Automated thermoplastic dispensing device

A device for dispensing molten thermoplastic material comprising a barrel member having an internal melting chamber communicating with an outlet opening through a nozzle, and a sleeve having one end secured at the barrel member and a through opening communicating with the end of the melting chamber opposite the outlet opening. The sleeve is adapted to receive a rod of solid thermoplastic material with one end portion of the rod in the melting chamber and the rod projecting through the sleeve along a predetermined path. The barrel member is heated to melt the end portion of the rod therein, and a drive adapted to be switched between a deactivated state and an activated state is provided for driving the rod of solid thermoplastic material into the melting chamber at a predetermined rate to expel molten thermoplastic material through the nozzle. Upon switching of the drive means from the activated state to the deactivated state the drive moves the rod of solid thermoplastic material a small distance out of the melting chamber to cause molten thermoplastic material in the nozzle to flow toward the melting chamber and restrict movement (i.e., dripping or stringing) of that molten thermoplastic material out of the nozzle.

TECHNICAL FIELD 
The present invention relates to devices for dispensing molten 
thermoplastic material, and in one important aspect to means in such 
devices for restricting molten thermoplastic material from leaking through 
a nozzle on such a device when it is not in use. 
BACKGROUND ART 
Devices are known for dispensing molten thermoplastic material that 
comprise a barrel member mounted on a frame and having an internal melting 
chamber communicating with an outlet opening through a nozzle, a sleeve 
having one end secured at the barrel member and a central opening 
communicating with the end of the melting chamber opposite the outlet 
opening which is adapted to receive a rod of solid thermoplastic material 
with one end portion of the rod in the melting chamber and the rod 
projecting through the sleeve along a predetermined path, and means for 
heating the barrel member to melt the end portion of the rod therein so 
that when the rod is pressed into the barrel member molten thermoplastic 
material will be expelled through the nozzle. U.S. Pat. Nos. 4,552,287 and 
4,457,457 describe such devices. The device described in U.S. Pat. No. 
4,457,457, also includes driving means in the form of an external 
compressed air power source adapted to be switched between activated and 
deactivated states for, when in the activated state, driving the rod of 
solid thermoplastic material into the melting chamber to expel molten 
thermoplastic material through the nozzle. While such devices are suitable 
for many purposes, they do not afford the precision needed to dispense 
molten thermoplastic material in many automated systems (e.g., robot 
operated systems) in that it is difficult to actuate the device in such a 
way that will produce a precise predetermined amount or rate of output 
from the device, and there is a tendency for some molten thermoplastic 
material to escape from the nozzle when the devices is not being activated 
to dispense material, which is undesirable or unacceptable for many 
automated applications. 
DISCLOSURE OF INVENTION 
The present invention provides a device for dispensing molten thermoplastic 
material which does afford the precision needed to dispense molten 
thermoplastic material in automated systems, can be actuated in such a way 
that will produce a variety of precise predetermined amounts or rates of 
output from the device, and which restricts molten thermoplastic material 
from escaping or "stringing" from the nozzle when the device is not being 
activated to dispense material. 
According to the present invention there is provided a device for 
dispensing molten thermoplastic material which, like the device described 
above, comprises a barrel member mounted on a frame and having an internal 
melting chamber communicating with an outlet opening through a nozzle, a 
sleeve having one end secured at the barrel member and having a through 
opening communicating with the end of the melting chamber opposite the 
outlet opening, the sleeve being adapted to receive a rod of solid 
thermoplastic material with one end portion of the rod in the melting 
chamber and the rod projecting through the sleeve along a predetermined 
path, means for heating the barrel member to melt the end portion of the 
rod therein, and driving means adapted to be switched between a 
deactivated state and an activated state for driving the rod of solid 
thermoplastic material into the melting chamber so that molten 
thermoplastic material will be dispensed through the nozzle. 
Unlike the device described above, however, in the device according to the 
present invention the driving means can be activated to drive the rod of 
solid thermoplastic material into the melting chamber at a predetermined 
rate to expel molten thermoplastic material through the nozzle at a 
predetermined rate, and includes suck back means operable upon switching 
of the drive means from the activated state to the deactivated state 
adapted for moving the rod of solid thermoplastic material a short 
distance out of the melting chamber to cause molten thermoplastic material 
in the nozzle to flow toward the melting chamber and restrict movement or 
dripping of that molten thermoplastic material out of the nozzle. 
