Note: Descriptions are shown in the official language in which they were submitted.
~158819
METHOD AND APPARATUS FOR SETTING A CLAMPING LOAD
BACRGROUND O~ THE INVENTION
The present invention relates to a method and apparatus
for setting the preload between die plates such as found in
molding, stamping and pressing machines. More particularly, the
invention ig concerned with.setting the preload between two die
plates that are operated by a clamping mechanism of given stroke.
In injection molding machines of the prior art, a pair
of die plates are reciprocated relative to one another to open
and close molds defined or carried by the plates. A toggle
linkage is customarily employed to develop the high clamping
forces holding the die plates together during the injection
process. The linkage reacts the clamping forces through a thrust
plate and a set of adjustable tie bars that extend between the
thrust plate and the die plates. The clamping load is sometimes
referred to as the preload because it is developed by the linkage
as soon as the molds are closed.
It is desirable to set the "die height" or preload on
the die plates when different molds or other changes are made in
the molding machine. The preload is directly related to the
a~unt by which the tie bars are strained when the toggle linkage
places the dies in a lock-up condition. Thus, setting the length
of the tie bars which are strained between the die plates and
thrust plate sets the preload. The effective tie bar length can
be set by means of a motor which rotates nuts on the ends of the
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tie bars and moves the thrust plate relative to the die plates.
U.S. Patent 2,498,264 to Goldhard discloses a set of motor-driven
nuts of this type and a control mechanism for deenergizing the
motor when a particular setting is obtained.
The strain of the tie bars associated with the preload
is a small quantity compared to the relative displacement of the
die plates in a molding operation; therefore, the mechanism for
adjusting that strain must be accurate. In the 601dhard patent
referenced above, a limit switch that can be adjusted in position
for various preloads is actuated by one of the die plates as the
tie bar length for a particular operation is reached. Since the
adjustment in length must be set to thousandths of an inch, the
limit switch actuated by the die plate, thereby measuring absolute
position of the plate, makes accuracy and repeatability difficult
to achieve.
It is accordingly a general object of the present in-
vention to provide method and apparatus for setting the preload
or die height between two clamped members in molding machines and
the like.
SUMMARY OF THE INVENTION
_ The present invention resides in a machine for molding,
stamping, pressing or the like in which a first die plate is
reciprocated between open and closed positions relative to a
second die plate in a cyclic machine operation. A clamping
25 mechanism for opening and closing the die plates is connected
between the first die plate and a thrust plate tied to the second
die plate for reacting the clamping load between the plates in a
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closed position. Drive means used for setting die height moves
the thrust plate and the second die plate relative to one another
and thereby sets a corresponding clamping load between the plates
in the closed position.
Apparatus for establishing a predetermined clamping load
between the die plates comprise$ sensing means operatively
connected with the drive means for measuring the movement of the
thrust plate and second die plate from one relative position to
another by the drive means. For example, the drive means may
include a high ratio mechanical drive having a large input dis-
placement and small output displacement that facilitates relative
movement of the thrust plate and die plate. A sensor associated
with the large displacement input accurately measures displacement
of the order needed to set the preload.
The invention also relates to the method by which the
preload is set in a machine having a clamping mechanism of given~
stroke. An adjustable linkage, such as a tie bar, is provided
between the clamping mechanism and one of the die plates to adjust
the stress in the linkage and the corresponding preload between
the plates in the closed position at one limit of the clamping
stroke. With the clamping mechanism at the limit, the linkage
between the clamping mechanism and one of the die plates is ad-
justed to a first length which brings the two die plates into a
known position relatlve to one another, for example, in contact
without any significant preload. Thereafter, the length of the
adjustable linkage is changed by a selected amount from the first
to a second length which preloads the die plates correspondingly a
the limit of the clamping stroke. The adjustment of the linkage i
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performed while the clamping mechanism is holding the die plates
in spaced relationship so that the adjustment is not opposed by
the loads established by the adjustment.
The setting of the linkage for a particular preload
can be accomplished automatically by providing motor means for
changing the length of the adjustable linkage and a sensor for
measuring the change in length as the motor is driven. By com--
paring the change as it occurs with a desired change and
corresponding preload, the motor may be automatically deenergized
at the proper adjustment.
