An electric heater having a ceramic core with a heating element
surrounding the core and a jacket surrounding the heating element
such that the heating element is secured to the jacket but there
is no chemical bonding between the heating element and the core.
In the method of producing the electric heater, the core is formed
in an uncured state and then surrounded with the heating element.
The heating element is surrounded by a jacket such that the heating
element is secured to the jacket, and the core is thereafter cured.
In producing an article such as the core of the heater from a thermosetting
type material, the material is preheated to within a range so as
to soften the material sufficiently for injection into a mold. The
mold is preheated to a temperature sufficient to cure the material
after injection into the mold. The material is then injected into
the mold and the injection pressure thereafter released while keeping
the mold clamped together for a time after release of the injection
pressure, and thereafter the mold is unclamped and the article is
removed from the mold.
What I claim is:
1. A heater comprising a rigid, substantial hollow core of heat
conductive material, said core having an elongated tapered shank
and an enlarged neck portion at the large end of the shank said
core being open at the end where the enlarged neck portion is located
said neck portion having an externally threaded portion that tapers
into the shank, a jacket of electrical and heat insulating material
surrounding the core, the inside surface of said jacket being of
the same taper as the shank and externally threaded neck portion,
an electrical heating element coil secured to the inside surface
of said jacket, said element coil following the threads of the externally
threaded neck portion and along the shank of said core, and being
contiguous therewith, there being a substantial freedom of attachment
between said heating element and said core.
2. The heater of claim 1 wherein the heating element is secured
to the jacket by means of a layer of cement which bonds the heating
element to said jacket.
3. The heater of claim 1 wherein the core is made of a cured moldable
4. The heater of claim 1 wherein the core is removable from said
heating element and jacket without damaging said element.
BACKGROUND OF THE INVENTION
This invention relates to an electric heater and, specifically
of a type for producing a relatively high heat concentration in
a small volume useful for the quick sterilization of probes and
the like. For example, such electric heaters may be found in medical
laboratories where probes and the like are used for handling test
specimens and must be sterilized periodically.
By way of background, electric heaters of this general type comprise
a core made of a ceramic material that is first machined to the
desired shape, and then fired or cured. The core has a shank portion
of constant cross section and an enlarged neck portion, generally
funnel shaped, for ease in inserting a probe or the like therein
for sterilization. A wire type heating element is wound around a
portion of the neck at the location of external threads formed thereon,
and also along a substantial portion of the shank. A cement having
high temperature and good bonding characteristics is coated over
the heating element, and an outer covering of felt also having high
temperature characteristics is wrapped around the cement. Electrical
leads are provided for applying electrical power to the heating
The procedure in making the prior art heaters was to first machine
the core to the proper shape and dimensions, and then fire or cure
it. Thereafter, the heating wire was wound onto the core, a coating
of cement applied over the wire which bonded the wire to the core,
and the felt covering than wrapped over the cement, such that the
cement bonded the felt covering, wire and core together.
While these prior art heaters were operable to an extent, they
were possessed with certain problems. One such problem was that
the heating element, and thus the heater, failed sooner than was
acceptable. This is believed to have been due to expansion and contraction
of the core upon energizing and deenergizing the heating element
which, because the heating element was bonded to the core, in turn
caused expansion and contraction of the heating element and finally
Another problem with the prior art heaters was that the core had
relatively high leakage characteristics so that the user would often
receive an electrical shock if the probe he was sterilizing and
holding in his hand touched the inner surface of the core. With
heaters of this type that operated at the higher voltages, such
as 240 volts, the shock was more severe and quite dangerous.
This invention, including the heater and method of making same,
has overcome these problems to provide a heater that has exceptionally
long life, exceptionally low leakage characteristics, and is of
generally higher quality than the electric heaters of this type
found in the prior art.
Summary of the Invention
The heater of this invention generally comprises a molded ceramic
core made from a material having high temperature and low leakage
characteristics. The core has a tapered shank portion with an enlarged
neck portion at one end. An electric heating element surrounds a
portion of the core and a jacket covers the heating element. The
jacket generally comprises an outer layer of high temperature felt
insulation which is bonded to the heating element by a layer of
cement therebetween. It is an important feature of this invention
that there is an absence of chemical bonding between the heating
element and the core. In a preferred embodiment of the invention
the heating element is a wire which is wound on an externally tapered
surface of the neck portion and along a length of the tapered shank.
Suitable leads are provided for connecting a source of electrical
power to the heating element.
