Note: Descriptions are shown in the official language in which they were submitted.
2030761
"CA~TIN~ APPARATUS"
This invention relates to casting apparatus and
in particular but not exclusively to the casting o~
low melting point alloys for use in mouldiny plastics
materials.
It is known from EP-A-0177257 to provide
casting apparatus in which molten metal is dispensed
into a die from a dispensing cylinder by action o~ a
piston slidable in the cylinder and driven by an
actuator comprising a hydraulic or pneumatic
cylinder~ The piston continues to pressurise the
molten metal after filling of the die during a period
in which the metal in the die cools and solidifies.
A microprocessor is in this arrangement
provided for controlling the sequential operation of
the actuator and its associated flow valve.
It is also known from GB-A-420728 to provide a
rotary driven actuator for urging a piston in a
dispensing cylinder and a control circuit is provided
for switching on and o~f a rotary motor which drives
the actuator.
It is an object of the present invention to
provide casting apparatus having an actuator which
controls the piston more precisely and more
conveniently than has previously been possible.
According to the present invention there is
disclosed casting apparatus comprising a dispensing
cylinder receiving molten metal in use, a piston
slidably received in the cylinder, a rotary driven -
linear actuator operable to drive the piston in the
cylinder, rotary drive means operable to drive the
actuator and communicating means communicating during
a dispensing strok2 of the piston between the
cylinder and a die ~or dispensing molten metal to the
die, wherein the apparatus further comprises control
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means operable to control the rotational speed of the
drive means to thereby control the speed of the
piston and thereby control the rate at which molten
metal is dispensed into the die.
An advantage of such apparatus is that the
piston speed can be directly controlled throughout
the dispensing stroke whereas in each of the prior
art apparatus referred to above there is no provision
for monitoring the actuator speed and correcting any
deviation in speed from a preferred value.
The apparatus of the present invention thereby
provides greater control and repeatability which is
of particular importance in maintaining quality of
casting.
Preferably the control means is provided with
data storage means receiving in use data defining a
desired characteristic of piston spsed as a function
of time during the dispensing stroke and is operable
to control the drive means such that the
instantaneous speed of the piston is substantially
equal to that of the desired characteristic during
filling of the die.
An advantage of such an arrangement is that the
piston speed can be varied during the dispensing
stroke in a repeatable manner which can be tailored
to suit the requirements of a particular die having a
specific internal shape. By adjusting the i
characteristic of piston speed it is possible to ~-
avoid imperfections in the resulting casting which
are associated with the elevation speed at which the ~-
level of molten metal rises within the die. By
adjusting the characteristic of piston speed it is
possible to maintain the elevation rate within
preferred maximum and minimum values and to avoid
turbulent ef~ects associated with rapid change in
elevation rate.
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Preferably the drive means includes a rotary
motor and slippable torque transmitting means
transmitting rotary motion from the motor to the
actuator whereby torque continues to be transmitted
to the actuator after the piston is arrested by back
pressure in the cylinder after the die is full.
Conveniently the torque transmitting means comprises
a clutch having means for adjusting the torque
transmitted to thereby adjust the pressure of molten
metal in the die when filled.
The clutch may be electromagnetically operated
and adjustable by mechanical or electromechanical
adjustment means.
Preferably the apparatus includes an actuator
sensor responsive to the speed of rotation at which
the linear actuator is driven, a motor sensor
responsive to the speed of rotation of the motor and -
control means connected to the respective sensors and
operable to control the speed of the motor.
The distance travelled by the actuator may
therefore be controlled using a closed feedback loop ;~
in an accurate manner.
Advantageously the torque transmitting means is
adjustable by means actuated by the control means. -
Where the torque limiting means is a clutch it may
therefore be electromagnetically actuated by signals -
from the control means. ,
Preferably the rotary driven linear actuator
comprises first and second cooperating screw threaded
members, the drive means being operable to provide
relative rotational movement between the first and
second members resulting in relative linear motion by
screw action and connecting means which connects the
first and second members to the cylinder and piston
respectively so as to provide actuating movement
therebetween.