A principal advantage of the present invention is that
the preload setting is obtained more accurately by measuring in-
cremental changes in the length of the adjustable linkage from a
known positional relationship of the die plates. Small relative
movements are measured rather than an absolute position of the
thrust plate. The method and apparatus allow changes in the
preload to be rapidly set in the machine, and the preload for
particular molds,clamps or other devices may be recorded and re-
established at any time without difficulty.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 schematically illustrates an injection molding
machine in which the present invention is employed, and shows the
machine with the die plates closed.
Fig. 2 illustrates the injection molding machine in
Fig. 1 with the die plates in an open position.
Fig. 3 illustrates the molding machine in Fig. 1 with
the length of the tie bars set to establish clearance between the
die plates when the clamping mechanism is at one limit of its
S e ro~e.
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Fig. 4 illustrates the molding machine of Fig.
1 during the step of adjusting the length of the tie bars.
Fig. 5 is a perspective view showing the back-
side of the thrust plate and the preload adjusting appara-
tus in the molding machine of Fig. 1.
Fig. 6 is a schematic diagram of machine controls
for automatically setting the preload.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
_
Figure 1 illustrates an injection molding machine,
generally designated 10, incorporating and utilizing the
concepts of the present invention. The machine 10 has a
stationary die plate 12, a movable die plate 14 and a thrust
plate 16, all of which are mounted in parallel relationship
with one another on four tie bars 20, 22, 24, 26 as shown
in Figs. 1 and 5. The die plate 12 includes a mold cavity
30 which is supplied with melted plastic or other settable
material from a heated injection cylinder 3Z. The injec-
tion cylinder receives the plastic material in powdered or
pellet form through the hopper 38 and a charge of the
material is loaded into the cylinder by means of a rotary
drive motor 36 at the end of the cylinder opposite from
the mold 30. The die plate 14 includes a parison pin 34
(Fig. 2) which defines the shape of an object to be formed
from the settable material in conjunction with the cavity
30.
During a molding operation the die plates are
moved to a closed position such as illustrated in Fig. 1
and are clamped together with a preload sufficient to with-
stand the injection pressures and prevent separation or
movement at the interface of the die plates. Flashing,
that is, a thin film of plastic emanating from the molded
part at the seam between separated or moved die plates,
is prevented by the preload which may be on the
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order of several hundred tons for large machines.
In the illustrated molding machine 10, the clamping
force is produced by a clamping mechanism including a hydraulic
cylinder 40 mounted in the thrust plate 16 and a toggle mechanism
actuated by the cylinder 40 and including a cross-head 42 and
upper and lower toggle linkages 44. l~hen the cylinder 40 has
been extended as shown in Fig. 1, the toggle linkages 44 in the
toggle position produce the preload between the die plates 12 and
14, and that load is reacted from the die plate 14 and the toggle
linkages through the thrust plate 16 to the tie bars 20-26 and
the die plate 12. The preload which is reacted through the tie
bars elongate or strains the bars by an amount which is directly
proportional to the load. The fixed nuts 48 in Fig. 1 on the one
end of the tie bars and the adjusting nuts 50, 52, 54 and 56 in
Figs. 1 and 5 on the opposite ends of the tie bars establish the
effective length of the tie rods supporting and strained by the
preload. By increasing or decreasing the length of the tie bars,
the preload is set since the toggle linkages 44 in conjunction
with the cylinder 40 have a given or fixed stroke and always clamp
the die plates in the toggle position at the one limit of the
stroke.
To this end, the adjusting nuts 50-56 shown in greater
detail in Fig. 5 are rotatably driven and adjust the length of the
tie bars 20-26 respectively. (The tie bar 20 is intentionally
omitted in the nut 50 in order to illustrate the threading therein .
