Note: Descriptions are shown in the official language in which they were submitted.
>~ s
~0336146
SPECIFICATION
accompanying
Application for Grant of U.S. Letters Patent
TITLE: ENERGY-CONSERVING INJECTION MOLDING
MACHINE
Field of the Invention
The invention relates to hydraulic injection
molding machines offering improved energy efficiency.
More particularly, the present invention relates to
hydraulic injection molding machines incorporating a
means for adjusting the output of the hydraulic pump
in accordance with the different hydraulic demands
imposed by the machine during different phases of its
operating cycle so that the output of the pump does
not significantly exceed the imposed demand in order
to conserve energy.
Background of the Invention
Injection molding is a process for convert-
ing material from one form or shape to another by
expending energy. Typically, material in the raw form
~,~
X033646
of solid pellets is loaded into an injection molding
machine wherein the pellets are first converted to a
molten state by introducing energy in the form of heat
and mechanical shear. Further energy is expended to
inject the plasticized material under pressure into a
mold having a cavity defining the final shape of the
part to be produced while the mold is clamped forcibly
closed. Additional energy is used to cool the materi-
al within the mold to return it to a solid state. The
mold clamp is then opened, the molded part ejected and
the mold reclosed in preparation for molding the next
part. The laws of physics require that the total
energy input to the molding machine equally balance
the energy output.
In an hydraulic injection molding machine,
energy is input to the machine in the form of elec-
trical energy. Much of this energy is converted into
hydraulic flow energy by means of an electric motor
drivably connected to a hydraulic pump. The fluid
delivered by the pump serves to operate various
hydraulic components including electro-hydraulic
control valves and hydraulic actuators. The pressure
or volume flow demands for hydraulic fluid vary
considerably not only from one given machine set-up to
another but, also during different phases of the
operating cycle of a machine in any given set-up. For
instance, a set-up requiring rapid injection of the
X033646
material into the mold will require a higher volume
flow rate than that needed for a set-up calling for
slower injection. Also, phases of the machine oper-
ating cycle such as the clamp open phase typically
require greater hydraulic flows than the part curing
phase. While a hydraulic injection molding machine
must be capable of supplying whatever r~i mum fluid
pressure and/or flow requirements are needed to meet
the machine's maximum rated capacity, significant
energy losses have been incurred when operating under
conditions where lesser hydraulic fluid pressures
and/or volume flow rates are required to operate the
machine than the pressure and/or volume flow actually
delivered by the pump.
In machines having a fixed-displacement
hydraulic fluid pump driven by a fixed-speed electric
motor, the pump must be driven to constantly deliver
sufficient flow capable of satisfying maximum machine
requirements even though the instantaneous hydraulic
demand may be significantly lower. The excess flow,
i.e., the difference between the actual flow delivered
by the pump and the instantaneous demand, is pumped
over relief valves. In doing so, energy is wasted.
Some of the wasted energy is given off in the form of
heat which causes undesirable heating of the hydraulic
fluid itself. Further energy must be expended when
the fluid temperature rises to a point where active
2033646
cooling is required to restore the fluid to a suitable
operating temperature. In an effort to overcome these
problems, there have been various attempts in the
prior art to match the output of the hydraulic pump
with the demand presented by the rest of an injection
molding machine.
One such attempted solution has been to
provide a hydraulic injection molding machine having a
fixed displacement hydraulic pump driven by an AC
motor with an adjustable speed drive such as a vari-
able frequency or inverter type drive. By varying the
speed control input to the drive, the motor speed and
therefore the pump delivery rate can be varied to more
closely approximate the actual hydraulic demand.
Unfortunately, the energy savings of such machines are
realized at -the expense of machine performance.
Because the moving components of the motor/pump
assembly cannot be accelerated or decelerated very
rapidly, the frequency response of the machine is
degraded. As a result, molded part quality may be
adversely affected due to variations in molding
parameters such as shot weight, injection velocity and
injection pressure.
The assignee of the present application has
marketed a line of hydraulic clamp machines under the
trademark VISTA. Those machines utilize a fixed RPM
AC motor to drive an axial piston, swash-plate design,
~0;~3646
variable displacement pump to insure that only the
amount of flow necessary to meet load conditions is
delivered by the pump thereby reducing energy usage
and hydraulic fluid heating. A hydraulic feedback
circuit is used to effect either pressure compensation
or flow compensation of the pump output on a select-
able basis by moving the angle of the swash-plate of
the pump to vary its stroke. Since such control
action does not require acceleration or deceleration
of high inertia rotating components of the motor/pump
assembly, response time is significantly improved as
compared to the fixed displacement pump/variable speed
AC motor machines discussed above. Further improve-
ments in transient response are realized by connecting
the pump output to a gas-charged accumulator. The
output of the accumulator is connected to a multi-
function servo-controlled proportional directional
valve which provides closed-loop control of injection
velocity, injection pressure, back pressure and melt
decompression. Nevertheless, the energy savings
realized in VISTA model machines has been less than
the savings made possible by the present invention.
Summary of the Invention
In view of the foregoing drawbacks of the
prior art, it is an object of the present invention to
provide an energy-conserving injection molding machine
which affords greater energy savings than prior art
~0336~6
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machines while maintaining excellent response charac-
teristics.
In accordance with a first aspect of the
present invention, an energy-conserving injection
molding machine which includes a hydraulic pump driven
by a brushless DC motor is provided. Such a motor
affords a considerable advantage in energy efficiency
as compared to the variable speed AC pump motors of
the prior art.