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Advantageously the first member i5 a screw
threaded shaft and the second member is a cooperatiny
nut which nut includes ball bearing means so that the
nut and shaft together comprise a ball scre~J.
An advantage of using such a ball screw is that
the screw action is accompanied by minimal ~rictional
losses and minimal lost motion so that the piston
position is accurately and smoothly controllable even
at very slow actuation speeds.
According to a further aspect of the present
invention there is disclosed a method of castiny
comprising the steps of placing a dispensing cylinder
receiving molten metal in communication with a die
and driving a piston in the cylinder by means of a
rotary driven linear actuator such that molten metal
is dispensed into the die during a dispensing stroke
of the piston, and operating a control means to
control the rotational speed of a rotary drive means
driving the actuator to thereby control the rate at
which molten metal is dispensed into the die.
Preferably the method includes the steps of
operating the control means to control the speed of ~; ;
the drive means such that during filling of the die
the piston speed is substantially equal to a desired
characteristic of piston speed as a function of time.
Advantageously the characteristic of piston
speed against time is selected such that the level of
molten metal in the die rises at an elevation rate
which increases from zero to a minimum preferred
value above which the elevation rate is thereafter
maintained during the filling of the ~ie.
The method may include the step of continuing
to apply torque to the rotary driven linear actuator `~
after the die has become filled with molten metal
until the metal in the die has substantially
solidified.
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An advantage of this method is that any chanye
of volume of the metal within the die ~an be
compensated by an influx or efflux o~ molten alloy
under pressure applied by the piston.
Preferably the method includes the step of
varying the rotational speed at wh.ich the rotary
driven linear actuator is driven during a time period
in which the die is progressively filled with molten
metal such that the level of molten metal within the
die rises at a predetermined rate.
An advantage of this method is that the
internal shape of the die can be taken account of i.n
controlling the rate of influx of molten alloy in a
manner which ensures that the die is completely
filled without formation of surface defects. ,.
Particular embodiments of the present invention
will now be described by way of example only and with
reference to the accompanying drawings of which:-
Figure 1 is a partly sectioned elevation of
casting apparatus;
Figure 2 is a partly sectioned elevation of the .,
actuator of the casting apparatus of Figure l;
Figure 3 is a sectional elevation of a die for
use with the casting apparatus of Figures 1 and 2; and
Figure 4 is a graph of piston speed against
time illustrating the characteristic of dispensed
flow rate suitable for filling the die of Figure 3.
In Figure 1 a casting apparatus 1 is suitable
for casting metal alloys having low melting
temperatures for example having solidus temperatures
in the range of 35C to 300C. The apparatus 1
has a tank 2 in which is held a quantity of molten
alloy 3 which fills the tank to a normal level 4. A
cylinder 5 is located within the tank 2 and immersed
in the molten alloy 3. The cylinder 5 extends
vertically upwardly from a mounting block 6 upon
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which it is supported and receives a piston 7 ~7hich
is axially slidable within the cylinder. The
mounting block 6 rests upon supports 8 and 9.
The cylinder 5 defines a cylindrical dispensing
chamber 10 and the mounting block 6 defines a conduit
11 communicating between the chamber and a three port
ball valve 12. The ball valve 12 has a ball 13
mounted on a stem 14 and located between valve seats
15 and 16 within a valve chamber 17 defined by the
ball valve.
The valve 12 is provided with a first port 18
which communicates with the interior of the tank 2 so
as to supply molten alloy to the valve, a second port
19 communica~ing with the conduit 11 so as to be in
communication with the dispensing chamber 10 and a
third port 20 communicating with a dispensing channel
21. The stem 14 is axially movable by action of a
ball valve actuator 22 such that the ball can be
positioned to close either the first port 1~ or the
third port 20.
The dispensing channel 21 extends out of the
tank 2 and communicate's with a flow valve 23 which is
actuated by a rotary flow valve actuator 24 to open
or close the flow path through the flow valve. A
further portion 25 of the dispensing channel 21
communicates between the flow valve 23 and a die 26.
