ELECTRIC SLEEVE HEATER WITH DIFFERENT-TEMPERATURE HEATING ZONES

MANCHON DE CHAUFFAGE ELECTRIQUE AVEC ZONES DE CHAUFFAGE A TEMPERATURES DIFFERENTES

English Abstract

An electrical heater for heating a machine part having
a substantially cylindrical surface has a cast metallic body
having an inner surface and an outer surface one of which has a
portion in direct contact with the surface of the machine part
and another portion offset therefrom and forming a space
therewith. A heating element imbedded in the body includes a
resistive wire, a layer of insulation surrounding the wire, and a
dimensionally stable jacket surrounding the insulation layer.

Claims

I CLAIM:
1. An electrical heater for heating a machine part
having a substantially cylindrical surface, the heater
comprising:
a cast metallic body having an inner surface and an
outer surface, one of the surfaces of the body having a portion
in direct contact with the surface of the machine part and
another portion offset therefrom and forming a space therewith;
and
a heating element imbedded in the body and including
a resistive wire,
a layer of insulation surrounding the wire, and
a dimensionally stable jacket surrounding the
insulation layer.
2. The electrical heater defined in claim 1, further
comprising a mass of insulating material filling the space.
3. The electrical heater defined in claim 1 wherein
the other surface portion is formed by an annular groove in the
body.
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4. The electrical heater defined in claim 1 wherein
the other surface portion is formed by an axial groove in the
body.
5. The electrical heater defined in claim 1 wherein
the body is formed with two axially extending grooves defining
respective such other surface portions and subdividing the body
into two regions each provided with a respective such heating
element.
6. The electrical heater defined in claim 1 wherein
the element is at least generally helical.
7. The electrical heater defined in claim 1 wherein
the body is formed with at least one axially extending groove
having a floor constituting the other surface portion.
8. The electrical heater defined in claim 7 wherein
the body is formed to each side of the groove with a radially
projecting rib.
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9. The electrical heater defined in claim 1 further
comprising a jacket on the one surface of the body.
10. An electrical heater for heating a machine part
having a substantially cylindrical outer surface, the heater
comprising:
a cast metallic body having a cylindrical inner surface
in direct contact with the surface of the machine part and formed
with a groove having a floor defining a surface portion offset
from the machine-part surface and forming a space therewith; and
a nonstraight heating element imbedded in the body and
including
a resistive wire,
a layer of insulation surrounding the wire, and
a dimensionally stable jacket surrounding the
insulation layer.
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Description

2158911
19654
ELECTRIC SLEEVE HEATER WITH DIFFERENT-TEMPERATURE HEATING ZONES
SPECIFICATION
FIELD OF THE INVENTION
The present invention relates to an electrical heater.
More particularly this invention concerns a sleeve or plug heat
that is set in a hole or fitted around a machine part to heat it.
BACKGROUND OF THE INVENTION
A cylindrical electric heater, whether tubular so it
can be slipped over a machine part to be heated or solid so it
can be fitted into a hole in a machine part to be heated,
typically has a cast metallic body having an inner surface and an
outer surface, one of which is in direct contact with the surface
of the machine part. A heating element, typically formed as a
helical coil, is imbedded in the body and has a resistive wire, a
layer of insulation surrounding the wire, and a dimensionally
stable jacket surrounding the insulation layer. Electricity is
passed through the wire to heat the element and the heat is
transmitted through the copper or brass body to the machine part
being heated.
Such a heater is normally of circular section. When
used, for instance, to heat an injector nozzle it is tubular and

21S8gl~
19654
is fitted over the cylindrical outer surface of the nozzle. In
other systems it is solid and is fitted into a cylindrical bore
in the part to be heated.
In many applications it is advantageous to heat
different zones of the machine part to different temperatures.
This is typically done by varying the pitch of the heating
element, making its pitch steeper in areas to be heated less and
shallower where more heat is required. Since, however, the body
is highly thermally conductive, such systems are only marginally
effective because the heat from the element will flow rapidly in
the brass or copper body.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to
provide an improved electrical heater for a machine part.
Another object is the provision of such an improved
electrical heater for a machine part which overcomes the above-
given disadvantages, that is which can produce sharply different
temperatures in different zones of the machine part it is being
used to heat.