Preferably device is adapted to drive the rod of solid thermoplastic 
material into the melting chamber at any one or a plurality of different 
predetermined rates, and the suck back means is adapted for moving the rod 
of solid thermoplastic material a single predetermined distance out of the 
melting chamber regardless of the rate at which the rod was being driven 
into the melting chamber in the activated state. 
Also, preferably the driving means comprises at least one drive roller 
which is rotatably mounted on the frame adjacent the end of the sleeve 
opposite the chamber with its axis transverse of the path and its 
periphery (which is adapted for engagement with the rod of solid 
thermoplastic material) positioned to afford driving engagement with the 
portion of the rod of solid thermoplastic material projecting along the 
path. A rotor in a reversible direct current motor is coupled by drive 
means to the drive roller, and motor control means are provided that can 
rotate the rotor of the motor in a forward rotational direction at 
different predetermined rates of speed so that through the drive roller 
the motor can expel molten thermoplastic material through the nozzle at 
different predetermined rates; and the suck back means comprises means in 
the motor control means for, when the motor is deactivated, sequentially, 
shorting electro motive forces in the motor to ground, and applying a 
predetermined amount of power to rotate the armature of the motor in a 
reverse rotational direction through a predetermined angle to move the rod 
of solid thermoplastic material said single predetermined distance out of 
the melting chamber. Such shorting of the electro motive forces in the 
motor is important, for if it were not done the application of a 
predetermined amount of power to operate the motor in the reverse 
direction would result in different angles of rotation of the motor 
armature due to the need before the rotation could begin to overcome 
different amounts of electro motive force remaining in the motor resulting 
from different rates of armature rotation prior to deactivating the motor.

DETAILED DESCRIPTION 
Referring now to the drawing, there is shown a device for dispensing molten 
thermoplastic material according to the present invention generally 
designated by the reference numeral 10. 
Generally the device 10 comprises a frame 12 adapted to be mounted by a 
bracket 13 on a movable support such as the arm of a robot, a barrel 
member 14 mounted on the frame 12 and having an internal melting chamber 
communicating with an outlet opening through a nozzle 16, a sleeve 18 
having one end secured at the barrel member 14 and a through opening 
communicating with the end of the melting chamber opposite the outlet 
opening, the sleeve 18 being adapted to receive a cylindrical rod 20 of 
solid thermoplastic material with one end portion of the rod 20 in the 
melting chamber and the rod 20 projecting through the sleeve 18 along a 
predetermined path, and means for heating the barrel member 14 to melt the 
end portion of the rod 20 therein, all being of generally the same 
structure as the corresponding components of the device described in U.S. 
Pat. No. 4,552,287 (the content whereof is incorporated herein by 
reference) modified to incorporate the temperature control described in my 
U.S. Pat. No. 4,816,642, the content whereof is also incorporated herein 
by reference. 
Additionally the device 10 includes novel driving means 22 adapted to be 
switched between a deactivated state and different forward activated 
states for driving the rod 20 of solid thermoplastic material into the 
melting chamber at different predetermined rates to expel molten 
thermoplastic material through the nozzle 16 at different predetermined 
rates, and suck back means operable upon switching of the drive means from 
any one of the forward activated states to the deactivated state adapted 
for moving the rod 20 of solid thermoplastic material a single 
predetermined distance out of the melting chamber to cause molten 
thermoplastic material in the nozzle 16 to flow toward the melting chamber 
and restrict movement of that molten thermoplastic material out of the 
nozzle 16. 