The dispensing channel 21 is formed in a fixed -~
member 27 which is separable from the mounting block
6 such that the block can be removed from the tank.
The fixed member 27 includes a domed surface portion
28 locating in a corresponding recess ~9 formed in
the mounting block 6 for ease of alignment of the
dispensing channel 21 with the third port 20 on
refitting the mounting block 6 within the tank 2.
The tank 2 and the dispensing channel 21 are
surrounded in insulating material 30 to maintain an
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even temperature throughout.
The piston 7 is driven by an actuating means 3
which is represented schematically in Figure 1 and is
shown in detail in Figure 2.
The actuating means 31 includes a screw
threaded shaft 32 on which is received a screw
threaded nut 33 including ball bearing means (not
shown) such that the shaft and nut together comprise
a ball screw 34. The screw threaded shaft 32
includes screw threads 35 of part circular
cross-section adapted to cooperate with the ball
bearing means of the nut 33. The construction o~
the ball screw 34 is such that relative rotational
movement between the nut 33 and the shaft 32 results
in linear relative movement as in the case of known
screw and nut combinations.
The nut 33 is rigidly connected to the piston 7
by means of a vertically extending tube 36 having a
threaded coupling 37 at its lower end 38 allowing the
piston to be bolted to the tube 36.
The screw threaded shaft 32 extends vertically
and coaxially within the tube 36 and has an upper end
39 received in a bearing 40 which is connected to a
tubular casing 41.
The casing 41 extends vertically with the tube
36 being coaxially received within the casing and the ~ -
bearing 40 facilitates rotation of the shaft 32
relative to the casing but prevents longitudinal
relative motion between the casing and the shaft 32.
A motor 42 has a driven shaft 43 arranged to
drive an input pulley 44 of an adjustable clutch 45
via a toothed drive belt 46 and the clutch has an
output shaft 47 rigidly connected in axial alignment
with the screw threaded shaft 32. The clutch 45
comprises a solenoid actuated disc clutch having
means for adjusting the pressure applied between the
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discs (not shown) to thereby provide means for
settiny the maximum torque transmitted by the clutch.
A take-off pulley 48 is connected to the oukput
shaft 47 of the clutch 45 and is connected by means
of a toothed belt 49 to an actuator tachometer 50 ~or
sensing actuator speed. A motor tachometer 51 is
connected to the motor 42 for sensing the motor speed.
A digital elect,ronic control unit 52 is
connected to receive output signals from the actuator
tachometer 50 and the motor tachometer 51 indicating
the rotational speeds o~ the screw threaded shaft 32
and the motor 42 respectively. As indicated
schematically in Figure 1 the control unit 52 is
connected to provide command signals to the motor 42
to regulate its speed, to the actuating solenoid of
the clutch 45, to the ball valve actuator 22 to
actuate the valve 12 and to the flow valve actuator
24 to control the actuation of the flow valve 23.
Upper and lower inductive proximity switches 53
and 54 respectively are connected to the casing 41 at
vertically spaced apart locations and are arranged to
sense the maximum and lower limits of normal travel
of the nut 33 along the screw threaded shaft 32. -
Output signals from the proximity switches 53 and 54 ~ -
are connected at inputs to the control unit 52.
The control unit 52 is provided with a memory
56 for recei~ing data defining the preferred
characteristic of speed of piston 7 as a function of
time during a dispensing stroke of the piston. The
selection of the preferred characteristic is
described below with reference to Figures 3 and 4.
In use to dispense moltsn alloy 3 into the die
26 the piston is raised as shown in Figure 1 to its
normal upper limit of travel with the chamber 10
~illed with molten alloy. The ball valve 12 is
moved into a position in which the ball 13 is fully
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`" 2030761.
g
raised by actuation of the ball valve actuator 22
thereby closing the first port 18. The ball valve
12 in this position provides communication between
the conduit 11 and the dispensing channel 21.
