Injection molding nozzle

An extruder barrel of a thermoplastic molding machine connected to an injection molding nozzle provided with a plurality of heat pipes distributed around the central bore of the nozzle, each of the heat pipes deriving its thermal input from the extruder barrel itself.

BACKGROUND OF THE INVENTION 
In injection molding of thermoplastic material, such material is fed in 
molten form from the extruder barrel of an injection molding machine 
through an injection nozzle, which is adapted to engage an injection 
bushing through which the material is supplied to the interior of the mold 
in which the material cools and solidifies to form the desired object. The 
quality of the material of the molded object is dependent to a 
considerable degree upon the extent to which the material is uniformly 
maintained at an optimum processing temperature throughout its residence 
time in the extruder barrel and injection nozzle. If the temperature of 
the injection nozzle is permitted to drop below the temperature of the 
material in the extruder barrel, the material delivered to the injection 
bushing may be too cold for optimum results, while if an attempt is made 
to heat the injection nozzle by separate heating means, the material may 
become too hot for such optimum results. Also, additional heaters are 
expensive and introduce system control problems. 
SUMMARY OF THE INVENTION 
The present invention meets the objective of supplying heated molten 
thermoplastic material, for injection molding purposes, automatically at 
the optimum processing temperature by means of an injection molding nozzle 
incorporating a specially designed heat pipe structure which is so located 
with respect to the extruder barrel that it obtains its heat input solely 
from the barrel structure itself, whereby it maintains the molten material 
throughout the length of the nozzle at the same temperature which it 
possessed while in the extruder barrel. 
The heat pipe structure preferably consists of a plurality of separate heat 
pipes formed in the body of the nozzle surrounding its central ejector 
passage. Each heat pipe wall is provided with a capillary wick structure 
which is wetted by the working fluid, preferably water. The body of each 
such heat pipe is preferably of a high strength steel, such as tool steel. 
However, since the iron in the steel may react with water to release the 
noncondensible gas hydrogen, the inner wall of each heat pipe is plated or 
otherwise coated with a material, such as nickle, which is compatible with 
water. Likewise the wick is made of a similarly compatible material such 
as monel. Each heat pipe is preferably furnished with a plug of a 
material, such as monel, through which any hydrogen released in the pipe 
may diffuse. A substantial portion of each wick structure extends within 
the portion of the nozzle located within the body of the extruder barrel, 
whereby all of the operating thermal energy for each heat pipe is derived 
solely from the body of the extruder barrel and thus, by the well known 
heat pipe principle, maintains the molten plastic material, throughout the 
length of the nozzle, at the same temperature as the molten plastic 
material within the extruder barrel itself.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
In FIG. 1, a standard type of heat conductive metal extruder barrel 4, 
which is part of the usual thermoplastic material injection molding 
machine, may be provided with any convenient heater means, such as an 
electrical heater band 6, for maintaining the barrel 4 at the desired 
temperature for holding the thermoplastic material within the bore 8 of 
the barrel 4 in its molten state at which the material is to be extruded 
from barrel 4. Such a barrel is quite massive and possesses such a high 
heat capacity that it maintains all of the thermoplastic material flowing 
through it at a constant optimum temperature for proper injection molding. 
The end of barrel 4, adjacent the outer end of bore 8, is constructed with 
an internally threaded socket 10 into which the inner end 12 of an 
injection molding nozzle 14 is threaded. The nozzle 14 may be provided 
with an hexagonal outer section 16 to accomodate a wrench for securely 
bolting the nozzle in place so as to insure an intimate thermal contact 
between the nozzle and the extruder barrel. The nozzle 14, which is of a 
high heat conductivity material, is provided with a central bore 18 which 
forms a continuation of bore 8 when nozzle 14 is bolted in place. The 
outer end of nozzle 14 is formed with an internally threaded socket 20 
into which a conventional type of injection tip 22 may be threaded. The 
tip 22 is provided with a bore 24 in line with bore 18. The molten plastic 
material to be injected into a mold flows through the bore 24 and is thus 
delivered to the mold. 
