BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an injection mold having a hot-runner
mold in which a needle nozzle is constantly nozzle-touched to the sprue of a cavity.
2. Description of the Prior Art
Fig. 3 shows one example of an injection mold having a conventional
structure in which a hot-runner mold 2 is below a cavity mold 1 for molding a preform.
A cavity mold 1 has an internal injection core 11 and a sprue 13 of
a cavity 12 formed around the injection core 11 in the bottom of 12. The gate of
the sprue 13 is opened in the flat face of a concave 14 formed in the bottom of
the cavity mold 1. A nozzle 22 of a needle-valve-nozzle 21 provided in the hot-runner
mold 2 is inserted into the concave 14, whereby molten resin can be injected and
filled into the cavity 12 from the hot-runner mold 2.
Although omitted in the drawings, usually, in the cavity mold 1, a
plurality of cavities 12 are provided in parallel and in each of the cavities 12,
the needle-valve-nozzle 21 is erected on a runner block 23 which is a laterally
elongated cylinder. The needle-valve-nozzle 21 is constructed of a nozzle cylinder
25 having a band heater 24 on its outer periphery, the nozzle 22 mounted on the
end of a nozzle cylinder 25 and having a flat end face, a needle 26 inserted through
the runner block 23 until its end reaches a nozzle orifice, and a pneumatic or hydraulic
cylinder 27 connected to a piston 28 at its bottom for opening or closing the nozzle
21. The needle-valve-nozzle 21 is opened or closed by moving up or down the piston
28.
The hot-runner mold 2 is constantly heated so as to keep the temperature
of molten thermoplastic resin in a runner, whereas the cavity mold 1 is constantly
cooled so as to rapidly solidify the molten resin which is injected and filled into
the cavity mold 1. For this reason, if the nozzle 22 is directly nozzle-touched
to the bottom of the cavity mold 1 in which the gate of the sprue 13 is formed,
the peripheral portion of the gate of the sprue 13 is heated by the nozzle 22 to
cause incomplete cooling and solidifying the resin in the sprue 13. Then, when a
preform is released from the mold, the resin becomes stringy or leaks, or the bottom
of the preform becomes whitened .
In order to prevent this phenomenon, as shown in Fig. 4, a method
is used in which a heat insulating sheet 3 is sandwiched between the bottom of the
cavity mold 1 and the end face of the nozzle 22, and consequently nozzle-touching
is achieved indirectly via the heat insulating sheet 3. The heat insulating sheet
3 generally used is made of a polyamide plastic sheet having a thickness of about
0.5 mm and has heat resistance, but it needs to be periodically replaced because
it has a short life. Moreover, variations are caused in the height of the nozzle
by tolerance at machining or assembling the cavity mold and the hot-runner mold,
so that it is necessary to adjust the height of the nozzle. The adjustment of the
height of the nozzle causes the difficulties of measuring, disassembling, and further
machining the nozzle.
JP200215410 discloses an injection mold apparatus having a hot runner.
In this arrangement, a slidably mounted seal ring is made to contact a gate bush
under the pressure of a thermal plastic resin when the molten resin passes through
a material passage. The seal ring closes the clearance space in the region of a
sprue gate in order to prevent the penetration of the molding material into said
clearance space.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above drawbacks of
the prior art. It is the object of the invention to provide a new injection mold
having a hot-runner mold in which the end part of a nozzle is constructed of a tip
having low thermal conductivity thereby to enable the nozzle to directly nozzle-touch
a cavity mold without a heat insulating sheet and in which the relationship between
the tip and a needle put in the tip eliminates the need for adjusting the height
of the nozzle.
The present invention relates to an injection mold having a hot runner
comprising: a cavity mold which has a gate of a sprue of a cavity in a concave formed
in the bottom thereof, a peripheral portion of the gate being formed into a flat
face, and a hot-runner which is provided with a needle-valve-nozzle having an end
face formed into a flat face, the hot-runner mold being arranged on the cavity mold
with the end of the needle-valve-nozzle inserted into the concave, wherein the nozzle
includes: a nozzle body having an opening formed in the end face of the nozzle;
and a short cylindrical tip that is formed of metal having lower thermal conductivity
than the nozzle body and has a flat end face and a nozzle orifice in the center
of the end face, the cylindrical tip being slidably fitted in an opening formed
in the end face of the nozzle in such a way that its end face is protruded from
the end face of the nozzle and directly nozzle-touched to the gate of the sprue
of the cavity.
The cylindrical tip has an inner peripheral wall face extending to
the nozzle orifice, the inner peripheral wall face being formed in a conical face
having the same angle as a conical end portion of a needle mounted in the needle-valve-nozzle
and being fitted on the end portion of the nozzle to close the nozzle orifice and
to support the cylindrical tip.