In accordance with a second aspect of the
invention, a variable speed motor, preferably of the
brushless DC type, is used to drive a variable dis-
placement hydraulic pump. The electronic machine
controller delivers a driving signal to the motor
drive to adjust the motor speed in accordance with
desired molding parameters. The motor speed is
selected to substantially match the output of the pump
to the hydraulic load expected during a particular
phase of the machine operating cycle given the molding
parameters entered into the controller by an operator.
The controller automatically outputs new driving
signals as the machine cycles through phases of
operation characterized by different hydraulic loads.
The displacement of the pump is governed by a pump
control which selectively regulates the flow delivered
by the pump in either a pressure compensation or flow
compensation mode. The pump control is capable of
~ Z033646
responding to load changes quickly and therefore
provides improved transient response. To further
improve transient response, the pump output is con-
nected to a gas-charged accumulator by way of a check
valve. These and other aspects and advantages of the
present invention will be apparent to persons of
ordinary skill in the art in view of the following
detailed description taken in conjunction with the
drawings wherein like reference numerals designate
like items.
Brief Description of the Drawings
Fig. 1 is a schematic diagram of an injec-
tion molding machine constructed in accordance with
the present invention.
Fig. 2 is a cross-sectional view of a
variable-displacement pump preferred for use as the
pump shown in Fig. 1.
Fig. 3 is a schematic diagram illustrating
the pump control of Fig. 1 in further detail.
Fig. 4 is a flow diagram illustrating the
manner in which the controller of Fig. 1 derives a
motor driving signal.
Detailed Description of the Preferred Embodiment
Fig. 1 illustrates a preferred embodiment of
an injection molding machine 10 constructed in accor-
dance with the present invention. Machine 10 includes
an injection unit 11 juxtaposed a clamping unit 12.
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--8--
The sequence, timing and quantitative values associ-
ated with the various operations performed by injec-
tion unit 11 and clamping unit 12 are carried out
under the direction of an electronic machine con-
troller 15 in accordance with molding parametersentered into controller 15 by an operator through an
operator interface 16. Closed-loop control is facil-
itated by a plurality of feedback signals 18 which
controller 15 receives from injection unit 11 and
clamping unit 12. Injection control 20 and clamp
control 21 serve as electro-hydraulic interfaces
between machine controller 15 and injection unit 11
and clamping unit 12, respectively. Injection control
20 and clamp control 21 are well known in the art and
comprise electrically controlled proportioning valves
and other hydraulic and electro-hydraulic devices-
arranged for generating the hydraulic pressures and/or
flows necessary for operating injection 11 and
clamping unit 12 in accordance with various operating
signals received from machine controller by way of
respective multiple signal paths 23 and 24, respec-
tively.
Hydraulic fluid is supplied to injection
control 20 and clamp control 21 from a pump 26 by way
of manifolds 27 and 28. Pump 26 communicates with a
reservoir 30 by way of a case drain line 31 and a
supply line 32 and is driven by an electric motor 34
9 ~1~33646
which receives electrical input energy at 35. Pref-
erably, motor 34 comprises a variable speed brushless
DC motor but, in accordance with certain aspects of
the invention, may also suitably comprise other
variable speed motors either AC or DC. Motor 34 is
mechanically connected to pump 26 by way of a rotat-
able shaft 33. In order to conserve energy by
matching the hydraulic output of pump 26 to the demand
during different phases of the operating cycle of
machine 10, the invention contemplates controlling the
speed of motor 34 and shaft 33, and thus the output of
pump 26, with a driving signal 40 generated by machine
controller 15 in a manner to be described in further
detail hereinafter. For present purposes it is
sufficient to note that machine controller 15 calcu-
lates the magnitude of driving signal 40 for each
phase of the operating cycle of machine 10 from the
particular molding parameters entered by an operator
through operator interface 16 during the setup of
machine 10. The molding parameters determine the
hydraulic demand expected during each phase of the
machine cycle.
The hydraulic flow delivered by pump 26 is
monitored, via a line 42, by a pump control 43. Pump
control 43, which will be described in further detail
with reference to Fig. 3, communicates with pump 26
via a line 44 to operate pump 26 to operate in either
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--10--
a pressure compensation mode or a flow compensation
mode in accordance with an electrical pressure or flow
compensation signal delivered to pump control 43 from
machine controller 15 by way of a signal path 46. In
order to improve hydraulic transient response, the
output of pump 26 which supplies manifolds 27 and 28
is preferably connected to a gas-charged accumulator
48 through a check valve 49.
Accumulator 48 has a ballast charged at an
initial internal pressure that resists the flow of
liquid into its interior. The design of such accumu-
lators is well known within the art. When the pres-
sure in the line to which accumulator 48 is attached
exceeds the internal pressure of the accumulator,
hydraulic fluid will flow into accumulator 48 com-
pressing the ballast. When the line pressure drops
below the internal pressure of the accumulator,
hydraulic fluid will flow from accumulator 48 into the
line. Thus, if pump 26 and motor 34 lag behind the
demand for flow rate created by valves opening in one
of the control circuits 20 or 21, accumulator 48
rapidly responds to temporarily make up the additional
demand. Thus, accumulator 48 provides more precise
and faster-responding matching of flow delivery with
flow demand.
Injection unit 11 includes an injection
screw 52 that is rotatably coupled to a hydraulic
-11- Z~;~3646
extruder motor 53 by way of a shaft 54. The rota-
tional direction and speed of motor 53 and thus, those
of screw 52 are governed by the hydraulic flow
directed to motor 53 from injection control 20 by way
of a pair of hydraulic lines 57 and 58. In order to
sense the hydraulic flow to extruder motor 53 during
the "extruder run" phase of the machine operating
cycle, line 58, which emanates from the output side of
an electrically controlled throttle (not shown)
located within injection control 20, is connected to
pump control 43 by way of a line 56. The purpose of
the latter connection will be made clear in connection
with the description of pump control 43 with reference
to Fig. 3 hereinafter.