In the method of this invention for producing the heating element,
a suitable moldable ceramic material having the requisite high temperature
and low leakage characteristics, as well as sufficient strength
when cured, is injected into a mold for producing the core of the
required size and configuration. A heating element, preferably in
the form of a heating wire, is wound onto the core after the core
is molded but prior to curing or firing the core. Thereafter, a
coating of cement characterized as having high temperature and good
bonding characteristics is applied over the heater winding, and
a high temperature insulation felt is wrapped over the cement wherein
suitable slots are provided for allowing the passage of electrical
leads therethrough for connection to the heating element and for
applying electrical power to the heater. Next, the core is cured
by generally applying increased amounts of heat to it over a period
of time so as to change the core from a plastic to a ceramic state.
It is a primary feature of this method of the invention that the
heating wire is wound onto the core prior to curing, for it is found
that when this sequence is followed, the life of the heater is greatly
It has also been found that the proper selection of material for
the core can greatly improve the leakage characteristics, and thus
resistance to electrical shock, of the heater. It has been found
that a ceramic molding compound is preferable. Such compounds are
preferably of the thermosetting type which contain a silicone binder
and which are set in a plastic state by the application of a certain
amount of heat and later cured to a ceramic state by the gradual
application of increased amounts of heat over time. An example of
such a compound is Dow-Corning QM-9-1113.
While this material has been found to have the desirable characteristics
for the core of the heater of this invention, it has been found
that a unique molding process is required to produce cores of the
high quality required. Hence, in accordance with the process of
this invention for molding this type of material, a quantity of
the material is preheated to within a temperature range causing
it to soften or become somewhat liquid, so that the material can
be injected under pressure under a suitable mold. The mold has male
and female portions held together by clamping means or the like
and is preheated to a temperature sufficient to set the injected
The preheated material is injected into the preheated mold under
appropriate pressure. The injection pressure is maintained for a
time after the material is fully injected, at which time the injection
pressure is released but the mold held clamped together for an additional
period of time after releasing the pressure. Thereafter, the mold
is removed from the molding machine and unclamped while heat continues
to be applied to the mold for a length of time after releasing the
clamp to firmly set the material. Finally, the mold portions are
separated and the part removed from the mold.
This sequence of steps, which will be hereinafter described in
more detail, has been found to be effective in the production of
high quality parts made from the above-referenced material.
Thus, it is an object of this invention to provide an improved
heater greatly improved life and leakage characteristics.
It is another object of this invention to provide an improved method
of producing such a heater.
It is a further object of this invention to provide a process for
molding a silicone ceramic molding compound of the above type for
the production of high quality parts therefrom.
These and other objects of the invention will become apparent from
the drawing and detailed description to follow.
Description of the Drawings
FIG. 1 is a side elevation view of an electric heater of this invention;
FIG. 2 is a view in section taken generally along the line 2--2
of FIG. 1;
FIG. 3 is an enlarged view in section taken generally along the
line 3--3 of FIG. 1;
FIG. 4 is an enlarged view in section taken generally along the
line 4--4 of FIG. 1;
FIG. 5 is a side elevation view in partial section of the core
used in the electric heater of FIG. 1;
FIG. 6 illustrates generally the apparatus and method used for
winding heater wire onto the core of FIG. 5; and
FIGS. 7 and 8 illustrate the step of wrapping the wire wound core
with a covering.
Detailed Description of a Preferred Embodiment
Referring to FIGS. 1 through 5 of the drawing, there is shown an
electric heater 20 of this invention, comprising a core 22 having
a tapered shank 23 with an enlarged neck portion 24 formed at one
end. The neck portion 24 has a tapered section 25 extending from
an end ring portion 26 to the largest end of the shank 23. External
threads 28 are formed on part of the tapered neck portion 25 and
extend to the enlarged end of the shank 23. The purpose of the threads
28 is to hold a heating element wire as will be explained.
The core 22 is hollow and is of relatively thin walled construction,
for example, approximately 0.040 of an inch, for greater heat transfer
as will be explained. The core is made from a material that withstands
high temperature, in this preferred embodiment up to approximately
1800.degree.F., is a good conductor of heat, and has low electrical
current leakage characteristics. In a particularly preferred embodiment
of the invention, a moldable ceramic compound having a silicone
binder is used. An example of such a material is Dow-Corning QM-9-1113.
The core 22 is molded from such a material by an injection process
and later cured to a ceramic state as will be more clearly described.
A heating element 30, in the form of a heater wire, is wound onto
the threads 28 and along the shank 23 of the core 22. The wire 30
is of any suitable type, it exact characteristics such as gauge
and the like, and the number of turns wound onto the core, depending
on the voltage requirements of the heater.