An operator programmes the control unit 52 with
the necessary data to achieve the optimum flow rate
of molten alloy into the die 26, the flow rate being
dependent on the internal shape of the die. This
data is stored in the memory 56. The clutch 45 is
set to provide a predetermined limit of torque such
that torque across the clutch below a threshold limit
will result in one-to-one transmission of drive
through the clutch and torque exceeding the limit
will result in slippage of the output shaft ~7
relative to the speed at which the clutch is driven
by the motor 42 whilst a set value of torque
continues to be transmitted.
The motor 42 is then driven at an initial speed
under the control of the control unit 52 such that
the clutch 45 is driven by the drive belt 46, the
output shaft 47 of the clutch being correspondingly
driven to thereby drive the screw threaded shaft 32
at an initial rate of rotation. Rotation of the
shaft 32 results in linear movement of the nut 33
relative to the shaft, the direction of rotation of
the motor being selected to be such that the nut
moves downwards relative to the shaft. Since the
nut 33 is rigidly connected to the piston 7 the
piston moves correspondingly downwards and displaces
molten alloy from the chamber 10. Alloy flows from
the chamber 10 through the conduit 11, through the
valve 12 into the dispensing channel 21, through the
flow valve 23, through the further portion 25 of the
dispensing channel 21 and into the die 26.
Throughout this flow path the insulation 30 retains
the molten alloy at a temperature above its solidus
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temperature.
As the molten alloy is progressively dispensed
from the chamber 10 the rate at which alloy is
dispensed and the precise volume which has been
displaced is monitored by the control unit 52 by
means of signals received from the actuator
tachometer 50 which provides a signal directly
proportional to the rotational speed of the shaft 32,
the travel of the piston being calculated by
integrating the signal.
The control unit 52 adjusts the speed o~ the
motor 42 during the dispensing operation in order to
achieve a predetermined characteristic of piston
speed against time. Since the actual displacement
of the piston 7 is measured by the control unit and
the speed of the piston directly regulated by the
control unit this constitutes a closed loop feedback
system providing precise control of the dispensing
operation.
When the die 26 is completely filled with
molten alloy the incompressibility of the liquid
alloy 3 arrests the motion of the piston 7 such that
the torque experienced across the clutch 45 exceeds
the threshold limit to which it is set and the clutch
~5 therefore begins to slip. The halting of the piston
7 is sensed by the control unit 52 which then sets
the motor 42 to run at a speed which maintains a set
level of torque across the clutch 45 so as to bias
the piston 7 downwardly and maintain a constant level
of pressure in the dispensing flow path. The metal
alloy within the die 26 is allowed to cool and
solidify and in doing so undergoes a change of
volume, either an increase or decrease, depending on
the composition of the alloy. ~ decrease in volume
will be accompanied by a slight influx of molten
alloy to the die and the piston will move down
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accordingly by a small amount.
After solidification the control unit actuates
the flow valve actuator 24 to turn off the flo~l valve
23 and the die 26 is removed from the casting
apparatus 1 and replaced with the sarne die after
removal of the solidified casting or with a further
empty die.
The control unit 52 then actuates the ball
valve actuator to lower the ball 13 so as to close
the third port 20 and open the first port 18. The
direction of rotation of the motor 42 i8 reversed and
the piston 7 is then raised to create suction within
the chamber 10 and draw molten alloy 3 from the tank
2 which constitutes a reservoir. The dispensing
operation can then be repeated as described above
after first opening the valve 23.
During extended periods between casting
operations the piston 7 may be raised and lowered
with the ball valve in its fully lowered position
such that molten alloy from the tank 2 is cyclically
drawn into and expelled from the chamber 10 in order
to ensure that adequate agitation of the alloy is
maintained to prevent dissociation of its constituent
metals. During this recycling the upper and lower
proximity switches 53 and 54 provide inputs to the
control unit 52 as to when the piston has reached its
operating limits of travel.