Injection screw 52 is housed within a barrel
60 having a nozzle end 61. Barrel 60 communicates
with a hopper (not shown) which may be filled with a
moldable material usually in the form of solid pellets
or granules. As screw 52 rotates in one direction
within barrel 60, the barrel is filled with the
material which is transformed into a molten plastic
state by the mechanical shearing action of screw 52.
Plasticizing of the molding material may b~e assisted
by one or more electric heaters (not shown) mounted in
thermal contact with barrel 60. As moldable material
accumulates within barrel 60 from its nozzle end 61
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rearwardly (to the left in Fig. 1), screw 52 is forced
axially rearwardly within barrel 60.
Screw 52 may be selectively extended or
retracted within barrel 60 at a desired rate by means
of a pair of hydraulic actuators 63 and 64 which are
operably mounted between a fixed platen 66 and a
platen 67 mounted for axial movement relative platen
66. Actuators 63 and 64 communicate hydraulically
with injection control 20 by way of a pair of lines 69
and 70 which, when pressurized, cause screw 52 to
retract and extend, respectively, within barrel 52.
To monitor the pressure under which screw 52 is
extended, a pressure transducer 72 connected to line
70 sends a feedback signal to machine controller 15 by
way of a signal path 73. The axial position of screw
52 within barrel 60 as well as its rate of travel are
monitored using a distance transducer 75 connected in
distance sensing relation to movable platen 67 and
communicating with machine controller 15 by way of a
signal path 76. Both platens 66 and 67 are mounted
upon an axially movable sled 77 so that barrel 52 can
be selectively retracted from or moved into engagement
with clamping unit 12. Axial movement of sled 77 is
controlled by a hydraulic actuator 78 which is mechan-
ically coupled to the sled and hydraulically coupled
to injection control 20 by way of lines 80 and 81.
-13- ~6
With continued reference to Fig. 1, clamping
unit 12 comprises a pair of opposed clamp faces 84 and
85 which support matable sections 87 and 88 of a mold
89. Mold 89, which forms no part of the present
invention, is typically supplied by the user of
machine 10 and includes an interior cavity 91 whose
shape defines the shape of a part produced by machine
10. Clamping unit 12 further includes a pair of
hydraulic actuators 94 and 95 for selectively moving
mold sections 87 and 88 into mating relation (as
shown) in order to close mold 89 or to separate
sections 87 and 88 in order to open mold 89. Actua-
tors 94 and 95 are operated by hydraulic fluid sup-
plied from clamp control 21 by way of lines 97 and 98
as shown. A distance transducer lOo, which may
suitably comprise a linear potentiometer, is mounted
in distance sensing relation between clamp faces 84
and 85 in order to permit machine controller 15 to
monitor the distance therebetween by way of a signal
path 101. A conventional ejector mechanism 104 is
connected to clamp control 21 via one or more hydrau-
lic lines 105 to selectively extend one or more
ejector pins 106 into cavity 91 to facilitate removal
of a molded part therefrom when mold 89 is opened.
Clamping unit 12 further includes means for
selectively holding mold 89 forcibly closed. While a
mechanical toggle mechanism may suitably be used for
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this purpose, the embodiment of Fig. 1 employs hydrau-
lic clamping whereby a piston 110 may be selectively
urged against the rear of clamp face 85 under high
hydraulic pressure. That pressure is developed in the
interior volume 112 of a cylinder 113 which slidably
receives piston 110 in hydraulically sealed relation.
Interior volume 112 is selectively fluidly connected
to or sealed from a prefill reservoir 116 by means of
a valve 117. Valve 117 is actuated by clamp control
21 by way of a pair of lines 119 and 120. Valve 117
includes a movable valve stem 122 which is sealably
engageable with a seat 123 formed in the wall of
cylinder 113. After volume 112 has been filled by
opening valve 117, valve 117 is closed to seal valve
stem 122 against seat 123. In order to hold mold 89
clamped forcibly closed, interior volume 112 is
hydraulically charged under high pressure by clamp
control 21 through a hydraulic line 125 coupled to an
inlet port 126 formed in the wall of cylinder 113.
The hydraulic pressure within volume 112 is monitored
at line 125 by a pressure transducer 128 which
communicates with machine controller 15 by way of a
signal path 129.
A typical operating cycle of an injection
molding machine such as machine 10 comprises a number
of phases including, a "clamp close" phase, an "injec-
tion" phase, an "extruder run" phase, a "clamp open"
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phase and an "ejection" phase. Each phase is distinct
and imposes a different hydraulic demand. A sequence
program within controller 15 determines the sequence,
timing and quantitative values associated with those
phases by outputting various operating signals 23 and
24 to electro-hydraulic controls 20 and 21 at appro-
priate times. Energy is conserved by regulating the
output of pump 26 so as not to significantly exceed
the demand associated with each phase. In accordance
with the invention, further energy savings are
realized by the novel use of a brushless DC motor for
driving a variable displacement pump in an injection
molding machine as well as by adjusting the speed of
the motor in accordance with particular driving
signals. In the preferred embodiment of the inven-
tion, motor 34 is a brushless DC type manufactured by
Powertec of Charlotte, North Carolina and is a 75 HP
version of their DPFG288T. The driving signals are
calculated to not only match pump output with demand
for each phase but, most preferably, also to operate
the hydraulic pump/motor combination for best possible
efficiency except when pump control 43 makes adjust-
ments to effect pressure compensation or flow compen-
sation.
