Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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This inven~ion relates to a method of and apparatus for
enamelling wire; by 'wire' is meant any elongate electrical
conductor especially round wires and ~lat strips of copper and
aluminium up to about 10 mm2 but including also electric resist-
ance wires; and by "enamelling" is meant the application of an
adherent electrically insu~$ing thin film of curing synthetic
resin to the wire surface.
In all conventional wire enamelling techniques, the resin
is applied to the wire in solution in a volatile solvent, and the
wire with the solution thereon is passed into an oven (usually
gas fired, but sometimes electrically heated) to evaporate the
solvent and cure the resinous coating remaining.
Recently the development of 'solventless' or at least 'semi-
solventless' enamels has become a serious possibility, but use of
ovens for curing has not hitherto been questioned. (For the pur-
pose of this specification, an enamel is considered 'solventless'
if it contains no appreciable quantity of vQlatile solvent (and
the inclusion of solvents which polymerise as part o~ the curing
process is not excluded) and 'semi-solventless' i~ it contains
20 volatile solvent in an amounk insufficient to render it liquid -~
at 25C).
Such ovens have heat transfer characteristics that are not
as good as those that can be obtained with some other heat trans-
fer techniques, and it is an object of the invention to provide a
wire enamelling process utilising more efficient heat transfer
equipment, and apparatus ~or use in the method.
~ he method of the invention is applicable especially to
enamelling with solventless and semi-solventless enamels, but can
also be used with some conventional solvent-based enamels.
~0 In the method of the invention, the wire (which may be bare
or may be already coated with enamel or covered with another
insulating material) is pre-heated to an elevated temperature
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and, whilst it is hot, at least one coating o~ liquid resinous
material is applied to it. The heat content of the wire is
s~cient to eliminate any volatile solvent by flash evaporation
and to cure the enamel coating to a non-tacky condition, and the
coating is thereafter fully cured by immersion in a ~luidised
bed of hot inert particles.
Any pre-existing enamel coating may have been applied by
the method of the invention or by a conventional method.
Preferably the pre-heating of the wire is carried out also
by use of a fluidised bed of hot inert particles, but other
methods can be used, for example in the case of bare wire direct
application of a flame or direct electrical resistance heating
or induction heating.
When, as is usualJ more than one coating of enamel is
requiredJ it is preferable to apply all the coatings in a single
pass through the apparatus. Ideally all the coatings would be
applied in succession without intermediate heating, a single
fluidised-bed heating step serving to complete the cure of all
the layers, but this will not always be possible: in some cases
the pre-heating temperature required would be too high (for
instance because the wire would melt or otherwise lose strength
or because the first coating of enamel would be pyrolysed),
and in other cases volatile by-products of the curing reaction
might be prevented from escaping. When such problems are encount-
ered or an~icipatedJ they may be overcome by using two or more
pre-heating steps for individual coatings or for groups of
consecutive coatings. In this case the fluidised bed effecting
cure of one coating or groups of coatings preferably serves also
as the pre-heating means for the next coating or group. Where
an enamel coating passes ~hrough more than one fluidised heating
bed it should normally emerge from the first and any intermediate
bed less than fully cured, so as to avoid risk of degradation
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during curing of subsequent coatings.
Although in the preceding paragraph reference has been
made to two or more fludised beds, it will be understood that
equivalent results can be achieved by using a single ~luidised
bed suitably par~itioned or with sui~ably designed enamel
applica~ors immersed in it, an essential requirement being that
particles from the bed are not allowed to contact enamel that is
in a liquid or tacky condition.
The particles of the or each fluidised bed are preferably
mineral particles of high thermal capacity ; either solid
particles or hollow ones can be used. An example of a suitabl e
solid material is silica, for example in the form of fine sandJ
and an example of suitable hollow particles is the material
known as 'ballotini', consisting of small hollow glass beads.
They may be blown withair, or with a protective gas (nitro-
gen or steam for example): the air or other gas is preferably
recycled to conserve heat.
In the case of solventless enamels, it may sometimes be
possible by the method of the invention to obtain a solid film ~ -
in contact with the hot wire whilst still immersed in liquid
enamel, and so to obtain a thicker, and yet more uniform, coating
than can be obtained by conventional techniques.
The apparatus in accordance with the invention comprises
a pre-heater, at least one enamel applicator, and at least one
fluidised-bed curing chamber arranged for operation as described.
The invention will be further described, by wa~v of exampleJ
with reference to the accompanying drawings in which each of
Figures 1 - 3 illustrates a different way of carrying out the
invention.
In the apparatus shown in Figure 1, wire 1 taken from a
conventional pay-off stand or (in accordance wi~h our patent
no. 944313) direct from a low speed wire drawing machine (neither
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of which is sho~n) is first pre-heated by passage through a
heated fluidised bed 2. The hot w~re passes through a suitable
fluid-ti~ht seal ~ into a trough~o~liquid enamel 5, which is
continuously circulated from a heated reservoir 6 using a pump 7
in conjunction with a constricted return pipe 8 to maintain a
controlled level in the ~rough.
Immersed in the enamel in the trough are a series of wiping
dies 9 o~ increasing diameter which produce an effective wiping
action despite being immersed because the wire is hot enough to
ensure that the enamel film that enters a die wet reaches a set
condition before it emerges from the die. The wire passes from
the trough through a further seal 10 into a second heated fluid-
ised bed 11; a short length of wire downstream of the seal is
protected by a screen 12 to avoid contact of wet enamel film
with the fluidised bed particles.
The coated wire emerges from the fluidised bed a~ 13 and
passes to any conventional take-up mechanism (not shown).
The form of apparatus shown in Figure 2 is similar~ except
that the enamel is pumped via manifold 14 to a series of individ-
20 ual applica~ors 15 and the trough 4 serves simply to collectexcess enamel and return it to the reservoir. In this case it is
only necessary for the enamel of each coat to be set before it
reaches the next applicator (or, in the case of the last appli-
cator~ before it enters the fluidised bed 11).
In the arrangement shown in Figure 3, a series of heated
fluidised beds 16, 17 ... 17, is used: fluidised bed 16 is used
to pre-heat the wire, which is passed to an applicator 18, and
each of the fluidised beds 17 serves to re-heat the coated wire
before it enters a respective one of the further enamel applica-
tors 19 ... 19, 20. The temperatures of the beds 17 are chosento avoid complete curing of the coatings therein, and all the
coatings are cured together in the fluidised bed 11 as in the
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other examples. For simplicity, the applicators 18, 19 ... 19,20 have been shown as metering applicators fed through ducts 21,
but ot~er types of applicator can be used and provision made for
returning excess enamel if required.
If the temperatures of the fluidised beds 17 are chosen to
obtain a considerable degree of cure therein, it may be desir-
able, or even necessary, to cool the wire somewhat between the
hot fluidised beds 17 and the subsequent applicators; this can
be done (for example) by use of a cool fluidised bed, an air ~et,
or by spacing the applicators sufficiently from the outlets of
the fludised beds.
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