Method and apparatus for drying coatings or films

A technique is disclosed for drying and curing a liquid film applied to a former which is reactive to radio frequency energy so as to generate heat. A liquid film, such as latex, is applied to a metallic former which is then disposed in the proximity of an applicator which propagates radio frequency energy. In response, by a process of induction the former generates heat and thereby advantageously dries said film in an inside-to-outside manner. A superior product is produced which is substantially free of defects and blemishes.

FIELD OF THE INVENTION 
The present invention relates generally to techniques for drying films or 
coatings, and more particularly to a technique for the efficient 
production of substantially blemish-free latex-dipped products, such as 
gloves and condoms. 
BACKGROUND OF THE INVENTION 
Conventional techniques for producing natural latex and synthetic latex 
condoms and gloves involve, generally, the steps of dipping a former into 
a vat or tub of liquid latex to apply a relatively thin film to the 
former, placing the former with the applied coating into a conventional 
oven to dry and cure the product, removing tile cured product from the 
former and repeating the process. A wide variety of additional steps may 
typically be included in the process to produce specific features or a 
higher quality product, such as, for example, multiple dipping 
applications to produce a laminated or thicker film or forming rings at 
the edges of the product. Mass production of latex products is typically 
accomplished in a batch process or by using a continuous chain or belt 
that carries a plurality of formers. The chain is continuously driven at a 
steady rate through each station where a step of the process is performed. 
The batch process uses a plurality of formers rigidly mounted on a plate, 
the plate being advanced in discrete steps. Desired qualities of such 
latex products include uniformity of thickness, purity of composition, 
tensile strength and resistance to deterioration. 
Although defects or impurities may be introduced at any step of the 
process, many product defects are caused during the drying and curing 
steps. Contamination, "mud-cracking" and "skinning oven" are examples of 
product defects that may be introduced during the conventional drying and 
curing stages of the production process. Mud-cracking, or gel-cracking is 
a product defect caused during prior art drying processes due to the fact 
that the outer surface dries first, or skins over. As drying progresses 
inwardly, the material beneath the surface dries and shrinks. The surface 
skin, which is developing, attaches to the gelling and shrinking material 
beneath the skin and is ruptured into fissures as the material beneath the 
surface skin shrinks. In accordance with the present invention, this prior 
art defect is overcome if the drying first takes place at the inner 
surface next to the former because this gelled material cannot shrink 
since its size is maintained by the former. As drying progresses 
outwardly, shrinking of gelled material is prevented by the rigid former 
and the solid material beneath the ungelled portion. 
Skinning over is another product defect caused during prior art processes 
where the outer layers of the film commence drying and form a solid 
surface that acts as a barrier which prevents moisture and other volatiles 
trapped within the inner layer from escaping as the inner portion of the 
film drys. The unwanted trapped substances result in the production of 
discontinuities that weaken and compromise the integrity of the final 
product. 
Conventional ovens, usually in the form of long ducts which form tunnels, 
typically use steam heated radiators which heat the air forced through 
tile ducts. Alternatively, hot gases of combustion from gas burners, mixed 
with incoming air, may be forced through the ducts. Sometimes radiant heat 
is used to dry and cure the product on the formers. Since drying of the 
product during such conventional prior art heating processes initiates 
with the outer surface of the film, those processes prevent evaporation of 
volatiles which are trapped beneath the outer skin of the film. In 
addition, hot air forced through the ducts carries dirt and dust which 
impinges on and blemishes the product. 
Of an entire production cycle, a relatively short amount of time is spent 
in the steps of applying the liquid latex. The majority of the cycle time 
involves drying and curing in the ovens, thereby increasing the likelihood 
that product defects will be introduced during the drying stage. For 
example, a typical mass production apparatus for producing condoms may be 
comprised of a first dipping step, a second dipping step, ring formation, 
drying and curing. The entire production cycle may take twenty-five 
minutes, and due to the general inefficiency of the ovens, the curing and 
drying steps may comprise twenty minutes or 80% of the entire cycle time. 
A great deal of the heat from the ovens is, during a mass production 
process, released into the atmosphere of the factory, creating an 
extremely uncomfortable environment for the factory personnel. In 
addition, the gases and fumes created during the drying process often 
interfere with the performance and comfort of the factory personnel. 
U.S. Pat. Nos. 5,049,323 to Giles, Jr., 5,082,436 to Choi et al., and 
5,101,085 to Minnich are generally directed to heating techniques which 
use radio frequency energy to thermostamp and cure thermoplastic 
composites. Radio frequency sensitizing agents are added to the uncured 
composite which is then heated by subjecting it to radio frequency energy. 