The nuts 50-56 are driven simultaneously by means of a drive motor
_ 58 and a chain-and-sprocket system including sprockets 60, 62, 64
and 66 mGunted respectively on the adjusting nuts 50, 52, 54, 56
and a common chain 68. The adjusting nuts 50-56 together with
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the sprockets 60-66, the drive chain 68 and the drive sprocket
70 on the shaft of the motor 58 constitute a high ratio mechanical
drive or reduction drive from the motor 58 primarily due to the
threading between the nuts and the tie bars and the relative
sizes of the sprockets 60-66 and the drive sprocket 70. As a
result a relatively large rotational displacement, that is,
several rotations of the motor drive shaft, produce a relatively
small axial displacement of the adjusting nuts and the thrust
plate 16 on the tie bars. Of course, the effective length of the
tie bars changes correspondingly, and an accurate setting of the
preload is brought about as described in greater detail below.
To establish a predetermined preload between the die
plates 12 and 14,the adjusting nuts 50-56 are rotated by the
drive motor 58 until the die plates 12 and 14 are clearly separate~
when the toggle linkages are extended in the toggle position as
shown in Fig. 3. Preferably the hydraulic cylinder 40 is first
actuated to draw the die plate 14 away from the die plate 12 as
shown in Fig. 2 so that a relatively low power motor 58 can
rotate the adjusting nuts in the counter-clockwise direction and
backoff the adjusting nuts without a clamping load. It will be
understood that since the sprockets, drive chain and nuts con-
stitute a high ratio drive, and adjustment of the die plates can
be carried out at a no load condition, the motor 58 only needs
to produce enough power to overcome friction and make the desired
adjustment.
After the die plates have been separated, the drive
_ motor 58 is energized to rotate the adjusting nuts 50-56 and move
the thrust plate 16, the die plate 14, and extended toggle
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linkages 44 toward the stationary die plate 12. When the die
plate 14 makes contact with the plate 12 without any substantial
preload, that is, when the die plates are "kissing", the drive
motor 58 is deenergized. Preferably the motor 58 is allowed to
drive the plate 14 against the plate 12 until the motor stalls
and then the motor is deenergized. In this fashion a uniform
reference point is established for subsequent steps in setting
the preload.
It will be understood that further adjustment of the
drive nuts 50-56 beyond the positions in which the die plates 12
and 14 are brought into "kissing" relationship, results in ex-
tension or strain of the tie rods when the die plates are closed,
and locked up, and produces a corresponding preload of the die
plates against one another. Such adjustment of the tie rods can
be measured with hig~ accuracy by means of a displacement sensor
connected with the input end of the high ratio drive mechanism
between the motor 58 and the tie rods 20-26. The sensor aoes not
need to be a high accuracy displacement sensor because the dis-
placement of the chain 68 by the motor 58 is relatively large
compared to the displacement of the adjusting nuts and thrust
Plate 16 on the tie rods, the displacements being related by the
ratio of the mechanical drive.
To this end a displacement sensor comprised by a toothed
plate 80 and a proximity switch 84 is driven from the chain 68 by
2~ means of a sprocket 82. The proximity switch may be an optical
or magnetic sensor which detects each tooth of the plate and
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produces a corresponding output pulse as the plate is
rotated by the sprocket in front of the switch. Each
pulse represents an incremental displacement of the chain
68 and an infinitesimal change in the effective length of
the tie bars 20-26. The difference in diameter of the
toothed plate 80 and the sprocket 82 improves the resolu- ;
tion of chain displacement sensed by the switch 84, and
the proportional displacement and adjustment of the nuts.
The proximity switch 84 is only used to measure
the nut adjustment from the reference position established
by bringing the die plates into kissing relationship and
the final position that establishes the preload. Since
the drive motor 58 is not capable of driving the adjusting
nuts 50-56 while the die plates 12 and 14 are in kissing
relationship, it is necessary to open the die plates by
means of the clamping mechanism as shown in Fig. 4. The
toggle linkages 44 are pulled from their toggle position
and then the drive motor 58 is energized. ;~
The movement of the adjusting nuts 50-56 and the
thrust plate 16 relative to the tle bars 20-26 is measured
by counting the pulses from the proximity switch 84. With
a direct relationship existing between the number of pulses
and nut displacement, and since the displacement can be
related to the preload or lockup load by virtue of the
spring constant of the system, the output pulses of the
switch 84 are a measure of the preload that will exist
between the die plates 12 and 14 when the toggle linkages
44 are subsequently placed in their toggle position. Thus,
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a table of values representing the number of pulses generated by
the proximity switch can be established for given preloads of
the molding machine 10.