A layer of cement 32 is applied over the heater wire 30, the cement
being of a type capable of withstanding temperatures produced during
operation of the heater, in this embodiment as much as approximately
1800.degree.F., having good bonding characteristics, and such as
not to attack the heating element. An example of such a cement is
No. 8 Sauereisen cement.
Surrounding the cement layer 32 is a jacket or covering 34 of felt
insulation. In this preferred embodiment, it too should have temperature
characteristics sufficiently high to withstand temperature of over
1800.degree.F. and should also possess good heat insulation characteristics
so as to hold the heat generated by the element 30 to within the
hollow core 22. An example of such of covering material is a ceramic
fiber insulation made by Carborundum and identified by the name
Also provided are leads 35 and 36 of high temperature wire extending
through the covering 34 to connect opposite ends of the heating
element 30 to a suitable source of electrical power.
It is an important feature of this invention that the cement 32
bonds the wire heating element 30 to the covering 34, but does not
bond the heating element to the core 22. Thus, with the core 22
in place within the covering 34, the heating element 30 is in contact
with the tapered neck portion 25 and shank 23, but is not chemically
bonded to it. In this described embodiment of the invention, the
core 22 may actually be removed by sliding it out the large end
of the covering with the heating wire 30 left embedded in the cement
layer 32 and bonded by the cement layer to the covering 34. Since
there is no bonding of the heating element to the core, expansion
and contraction of the core upon energizing and deenergizing the
heater, does not cause a corresponding expansion and contraction
of the heater wire which is believed to be a major cause of failure
of the heater wire in prior art heaters of this type. Also, the
use of the preferred core material as described above, provides
the core with very low leakage characteristics to greatly reduce
electrical shock to the user.
The method of this invention for producing the heater 20 will not
After the core material is molded and set to its plastic state
to form the core 22, a procedure that will be more fully described,
the heater wire 30 is wound onto the threads 28 of the neck portion
25 and along the tapered shank 23 by means of an apparatus of the
type shown schematically in FIG. 6.
Thus, in FIG. 6 there is shown a spindle 40 on which the enlarged
neck portion of the core 22 is mounted, the spindle 40 being rotatably
mounted on the support structure 41. A clamping device 43 holds
the core 22 in immovable relationship relative to the spindle 40.
Also rotatably mounted to the support structure 41 is a lead screw
45. Both the spindle 40 and lead screw 45 are driven by means of
a variable speed motor 47 and pulley arrangement 48, as shown. Suitable
controls, such as a foot pedal or the like (not shown) are provided
for controlling the speed of the motor and thus the speeds of the
spindle 40 and lead screw 45.
A spool 50 of heating element wire is mounted in a manner commonly
known in the art, and the wire is fed over the lead screw 45 and
into the thread of the threaded neck portion 25 nearest the large
end of the core 22. The end of the wire is angled from this thread
under the clamp 43 and past the enlarged end of the core, leaving
a pigtail 52 of suitable length. Tape 54 is used to further secure
the pigtail 52 to the enlarged end of the core. With the wire so
positioned, the motor 47 is operated to drive the spindle 40 and
feed screw 45 in a direction to wind the wire on the threads 28
and along the tapered shank 23 of the core as it moves along the
lead screw 45. The wire is wound onto the shank for a length depending
on the voltage requirements of the heater. When the wire is wound
the proper length, the other end is taped as shown at 55 and the
wire cut to leave a pigtail 57 of suitable length.
The wound core is then removed from the spindle 40 and a coating
of cement, of a type heretofore described, is applied over the wire.
After the cement had dried, wire leads 35 and 36 are then connected
to the pigtails 57 and 52, such as with crimped type connectors,
and the felt covering 34 is wrapped around the cemented core, the
pigtails 57 and 52, and portions of the leads 35 and 36, such that
end portions of the leads 35 and 36 extend outwardly therefrom for
connection to a suitable source of electric power.
After the core has been wound, cemented, and wrapped, clamps are
placed around the wrapping 34 to hold it in place while the wrap
To cure the core the leads 35 and 36 are connected to a suitable
source of supply and a gradually increasing voltage is applied to
the heating element until fully rated voltage is reached. In this
manner, the temperature of the core is increased very slowly over
time until it finally reaches its ceramic state. As the core is
heated during curing, gases are given off which must be allowed
to escape. If the core is heated too quickly during curing, the
heated gasses trapped and produce fractures in the core. With the
Dow-Corning QM-9-1113 material described above, it has been found
that proper curing is obtained where the temperature of the core
is steadily increased from room temperature to approximately 1100.degree.