The body of nozzle 14 surrounding the bore 18 is formed with a plurality of 
heat pipes 26, shown more clearly in FIG. 2. Typically, six of such heat 
pipes may be used, although any convenient number, such as four, may be 
used. In the preferred embodiment illustrated, the body of the nozzle is 
made of a high strength tool steel which has been found to have the 
desired high heat conductivity and strength. Although any vaporizable and 
condensible liquid, such as mercury, may be used as the vaporizable heat 
transferring medium in the heat pipes 26 it is far more preferable to use 
water as the working liquid. 
One drawback to the use of water is the fact that there is a tendency for a 
reaction occur between the iron in the steel and the water in which the 
iron combines with the oxygen of the water leaving a residue of hydrogen 
which is an incondensible gas under the conditions of operation of the 
heat pipe. The presence of hydrogen in the heat pipe is highly deleterious 
to its effective operation. For the purposes of this invention any 
material, such as iron or an alloy of iron, which tends to release 
hydrogen from water is termed "water incompatible material." 
However, the use of the desirable high strength steel is made practicable 
by plating or otherwise covering the interior wall of each heat pipe by a 
material which does not tend to dissociate water. Examples of such a 
material are nickle, copper, and alloys of nickle and copper, such as 
monel. Such materials are defined hereinas "water compatible materials." 
As shown in FIG. 4, the inner wall of each heat pipe 26 is plated with a 
water compatible material 28, preferably nickle. Such plating is 
sufficiently heavy to be impermeable to water and water vapor. Into each 
heat pipe is inserted a wick structure 30 consisting of a water compatible 
cylindrical metal screen forced into and tightly pressed against the wall 
of its heat pipe. The wick is preferably of monel. 
The normal optimum temperature at which molten plastic material is injected 
into molds lies within a range of about 200 to 400 degrees centrigrade. 
The vapor pressure of water at these temperatures, although quite high, is 
readily and safely contained with the heat pipe which combines the 
strength of the high strength steel of the nozzle 14 and the water 
compatibility of the plating 28. 
After each heat pipe 26 is evacuated, a limited amount of water, typically 
about 30% of the volume of the pipe, is inserted into each of them. 
Following the insertion of the water, the outer end of each heat pipe is 
sealed by means of any well known type of sealing plug 32 which is rammed 
into the open end of each heat pipe, is locked in place, and seals the 
heat pipe against escape of water vapor. It has been found that nickle and 
its alloys, such as monel, are slightly permeable to hydrogen. Therefore 
each plug 32 is preferably of monel. Under the operating conditions of the 
structure described above, any small amount of hydrogen which may be 
released in a heat pipe 26, will diffuse out of the heat pipe through its 
plug 32, keeping the interior of each heat pipe virtually hydrogen free. 
It should be noted that the length of each heat pipe 26 is such that a 
substantial length of the pipe extends well within the portion of the 
nozzle 14 which is in good thermal contact with the extruder barrel 4. As 
a result, the length of each heat pipe within the threaded end 12 is 
closely maintained at the same temperature as the temperature of the 
molten plastic entering the nozzle 14 from the extruder barrel bore 8. In 
accordance with the well known heat pipe operation, water is vaporized by 
heat flowing into the inner end of each heat pipe from the extruder 
barrel, water vapor travels to each portion of the heat pipe from which 
heat is being extracted and the vapor condenses at each such portion to 
yield up its heat of condensation to maintain the entire length of the 
heat pipe at the same temperature. The vaporization of water from the 
inner end of the wick structure 30 creates a capillary attraction to draw 
condensed water from the rest of the wick structure back to the evaporator 
portion of the wick thus completing the cycle of water flow to maintain 
the heat pipe action. 
It has been found that not only does the novel injection nozzle maintain 
the entire length of its bore 18 at the same temperature as that of the 
incoming molten plastic but also that there is virtually no variation of 
temperature around the circumference of the bore 18 due to the fact that a 
plurality of heat pipes are used to distribute the heat. 
It is to be understood that the structure described in detail above 
represents a preferred embodiment of the invention and that various 
modifications of such structures may be made within the scope of the 
appended claims.