Hot nozzle for runnerless mold

Disclosed herein is a hot nozzle for use with a mold for runnerless injection molding of plastic material. The hot nozzle includes a body formed of a conductive metal having a predetermined electrical resistance. The body has a nozzle portion and a base portion, and has a pair of diametrically disposed longitudinal slits extending substantially over the length thereof except at least a portion of the forward end of the nozzle portion in such a manner as to divide the nozzle portion substantially into two sections. The base portion has on opposite ends separated by the slits a pair of power receiving mechanisms for connection to an external source of power. With this arrangement, the nozzle portion itself, particularly at the front end thereof, is heated by voltage applied across the opposite ends of the base portion. Further, the outer surface of the nozzle body is covered with a dielectric and adiabatic film so as to prevent leakage of resin from the nozzle through the slits and ensure both electrical and thermal insulation of the heating nozzle portion.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a hot nozzle for use with a mold for 
runnerless injection molding of plastic material, and more particularly to 
such hot nozzle structure in a hot runner system using a hot runner type 
runnerless mold. 
2. Description of the Prior Art 
Conventionally, many of plastic products used in daily necessaries, 
domestic electric apparatus, automobiles and other industry are 
manufactured by injection molding. In such injection molding, hot molten 
resin is injected under high pressure through runners into cavities of a 
mold, and after cooled, the molded product is removed from the mold. 
Inevitably, the molded product includes resin hardened in the runners, and 
therefore requires an additional operation to sever and remove the 
hardened resin. Such an operation is time- and labor-consuming work, and 
tends to reduce yields of product. Reclamation of the resin further 
requires cumbersome operations such as crushing. Particularly, all of 
these disadvantages are evident in multi-cavity molds. 
Thus, it has been desired to provide a mold with no runner, or one having 
runners but eliminating the need for removing such runners at every 
molding operation. The hot runner system has been devised in order to meet 
this particular desire. 
One known hot runner system is shown in FIG. 8 and as may be seen, a fixed 
mold half 101 and a movable mold half 102 define therebetween a plurality 
of cavities 103 (only one is shown in FIG. 8). The fixed mold half 101 has 
formed therein a longitudinal bore 104 in place of a runner which 
communicates with the cavity 103. A torpedo spreader 105 is mounted on the 
fixed mold half 101 at the inlet of the bore 104, with its elongated 
portion 105a protruding into the bore 104. The torpedo spreader 105 has 
formed therein passages 107 (only one is shown in FIG. 8) communicating 
with a molten resin dispensing manifold 106 through which molten resin is 
supplied from a nozzle of the injection molding machine (not shown). The 
molten resin passes through the passages 107 into a space 108 defined in 
the bore 104 between the fixed mold half 101 and the elongated portion 
105a of the torpedo spreader 105, and flows into the cavity 103. The 
torpedo spreader 105 has formed therewithin an insertion hole 109 
extending substantially up to the top end of the elongated portion 105 a 
for accommodating a heater 110 which can be energized by an external 
source of power through a lead wire 111. 
In such a hot runner system employing a so-called internal heating system, 
the resin in the bore 104 corresponding to a runner is heated by the 
heater 111 through the elongated portion 105a of the spreader 105 to be 
always maintained in a molten state and hence, the molded article has no 
runner when removed from the cavity 103. 
It should be noted, however, that in this hot runner system having an 
internal heating mechanism, the molten resin in the spacing 108 contiguous 
to the fixed mold half 101 tends to be hardened and stagnant as it is 
cooled by the fixed mold half 101. If the temperature of the heater is 
raised to prevent such stagnation, the resin in the vicinity of the 
elongated portion 105a of the spreader 105 will be thermally decomposed 
and clung to that portion. Thus, temperature control is difficult and 
power consumption is increased. Additionally, as resin is always stagnant 
in the bore 104, change of colors of resin will cause mixture of a color 
with the previous one, resulting in production of defective articles. 