In the preferred embodiment, pump 26 pref-
erably comprises a variable displacement pump such as
a vane pump or a piston pump. A preferred variable
` Z0336~6
-16-
displacement pump of an axial piston, movable swash-
plate design is available from Rexroth of Bethlehem,
Pennsylvania as Model AA4VS0 and is illustrated in
further detail in Fig. 2. Pump 26 comprises a housing
133 which is capped by a port plate 134 which captures
a rotatable piston assembly 136. Port plate 134
includes an inlet port 135a which connects to reser-
voir 30 of Fig. 1 by way of line 32 and an outlet port
135b which connects to manifold 27 and 28 as well as
accumulator 48 by way of check valve 49. Piston
assembly 136 comprises a cylinder barrel 137 defining
cylinders which receive reciprocable pistons 138
tiltably supported by shoes 139 which slide along the
face of a tilted swash-plate 140. Reciprocal movement
of pistons 138 is caused by rotation of drive shaft 33
which is rotatably linked to motor 34. The angle of
swash-plate 140, and therefore the stroke of pistons
138 is variable under the control of a cylinder 142
which is connected to pump control 43 by way of line
44 in a manner which will be described in further
detail with reference to Fig. 3. Cylinder 142 engages
swash-plate 140 in order to permit changing the tilt
angle of the swash-plate in accordance with a hydrau-
lic signal applied to cylinder 142. Cylinder 142 is
oriented along an axis perpendicular to the plane of
Fig. 2 to reciprocate toward and away from the viewer.
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That reciprocal motion effects tilting of swash-plate
140 through cam action.
In operation of pump 26, rotation of shaft
33 causes pistons 138 to slide generally axially
inside cylinders 137 as shoes 139 slide along the face
of tilted swash-plate 140. As a piston retracts
within its respective bore, hydraulic oil fills the
cavity above the piston through inlet port 135a. At
maximum retraction of piston 138, the piston begins to
align with outlet port 135b. Continued rotation of
piston assembly then extends the piston in order to
discharge hydraulic fluid under pressure from port
135b. Thus, the instantaneous volume flowrate
delivered by pump 26 depends on the speed of rotation
of shaft 33 as well as upon the angle of swash-plate
140, the latter of which may be varied due to the
action of cylinder 142 in order to selectively effect
either pressure compensation or flow compensation.
With additional reference now to Fig. 3,
pump control 43 will now be described in further
detail. Pump control 43 comprises a flow compensation
valve 150, a pressure compensation valve 157, an
orifice 161, an electrically actuated mode selector
valve 165 and an electrically settable pump pressure
control valve 170, all connected as shown in Fig. 3.
The high pressure output of pump 26 is connected to
valve 150, 157 and 165 by way of line 42. Fluid is
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supplied to pump 26 from reservoir 30 by way line 32.
on the low pressure side, both pump 26 and pump
control 43 are connected to reservoir 30 by way of
lines 31 and 59, respectively.
In order to operate pump 26 in a flow
compensation mode, which is typically desired during
the extruder run phase of the machine operating cycle,
controller 15 applies flow compensation signal via
multiple signal path 46. As a result, the spool of
lo valve 165 shifts to the right in Fig. 3 causing
pressure from line 56 to be applied to the right pilot
side of valve 150 by way of an orifice 152. The
output pressure from pump 26 appears by way of line 42
on the left pilot side of valve 150. As noted previ-
ously, line 56 emanates from an electrically con-
trolled extruder motor throttle valve located inside
injection control 20. In the flow compensation mode,
valve 150 operates to maintain the pressure drop
across that throttle at a desired constant value.
That value is electrically settable by machine con-
troller 15 in accordance with a flow compensation
signal applied to the electrical input of valve 150.
That flow compensation signal is applied to pump
control 43 also by way of the multiple signal path 46
shown in Fig. 1.
In the event the pressure differential
between the two pilots of valve 150 exceeds the
.
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electrically set desired value, the spool of valve 150
will shift toward the right in Fig. 3, thereby
applying pressure to the bottom of pump cylinder 142.
As Fig. 3 more clearly illustrates, cylinder 142 is
mechanically coupled to tiltable swash-plate 140.
Cylinder 142 includes a spring which exerts a force
normally operable to maintain swash-plate 140 in a
position which provides pump 26 with a maximum stroke.
Because the construction of pump 26 is such that it is
most efficient when operating at or near m~x;rum
stroke, it is desirable to operate the pump there
whenever possible. If, however, upon the shifting of
valve 150, sufficient pressure is applied via line 44
to the bottom of cylinder 142 to overcome the spring
force, cylinder 142 will operate to move swash-plate
140 to shorten the stroke of the pump as necessary to
restore valve 150 to its equilibrium position.
Accordingly, pump 26 will deliver only the amount of
fluid necessary to maintain a desired pressure drop
across the throttle valve within control 20 which
controls extruder motor 53.
The pressure compensation mode is selected
by controller 15 during all phases of the operating
cycle of machine 10 other than the extruder run phase.
To do so, controller 15 deenergizes valve 165 to
return it to the normal position indicated in Fig. 3
and applies a pressure compensation signal to valve
2(~336~6
-20-
170. The pressure compensation signal is calculated
by controller 15 in a manner to be described in
further detail hereinafter and is applied to pump
control 43 by way of multiple signal path 46 as
illustrated in Fig. 1. In the pressure compensation
mode, the output delivered by pump 26 is transmitted
to both pilots of valve 150 thereby maintaining valve
150 in the position illustrated in Fig. 3. The output
of pump 26 is also applied to the pilot on the left
side of valve 157 as well as to the left side of
orifice 161. The left side of orifice 161 is con-
nected to both the right pilot of valve 157 and to the
input side of valve 170. As long as the pressure at
the input of valve 170 does not exceed the desired
value presented to valve 170 in the form of the
pressure compensation signal from controller 15, no
flow passes through valve 170. As a result, there is
no flow through orifice 161 and the pump output
pressure appears equally at both pilots of valve 157,
thereby maintaining valve 157 in the normal position
illustrated in Fig. 3. During that time, the spring
within valve 142 will maintain the swash-plate 140 of
pump 26 in a position to provide nearly maximum stroke
for most efficient operation.