U.S. Pat. No. 5,116,551 to Davidson et al. involves drying of a 
liquid-containing coating by application of microwave radiation. Each of 
these prior art techniques, however, are directed to application of an 
energy source that directly and primarily affects the material of the 
product being produced. As a result, if such techniques were used to dry, 
for example, latex dipped products, the outer layer of the film would tend 
to dry first, trapping unwanted volatiles and residue beneath the dried 
outer skin of the film, resulting in several undesirable product defects. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a technique for 
drying and curing a liquid latex or other liquid film on a former which 
ensures that the innermost layer of the film drys first, thereby allowing 
volatiles ordinarily trapped within to escape through the outer undried 
portion of the film. 
It is another object of the present invention to provide a technique for 
drying and curing a liquid latex or other liquid film which rapidly drys 
or cures the film. 
It is another object of the present invention to provide a technique for 
drying and curing a liquid latex or other liquid film that results in 
relatively insignificant quantity of heat escaping into the proximate 
environment. 
It is another object of the present invention to provide such a technique 
that may be readily adapted to replace the drying and curing stations of a 
conventional mass production apparatus. 
The above and other objects and advantages are realized in accordance with 
the present invention which provides an efficient technique for drying and 
curing latex or other liquid films or coatings to produce high quality 
products and a reduction in product defects. The present invention 
represents a fundamental advance over the prior art techniques in that it 
recognizes that, in order to promote substantially inside-to-outside 
drying of a film applied to a former, the drying energy applied must be 
effective to heat the former yet substantially "transparent" or directly 
ineffective with respect to the film or coating itself. 
According to a preferred embodiment of the present invention, 
inside-to-outside drying of a film applied to a former is accomplished by 
radio frequency induction heating. The former, when subjected to radio 
frequency energy generates heat which operates to dry the film commencing 
from the innermost portion of the film adjacent the former and progressing 
toward the outer surface of the film. The technique of the present 
invention, therefore, ensures inside-to-outside drying and results in the 
efficient production of a uniform high-quality product.

DETAILED DESCRIPTION OF THE INVENTION 
Turning to FIG. 1, there is shown an apparatus 10 for drying and curing 
liquid films applied to a former in accordance with a preferred embodiment 
of the present invention. A radio frequency generator 12 is provided with 
a pair of applicator arms 14, 16 which serve to propagate radio frequency 
energy which heats the formers 18 by induction. The applicator arms 14, 
16, are comprised of a length of metallic tubing such as copper, which is 
an effective conductor of radio frequency current. 
According to a preferred embodiment of the invention, the metallic tubing 
is encased or wrapped in an electrically insulating material, such as 
plastic. Although the insulating material does not itself enhance the 
performance of the apparatus, it protects workers or operators from the 
severe electrical shock that would result if the tubing were touched while 
energized. Similarly, the insulators protect against electrical shocks or 
damage that might be caused if the conveyor mechanism or other elements of 
the system inadvertently came into contact with the tubing while it was 
energized. 
The upper 14 and lower 16 applicators are preferably arranged in a 
serpentine configuration along the length of the generator 12, and are 
spaced apart to form a channel that, as seen in FIG. 2, closely conforms 
to the periphery of the former 18 coated with a liquid film, such as 
liquid latex. The width of the channel is, at all points, greater than the 
width of the former 18 coated with film, so as to ensure that the 
applicator 14, 16, including the insulating material, does not come into 
contact with the former 18 or the applied film. 
It is preferred that the applicators 14, 16 are arranged so as to define a 
channel which relatively closely accommodates the contour of the former 
18, since the effectiveness of radio frequency energy transmitted by the 
applicators 14, 16 is a function of the distance between the former 18 and 
the applicators 14, 16. Thus, in general, the radio frequency energy 
transmitted by the applicators 14, 16 will be more effectively and 
efficiently utilized as the applicators 14, 16 are located more closely to 
the former 18. For this reason, the applicators 14, 16 as illustrated in 
FIGS. 1 and 2 are bent slightly toward each other near the chassis in 
order to more closely follow the contour of the tip of the elongated 
formers 18. 
The applicator tubing 14, 16 may be secured at both ends by a pair of 
collars 20 or sleeves which are threadedly or boltedly engaged to the 
chassis of the generator 12. The tubing 14, 16 may be cooled, particularly 
during prolonged use, by liquid circulation in a conventional manner. Such 
a cooling liquid could be circulated through a separate, preferably 
electrically insulated, tubing wrapped in a spiral manner about the length 
of the applicator tubing 14, 16. Alternatively, the cooling liquid could 
be circulated within the applicator tubing 14, 16 itself, entering at one 
end and exiting at the opposite end of the applicator tubing 14, 16. The 
liquid may be cooled so as to maintain the temperature of the tubing 14, 
16 within a predetermined acceptable range by a number of conventional 
techniques. 