Further, such a hot runner system has other disadvantages that the 
spreader body is difficult to machine, and that the construction is itself 
complicated. 
The prior art hot runner system has also employed a socalled external 
heating mechanism in which a nozzle has therewithin a passage for molten 
resin, and a band heater is attached to the outside of the nozzle, or a 
heater is incorporated in the nozzle as a unit. 
In this arrangement, however, heat of the heater tends to escape through 
the nozzle-mounting bore to the fixed mold half, resulting in increase in 
power consumption of the heater. In order to heat resin over the length of 
the nozzle-mounting bore, the heater must be provided over the length of 
the nozzle, and usually another heater is provided to heat the top end of 
the nozzle and facilitate the flow of resin into the cavity. Consequently, 
complexity of the structure and difficulty in temperature control are 
disadvantageously increased. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a hot nozzle which can 
substantially prevent escape of heat of resin in the nozzle to the mold 
and is free from stagnation of cooled resin and consequent mixture of 
colors during color changeover. 
Another object of the present invention is to provide a hot nozzle having a 
heating system which is simple in construction. 
A further object of the present invention is to provide a hot nozzle having 
a heating system in which accurate and easy temperature control may be 
accomplished. 
A still further object of the present invention is to provide a hot nozzle 
having a heating system whose power consumption for heating is small. 
According to the present invention, there is provided a hot nozzle for use 
with a runnerless mold having at least one mold cavity and a longitudinal 
bore communicating with the mold cavity. The hot nozzle comprises a body 
formed of a conductive metal having a predetermined electrical resistance. 
The body has a nozzle portion adapted for insertion into the bore of the 
mold and a base portion communicating with the nozzle portion to provide 
for the flow of molten resin to the mold cavity. The body also has a pair 
of diametrically disposed longitudinal slits extending substantially over 
the length thereof except at least a portion of the forward end of the 
nozzle portion in such a manner as to divide the nozzle portion 
substantially into two sections. The base portion has on opposite ends 
separated by said slits a pair of power receiving means for connection to 
an external source of power. With this arrangement, the nozzle portion 
itself, particularly at the front end thereof, is heated by voltage 
applied across the opposite ends of the base portion. Further, the outer 
surface of the nozzle body is covered with a dielectric and adiabatic film 
so as to prevent short and leakage of resin from the nozzle through the 
slits. 
In the present invention, the nozzle body is so constructed that the nozzle 
portion is heated mainly at the front end thereof and hence, heating of 
such a nozzle portion can be accomplished in a simpler construction than 
heating by a heater incorporated in a nozzle or a band heater attached to 
the outside of the nozzle. Particularly, when the wall of the nozzle 
portion is formed thin, the heat capacity of the nozzle portion becomes so 
small as to permit rapid rise and fall of the temperature by energizing 
and deenergizing the nozzle. Thus, both easy and accurate temperature 
control may be accomplished. 
In accordance with the preferred embodiment of the invention, the outer 
surface of the nozzle body is coated with a dielectric and adiabatic film 
in such a manner as to cover the slits and hence, short and leakage of 
resin through the slits may be prevented, and also escape of heat from the 
nozzle portion may be prevented, there being no need for additional 
heating of the nozzle portion for compensating the dissipated heat. Thus, 
the provision of film on the nozzle body is effective to prevent thermal 
decomposition of resin which may be caused by additional heating of the 
nozzle portion and prevent consequent stagnation of resin in the nozzle 
which will cause mixture of colors at color changeover. Further, 
prevention of heat escape from the nozzle portion is also effective to 
lower power consumption for energy saving. 
In the preferred embodiment of the invention, the film is formed by 
depositing ceramic powder onto the outer surface of the nozzle body. 
In an alternative embodiment of the invention, the nozzle portion is 
surrounded by an adiabatic protection pipe such as of ceramic which may 
more effectively prevent dissipation of heat from the nozzle portion. 
