Method for forming a metal termination on a wound capacitor section

A metal termination is formed at the end of a wound capacitor section by rotating the section while spraying a molten metal. A shield plate is placed between the spray nozzle and a portion of the section end so that each incremental surface region of the section end being sprayed is shielded from the molten metal spray during part of each period of rotation. This causes a periodic cooling of each surface region of the section end during spraying so that any desired thickness of termination may be quickly deposited without risk of damaging the section end which may contain plastic or paper dielectric sheet materials that are easily deformed or dissintegrated by conventional molten metal spraying methods.

BACKGROUND OF THE INVENTION 
This invention relates to a method for forming a metal termination at the 
end of a wound capacitor section and more particularly to the metal spray 
deposition of capacitor terminals. 
It is well known to apply by spraying low melting temperature metals such 
as zinc, alloys of aluminum and solders to the ends of a wound capacitor 
section. This method is indeed the only practical termination method for 
use in some wound capacitors having metallized thermoplastics dielectric 
layers. The spray method conventionally employed is critical in that the 
molten metal particles impacting the end of such a capacitor must impact 
the exposed edges of an electrode with sufficient force and at a 
sufficient temperature to remove the oxide from the electrode edge and to 
make a sound electrical connection between the sprayed metal and the 
electrode. At the same time, the molten metal particles must not impact 
the thermoplastic dielectric material with so great a force and at so high 
a temperature that substantial physical distortion and damage occurs along 
the thermoplastic edge. It is particularly difficult to apply a thick and 
strong metal termination to such a capacitor because the temperature of 
the section end rapidly reaches destructively high values. It is 
conventional to apply a series of thin layers with long waiting times 
between layers to achieve the desired thickness and strength. 
It is therefore an object of this invention to provide a method for the 
metal spray deposition of a termination to a wound capacitor requiring a 
minimum of handling and having a minimum number of steps. 
It is a further object of this invention to provide a rapid low cost method 
for depositing a metal termination to an end of a wound capacitor section 
which section has heat sensitive dielectric layers. 
SUMMARY OF THE INVENTION 
A method for forming a metal termination at the end of a wound capacitor 
section comprises rotating the section about an axis that is essentially 
parallel to the axis of the section, placing a shield plate adjacent to 
and spaced from an end of the rotating section and spraying a molten metal 
onto the section end. The shield plate is offset with respect to the axis 
of rotation to expose each region of the section end to the sprayed metal 
for substantially less than the full period of a revolution of the 
section. The method of this invention is particularly advantageous for 
depositing a metal termination to the end of a capacitor section which 
section has two metal sheet electrodes that are spaced by and may be 
supported by thermoplastic dielectric layers. Some thermoplastic 
dielectric materials typically employed in wound capacitors are 
polypropylene, polyethyleneterephthalate, polycarbonate, polystyrene, and 
certain fluorocarbons. The alternate deposition and cooling that is 
effected in the method of this invention is capable of preventing 
substantial distortion of the thermoplastic material at the section end 
being sprayed. Such distortion and melting typically results in poor and 
inefficient electrical connection between the sprayed termination and the 
electrode and often results in a short between the adjacent of the 
electrodes. At the same time, a thick strong termination layer may be 
deposited by this novel method within a matter of seconds with a minimum 
of handling. The result is an improved quality termination having a lower 
cost of manufacturing. 
This invention recognizes the principle that during metal spray deposition, 
the more frequently the spray deposition is interrupted by a cooling 
period, the lower the average temperature of the object being sprayed 
becomes. Also accompanying this trend is a lower value of peak temperature 
that is reached by the object being sprayed. The number of alternate spray 
deposit and cooling cycles employed to deposit a given thickness of metal 
coating in a given time may readily be increased by increasing the rate of 
rotation at no additional cost.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The wound capacitor section 10 shown in FIG. 1 comprises dielectric layers 
12 and 13 having deposited thereon submicron thick aluminum films 14 and 
15, respectively. The dielectric layers 12 and 13 are polypropylene. Other 
dielectric material would also be suitable such as other plastics or 
paper. The films 14 and 15 serve as the capacitor electrodes in the 
capacitor section 10 and are positioned in the conventional "extended 
foil" manner so that each of the electrodes extends outwardly beyond the 
other at one of the section ends. This is accomplished here by providing 
unmetallized margins 16 and 17 at opposite edges of the two layers 12 and 
13, respectively. 
A spray deposited metal terminal 20, as shown partially cut away in FIG. 1, 
makes contact to electrode film 14, the cut away portion revealing section 
end 22. Another terminal 23 contacts film 15 at the opposite end of the 
wound section 10. The vertical or thickness dimensions of the various 
layers and films in FIG. 2 are greatly exaggerated to clearly show the 
construction. The plastic layers 12 and 13 are typically less than 0.001 
inches thick while electrode films 14 and 15 may extend 0.1 inches 
outwardly of each other such that the unmetallized layer margins 16 and 17 
are 0.1 inch wide. The spray deposition of terminals 20 and 23 will not 
result in a conductive metal path being formed between the two electrodes 
14 and 15 unless the hot molten metal being sprayed melts or disintegrates 
the edges of the dielectric layers. 