To use the preload table of values it will be, of course
necessary to determine how much preload is required for a given
set of molds. When the smallest preload that properly produces
articles from a particular set of molds has been determined, the
machine can be set and reset time and time again at that preload
by carrying out the adjusting process described above and matching
the pulse count from the proximity switch with the table value.
The preloads are accurate since the pulse count represents the
large displacement input of the high ratio drive, and the accurate
setting is made without a great deal of difficulty from the output
signals of the switch.
The capability of setting the preload is highly desirabl~
since many different molds are used in injection molding machines,
and each set of molds requires a unique preload to properly form
the molded articles. An alternative is to preload the die plates
at the maximum tonnage of the machine all of the time; however,
such a procedure needlessly reduces the life of the mold, and may
also reduce the life of the machine by requiring the machine to
cycle at higher stress levels which accelerate wear and failure
due to fatigue. Consequently, the ability to adjust the preload
accurately and to repeatedly set a particular preload in the
machine offers greater flexibility of the machine in the hands of
the customer and a longer useful life of the molds and machine.
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Fig. 6 is a schematic diagram of machine controls that
allow the preload of the molding machine 10 to be set automatic-
ally after a desired preload has been programmed in the machine. A
sensor 100 is provided to measure the relative movement of the
adjusting nuts 50-56 along the tie bars 20-26. For example, the
toothed plate 80, sprocket 82 and the proximity switch 84 in Fig.
5 provide an accurate measure of the nut displacement and hence,
are quite suitable for the sensor 100 in Fig. 6. As a pulse
generator, the proximity switch 84 produces a pulsed output signal
proportional to the displacement, and that signal is transmitted
to a pulse counter 102 for conversion to a digital or analog
signal of corresponding value. The pulse counter is only energize
to record the pulse count during the interval in which the drive
motor 58 is energized to move the thrust plate 16 from the
reference position that brings the die plates 12 and 14 into
kissing relationship and the adjusted position which produces the
desired preload between the die plates. In this manner the pulse
count represents the relative movement of the thrust plate from
the reference position rather than an absolute position.
The pulse count is then compared with a count stored in
a preset memory 104 that has been programmed with a pulse count
corresponding to a desired preload. Comparison of the programmed
count in the memory 104 and the actual count in the counter 102
is made by the comparator circuit 106 to determine when the
counts are the same, and when a coincidence of the counts is
detected, the comparator deenergizes the drive motor 58 automatic-
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ally through the mo~or control 10~. When the motor is deenergizedrotation of the nuts 50-56 stops and the thrust plate 16 is then
in a position along the tie bars 20-26 that produces the pre-
determined preload when the toggle linkages 44 of the clamping
mechanism close the die plates and hold the plates in a preloaded
state at the toggle position.
~ ccordingly, an apparatus and method have been disclosed
for preloading the die plates of a molding machine in a simple,
high accuracy and speedy process. Accuracy is achieved by utiliz-
ing a sensor which measures only relative movement of the ad~ustinlinkage, that is, tie bars which connect the clamping mechanism
with the die plates. The sensor also provides an accurate measure
for.preloading the plates since it operates upon the large dis-
placement input of a high ratio mechanical drive adjusting that
15 linkage. The speed with which an operator of the machine can set
a particular preload is improved by the simplicity of the controls
and their use in the process.
While the present invention has been described in a
preferred embodiment, it should be understood that numerous modifi
20 cations and substitutions can be had without departing from the
spirit of the invention. For example, although the invention
has been disclosed in a molding machine, its utility is not
limited in this respect but has applica~ility to stamping, cutting
and other types of pressing machines in which a clamp mechanism
25 is required to load the die plates. Toggle lin~ages 44 are typi-
cally used in machines of this type, but other types of clamping
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mechanisms having a given stroke may also be used. The proximity
switch serving as a pulse generator for measuring the incremental
displacement of the adjusting nuts and thrust plate from one
position to another with high accuracy is but one of a plurality
of displacement transducers that can serve the measuring functon.
Accordingly, the present invention has been described in a
preferred embodiment by way of illustration rather than limitation