In a further embodiment of the invention, the outer diameter of the 
protection pipe is smaller than the inner diameter of the bore of the 
fixed mold half so that a clearance is provided between the pipe and the 
inside wall of the bore. Thus, heat dissipation may be more effectively 
prevented. 
The present invention will become more fully apparent from the claims and 
description as it proceeds in connection with the drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1 and 2 of the drawings, shown therein is an 
injection molding mechanism of the type which is commonly to be found 
employed in injection molding machines. As shown therein, the injection 
molding mechanism includes an injection molding machine nozzle 10 and a 
manifold 12 adapted for feeding molten resin injected from the nozzle 10 
into an internal passage 13 through an inlet 11. The manifold 12 is held 
by a supporting frame 16 having a nozzle-inserting opening 14 for 
receiving the nozzle 10 therein. The manifold 12 is provided with a 
plurality of resin dispensing passages 22 formed in flow communication 
with mold cavities 20 in a mold 18 which will be mentioned later. A spacer 
24 is interposed between the supporting frame 16 and the mold 18 to 
provide the mounting space for the manifold 12. 
The manifold 12 mounts a plurality of hot nozzles 26 (four nozzles shown in 
FIG. 1) of the invention in flow communication with the respective 
passages 22. Specifically, as shown in FIG. 3, each of the hot nozzles 26 
is carried at one end on a pair of retaining plates 30 secured to the 
manifold 12 through bolts 28, the other end protruding into the mold 18. 
The mold 18 includes a fixed mold half 32 and a movable mold half 34 which 
cooperate to define therebetween the mold cavities 20 when in closed 
position. The fixed mold half 32 has longitudinal bores 36 for receiving 
the respective hot nozzles 26 therein. The bores 36 communicate with the 
respective cavities 20 through gates 21 which are formed to tapering 
configuration. The movable mold half 34 may be moved in and out of contact 
with the fixed mold half 32 by driving means (not shown) through a 
supporting frame 38, and is provided with ejector pins 40 for ejecting 
molded articles from the cavities 20. The arrangement of such a movable 
mold half 34 is well known in the art and hence has not been described in 
detail. 
Referring next to FIG. 4, shown therein is the pertinent construction of 
the hot nozzle 26. As shown therein, the hot nozzle 26 includes a body 27 
formed of a conductive metal having a predetermined electrical resistance. 
The body 27 has a base portion 27a supported by the retaining plates 30 
and a nozzle portion 27b adapted to be inserted into the longitudinal bore 
36 of the fixed mold half 32 and having a conical front part tapering to 
the front to suit the bore 36. It should be noted that the wall of the 
nozzle portion 27b is formed particularly thin. The body 27 has formed 
therein a longitudinal passage 42 for the flow of molten resin to the mold 
cavity 20. As shown in FIGS. 5, 6 and 7, the body 27 has a pair of narrow 
longitudinal slits 44 extending therethrough except the front end of the 
nozzle portion 27b in diametrically opposed relationship, so that the body 
27 is divided into substantially two sections thereby. The sections 
divided by the slits 44 have at the ends thereof in the regions normal to 
the slits 44 and opposing to each other a pair of power connecting 
portions 58, 60 having screw holes 58a and 60a for securing terminals 50, 
52 of lead wires 46, 48 through screws 54, 56, respectively. As best shown 
in FIG. 3, the lead wire 46 is connected along with lead wires 46 of the 
other hot nozzles 26 to an external power supply through an 
interconnecting lead bar 62; and in the same way the lead wire 48 is 
connected along with lead wires 48 of the other hot nozzles 26 to the 
ground through another interconnecting lead bar 64. The voltage from the 
power supply is regulated by voltage regulating means operatively 
connected with sensors for detecting the temperature of the front part of 
the nozzle portion 27b and/or the temperature of resin within the nozzle 
portion 27b. Thus, the voltage is regulated on the basis of the 
temperature detected by the sensors so that the temperature of resin 
within the nozzle portion 27b can be always maintained at a constant 
level. Such temperature regulating means is well known in the art and 
hence has not been described in detail. 