The driving means comprises at least one, and as illustrated, two drive 
rollers 24 each having an axially ribbed concave periphery adapted for 
engagement with by indenting one side of the rod 20 of solid thermoplastic 
material and rotatably mounted about shafts 26 on the frame 12 in spaced 
relationship adjacent the end of the sleeve 18 opposite the chamber with 
its axis transverse of the path and its periphery positioned to afford 
driving engagement with a portion of the rod 20 of solid thermoplastic 
material projecting from the sleeve 18 along the path between the sleeve 
18 and a guide tube 27. Each drive roller 24 is in opposed relationship to 
an idler roller 28 on the opposite side of the path that is similar in 
size and shape but has a smooth outer surface. Each idler roller 28 is 
rotatably mounted on a shaft 30 having ends received in slots in the frame 
12. The idler rollers 28 are biased toward the drive rollers 24 by the 
ends of a spring 32 mounted by having a central coil of the spring 32 
around a pin 33 on the frame 12 to insure good driving engagement between 
the drive rollers 24 and the rod 20. 
Also included in the driving means is a reversible direct current motor 34 
having a rotor 35, drive means in the form of a gear reduction assembly 
including a spur gear 36 on an output shaft driven by the rotor 35 (e.g., 
the 6 volt DC motor commercially designated Escap 22C11-216-5 together 
with the 128 to 1 reduction gear reduction assembly commercially 
designated Escap B24.0-128, both available from Stock Drive Products 
Designatronics, Inc., New Hyde Park, N.Y.) and engaged with spur gears 38 
fixed at ends of the drive rollers 24 for coupling the rotor 35 to the 
drive rollers 24, and an electrical circuit (see FIGS. 5A and 5B) that 
provides motor control means for deactivating the motor 34 and for 
operating the motor 34 to rotate the rotor 35 in a forward rotational 
direction at different predetermined rates of speed so that the motor 
rotates the drive rollers 24 in a direction to move the rod 20 of solid 
thermoplastic material into the melting chamber at different predetermined 
rates to expel molten thermoplastic material through the nozzle 16 at 
different predetermined rates; and wherein the suck back means comprises 
means in the motor control means sequentially operated upon deactivation 
of the motor 34 for shorting electro motive forces in the motor 34 to 
ground, and for applying a predetermined amount of power to the motor 34 
to rotate the rotor 35 in the motor 34 in a reverse rotational direction 
through a predetermined angle. Such shorting of the electro motive forces 
in the motor 34 to ground is important, for if it were not done the 
application of a predetermined amount of power to operate the motor 34 in 
the reverse direction would result in different angles of reverse rotation 
of the rotor 35 due to the need before such reverse rotation could begin 
to overcome different amounts of electro motive force remaining in the 
motor 34 resulting from different rates of forward rotor rotation prior to 
deactivating the motor 34. Use of such grounding, however, insures that 
the the rod 20 of solid thermoplastic material will be moved a single 
predetermined distance out of the melting chamber to both cause molten 
thermoplastic material in the nozzle 16 to flow toward the melting chamber 
and restrict movement or dripping of that molten thermoplastic material 
out of the nozzle 16, and to place that rod at a known location with in 
the melting chamber so that upon reactivation of the motor 35 to rotate 
the rotor 35 in the forward direction the amount of rotation required to 
start melted thermoplastic material flowing from the nozzle 16 will be 
known, which is important to place that melted thermoplastic material at a 
predetermined location on a substrate. 
An electrical circuit that provides at least a portion of the motor control 
means is illustrated in FIGS. 5A and 5B. In that circuit power is directed 
to the motor 34 by power transistors 40 (forward rotation) and 42 (reverse 
rotation). Transistors 40 and 42 are connected together in a complimentary 
emitter follower configuration and have their emitters connected to 
contacts of a relay 44 such that through the relay 44 either of the 
transistors 40 or 42 alone may be connected to the motor 34. When the 
relay 44 is de-energized its contacts are position so that the reverse 
rotation transistor 42 is connected to the motor 34, however, a clamp 
transistor 46 is turned on hard, diverting to ground base drive current 
for the transistor 42 so that transistor 42 is turned "off" and no power 
is transmitted to the motor 34. The motor 34 is activated to drive the rod 
20 into the chamber by energizing the relay 44 either by depressing the 
manual adhesive feed switch 48 or by a remote control device (such as may 
be incorporated in a robot) closing contacts to complete a connection 
through a plug 50. When the relay 44 is energized, forward rotation 
transistor 40 is connected to the motor 34. The magnitude of the voltage 
supplied to the motor 34 and the corresponding rate of armature rotation 
in the motor 34 is determined by the setting of a potentiometer 52 
connected to the base of the forward rotation transistor 40. The maximum 
voltage that can be applied to the motor 34 is approximately 6 volts and 
is limited by a zener diode 54 connected across the base input network of 
the forward rotation transistor 40. Power to rotate the rotor 35 in the 
motor 34 in the forward direction (and thereby dispense thermoplastic 
material from the nozzle 16) will be continuously supplied as long as the 
relay 44 is energized. 