As part of the process of mass production of latex articles, a train of 
formers 18 with a recently applied liquid film are paraded along the 
production line. The formers are typically attached in a conventional 
manner to an endless conveyor apparatus 22 disposed so as to pass the 
formers through the channel. The conveyor apparatus 22 may include a belt 
or track that allows rotation of the formers 18 about their longitudinal 
axis as they proceed through the channel. 
Turning now to FIG. 3, there is shown, by way of example, a cross-sectional 
view of a former 18 according to a preferred embodiment of the invention. 
The former 18 illustrated in FIG. 3 is shaped so as to provide a mold for 
a condom. According to an important aspect of the present invention, the 
core 24 of the former is composed of a substantially magnetic material, 
such as steel or iron, which will readily heat in response to the 
application of radio frequency energy. Although a wide variety of 
materials, including various alloys, may be used as the core 24 of the 
former 18, it is important that the material selected, in order to be 
effective in accordance with the present invention, readily generate heat 
when subjected to radio frequency energy. Thus, the former 18 receives 
radio frequency energy and transforms it into heat which drys the applied 
film. 
Since the former is dipped or otherwise comes into direct contact with a 
liquid film, such as liquid latex and water spray, a non-oxidizing 
platting 26 such as chromium or nickel is applied to the core 24 of the 
former 18 to prevent rust. A suitable plating 26 for the production of 
condoms is a plating 26 of chromium upon the core 24. Although the plating 
26 itself is not particularly radio frequency reactive, i.e., effective at 
generating heat in response to radio frequency energy, the plating 26 
conducts heat generated by the core 24 and is thin so that it does not 
substantially hinder heat from the core of the former being transmitted to 
the film 28 on its surface. A suitable thickness of plating 26 would be in 
the range of about 0.001 to about 0.01 inches, and would preferably be 
about 0.003 inches. 
The former 18 illustrated in FIG. 3 has been coated with a liquid film 28, 
such as liquid latex. The film 28 is typically applied to the former 18 by 
a conventional dipping process, but may be applied by any other technique 
that coats the surface of the former 18 with a liquid film 28. According 
to an important aspect of the invention, when the former 18, with an 
applied coating of a liquid film 28, is subjected to radio frequency 
energy, the film 28 is unaffected directly by the magnetic field. The 
former 18, however, and particularly the magnetic core 24, being reactive 
to the radio frequency energy, generates heat which is transmitted to the 
film 28 applied to the former 18. The heat generated by the former 18 is 
transmitted initially to the portion of the film 28a adjacent to the 
surface of the former, i.e., the "inside" of the film 28a, and then to the 
middle and eventually to the outer layer of the film 28b. Thus, by the 
present invention, the film 28 applied to the former 18 is dried in a 
substantially inside-to-outside manner, thereby allowing liquid molecules, 
gases and other undesirable by-products to escape through the outer 28b 
undried layers of the film. This inside-to-outside drying process 
represents a substantial advance over prior art techniques in that it 
overcomes the inherent deficiencies and likelihood of defects resulting 
from the conventional outside-to-inside drying process. 
Turning back to FIGS. 1 and 2, the formers 18, which have been coated with 
a liquid film 28, progress along the production line and approach the 
channel of the applicators 14,16. As the formers 18 near the vicinity of 
the channel, radio frequency energy produced by the generator 12 and 
propagated by the applicators 14, 16 is "received" by the core 24 of the 
former. In response, the core 24 begins to heat and continues to heat as 
the former 18 passes into the channel defined by the applicator arms 14, 
16. Due to the heat generated by the core 24 of the former 18, the film 28 
begins to dry, starting from the inner layers 28a in direct contact with 
the former 18 and progressing toward the outer layers 28b. During the 
drying process, volatiles and other liquids within the inner layers 28a of 
the film evaporate through the undried outer layers 28b. As a former 18 
leaves the channel, and during its passage enroute to another station in 
the production line, the film including the outer layers 28b upon the 
former 18 is substantially dried from the heat generated by the core 24 of 
the former 18 in response to the radio frequency energy. 