Preferably, the nozzle body is made of steel for a mold and the tip
is made of a titanium alloy having lower thermal conductivity than the steel for
a mold.
BRIEF DESCRIPTION OF THE DRAWINGS
- Fig. 1 is a longitudinal cross-sectional view of the main part of an injection
mold having a hot-runner mold in accordance with the present invention;
- Fig. 2 a side view of a nozzle end portion of the injection mold shown in Fig.
1;
- Fig. 3 is a longitudinal cross-sectional view of an injection mold having a
hot-runner mold of a conventional structure in a prior art; and
- Fig. 4 is a longitudinal cross-sectional view of a nozzle-touching portion of
the injection mold in Fig. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Fig. 1 and Fig. 2 show one embodiment of the main part of this invention
and the parts having the same structures as shown in Fig. 3 and Fig. 4 are denoted
by the same reference numerals.
The nozzle 22 of a needle-valve-nozzle 21 shown in the drawings is
constructed of a nozzle body 22a having an opening 22b formed in its end face and
a tip 4 which is fitted in the opening 22b and the flat end face of which is protruded
from the end face of the nozzle body 22a by a height (h) larger than at least a
thickness of a heat insulating sheet. This tip 4 is a cylindrical body made of metal
such as titanium alloy having lower thermal conductivity than steel for a mold forming
the nozzle body 22a. For example, the steel for a mold has a thermal conductivity
of 0.073 cal/cm2/sec/°C/cm and titanium has a thermal conductivity of
0.041 cal/cm2/sec/°C/cm. With this level of difference in the thermal
conductivity, it is possible to achieve the object of the invention.
A nozzle orifice 4a is made in the center of the end face of the tip
4, and an inner peripheral wall face 4b extending to the nozzle orifice 4a and the
end inner peripheral wall face 22c of the nozzle body 22a are formed in a conical
face having the same angle as a conical needle end portion 26a.
Moreover, the outside diameter of the tip 4 is made smaller than the
inside diameter of the opening 22b by a fine sliding clearance. Thus, by this fine
sliding clearance, the tip 4 is not fixed in the opening 22b but can be fitted on
and supported by the needle end portion 26a.
In this kind of nozzle 22, the end portion 26a of a needle 26 is located
at the center of the opening 22b and hence only by fitting the tip 4 into the opening
22b, the nozzle 22 can be directly nozzle-touched to the cavity mold 1, that is,
as shown in Fig. 3, when a hot-runner mold 2 is set under a cavity mold 1 and the
nozzle 22 is inserted into a concave 14 and then the needle 26 is moved up along
with a piston 28 by the action of a cylinder 27, the flat end of the tip 4 is put
into contact with the flat face around the gate of a sprue 13, whereby the nozzle
22 is directly nozzle-touched to the cavity mold 1.
At this time, the tip 4 is pressed and slid in the opening 22b by
the needle 26 until it contacts the cavity mold 1. Thus, even if variations in height
are caused between a plurality, of needle-valve-nozzles 21 mounted in the hot-runner
mold 2 by tolerance in machining and assembling, the tips 4 are protruded from the
end faces of the nozzle bodies 22a according to the variations thereby to eliminate
the need for performing such a height adjustment as is required in the related art.
Further, even if the needle 26 is moved down and its end portion 26a
is separated from the tip 4 at the time of injecting and filling molted resin, the
inner peripheral wall face of the tip 4 is pressed by the injecting pressure of
the resin and hence the nozzle-touching is maintained. In short, since the tip 4
alternately receives the pushing pressure and injecting pressure applied to the
needle-26, it keeps the nozzle-touching. Thus, the tip 4 is only slidably fitted
into the opening 22b of the nozzle body 22a, the nozzle-touching force can be kept
and the resin leakage can be prevented.
Still further, an air gap is produced between the end face of the
nozzle body 22a and the bottom face of the cavity mold 1 around the gate of a sprue
13 by the tip 4 protruded from the end face of the nozzle 22. This air gap and the
low thermal conductivityof the tip 4 significantly reduce thermal conduction from
the needle-valve-nozzle 21 to the cavity mold 1, thereby eliminating the need for
providing the heat insulating sheet previously used.
In addition, the tip 4 made of the metal having the low thermal conductivity
has excellent durability that can not be compared with the heat insulating sheet
made of heat-resistant plastic, so it is not necessary to periodically replace the
tip 4. Moreover, even if the metal having the low thermal conductivity is more expensive
and harder to machine than a metal material usually used, it is only used as the
tip 4 for a part of the nozzle 22 and the tip 4 is simplified in structure and can
be easily machined. Therefore, this tip 4 produces also an economic effect.
In the present specification "comprises" means "includes or consists
of" and "comprising" means "including or consisting of".
