Self-tightening electrical sleeve heater

An electrical sleeve heater has a tubular outer wall and a heating coil inside the outer wall. An outer sleeve inside the heating coil formed with an axially throughgoing slot having angularly confronting edges is made of a material having a high coefficient of thermal expansion. An inner sleeve inside the outer sleeve is formed with an axially throughgoing slot and with outwardly projecting lips engaged through the outer-sleeve slot bearing angularly on the outer-sleeve slot edges. It is made of a material having a low coefficient of thermal expansion so that when the sleeves are heated the outer-sleeve edges press the inner-sleeve lips together and thereby decrease the radial inner diameter of the inner sleeve.

FIELD OF THE INVENTION 
The present invention relates to an electrical sleeve heater. More 
particularly this invention concerns such a heater used to heat an 
injection-molding nozzle or the like through which passes a material that 
must be maintained hot. 
BACKGROUND OF THE INVENTION 
A standard heater used on, for example, a nozzle of an injection-molding 
machine comprises a tubular inner wall, a helicoidal heater element, e.g. 
a resistive heating wire, surrounding the inner tube, a tubular outer wall 
surrounding the heater element, and a mass of insulating material, e.g. 
magnesium oxide, filling the annular chamber between the tubes around the 
heater element. Such a sleeve heater is slipped over the outer cylindrical 
surface of the normally tubular nozzle and the heater element is connected 
to a source of electricity. When electricity is passed through the heating 
element, it heats and this heat is transmitted through the oxide 
electrical insulation to the inner wall and thence to the outer surface of 
the object, i.e. the nozzle, the heater is fitted around. 
It is, obviously, essential that the sleeve heater fit as snugly as 
possible around the object being heated for best possible heat conduction 
between its inner wall and the outer surface of the object. At the same 
time it must be loose enough that it can be slipped on and of since such a 
heater is subject to considerable stress and must be removable. Thus there 
is a tradeoff between the tight fit necessary for good heat transmission 
and the looseness needed to make removal and replacement possible. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
electrical sleeve heater. 
Another object is the provision of such an improved electrical sleeve 
heater which overcomes the above-given disadvantages, that is which can be 
easily removed from and fitted over the object yet that will be tight when 
in use. 
SUMMARY OF THE INVENTION 
An electrical sleeve heater has according to the invention a tubular outer 
wall and a heating coil inside the outer wall. An outer sleeve inside the 
heating coil formed with an axially throughgoing slot having angularly 
confronting edges is made of a material having a high coefficient of 
thermal expansion. An inner sleeve inside the outer sleeve is formed with 
an axially throughgoing slot and with outwardly projecting lips engaged 
through the outer-sleeve slot bearing angularly on the outer-sleeve slot 
edges. It is made of a material having a low coefficient of thermal 
expansion so that when the sleeves are heated the outer-sleeve edges press 
the inner-sleeve lips together and thereby decrease the radial inner 
diameter of the inner sleeve. 
Thus with this system the inner diameter of the inner sleeve can be 
slightly more than the outer diameter of the object it is being used to 
heat so that it can be installed and removed easily. When the electrical 
heating coil is energized, however, the heated outer sleeve will expand, 
forcing together the lips of the inner sleeve and decreasing its diameter 
so that it will decrease in radial inner diameter and tightly hug the 
object. The system therefore only tightens itself to the object being 
heated when it is actively heating this object, and does this using 
nothing other than thermal expansion, that is without any mechanical 
clamping system. 
According to the invention the lips are formed at edges of the inner-sleeve 
slot. The material of the inner sleeve includes titanium and that of the 
outer sleeve includes aluminum. In addition the outer wall is an unslotted 
tube formed of a material with a low coefficient of thermal expansion, 
again titanium or a titanium alloy. Thus the outer sleeve is in effect 
braced radially outward against the outer wall, normally via a mass of 
thermal insulation surrounding the heating element between the outer 
sleeve and outer wall, so that, when the outer sleeve expands, it has 
nowhere to go but inward. 
The outer wall is formed of a pair of similar centrally joined tube 
sections having interfitting ends. Thus the device can be assembled 
easily, normally with the power leads for the heating element extending 
centrally out from between the outer-wall sections.

SPECIFIC DESCRIPTION 
As seen in FIGS. 1 and 2 a sleeve heater 1 according to the invention 
basically comprises a cylindrically tubular inner wall 2, a helical 
heating element 3 surrounding this wall 2, and a cylindrically tubular 
outer wall 4 coaxially surrounding the element 3 and inner wall 2. An 
insulating mass 10, for example of magnesium oxide, typically fills the 
annular space between and around the turns of the heater coil 3 between 
the inner and outer walls 2 and 4. 
In accordance with the invention the cylindrically tubular inner wall 2 is 
formed as shown in FIGS. 3 and 4 by a relatively thin inner sleeve 5 
formed with an axially through-going straight slot 7 and a somewhat 
thicker outer sleeve 6 formed as shown in FIGS. 5 and 6 with its own 
axially through-going straight slot 8 aligned radially with the inner slot 
7. In addition the inner sleeve 5 is formed along the edges of the slot 7 
with radially outwardly projecting straight lips 9 that fit within the 
slot 8 of the outer sleeve 6 so that the edges of the sleeve 6 defining 
this slot 8 angularly abut the lips 9. 
The inner sleeve 5 is wholly formed of a metal such as titanium or a 
titanium alloy that has a low coefficient of thermal expansion. The outer 
sleeve 5, however, is formed of a metal such as aluminum or an aluminum 
alloy that has a much higher coefficient of thermal expansion. In addition 
the outer sleeve 6 is actually formed of two generally identical sleeve 
halves that fit together axially centrally adjacent a location where a 
wire 11 connected to an unillustrated electric-power source and leading 
from the heater element 3 exits the heater. Outer ends of the outer wall 
4, which is made of the same low-expansion material as the inner sleeve 5, 
are provided with flanges 12 that are fixed to outer ends of inner sleeve 
5 and that axially contain the heating element 3. 
Thus when the heater 1 gets hot, the outer sleeve 6 will expand more than 
the inner sleeve 5 and will push together the lips 9 because it cannot 
expand outward due to the angularly continuous or tubular construction of 
the outer wall 4. This action will compress the inner sleeve 5 radially 
inward into tight engagement with a cylindrical outer surface of an object 
shown in dashed lines 13 in FIG. 2. When the heater 1 cools off, the 
effect is reversed so that the heater 1 can easily be removed, if 
necessary.