In the event that the pump output pressure
appearing by way of line 42 through orifice 161 to the
input (right) side of valve 170 exceeds the pressure
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compensation value applied to valve 170, valve 170
will begin to discharge to reservoir 30 via lines 59
and 31. That flow will cause a pressure drop to
appear across orifice 161, thereby causing valve 157
to shift rightwardly. As a result of that shift,
valve 157 connects line 42 with line 44, thereby
supplying hydraulic fluid pressure to the bottom of
cylinder 142. When that pressure is sufficient to
overcome the spring within cylinder 142, pump 26 is
destroked in the manner described previously. The
output of pump 26 will thereby be reduced tending to
decrease the pressure drop across orifice 161 until
the pressure appearing at the input of valve 170 no
longer exceeds the pressure compensation value. Valve
170 then stops the flow to reservoir 30 and valve 157
is restored to the normal position illustrated in Fig.
3. Accordingly, in the pressure compensation mode,
pump 26 delivers only the amount of flow necessary to
maintain the desired pressure indicated by the pres-
sure control signal controller 15 provides to valve
170.
Desired molding parameters expressed as
setpoints in engineering units are entered into the
memory of controller 15 by an operator through opera-
tor interface 16. In the preferred embodiment,controller 15 is a CAMAC XTL controller manufactured
by Cincinnati Milacron, Plastics Division of Batavia,
-
-22- ~ 0 33 ~ 6
Ohio. The method for entering molding parameters into
this controller is detailed in Vista Hydraulic-CE
Users Manual, Publication No. PM-430 which is express-
ly incorporated herein by reference in its entirety.
The CAMAC XTL has default values for all the setpoints
in the machine for a standard machine cycle. All
setpoints that can be accessed by the operator are
displayed in menus 11-28 of the above-reference
manual.
10Controller 15 includes a program which
converts each setpoint from engineering units into
machine units which represent the values of the output
signals generated by controller 15. Those signals
include the operating signals delivered to electro-
15hydraulic controls 20 and 21 by way of signal paths 23
and 24, the pressure and flow compensation signals
delivered to pump control 43 by way of signal path 46
and the driving signal 40 delivered to the drive of
motor 34. The signal values are calculated by con-
troller 15 from the flowrates necessary for machine
operation during a phase. For example, the flowrate
required to effect an injection velocity of 15 inches
per second is computed by the formula:
in 1 qal 60 s
F = 15 s x 231 in3 x A in2 x min
where 15 is the setpoint, F is the flowrate in gallons
per minute (GPM), A is the area of the actuator to be
moved and the remaining terms are conversion factors.
-23- ~033~6
The required signal value (e.g., voltage) is then
calculated as a linear proportion of the ratio of
calculated flowrate to maximum flowrate. To continue
the example, the voltage to the valve should be:
D
V = cal max
where DCal is the calculated flowrate F, DmaX is the
maximum displacement of pump 21 and Vmax is the
voltage which drives the appropriate control valve
fully open. As is well known in the field the cal-
culated voltages are preferably adjusted by offset
values to account for leakages and other small losses
in the machine. Once each signal value is calculated,
it is stored in the program sequence memory of con-
troller 15. It is important to note that the value of
- a given signal is calculated and the value calculated
- for each phase is stored in the sequence memory in a
location selected to cause the appropriate signal
value to be output from controller 15 during the
corresponding phase.
once all setpoints have been converted into
machine units, the sequence program memory of control-
ler 15 contains all signal values needed to sequence
the machine through each phase of a complete injection
molding cycle. To sequence the machine through an
injection molding cycle, controller 15 outputs the
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various signal values from sequence program memory in
appropriate sequence and time relation.
Driving signal 40 controls the speed of
motor 34 and therefore the output of pump 26 to ensure
the pump delivers only the flow needed for setpoint
performance to meet the hydraulic demand imposed
during each phase of the machine cycle in accordance
with the molding parameters entered by an operator.
To compute the driving signal, the calculated required
flowrates for each phase are related to the maximum
voltage to operate motor 23 at full speed. For
example, after calculating the flowrate as discussed
above, the driving signal required to drive motor 34
is calculated according to the formula:
D
D
max
When DCal is the setpoint converted flowrate, DmaX is
the maximum pump displacement and Vmax is now the
voltage necessary to drive pump 21 at its maximum
displacement. If the relationship between the drive
voltage and pump output is not linear, a calibration
table containing precise linear adjustment factors
would be entered into the memory of controller 25 to
correct the calculated driving signal values. The
driving signals for each phase are then stored in
sequence program memory with the valve machine
-25- ~33~6
parameters for execution by controller 15 as explained
above.
Once controller 15 outputs operating signal
values from the program sequence memory to controls 20
and 21, controller 15 monitors the response of
clamping unit 12 or injection unit 11 to insure the
setpoint has been achieved. To do so, controller 15
first reads the value of a feedback signal 18 from a
transducer associated with the control action. For
example, pressure transducer 72 is coupled to hydrau-
lic line 70 leading to screw actuators 63 and 64. To
verify an injection pressure, controller 15 reads the
signal from pressure transducer 72 to confirm the
actual pressure in the line 70 corresponds to the
desired pressure. If the signal from pressure trans-
ducer 72 indicates too much or too little pressure,
then controller 15 varies the injection operating
signal to injection control 20 to reduce or increase
the flow through the valve within the circuit.
