Apparatus and method of making filaments

An improved apparatus and method for making solid filamentary elements from a pendant drop of a high melting point material (e.g. hafnium, zirconium). The improvement constitutes providing a protective layer in the form of a thin film on the circumferential edge of the casting wheel prior to engagement of the drop and release of the elements. The layer material serves to reduce heat flow from the drop material to the wheel as well as prohibit bonding of the drop material thereto. The preferred layer materials are aluminum, carbon (e.g. graphite), boron nitride, and borosilicate glass.

BACKGROUND OF THE INVENTION 
The invention relates to the making of filaments and particularly to 
filament production wherein an elongated rod has a pendant drop formed 
thereon which is engaged by a rotating heat extractor, such as a copper 
wheel-like member (e.g. disc). 
Even more particularly, the invention relates to the above subject matter 
wherein filaments are produced from rods of high melting point material, 
e.g. those having melting points exceeding 1200.degree. Celsius. 
With regard to the present invention, the term filament is meant to define 
slender, elongated metallic elements having a transverse dimension less 
than the element's length dimension. Examples of such elements include 
sheet, ribbon, or wire. One particular use for these products is as the 
combustible fill material within a photoflash lamp such as described in 
U.S. Pat. Nos. 3,535,063 (L. F. Anderson et al), 3,897,196 (J. P. Saunders 
et al), and 4,008,040 (D. E. Murray et al), all of which are assigned to 
the assignee of the present invention. The combustible fill material 
readily ignites upon activation to provide the intense flash typically 
associated with photoflash applications. At least two methods are 
presently used to produce elongated filaments from rods of metallic 
material. The first involves forming a suspended drop on the end of a 
vertically oriented rod and engaging the drop from below with a rotating 
copper disc. Such a process is described in U.S. Pat. No. 3,896,203 
(Maringer et al). The disc has a chill surface which has a coefficient of 
thermal conductivity sufficient to withdraw heat from the molten drop in 
such rapid fashion so as to form the filamentary element on the surface of 
the drop. The filament is thereafter emitted and collected for further 
use. 
A second method of producing metallic filaments from rods involves 
formation of what is termed a sessile drop. By a sessile drop is meant one 
which is substantially upwardly projected using suitable means to engage a 
rotating casting disc located above the drop. In other words, the rod and 
disc are inverted in comparison to the arrangement used in the 
aforementioned suspended drop process. With regard to the present 
invention, the term pendant drop will be used and is meant to include both 
suspended and sessile drops. 
One particular problem which occurs in the production of filamentary 
elements from a pendant drop of molten material concerns the damage (e.g. 
pits, scratches) which occurs on the casting disc's circumferential edge. 
The resulting erosion of this surface adversely affects filament release, 
which results in discontinuity of size and surface configuration of these 
elements. Additionally, the higher the temperature of the disc's edge, the 
more rapidly this edge erodes. 
From the foregoing background, it can be understood that an apparatus and 
method which produce dimensionally consistent filaments from a pendant 
drop of high melting point material and which assure substantial 
prevention of damage to the circumferential edge of the casting wheel 
would constitute significant advancements in the art. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is, therefore, a primary object of this invention to enhance the 
production of filamentary elements by providing an apparatus and method 
which overcome the several aforementioned problems inherent in many 
apparatus and methods of the prior art. 
According to one aspect of the invention, there is defined an improved 
method of producing solid filamentary elements from a pendant drop of 
molten material. The method involves engaging the drop with the 
circumferential edge of a rotating wheel-like, heat-extracting member 
(e.g. copper disc), solidifying the molten material on the edge to form 
the filamentary elements, and thereafter releasing the elements from the 
edge. The improvement to the method comprises applying a thin protective 
layer of material to the circumferential edge prior to said engagement 
with the drop and subsequent to release of the filament, said material 
adhering during these periods of engagement and release. 
In accordance with another aspect of the invention, an improved apparatus 
for producing filamentary elements is defined. The apparatus includes 
means for heating the filament material to form a molten pendant drop, a 
wheel-like, heat-extracting member having a circumferential edge thereon 
for engaging the molten drop, and means for rotating the heat-extracting 
member to effect said engagement and the resulting solidification of 
molten material thereon and release of the formed filamentary elements 
therefrom. The improvement to the apparatus comprises means for applying a 
thin protective layer to the edge of the heat-extracting member prior to 
engagement with the molten drop and after release of the solidified 
filaments, this material adhering during the engagement and release of the 
filaments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
For a better understanding of the present invention together with other and 
further objects advantages and capabilities thereof, reference is made to 
the following disclosure and appended claims in connection with the 
above-described drawings. 
In FIG. 1 is shown an apparatus 10 for forming solid filamentary elements 
11 (one shown). The apparatus includes means 13 for heating the rodlike 
material 15 to form a pendant drop 17 thereon. Material 15 is preferably a 
high melting point (e.g. above 1200.degree. Celsius) material, two 
examples being hafnium and zirconium. Heating means 13 is preferably an 
induction coil but may instead comprise a gas torch, an electrical 
resistance heater, a focused arc, etc. 
Apparatus 10 further includes a rotating wheel-like, heat-extracting member 
19 (e.g. a copper disc) which rotates in the direction indicated as "a". 