After the terminals have been spray deposited at both ends of the capacitor 
section, lead wires such as 24 and 25 may be soldered or welded directly 
to the sprayed terminals. The electrode films 14 and 15 are typically 
deposited by vacuum evaporation and are usually less than 1000 A in 
thickness. The vacuum deposited films are usually of aluminum or zinc. The 
terminations are usually zinc or aluminum-alloys. The method of this 
invention is also advantageously employed for forming terminations of 
wound capacitors having off set foil electrodes. Such a foil capacitor is 
disclosed in the U.S. Pat. No. 3,049,651 issued Aug. 14, 1962 and assigned 
to the same assignee as is the present invention. 
In FIG. 3 a wound capacitor section 20 is shown mounted and firmly held by 
a rotating chuck 30. Chuck jaws 31, 32 and 33 grip the capacitor 20 such 
that rotation of the chuck causes the capacitor section to rotate about 
section axis 21 in a direction as shown by arrows 34 and 35. A shield 
plate 36 is mounted adjacent to but spaced from capacitor section end 22 
and is offset with respect to the axis of rotation so as to expose only a 
portion of capacitor section end 22 to the sprayed metal 37 being 
discharged from the metal spray nozzle 38. Any of the various standard 
metal spray equipments will be suitable including those employing a plasma 
arc, flammable gas, compressed air and combinations thereof. 
The above described method of holding and rotating the capacitor section is 
particularly suitable for sections having large diameters. For example, a 
capacitor section of diameter 2.0 inches and length of 4 inches was held 
in a chuck and rotated at 200 rpm. The spray nozzle was adjusted to 
produce a 11/2 inch diameter spot at 5 inches distance from the capacitor 
section. A shield plate was employed to cover approximately half of the 
section end. The metal shield was effective in shielding about half of the 
section end at any instant of time from sprayed metal and from radiant 
heat emanating from the spray gun. Thus, each incremental region in the 
section end was alternately subject to metal spray deposition and to 
cooling. The rotation of the section in the ambient air enhances the 
cooling. 
As illustrated in FIG. 4, to achieve even more efficient cooling, the 
shield plate 36 was constructed with an inlet port 39 remote from the 
section leading to a cavity 39a having outlet ports 39b facing the section 
end 22. A cooling gas, namely air, was admitted to the inlet port under 
pressure to further accelerate the carrying away of heat from the section 
end being sprayed. A termination of about 0.015 inch thickness was 
deposited on the section end within 6 seconds. Faster cooling yet may be 
accomplished by the use of refrigerated gases, for example, gas from 
compressed liquid nitrogen or carbon dioxide. 
In an alternative embodiment of this invention a plurality of capacitor 
sections 40a through 40f are shown in FIG. 5, all mounted in a rotatable 
disk 50. The axes 41a through 41f, respectively, of the capacitor sections 
40a through 40f, are all essentially parallel to each other and to the 
axis 51 of rotation of disk 50. Note that this is similar to the above 
described embodiment wherein the axis of rotation at section 20 
corresponds to the axis 21 of the capacitor section. The direction of 
rotation is shown by arrow 54. In this way the angle of incidence of the 
sprayed molten metal upon the ends of the capacitors is essentially 
normal. A shield plate 56 is mounted in a plane orthogonal with the axis 
51 and is spaced from the ends of the capacitor sections. The lower edge 
57 (as shown) of the shield plate 56 is adjusted to approximately 
intersect the axis 51 of the rotating disk 50. In this way the end (e.g. 
42b) of each section is subjected to metal spray deposition for 
approximately half of each period of revolution and is cooled during the 
other half of each period of revolution. 
Of course, the plate may be withdrawn in a direction away from the axis 51, 
e.g. upward as shown, to achieve an increase in the exposure to spray 
deposition over the cooling that takes place during each revolution. 
Alternatively, the shield plate may be positioned over the entire surface 
of the disk 50 but may have one or more slots through which the spraying 
is effected. Thus, the various mounting positions and/or geometries of the 
shield plate permit adjustment of the spray time to cool time ratio over 
the entire range between the extremes from 1 to 0. 
The method by which a plurality of capacitor sections may be simultaneously 
sprayed as illustrated in FIG. 5 is particularly suitable for providing 
terminations to small capacitor sections such as those having a diameter 
less than 1/2 inch. The means by which the capacitor section is mounted in 
such a rotating member is not critical. It is only necessary that the 
mounting be firm enough to prevent "blowing away" the section by the force 
of the spray metal. For example, each section may be mounted as 
illustrated in FIG. 5 by wedging or force fitting the section into a hole 
in the disk 50. 
In a further embodiment (not illustrated) of the method of this invention 
both ends of one or more wound capacitor sections may be simultaneously 
terminated. For example, the apparatus of FIG. 5 may be modified by adding 
a second shield plate near the other (right hand) ends of the capacitor 
sections and mounting a second spray gun facing the other ends. The 
oppositely directed guns and shields may conventionally be positioned 
symmetrically with respect to the rotating disc 50.