Pressure transducer 128 is provided to monitor
clamping pressure in an analogous manner. Likewise,
distance transducers 75 and 100 permit controller 15
to effect closed-loop control over injection velocity
and clamp position, respectively
A flowchart depicting the generation of the
signal values for machine 10 by controller 15 is shown
in Fig. 4. The first block shows the entry of the
-26- 20~3~6
molding parameters for the various phases to the
memory of controller 15 through operator interface 16.
This procedure is performed according to the manual
previously incorporated into this application.
S For each molding parameter, a program within
controller 15 calcuIates the pump flow rate, second
block of Fig. 4, required to achieve the molding
parameter. This calculation is performed in accor-
dance with the formula for DCal previously discussed
above.
Using the calculated flow rate, the program
performs the third block of Fig. 4 which is the
calculation of the operating signal values. These
values correspond to operating signals 23 and 24
- 15 delivered to injection and clamp controls 20 and 21 to
control the values within these controls.
In a similar fashion, the program uses the
calculated flow rate to perform the fourth block of
Fig. 4, Calculate Driving Signal Values. The program
determines the magnitude of driving signal 40 neces-
sary to drive motor 34 at the correct speed for
powering pump 26 to achieve the calculated flow rate.
This calculation can be adjusted by use of a cali-
bration table for piecewise linear correction as
previously discussed if needed.
Block five of Fig. 4 requires the use of the
calculated flowrate and an appropriate pressure limit
-27- Z033~6
factor. This factor is part of the program and
preferably is a few hundred psi (pounds per square
inch) above the pressure generated at the calculated
flowrate. The summation of the calculated flowrate
pressure and pressure limit factor yields the value of
the pressure compensation signal output to valve 170
for pressure compensation. A signal value for
shifting mode selector valve 165 to the pressure
compensation mode is also generated.
Likewise, the portion of the program which
implements block five of Fig. 4 also calculates the
flow compensation value using the calculated flowrate
and a flow limit factor of a few hundred psi. A
signal value for setting mode selector valve 165 in
the flow compensation mode is also generated.
In block 6 of Fig. 4, the program stores all
the generated values to the program sequence memory.
Controller 15 outputs the appropriate signals corre-
sponding to these values to cycle machine 10 through
an injection molding cycle.
In operation, a typical machine operating
cycle is made of a number of distinct phases, each of
which normally requires different hydraulic pressures
and/or flow. These phases include: clamp close,
injection, extruder run, clamp open and ejector
phases.
-28- X0~3~6
In operation, machine 10 sequentially
executes a number of successive phases under the
direction of controller 15. The first phase typically
executed is the clamp close phase. Machine controller
15 retrieves the driving signal value for motor speed
from its program sequence memory and output it as
driving signal 40 to motor 34. Motor 34 varies its
speed to conform to driving signal 40 and rotate its
output shaft 33 at the commanded speed. This rota-
tional output drives pump 26 to provide the flowratecalculated for the close clamp phase. The flow from
pump 26 travels through manifolds 28 and 27 to injec-
tion and clamp controls 20 and 21. Controller 15 also
retrieves the pressure/flow compensation signal values
and outputs flow/pressure compensation signals to pump
control 43. These signals put pump control 43 in a
pressure compensation mode and set the appropriate
pressure into valve 170.
In conjunction with the output of driving
signal 40, machine controller 15 also retrieves
operating signal values for the proportional control
of valves within injection and clamp controls 20 and
21. The operating signals output to injection control
20 along multi-signal path 23 close all valves in
injection control 20 since no movement of injection
mechanical components during this phase is required.
-` 203364 6
-29-
Operating signals for clamp control 21 from
machine controller 15 reach clamp control 21 over path
24 and set the proportional control of the clamp close
valve. When open, this valve permits hydraulic flow
from manifold 28 to enter line 97. This flow through
line 97 pushes hydraulic actuators 94 and 95 toward
their fixedly mounted ends. This movement forces the
hydraulic fluid behind the actuators through line 98
to reservoir 30. As they traverse, actuators 94 and
95 pull rear clamp face 85 toward front clamp face 84.
Rear mold section 88 coupled to rear clamp face 85
mates with front clamp section 87 attached to front
clamp face 84 to close mold 89.
As rear clamp face 85 approaches front clamp
face 84, controller 15 monitors the distance traversed
by clamp face 85 by a signal from distance transducer
100 by way of signal path 101. As rear clamp face 85
nears front clamp face 84, controller 15 outputs
operating signals to vary the proportional control of
the clamp close valve within clamp control 21 to
gradually close the valve. This slows the movement of
rear clamp face 85 towards front clamp face 84 and
protects mold ~9 from impact damage.
To build and hold a pressure for holding
clamp face 84 and 85 together, machine controller 15
retrieves another driving signal value and outputs a
new driving signal to motor 34. This signal drives
` -- 2033646
-30-
pump 26 at the necessary flowrate for building the
clamp holding pressure. Likewise, controller 15
outputs new flow/pressure compensation signals to
adjust the pressure compensation mode of pump control
43 to the new flowrate.
Machine controller 15 now outputs an operat-
ing signal to clamp control 21 through path 24 to open
a clamp holding valve. As the clamp holding valve
opens, the flow of hydraulic fluid from pump 26
through manifold 28 flows through line 119 to clamp
drain valve 117 which evacuates the valve through line
120 to reservoir 30. Valve stem 122 is pulled into
valve seat 123 by valve 117.
Machine controller 15 now retrieves another
operating signal value and outputs an operating signal
to clamp control 21 through path 24 to open the clamp
pressure valve within control 21. The clamp pressure
valve permits hydraulic fluid to flow from pump 26 to
volume 112 through inlet port 126 of cylinder 113.