Figures 3 and 4 illustrate the manner in which
the movement of piston 7 is controlled in order to
tailor the characteristic of piston speed against
time to suit a particular application. In the
example of Figure 3 an alternative die 60 defines a
cavity 61 having a shape resembling that of an
inverted bottle. The shape of cavity 61 is
symmetrical about a vertical axis and includes a
cylindrical upper portion 62 of uniform
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cross-section. A lower portion 63 of ca~ity 61 has
a reduce~ cross-section and is connected to the
cylindrical upper portion 62 by a flared portion 6~
which merges smoothly and continuously with the upper
and lower portions 61 and 62 respectively~
The die 60 is filled through an inlet 65
communicating with the lower portion 63 and is
connected in use with the nozzle 55 of the apparatus
of Figures 1 and 2.
Figure 4 illustrates graphically the preferred
characteristic of speed S of piston 7 as a function
of time t during a dispensing operation in which
molten alloy is dispensed from the cylinder 5 into
the cavity 61. During an initial time period A the
piston begins to move smoothly and with gradually
increasing speed from its rest position and the flow
rate of molten alloy into the cavity 61 increases in
direct proportion to this speed S. This gradual
pick-up in flow rate ensures that the rising level of
molten alloy remains substantially free from
turbulence without the formation of jets. At the
end of period A the elevation rate at which the level
of molten alloy within the cavity 61 is rising will
have reached a preferred minimum value above which
the elevation rate is to be maintained during the
filling of the cavity. If the level rises at a
slower rate then surface imperfections in the casting
result from the tendency to freezing of the meniscus
formed at the interface between the alloy surface and
the walls of the die 60. The flow of metal over the
frozen meniscus results in unwanted lines being
formed on the resulting casting.
As the level of alloy rises within the flared
portion 64 it is necessary for the piston speed S to
be accelerated during a further time period B in
order that the elevation rate should remain constant
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in spite of the increasing cross-sectional area
encountered by the alloy. When the level reaches
the cylindrical upper portion 62 however the uniform
cross-section of this portion requires a constant
piston speed S during a further time period C.
When the cavity 61 becomes entirely filled with
molten alloy the piston speed S drops rapidly during
a further time period D. Contraction of the alloy
during coolin~ occurs during a further time period E
requiring further motion of the piston as further
alloy is injected into the die under continued piston
pressure. Finally when no further alloy is able to
enter the die some motion of the piston during a
final time period F will result from imperfect
sealing between the piston and cylinder.
Throughout periods B and C of the above
dispensing operation the speed S of the piston 7 is
controlled such that the alloy level rises at an
elevation rate which is greater than or equal to the
preferred minimum elevation rate and which is not
greater than a preferred maximum elevation rate
depending upon the particular shape of the cavity
61. If the elevation rate is too high then
excessive pressures within the die result at the end
of period C when the die becomes entirely filled and
this can result in unwanted flashes being formed on
joint faces of the die. When filling more complex
die cavities there is an additional hazard in filling
too ~uickly in that unwanted voids can result in the
casting due to entrapment of air.
The preferred characteristic of S as a function
of t for a given die may be arrived at empirically
with the speed characteristic being adjusted to
remove surface imperfections or voids found in
castings resulting from trial dispensing
operationsO Once a satisfactory casting has been
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produced from a given speed characteristic it has
been found that subsequent castings using th~ same
speed charac~zristic show a high degree of
reproducability with consequent high ~ields of high
quality castings.
The characteristic of S as a function of t may
also be arrived at for new die shapes by theoretical
calculation based on empirical values obtained from
dies of simple shape. Such calculations may
conveniently be expressed as algorithms processed in
a computer in which informati.on concerning the shape
of the die cavity is entered ~irectly from computer
aided design data.
The memory 56 may comprise an electrically
erasable programmable read-only memory or
alternatively may comprise other known data storage
means.
The apparatus and method of the present
invention may be used with a cylinder and piston of ;~
different diameter and capacity where appropriate for
dies of different volume so that it will be necessary
to input to the control means the relevant parameters -
of piston and cylinder currently in use.
The torque transmitting means may alternatively
be a magnetic clutch.
In an alternative apparatus and method the
piston speed may be regulated without including the
step of sensing the haltin~ of the piston 7 by the
control unit 520 The motor may instead be set to
run at a reduced speed which maintains a constant
level of pressure in the filled die under the control
of the control unit 52 after a predetermined time
interval equal to the time taken to fill the die.
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