As should be evident, the generator will preferably include a set of 
controls, such as knobs and switches, as well as digital or deflection 
meters in order to monitor and control performance of the apparatus. For 
example, a knob may be provided to adjust the power or current through the 
applicator arms, while a meter may be provided to display the present 
power or current characteristics. Another knob and meter may be provided 
to adjust and monitor the frequency of the radio frequency energy produced 
by the generator. The invention may be practiced within a rather wide 
radio frequency range, from possibly as low as 25 Hz to as high as 50 Mhz, 
as long as the energy produced by the generator operates to heat the 
former. The invention may be practiced, for example, at a radio frequency 
of about 1 Mhz. In addition to an on-off switch, another switch may be 
provided to put the generator in a dwell mode, whereby radio frequency 
energy to the applicators is temporarily disrupted. Such a feature would 
allow an operator, for example, to adjust the applicator arms without risk 
of electrical shock or damaging the generator equipment. 
In practice it is found that when a latex film with a thickness of between 
about 0.001 to about 0.04 inches, i.e. the typical range of thickness to 
produce condoms or gloves is applied to a magnetic former, the film may be 
dried with superior results after application of radio frequency energy of 
only a few seconds. Similarly, curing of such a dried film may be 
performed in accordance with the present invention usually by subjecting 
the former to radio frequency energy for merely a few seconds. It should 
be noted that since the metallic former remains hot for some time after 
application of the radio frequency energy, drying and curing of the film 
upon the former continues after the former leaves the drying or curing 
station. Thus, the heat within the former operates to continue drying and 
curing the product. 
According to an important aspect of the invention, substantially all of the 
heat generated by the core 24 is utilized by the formers 18 to dry the 
film 28. In contrast to typical prior art techniques which result in the 
escape of substantial quantities of heat, the technique of the present 
invention does not appreciably increase the ambient temperature in the 
proximity of the generator. As a result, workers in the factory operating 
the production line can work in relative comfort at temperatures well 
within an acceptable range. Thus, the present invention provides not only 
a superior product, but does so in an energy efficient manner that allows 
relatively steady maintenance of ambient conditions, and makes feasible 
close control of ambient temperature and humidity (air conditioning) 
further enhancing the ability to produce a superior product. 
Turning now to FIG. 4, there is shown a flow diagram depicting, by way of 
example, the production cycle for a latex article utilizing the present 
invention to dry and cure the latex article at the appropriate stations. 
It should be understood that the illustrated production cycle is shown by 
way of example, and several additional production techniques and steps may 
be included and others removed, while the cycle still utilizes the 
superior drying and curing technique of the invention. As can be seen, the 
production cycle is continuous, typically using many--oftentimes 
thousands--formers which serve as a mold for the products produced. 
At the first station 30 of the production cycle, the formers, typically 
carried upon a continuous belt or similar conveyor apparatus, are cleaned 
and rinsed, removing debris or by-products from the surface of the 
formers. At the next station 32 a liquid film is applied to the formers by 
dipping them into a bath or vat of liquid, such as latex. The formers may 
be rotated to ensure that a smooth and uniform coating is applied to the 
surface of the former. 
At the next station 34, the formers are subjected to radio frequency 
energy, such as by an apparatus illustrated in FIG. 1. The formers, being 
composed of radio frequency reactive material generate heat in response to 
the radio frequency energy. The heat generated by the former is 
transmitted to and drys the liquid film, commencing with the inner layers 
and progressing toward the outer layers, thereby advantageously 
facilitating evaporation during the drying process. 
Due to the overall efficiency of the radio frequency technique of the 
present invention, the heating process may be completed in merely a few 
seconds and the former will shortly pass to the next station of the 
process cycle. In a typical mass production process cycle, formers are 
moved along on a conveyor apparatus traveling about 50 to 100 feet per 
minute. At such a conveyor speed, the applicators, in order to subject the 
formers to radio frequency energy for several seconds, would be 
approximately 3 to 10 feet in length--a substantially shorter distance 
than with a conventional oven, which would typically require the former to 
traverse a distance of more than one hundred feet before the film is dry. 
Thus, according to another advantageous aspect of the present invention, 
the drying time is substantially reduced thereby also substantially 
reducing the likelihood that product defects will be introduced at the 
drying station. 
At the next step 36 of the production cycle, the former, with a freshly 
applied and dried film, may be cooled in preparation for application of a 
second coating of film. The former is then again dipped into a bath of 
liquid at the next station 38 in order to apply a second layer of film. 
The former, with a second layer of freshly coated film, proceeds to the 
next station 40 where the former passes through another channel defined by 
applicator arms which propagate radio frequency radiation which is 
"received" by the core of the former. Again, in response to the radio 
frequency energy, the former generates heat which drys the freshly applied 
coating of film by the advantageous inside-to-outside drying technique of 
the present invention. 
