Heated injection nozzle

A nozzle for the injection of liquefied plastic material into the gate of a mold cavity has a tubular metallic body forming a sprue, the outer surface of this body being in contact with an annular heater in the shape of a closed shell whose interior is filled with a matrix of compacted MgO powder having one or more resistance wires embedded therein. The heater is held in position by a nut, threaded onto a gate-side extremity of the body, against a shoulder or a tapering surface portion thereof and is separated by an air space from a surrounding supporting structure.

FIELD OF THE INVENTION 
Our present invention relates to an injection-molding machine of the 
hot-runner type and, more particularly, to a nozzle for feeding liquefied 
plastic material from a sprue channel to an injection gate of a mold 
cavity. 
BACKGROUND OF THE INVENTION 
To maintain the fluidity of the oncoming plastic material during closure of 
the injection gate, enabling its use in a following molding cycle, it is 
desirable to form a terminal part of the sprue channel as an axial bore of 
a nozzle of good thermal conductivity closely surrounded by an electrical 
heating element. The heating element should be removably mounted on the 
nozzle for ready replacement in the event of a defect and should also be 
physically separated from a nozzle-supporting plate acting as a heat sink. 
For the sake of compactness, its axial and radial dimensions should be as 
small as possible. 
OBJECT OF THE INVENTION 
The object of our present invention, therefore, is to provide an improved 
injection nozzle satisfying these requirements. 
SUMMARY OF THE INVENTION 
A heater-equipped nozzle according to our invention is provided with a 
highly heat-conductive tubular insert defining a terminal part of the 
sprue channel, this insert being received in an annular recess of the 
nozzle body and having a tip separated from a downstream extremity of that 
body by a surrounding clearance which is occupied by an annular heat 
shield preferably also extending into a space between that tip and a mold 
plate engaged by the nozzle extremity. 
Advantageously, in accordance with our present invention, the nozzle heater 
has an annular shell filled with a compact comminuted mass of refractory, 
electrically insulating material, preferably magnesium oxide, in which one 
or more resistance wires are embedded. 
Such a shell can be fastened to the tubular nozzle body with the aid of a 
nut engaging the threaded extremity of that body proximal to the injection 
gate. The shell may be pressed by the nut against an outer peripheral 
shoulder of the body defining its axial position; such an abutment, 
however, becomes unnecessary if the body is tapered toward its threaded 
extremity and the shell has a complementary taper.

SPECIFIC DESCRIPTION 
In FIG. 1 we have shown part of a hot-runner structure 22 serving to supply 
liquefied thermoplastic resin from a nonillustrated pressure chamber via a 
sprue channel 18 to an injection gate 15 of a mold plate 12. Gate 15 is 
alternately openable and closable by a rod or pin 29 which is rigid with a 
double-acting piston in a nonillustrated fluid cylinder whose operation is 
synchronized with the movement of another mold plate (not shown) coacting 
with plate 12 to form a mold cavity 14 as is well known per se. For a 
particularly advantageous construction of such a piston-and-cylinder 
assembly, reference may be made to our copending application Ser. No. 
867,506 of even date. 
A cooled intermediate or backing plate 11, inserted between the mold plate 
12 and the hot-runner structure 22 as more fully illustrated in our 
aforementioned copending application, supports a nozzle 24 provided with 
an annular skirt 24a surrounding tubular nozzle body 24b from which it is 
separated by a cylindrical air gap. A tubular insert 27 of highly 
heat-conductive material, such as beryllium/copper, is seated in nozzle 
body 24b and extends into mold plate 12 as a guide for the free end of 
valve rod 29. An annular space existing between nozzle body 24b, the tip 
of insert 27 and mold plate 12 is occupied by a sheath 28 of resinous 
material which may be prefabricated or is formed from the overflow of 
injected resin in the first operating cycle or cycles. Thus, there is only 
minimal contact between insert 27 and a downstream extremity 24c of nozzle 
body 24b engaging the relatively cold mold plate 12, the sheath 28 serving 
as a further thermal insulator. 
Nozzle extremity 24c is formed with male screw threads 24d engaged by a nut 
49 which bears upon a heating element 25 surrounding the nozzle body 24b. 
Heating element 25 comprises a closed annular metallic shell 25a with a 
frustoconical peripheral surface complementary to a tapering contact 
surface of body 24b. Thus, pressure of nut 49 firmly maintains the element 
25 in its illustrated position near the discharge end of nozzle 24. 
The interior of shell 25a is filled with comminuted magnesium oxide 25b 
having a resistance coil 25c embedded therein. A cable 50 passing through 
a bore 11a of backing plate 11 supplies heating current to coil 25c. 
The thin-walled skirt 24a of nozzle 24, serving to brace the hot-runner 
structure 22 against backing plate 11, has an elevated thermal resistance 
so as to minimize the heat flow from structure 22 to plate 11. In the 
embodiment illustrated in FIG. 1, the skirt 24a spacedly surrounding 
nozzle body 24b terminates short of heater 25 which is separated by 
another annular air space from the surrounding plate 11; that air space, 
it will be noted, is defined by a bore in plate 11 whose diameter equals 
the outer diameter of the aforementioned cylindical air gap. As shown in 
FIG. 2, however, this skirt could be extended to overhang all or part of 
the heater. 
FIG. 2 further illustrates a modified nozzle 24' which differs from nozzle 
24 of FIG. 1 in that its body 24b' is cylindrical instead of tapered and 
is provided with an annular shoulder 24e forming a stop for a similarly 
modified heating element 25'. Heater 25' comprises an annular shell 25a' 
of rectangular rather than trapezoidal cross-section held by nut 49 
against shoulder 24e. The construction of this heater is otherwise 
identical with that of element 25, including a wire coil 25c embedded in a 
matrix 25b of MgO powder. 
The extension of a skirt 24a' of nozzle 24 over the major part of the 
peripheral surface of heater 25' reduces the amount of thermal radiation 
impinging from the heater upon the surrounding backing plate 11. In FIG. 2 
the plate 11 has been omitted along with other parts of the assembly shown 
in FIG. 1, including insert 27 receivable in an annular recess 24f of the 
nozzle body.