This fluid flow generates pressure within volume 112
pushing piston 110 and clamp cylinder 113 apart. This
pressure also secures valve stem 122 in valve seat
123. Machine controller 15 reads the pre~ssure gen-
erated within volume 112 from pressure transducer 128
through signal line 129. When the pressure indicated
from pressure transducer 128 has reached a prede-
termined level previously entered by the operator,
- 2033646
-31-
machine controller 15 closes the clamp pressure valve
to hold the fluid in volume 112. To do this, control-
ler 15 outputs an operating signal through path 24 to
the clamp pressure valve within clamp control 21. The
pressure exerted by the fluid within volume 112
maintains the pressure to hold the clamp and mold
together for the injection and cooling phase of the
machine cycle without requiring additional flow from
pump 26.
The next phase executed is the injection
phase. Machine controller 15 retrieves a new driving
signal value from its program sequence memory and
outputs a driving signal 40 to motor 34 to alter the
motor speed and vary the pump output to match expected
flow demand. Machine controller 15 also reads the
flow/pressure compensation signal values and outputs
the corresponding flow/pressure compensation signals
to pump control 43. Since the valves in clamp control
21 were closed by operating signals from machine
controller 15 at the close of the clamp close cycle,
flow is not directed to clamp control 21 but rather is
available for moving the injection mechanical compon-
ents.
Machine controller 15 retrieves an operating
signal value to proportionally control the valves in
injection control 20 and passes the operating signals
by path 23 to injection control 20. The first
~ 2033~
-32-
proportional valve control exerted by machine control-
ler 15 in the injection phase opens an injection
forward valve which permits hydraulic flo~ from
manifold 27 to push against hydraulic actuator 78
through line 81. Fluid evacuating actuator 78 leaves
through line 80 to reservoir 30. Actuator 78 pushes
injection unit 11 towards front clamp face 84. The
nozzle end 61 of barrel 60 mates within an opening in
front clamp face 84. As injection unit 11 approaches
lo front clamp face 84, machine controller 15 outputs
operating signals to injection control 20 to slowly
close the valve permitting hydraulic flow to actuator
78. This is done to prevent injection unit 11 from
crashing into front clamp face 84.
Machine controller 15 now outputs a new
driving signal 40 corresponding to a driving signal
value in program sequence memory to motor 34. This
signal changes the motor speed so pump 26 now provides
the correct flow for injection of material into mold
89 at a desired rate and under a desired pressure.
Controller 15 also outputs flow/pressure compensating
signals to pump control 43 in accordance with the
flow/pressure compensation values stored in program
sequence memory.
Machine controller 15 now outputs an operat-
ing signal ~hrough path 23 to injection control 20 to
open the screw forward valve. Fluid flows to
2033646
-33-
actuators 63 and 64 to push injection screw 52 within
barrel 60 forward toward front clamp face 84. As
injection screw 52 moves forward, it expels the
plasticized material previously melted within barrel
60 through nozzle 61 into volume 91 of mold 89.
The velocity of the plasticized material
entering mold 89 is controlled by machine controller
15 outputting appropriate drive signals to motor 34.
These drive signals vary the speed of motor 34 which
correspondingly change the flow rate of pump 26. As
these flow rates vary so does the force exerted
against actuators 64 and 63 which are coupled to screw
52. Machine controller 15 varies these drive signals
to alter the injection-velocity based upon the pres-
sure reading received from pressure transducer 72
through line 73. The,signals from pressure transducer
72 are used to confirm the proper injection velocities
previously entered by the operator are being achieved.
Machine controller 15 outputs an operating signal to
injection control 20 to close the screw forward valve
when a signal (from. distance transducer 75) is
received via path 76 indicating screw 52 is at the end
of its traverse.
If a sprue break action has been program~med,
machine controller 15 varies the pump flow by output-
ting a new drive signal 40 to motor 34. Controller 15
then sends operating signal to injection control 20
~ 20336~6
-34-
through path 23 to open a valve which permits hydrau-
lic f~ow to reverse the movement of actuator 78. This
flow will retract injection unit 11 from front clamp
face 84. This action is used to prevent heat transfer
S from barrel 60 into mold 89. If heat transfer does
not prevent the material within volume 91 from cooling
then this sprue break action is usually not required.
Machine controller 15 now outputs an operat-
ing signal to injection control 20 to gradually open a
valve connecting line 69 and line 70. This allows
actuators 63 and 64 to respond to mechanical pressure
exerted against screw 52 as will be explained below.
The next phase controller 15 executes is the
extruder run and cooling phase. Controller 15 now
varies the pump output by a new driving signal 40 to
motor 34. Controller 15 also outputs flow/pressure
compensation signals to shift mode selector valve 165
of pump control 43 leftwardly to select the flow
compensation mode. Another signal is delivered to
flow compensation valve 150 and indicates a prede-
termined flow limit.
Machine controller 15 then outputs an
operating signal to a throttle valve which permits
hydraulic flow through line 58 to hydraulic or
extruder motor 53 and then through line 57 to reser-
voir 30. This motor turns injection screw 52 in
response to this flow. Pellets from hopper (not
` ~ 2033646
-35-
shown) enter barrel 60 at this time. As they do, they
encounter the turning screw 52 and the mechanical
shear produced by screw 52 melts the pellets. The
melted material will be pushed forward by the vanes in
screw 52 towards nozzle 61 of barrel 60. As this
plasticized material accumulates in the front of
- barrel 60, it will exert a back pressure against screw
52. Since screw 52 is no longer held in position by
flow through actuators 63 and 64, the screw will
retract and move actuators 63 and 64.