After being subjected to radio frequency energy for several seconds, the 
former, with a second layer of film emerges from the channel and passes to 
the next station 42, drying further from the heat of the former while 
enroute. A ring is formed at the base of the product--a standard practice 
when producing latex condoms or gloves--by using a conventional technique 
42. 
The former, now with a dried layer of latex, is subjected again to radio 
frequency energy at the next station 44. As before, the radio frequency 
energy heats the former which results in the transmission of heat to the 
film. At this station 44, however, the additional application of heat from 
the former to the dried film serves to cure the film. As the film is 
cured, it is transformed into a relatively strong elastic product through 
molecular crosslinking and loses its tackiness, becomes relatively 
insoluble and more resistant to deteriorization. 
At the next station 46, the dried and cured product is tested, in apparatus 
which may consist of conventional high-voltage testing, or a saline bath 
test, and then the former continues to the next station 48 where the 
product leached according to conventional techniques. During leaching, 
excess unreacted chemicals are removed. 
At the next station 50 the products, still upon the formers, are sprayed, 
typically with water, in order to rinse the product. Excess water is 
removed and the product is dried at the next station 52, preferably again 
by application of radio frequency energy in accordance with the present 
invention. The former progresses to the next station 54 where the product 
is powdered, then removed from the former at the last station 56. The 
former then cycles back to the first station 30 again, in order to produce 
another new product. 
Turning now to FIG. 5, there is shown an apparatus in accordance with an 
alternative embodiment of the present invention for drying and curing 
liquid films applied to a glove-shaped former in order to produce a glove. 
A pair of applicator arms 14, 16 extending from a radio frequency 
generator (not shown) are disposed in a generally serpentine 
configuration. The applicator arms are spaced apart to define a channel 
between which formers 18 coated with a liquid film may pass. The glove or 
hand-shaped formers 18 may be supported by an endless chain or belt which 
is driven so as to convey the plurality of formers to the various stations 
of the process. 
If the glove-shaped formers 18 are rotated as they progress along the 
production line, the channel defined by the applicator arms 14, 16 should 
be wide enough to accommodate the greatest width of the glove-shaped 
former 18 after a liquid film or coating has been applied. Alternatively, 
if the former 18 is not rotated as it passes between the applicator arms 
14, 16 the channel should be wide enough to accommodate the width of the 
former 18 with a coating of liquid film when the former 18 is disposed in 
a predetermined position with respect to the applicator arms. 
According to an alternative embodiment of the invention, there is shown in 
FIG. 6 a process for drying or curing a liquid film applied to a former 18 
which is performed in a batch process. A plurality of formers 18, which 
are rigidly affixed to a platform 60, are moved and oriented along the 
production process in a plurality of discrete steps. In the illustrated 
embodiment, a set of four glove-shaped formers 18 are bolted to a platform 
60. As shown, after a coating of a liquid film is applied to the set of 
formers 18, an apparatus (not shown) which moves and positions the 
platform 60 along the production line moves the platform 60 to the drying 
or curing station. The station includes a radio frequency generator and a 
set of helical applicators 62 which propagate radio frequency energy 
produced by the generator. 
The formers 18 are composed of a core of substantially magnetic material 
which will readily heat in response to the application of radio frequency 
energy transmitted by the applicators 62. Preferably, the helical 
applicators 62 are shaped so as to rather closely follow the contour of 
the former 18 when inserted within the applicator 62. As seen in FIG. 7, 
the applicators 62 are generally oval-shaped in order to more closely 
accommodate the generally oval cross-sectional shape of the former 18. 
As the platform 60 of formers 18 approaches the drying or curing station, 
in a discrete step the platform 60, with the set of formers 18 rigidly 
attached, is positioned so as to maintain the set of formers 18 within the 
corresponding set of helical applicators 62. The formers 18 will be 
maintained in this position for a predetermined period during which radio 
frequency energy propagated by the helical applicators 62 operates to heat 
the formers 18 by radio frequency induction. Typically, sufficient heat 
will be generated by the formers 18 in response to only a few seconds of 
radio frequency energy to dry or cure the applied liquid coating. After 
the predetermined period, in another discrete step, the platform 60 is 
moved so as to remove the set of formers 18 from the corresponding set of 
helical applicators 62 and on to the next station of the production 
process. 
As can be seen from the foregoing detailed description, the present 
invention provides a unique, efficient and superior technique for drying 
and curing a liquid film applied to a former. The process is performed by 
subjecting a former with a magnetic core to radio frequency energy. In 
response, the former heats and drys the applied coating from 
inside-to-outside and if the former is subjected to additional radio 
frequency energy the dried coating will cure on the former.