21S8914
19654 3
SUMMARY OF THE lNv~NllON
An electrical heater for heating a machine part having
a substantially cylindrical surface has according to the
invention a cast metallic body having an inner surface and an
outer surface one of which has a portion in direct contact with
the surface of the machine part and another portion offset
therefrom and forming a space therewith. A heating element
imbedded in the body includes a resistive wire, a layer of
insulation surrounding the wire, and a d.mensionally stable
jacket surrounding the insulation layer.
Thus with this system the actual heat transfer between
the heated surface of the heater and the surface to be heated of
the machine part is reduced. When the space is empty the heat
exchange in the one surface portion is conductive and highly
efficient while at the other surface portion is radiant and
fairly inefficient. To further reduce heat exchange a mass of
thermal-insulating material fills the space.
According to this invention the other surface portion
is formed by a floor of an annular groove in the body. It can
also be formed by an axial groove in the body. When the body is
formed with two axially extending grooves defining respective
such other surface portions and subdividing the body into two
regions, each such portion can be provided with a respective such
heating element.

215891~
19654 Li
The element in accordance with this invention can be at
least generally helical and can be of reduced pitch in the other
surface portion to reduce the heat transmission in this zone even
further. The body can be formed to each side of the groove with
a radially projecting rib. A jacket is provided on the one
surface of the body.
BRIEF DESCRIPTION OF THE DRAWING
The above and other objects, features, and advantages
will become more readily apparent from the following description,
it being understood that any feature described with reference to
one embodiment of the invention can be used where poscible with
any other embodiment and that reference numerals or letters not
specifically mentioned with reference to one figure but identical
to those of another refer to structure that is functionally if
not structurally identical. In the accompanying drawing:
FIG. 1 is an end view of a heater according to the
invention;
FIG. 2 is a section taken along line II--II of FIG. l;
FIG. 3 is an end view of another heater;
FIG. 4 is a section taken along line IV--IV of FIG. 3;
FIG. 5 is an end view of another heater:
FIG. 6 is a section taken along line VI--VI of FIG. 5;
FIG. 7 is an end view of another heater; and

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19654
FIG. 8 is a section taken along line VIII--VIII of FIG.
7.
SPECIFIC DESCRIPTION
As seen in FIGS. 1 and 2 a sleeve heater 1 according to
this invention is intended to fit around a cylindrical part 19,
here an extruder nozzle, centered on an axis A. A resistive-type
heating wire 2 is imbedded in insulation 3 and surrounded by a
dimensionally stable metallic jacket 4. It is shaped into a
helix with a small pitch at each end and a large pitch in a
central region and is imbedded in a tubular body 5 of brass or
copper. This assembly is surrounded by another jacket 7 here of
stainless steel. A two-conductor cable 8 is connected to the
wire 2 for passing an electric current through it and thereby
generating heat that is transmitted to the cylindrical surface of
the body 19.
According to the invention the heater 1 forms a passage
6 centered on the axis A and having cylindrical small-diameter
end regions 9 where it directly contacts the outer surface of the
machine part 19 to be heated, and a cylindrical central larger-
diameter region 10 that is spaced from this part 19 and that isformed by the floor of a wide radially inwardly open annular
groove. Insulation 18 may be provided in the region 10 to reduce
heat transfer. Thus the part 19 will be heated substantially at
zones corresponding to the direct-contact end regions 9, where
~; _

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19654
the wire 2 is wound in a tight helix, and is heated less in the
central region where direct heat conduction is largely
eliminated.
In the arrangement of FIGS. 3 and 4 the nondirect-
contact region is formed by an axially extending inwardly opengroove 11 that is machined in the brass body 5. This system is
handy for injector nozzles having passages for two different
resins that need to be heated to different temperatures.
The arrangement of FIGS. 5 and 6 has two separate
resistive wires 12 and 13 supplied by separate cables 14 and 15.
The body 5 is formed with two separate diametrically opposite,
axially extending, and radially inwardly open grooves 11 that
subdivide it into two sections 16 and 17. The wire 12 runs only
in the section 16 and the wire 13 only in the section 17 and the
floors of the grooves 11 form noncontact regions that thermally
separate the sections 16 and 17.
A similar system is shown in FIGS. 7 and 8, but here
the body 5 is formed to each side of each notch 11 with a
radially inwardly projecting pointed ridge S' that engages in a
respective slot of the unillustrated machine part being heated.
As in FIGS. 5 and 6, this arrangement is ideal for a nozzle that
has separate passages for resins needing to be heated to
different temperatures.

Representative Drawing