As machine controller 15 senses the shift in
pressure from actuators 63 and 64 through pressure
transducer 72 along path 73, it exerts operating
signals to the screw control valve through injection
control 20, to close and hold the fluid in actuators
63 and 64. As the actuators resist this pressure
exerted by the plasticized material a solid shot of
plasticized material is built in barrel 60. Once the
pressure reaches a predetermined level that was
previously entered by the operator, machine controller
15 exerts a new operating signal to the screw control
valve which reverses the flow through lines 69 and 70
to retract screw 52. Plasticized material now fills
barrel 81 as screw 52 rotates. When screw 52 has
traversed barrel 60, as indicated to controller 15 by
distance transducer 75, controller 15 exerts a signal
through path 23 to close the screw control valve
~ ~ 2033646
--36--
within injection control 20. Another operating signal
is also output to injection control 20 to close the
extruder throttle valve connected to hydraulic motor
53 to stop the rotation of screw 52.
During the time that screw 52 has been
retracted and rotated to create a shot of plasticized
material, the material within volume 91 is cooled. As
this material cools within the cavity 91 of mold 89,
the injection molded part is formed.
Machine controller 15 now initiates the
clamp open phase. At the beginning of this phase,
machine controller 15 once again adjusts the speed of
motor 34 so the proper flowrate from pump 26 is
available for the clamp open phase. Flow/pressure
compensation signals are also output to pump control
- 43 to shift flow/pressure compensation value back to
pressure compensation mode. Controller also sends
pump control 43 another signal to set a new pressure
into valve 170 (Fig. 2).
Machine controller 15 outputs an operating
signal to clamp control 21 to open the clamp hold
valve. Hydraulic fluid within volume 112 now leaks
from volume 112 to prefill reservoir 116 behind
cylinder 113. After waiting a predetermined time,
clamp control 21 receives an operating signal from
machine controller 15 which opens a valve so hydraulic
fluid from line 120 flows to drain valve 117 reversing
~ 2033646
-37-
the movement of valve stem 122. When valve stem 122
is pushed into volume 112, a more direct and larger
diameter path is provided for the fluid within volume
112 to return to prefill reservoir 116. Once valve
stem 122 has been fully extended into volume 112,
machine controller 15 exerts an operating signal
through path 24 to clamp control 21 to stop the flow
from hydraulic line 120 to valve 117.
Machine controller 15 now outputs a new
driving signal 40 and new flow/pressure compensation
signals to setup pump control 43 and pump 26 for
opening the clamp. With the clamp holding pressure
now dissipated, machine controller 15 exerts an
operating signal through path 24 to clamp control 21
to provide flow through hydraulic line 98 to actuators
94 and 95. ~ydraulic line 97 drains the actuators to
reservoir 30. The flow against actuators 94 and 95
pushes rear clamp face 85 and the attached rear mold
section 88 away from front clamp face 84 and front
attached mold section 87. Again, machine controller
15 monitors the distance moved by rear clamp face 85
through a signal received from distance transducer
100. As rear clamp face 85 approaches the end of the
distance actuators 94 and 95 can traverse, machine
controller 15 exerts an operating signal to clamp
control 21.to slow the flow to actuators 94 and 95.
This decrease will continue until flow is stopped and
~ ~ 2033646
-38-
rear mold piece 88 is separated from front mold piece
87.
Machine controller 15 now executes the
ejection phase of the cycle. After adjusting pump
output and pump control, machine controller 15 exerts
an operating signal through path 24 to clamp control
21 to drive ejector 106 through line 105. The ejector
pushes against the part formed in mold 89 during the
cooling phase and ejects it. The part falls into an
area underneath the clamping unit 12. A gate (not
shown) can then be opened and the formed part
retrieved. Machine controller 15 then exerts an
operating signal to clamp control 21 to reverse the
flow to ejector 106 and retract ejector 106 to com-
plete the machine cycle.
From the foregoing, it can be appreciatedthat pump control 43 in conjunction with variable
displacement pump 26 provide means for providing
prompt and precise control over the output of pump 26
in order to effect pressure and/or flow compensation
on a selectable basis. Moreover, the invention
provides means for matching of the output of pump 26
with the anticipated hydraulic demand associated with
each phase of the operating cycle by adjusting the
speed of motor 34 in accordance with a driving signal
calculated in accordance with desired molding
parameters. As a result, pump 26 and motor 34 can be
`` `-- 2033646
-39-
operated the majority of the time at or near m~imum
efficiency. The motor/pump combination selected for
use in the preferred embodiment is most efficient when
the pump operates at or near maximum stroke. However,
those skilled in the art will recognize that other
motor/pump combinations may be most efficient when
operating in other operating regions. Accordingly, to
maximum energy savings, the most efficient operating
configuration for a given motor/pump combination
should be selected as that in which the pump operates
at all times except to correct transient pertur-
bations.
In an alternative embodiment of the inven-
tion, pump 26 comprises a fixed displacement pump
while pump control 43 is eliminated and motor 34 is
selected to comprise a brushless DC motor. The speed
of the motor is controlled during each phase of the
machine cycle in accordance with a driving signal
whose value is calculated to substantially match the
flow delivered by the pump with the hydraulic demand
expected during each respective phase. While the
energy savings to be realized with this alternative
are not as great as those possible with the preferred
embodiment, improved energy efficiency as well as more
precise and broader range adjustment over motor speed
and therefore, pump output are obtained as compared to
` -~ 2033646
-40-
the variable speed AC motor fixed volume pump com-
binations known in the prior art.
While the invention has been described in
connection with a hydraulic injection molding machine,
those skilled in the art will appreciate that the
invention is applicable to other hydraulically powered
plastics processing machinés such as hydraulic injec-
tion reaction molding machines.
What is claimed is:
