turn, makes possible a corresponding increase in the speed of opening and 
closing travel. The combined features of the higher flow speed through the 
power piston and the much smaller weight of the moving part of the power 
piston valve thus make it possible to greatly increase the operating speed 
of this closing unit, thereby correspondingly shortening the duration of 
an injection molding cycle, for a higher productivity of the injection 
molding machine. 
A preferred differential ratio for the piston rod diameters at the rod 
portions 92 and 92' is 15 to 12. As stated earlier, this same ratio is 
preferably also maintained with respect to the diameters of the auxiliary 
piston 94 and the associated piston rod portion 92', in order to obtain 
identical opening and closing travel conditions. 
It should be understood, of course, that the foregoing disclosure described 
only preferred embodiments of the invention and that it is intended to 
cover all changes and modifications of these examples of the invention 
which fall within the scope of the appended claims.