F. in a 24-hour period. At the end of the 24-hour period, full rated
voltage is applied to the heating element to raise the core temperature
to the fully operating temperature of 1600.degree. F. in approximately
15 minutes. Full voltage is applied for approximately another 11/2
hours to fully cure the core.
It is a primary feature of this invention that the core is cured
after the heating wire is wound onto the core. During the curing
process, the core loosens from the cement 32 and wire 30, while
the wire remains embedded and bonded into the cement. This is believed
to be caused by a combination of shrinkage and the leaching action
of the core material, and is believed to be aided by the tapered
shape of the core. Thus, after curing, the core is no longer chemically
bonded to the wire and, in fact, can be easily removed by simply
sliding it out the enlarged end of the jacket. The advantages achieved
thereby are quite significant, for while with the prior art beaters
of this type expansion and contraction of the core during normal
operation of the heater produced expansion and contraction of the
wire heating element and finally breakage or failure of the element,
with the heater and method of this invention such expansion and
contraction of the core does not produce a corresponding effect
on the wire heating element since it is not bonded thereto, with
the result that the life of the heater is greatly increased.
The process for molding the core 22 will now be described. It has
been found that there are certain critical steps in molding high
quality parts from the material described above as being a moldable
ceramic having a silicone binder, and particularly Dow-Corning QM-9-1113.
While parts can be produced by simply injecting the material under
pressure into a suitable mold at the proper injection and setting
temperatures, the resultant parts are often poor quality, and characterized
as having rough surfaces, scratches, poor strength, fractures or
nonuniformity of material. Hence, there are certain steps which
are critical in the molding process for producing high quality parts.
In the preferred molding method of this invention, a quantity of
the material sufficient to produce the parts to be molded is placed
into the injection nozzle of a suitable injection molding machine
where it is preheated to a temperature sufficient to make the material
liquid of flowable under the injection pressure of the machine.
A suitable mold having male and female portions is provided for
receiving the injected material and for producing the desired parts.
The mold is preheated to a temperature sufficient to produce setting
of the material after injection into the mold. These temperatures
may be easily determined for a given material, and for the Dow-Corning
material described above, and the particular core configuration
of the core 22, these temperatures are approximately 160.degree.
F. and 500.degree. F., respectively.
With the male and female portions of the mold firmly clamped together
and properly positioned in the molding machine, the preheated material
is injected under suitable pressure into the mold. After injection
of the material into the mold, the material begins to expand, and
it has been found desirable in producing high quality parts to maintain
the injection pressure for a period of time after initial injection
of the material into the mold until expansion of the material is
substantially completed. In this preferred embodiment of the invention,
it has been found that the injection pressure should be maintained
for approximately 30 seconds. After release of the injection pressure,
the mold should continue to be heated at the setting temperature
and should remain clamped for an additional period of time after
release of the pressure. In this preferred embodiment in making
the core 22, it has been found that the mold should be kept clamped
for an additional 30 seconds after release of the pressure. After
this period, the mold is unclamped, removed from the molding machine,
and heated at the setting temperature for a period sufficient to
complete the setting of the material. In this preferred embodiment,
it has been found that the mold should be heated an additional 2
minutes after unclamping the mold sections. Finally, the part is
removed from the mold.
An important step in this process for producing high quality parts
is not only to hold the injection pressure for a finite period of
time until expansion of the material in the mold is substantially
completed, but also to hold the mold clamped for a finite period
of time after releasing the injection pressure and while continuing
to apply sufficient heat to produce further setting of the material
within the mold.
Although it is not completely known what occurs within the material
while the mold sections are clamped together and after the injection
pressure is released, it is believed that the clamped mold prevents
an expansion of the material while it is setting that would otherwise
weaken its molecular bond and produce parts of inferior strength.
For example, cores 22 were molded using the above recited parameters
except the mold was not held clamped a sufficient time after release
of the injection pressure. The result was that 95-98% of the parts
were rejected for poor quality. They were characterized as having
fractures, discolorations, poor finish, insufficient strength, or
they would rupture during curing.
Thus, there has been described an improved heater having exceptionally
long life and low leakage characteristics, and an improved method
of producing same.
Various changes and modifications may be made within this invention
as will be readily apparent to those skilled in the art. Such changes
and modifications are within the scope and teaching of this invention
as defined by the claims appended hereto.