The suck back means are provided in that when the relay 44 is first 
energized normally open contacts 56 & 57 close and set a flip-flop 58 so 
that pin 36 of the flip-flop 58 goes negative causing a 0.01 .mu.f 
flip-flop capacitor 60 connected to the output of an inverter 62 to 
discharge through a 330 ohm resistor 63 and an output of the inverter 62. 
The flip-flop 58 remains in this state as long as the relay 44 is 
energized. When the relay 44 is de-energized by either breaking the 
connection at the manual adhesive feed switch 48 or in the remote control 
device connected by the plug 50) the reverse rotation transistor 42 is 
again connected to the motor 34, the flip-flop 58 is reset by normally 
closed contacts 56 and 66 causing pin 6 of the flip-flop 58 to go positive 
which, through the inverter 62, causes a pulse of current to flow though 
the 330 ohm resistor 63 (i.e., the 0.01 .mu.f capacitor 60 and the 330 ohm 
resistor 63 form a differentiating network). This ultimately causes a 
positive pulse of about 5 .mu.s to occur at the pin 2 input of a one-shot 
72. The Q NOT output of the one-shot 72 goes negative, causing the output 
of an inverter 74 to go high. This, in turn, causes the input of the clamp 
transistor 46 to go high by approximately 0.6 volt which causes the 
transistor 46 to turn "off". When the transistor 46 turns "off" 
base-emitter current flows in the reverse rotation transistor 42 causing 
reverse drive voltage to be applied to the motor 34. Note that a full 6 
volts of DC power is applied to the drive motor 34. The rotor 35 of the 
motor 34 will be driven in reverse, thus retracting (or pulling back) the 
adhesive rod 20 in the barrel member 14, and causing a check valve (not 
shown) at the nozzle 16 to close quickly, preventing dripping from the 
nozzle 16 for a short time and breaking the "string" of adhesive extending 
from the nozzle 16 that normally otherwise occurs. The reverse rotation of 
the rotor 35 will continue for the time setting of the one-shot 72, which 
time period is determined by the setting of a 500 K ohm potentiometer 76 
connected between pin 15 of the one-shot 72 and a positive 5 volts power 
supply. The timing provided by the one-shot 72 is variable between 
approximately 25 micro seconds and approximately 1.6 seconds by adjusting 
the potentiometer 76. After the one-shot 72 runs out the system returns to 
its quiescent condition. The suck back of the rod 20 will not occur when 
the motor 34 is activated for such a short time period that back EMF in 
the motor 34 is not stabilized and no movement of the rotor 35 or rod 20 
occurs. Rather, the rod 20 must be advanced by at least a very short 
amount before the suck back occurs to prevent the rod 20 from being 
"backed" out of the barrel member 14. 
The circuit shown in FIGS. 5A and 5B does not illustrate the means 
described above for shorting electro motive forces in the motor 34 to 
ground prior to applying a predetermined amount of power to the motor 34 
to rotate the rotor 35 in the motor 34 in a reverse rotational direction 
to provide the suck back of the rod 20. That means for shorting can be 
provided by incorporating an additional one-shot to control a PNP 
transistor across the windings of the motor 34 which, with suitable diode 
steering, will provide such grounding for a predetermined time. 
The present invention has now been described with reference to one 
embodiment thereof. It will be apparent to those skilled in the art that 
many changes can be made in the embodiment described without departing 
from the scope of the present invention. Thus the scope of the present 
invention should not be limited to the structures described in this 
application, but only by structures described by the language of the 
claims and the equivalents of those structures.