This rotational movement is provided by a suitable drive means 21 (e.g. 
electric motor) which is operatively connected to disc 19 by a drive shaft 
23. As disc 19 rotates, the circumferential edge 25 of this member engages 
the molten pendant drop 17. Because disc 19 is cooled, part of the 
material (see No. 27 in FIG. 2) from drop 17 becomes solidified and is 
thereafter released from edge 25 in the form of filamentary element 11. 
The improvement to apparatus 10 involves providing a means 29 for applying 
a thin film protective layer 31 to edge 25 of disc 19 during the disc's 
rotation. Layer 31 is preferably of uniform thickness (e.g. 20 to 200 
nanometers thick). Examples of materials sucessfully used for layer 31 
include aluminum, carbon (e.g. graphite), boron nitride, and borosilicate 
glass. By protective is meant a material which: (1) forms a weakly bonded 
layer on the edge of the disc; (2) reduces heat flow from the molten drop 
17 to disc 19; (3) prevents bonding of the molten drop material in 
solidified form to the disc's edge 25; (4) does not alter in any manner 
the surface configuration of the solid filamentary element which would 
ordinarily result if layer 31 were omitted. In other words, layer 31 is of 
substantially the same degree of smoothness as the original surface of 
edge 25. With regard to the invention, it is preferred that layer 31 be 
applied in such a manner that it may be readily removed subsequent to the 
release of filamentary elements 11. The release point of elements 11 in 
FIG. 1 is immediately to the right of drop 17. In other words, it is 
highly preferred in the present invention that elements 11 have a 
relatively short dwell time (period of adherence to edge 25). For a disc 
having a diameter of 8 inches and a rotational speed of 500 revolutions 
per minute, a dwell time of only about 0.001 seconds is preferred. This 
period is equivalent to an arc on disc 19 having a dimension of about 0.35 
inches. A filamentary element produced under these conditions has a length 
of about 10 inches and a cross-sectional area from about 1.times.10.sup.-6 
to 1.times.10.sup.-5 square inch. 
As stated, layer 31 adheres to edge 25 during the engagement of drop 17 by 
edge 25 and the subsequent release of elements 11. As also stated, layer 
31 is applied in such a manner that it may be easily removed after the 
aforementioned release. One method of accomplishing the removal (shown in 
FIG. 1) comprises using at least one honing wheel 33 which engages edge 
25. Should disc 19 be tapered (e.g. V-shaped) and include a pair of angled 
sides (35 in FIG. 2), removal means 33 would include two honing wheels, 
one for each side 35. In this arrangement, each wheel would comprise a 
5-inch diameter abrasive (e.g. silicon carbide) disc which is glued to a 
layer of foam plastic which in turn is bonded to a steel disc. Wheels 33 
are preferably rotationally driven (e.g. by a variable speed motor 37) 
during the aforedescribed removal. The above represents only one form of 
removing layer 31 and is therefore not meant to limit the invention. In 
one trial run of apparatus 10, it was possible to remove layer 31 using 
the operator's thumb and wiping it along one of the sides 35. 
In FIGS. 3 and 4, various embodiments of layer applicators are shown. The 
embodiment of FIG. 3, which is similar to that shown in FIG. 1, comprises 
an enclosed container 39 having a removable top 41 and a slot or opening 
43 therein. A resistance heating coil 45 is located within the walls of 
container 39 and serves to heat the material 47 which will become the 
desired protective layer. Coil 45, preferably tungsten, heats material 47 
to the vapor state whereupon the vapor passes through opening 43 and 
adheres to edge 25 of disc 19. The preferred material 47 in FIG. 3 is 
aluminum. 
The embodiment of FIG. 4 comprises a container 49 in which the surface 
layer material 51, in dry powder form, is located. An auger 53, powered by 
an electric motor 55, passes through container 49 and an adjacent housing 
57 (shown partly in section). A slot 59, shaped to match the contour of 
disc 19, is formed within housing 57 and receives the powder material 51 
from auger 53. In operation, slot 59 becomes substantially filled with 
material 51 which presses firmly against the edge 25 and sides 35 of disc 
19 as the disc rotates therein (in direction "a"). The preferred material 
51 for use in FIG. 4 is boron nitride. 
There has thus been shown and described an apparatus and method for 
applying a thin film layer of material to the circumference edge of a 
rotating heat-extracting member (disc) in order to protect the edge from 
the damage typically associated with high temperature fiber casting 
operations. The layer prevents diffusion bonding of the molten material to 
the disc's surfaces in addition to providing a boundary layer to 
substantially reduce heat flow from the molten drop to the disc. The 
described layer materials as successfully used with the invention do not 
alter the surface of the formed elements in any manner and therefore 
assure a degree of smoothness on the disc's edge as originally provided. 
In the event that damage (e.g. pits, scratches, etc.) has occurred to the 
edge, the layer materials as defined are further capable of filling in 
such voids and therefore maintaining the above smoothness. It is 
understood from the foregoing description that the layer materials as 
defined will not chemically attack the resulting filaments nor will such 
materials, at the state of application, adversely affect the environment 
about apparatus 10. Accordingly, these materials are therefore 
"sacrificed" during the solidification and release stages of filament 
production and may be released as part of each filamentary element. In 
this event, a subsequent step in the process would be included to separate 
the two materials, if desired. 
While there have been shown and described what are at present considered 
the preferred embodiments of the invention, it will be obvious to those 
skilled in the art that various changes and modifications may be made 
therein without departing from the scope of the invention as defined by 
the appended claims.