A ceramic film 66 having both dielectric and adiabatic properties is coated 
over the whole outer surface of the nozzle body 27 except the power 
connecting portions 58, 60 and the inlet portion indicated at 42a and the 
outlet portion indicated at 42b of the resin passage 42 in such a manner 
as to close the slits 44. This film 66 can be formed by hot spraying to 
deposit fine ceramic powder on the outer surface of the nozzle body 27 in 
thickness ranging from 0.3 to 0.5 mm, and its heat-resistant temperature 
is about 800.degree. C. 
The nozzle portion 27b of the body 27 includes a protection pipe 68 fitted 
thereon in substantially coextensive relationship with the longitudinal 
bore 36 and adapted for protecting the film 66 against shock especially 
when the hot nozzle 26 is inserted into the mold. The protection pipe 68 
may be formed of metal but is preferably formed of ceramic for the purpose 
of increasing adiabatic property. It will be noted that the outer diameter 
of the protection pipe 68 is smaller than the inner diameter of the bore 
36 so that a clearance is defined between the protection pipe 68 and the 
inside wall of the bore 36 to further improve adiabatic property. 
Now, the operation of the hot nozzle 26 thus constructed is as follows. 
Molten resin injected from the machine nozzle 10 is fed through the resin 
dispensing passages 22 in the manifold 12 into the passages 42 formed in 
the respective hot nozzles 26, and then flows therethrough into the 
cavities 20. In this resin filling process, preferably a low voltage is 
applied and thence a heavy current is fed from the interconnecting lead 
bars 62, 64 to the hot nozzles 26 through the respective lead wires 46, 
48, and each of the hot nozzles 26 itself generates heat mainly at its 
front part where no slit is formed so as to keep good flow of resin at the 
inlet region to the cavity 20 and to keep the resin within the nozzle 
portion 27b in the heated state. As the wall of the nozzle body 27 is 
thin, its heat capacity is so small as to cause rapid rise and fall of the 
temperature by application of power, as mentioned above, and by power 
cutoff, as will be mentioned later, as well as to permit accurate 
temperature control by the temperature control means. 
When the mold cavities 20 are filled with resin, cooling water is 
introduced into the mold 18 to harden the resin therewithin. At the same 
time as the resin filling process is completed, application of power to 
each hot nozzle 26 is temporarily stopped, so that, when the resin in the 
corresponding cavity 20 is completely hardened, the resin in the front 
part of the nozzle portion 27b or in the inlet region to the cavity 20 is 
half-hardened. 
When the resin in the cavities 20 is completely hardened or molding is 
completed, the movable mold half 34 is moved apart from the fixed mold 
half 32, and at the same time the pins 40 are protruded into the 
respective cavities 20 to eject molded articles from the mold 18. As the 
resin in the inlet region to the cavity 20 is half-hardened, cobwebbing of 
resin may be prevented during removal of molded articles, or leakage of 
resin from the nozzle portion 27b may be prevented after the ejecting 
operation. 
When ejection of the molded articles is finished, the mold 18 is closed 
and, when the body 27 of the hot nozzle 26 is again energized and heated 
to a predetermined temperature, injection of resin is restarted, and the 
similar operations are repeated. 
Although the preferred embodiment utilizes the common interconnecting lead 
bars 62, 64 for energizing the hot nozzles 26, it will be appreciated that 
separate lead wires may be used to connect the respective hot nozzles 26 
to the power supply so that the temperature of the hot nozzles 26 may be 
individually controlled. 
While the invention has been described with reference to a preferred 
embodiment thereof, it is to be understood that the embodiment is not 
intended to limit the scope of the invention and modifications or 
variations may be easily made without departing from the spirit of the 
invention.