Method of forming zinc collar on insulator metal cap and mold therefor

Disclosed is a method and a mold for forming a zinc collar on an insulator metal cap. The zinc collar forming mold comprises a setting section for setting the metal cap in position on the mold and an upwardly opening cavity defined around the periphery of the setting section. For molding a zinc collar, the metal cap is immersed in a molten zinc, and the metal cap is set upright on the setting section of the mold with the lower half thereof being still in the molten state. Subsequently, a molten zinc is poured from the upper opening of the mold, and upon solidification of the molten zinc, a zinc collar can be formed around the external circumference of the metal cap.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of forming a zinc collar on an insulator 
metal cap and a mold therefor. 
2. Description of the Prior Art 
Suspension insulators are generally used in the form of insulator string 
comprising a multiplicity of serially connected insulators interposed 
between transmission lines and the arms of steel towers for supporting the 
transmission lines in order to secure insulation to the earth. However, if 
the surfaces of these suspension insulators are polluted and wetted, 
leakage current flows over the ceramic surfaces of the suspension 
insulators, whereby the metal caps undergo electrolytic corrosion to cause 
thinning thereof. Accordingly, the metal caps come to have reduced 
strength and they may occasionally be damaged by the load of the 
transmission lines. 
With a view to overcoming the above problems, a suspension insulator, for 
example, of the structure shown in FIG. 4 has been proposed. This type of 
conventional suspension insulator has a pin 2 in the cavity of the head 1a 
of the insulator body 1 and fixed with a cement 3, and also has a metal 
cap 4 having a socket 4a with which a pin 2 of another insulator unit can 
be engaged is fixed with a cement 5 over the circumference of the head 1a 
of the insulator body 1, wherein a zinc collar 6 is integrally formed on 
the metal cap 4 from the lower external circumferential edge to the bottom 
for the purpose of preventing such electrolytic corrosion of the metal cap 
4. 
In forming such zinc collar 6, the following method has conventionally been 
employed, wherein a metal cap 4 molded through casting of a metallic 
material such as iron is subjected to pretreatment (degreasing and acid 
washing) and then to galvanizing, followed by solidification of the thus 
deposited molten zinc with water cooling. The thus treated metal cap 4 is 
then dipped upright in a molten zinc 11 as shown in FIG. 5 so that 
approximately the lower half of the entire cap height may be immersed in 
the molten zinc 11, and removed therefrom to allow approximately the lower 
half of the metal cap 4 may be soaked with the molten zinc. Subsequently, 
as shown in FIG. 6, the metal cap 4 is set on a preheated mold 12 which 
can be separated into halves. A molten zinc 13 is poured from a sprue 12b 
of the mold 12, which passes through a gate 12c and flows into a zinc 
collar molding cavity 12a, followed by solidification of the molten zinc 
13 to form a zinc collar 6 on the metal cap 4 from the lower external 
circumferential edge to the bottom. 
Nevertheless, in the above conventional zinc collar forming method, the 
mold requires a high-accuracy approaching/separating mechanism, since the 
zinc collar 6 is designed to be formed using a pair of separable die 
halves, so that the mold assembly comes to have an extremely complicated 
structure. Moreover, since when the metal cap is released from the mold, 
the solidified zinc is snatched off at the gate 12c, burrs are formed on 
the zinc collar surface along the gate 12c, requiring intricate procedures 
such as deburring and subsequent finish polishing. Further, the molten 
zinc 13 also stays in the sprue 12b and the gate 12c, extra amount of zinc 
must be used. For such reasons, production costs inevitably jump up 
disadvantageously. 
In the conventional molding method, the zinc collar molding cavity 12a of 
the mold 12 has a closed structure, so that the solidification of the 
molten zinc 13 poured into the cavity 12a proceeds from the external and 
internal circumferential surfaces of the zinc collar 6 toward the internal 
portion thereof. Thus, voids (micro-pores) are liable to be formed in the 
internal portion of the zinc collar 6 and products can be formed in very 
low yield, disadvantageously. 
SUMMARY OF THE INVENTION 
This invention has been accomplished in view of such problems inherent in 
the prior art, and one object of this invention is to provide a method of 
forming a zinc collar on the insulator metal cap which uses a simplified 
mold structure without requiring any high-accuracy approaching/separating 
mechanism for the mold. 
Another object of this invention is to provide a method of forming a zinc 
collar, which can not only obviate intricate procedures of deburring and 
subsequent finish polishing since no burring which may otherwise be caused 
due to the presence of gate occurs on the surface of the zinc collar, but 
also minimize the amount of molten zinc. 
Still another object of this invention is to provide a method of forming a 
zinc collar which assures prevention of void forming in the internal 
portion of the zinc collar by allowing the molten zinc to solidify from 
the lower portion of the mold upward. 
Further object of this invention is to provide a mold having a simple 
structure suitable for forming a zinc collar on the insulator metal cap. 
According to the method of forming a zinc collar on an insulator metal cap 
of this invention, a galvanized insulator metal cap, in order to attain 
the above objects, is set upright on a preheated top pouring type mold 
with the molten zinc substantially on the lower half surface of the metal 
cap being maintained in the molten state, and a molten zinc is poured from 
the top opening of the mold into the zinc collar molding cavity, followed 
by solidification of the molten zinc, whereby a zinc collar can integrally 
be formed from the lower external circumferential edge to the bottom of 
the metal cap. 
Further, in the mold for forming such zinc collar on an insulator metal 
cap, a setting section is defined for fitting the metal cap upright onto 
the center of the upper mold body surface, and a zinc collar molding 
cavity opening upward is defined on the upper surface of the mold body 
around the periphery of the setting section. 
The objects and features of the invention may be understood with reference 
to the following detailed description of illustrative embodiments of the 
invention, taken together with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention will be described below by way of preferred embodiments. A 
first embodiment of the method of forming a zinc collar on an insulator 
metal cap and a mold to be used therefor of this invention will now be 
detailed referring to FIGS. 1 and 2. 
In the first embodiment, a metal cap 4 having been formed by casting and 
subjected to pretreatment in the same manner as described in the prior art 
method before formation of a zinc collar 6 is first immersed in a molten 
zinc heated to about 440.degree. to 500.degree. C. to effect galvanizing. 
Subsequently, the thus treated metal cap 4 is removed from the molten zinc 
and immersed in about 10.degree. to 70.degree. C. water to cool and 
solidify the molten zinc formed on the metal cap surface. Next, the thus 
galvanized metal cap 4 is again immersed upright with the socket 4a facing 
upward in a molten zinc 11 heated to about 450.degree. to 650.degree. C. 
substantially in the same manner as in the conventional method as shown in 
FIG. 5 so that approximately the half of the entire height of the cap may 
be immersed in the molten zinc 11 to heat the immersed portion 
approximately to the same temperature. 
Subsequently, the metal cap 4 is removed from the molten zinc 11 and then 
set on a top pouring type mold 16, with the molten zinc 11 substantially 
on the lower half of the metal cap 4 being still in the molten state, as 
shown in FIG. 1, followed by formation of the zinc collar 6 on the metal 
cap 4. 
Now, referring to the structure of the mold 16, the mold 16 has a 
block-shaped mold body 17, and a cylindrical protrusion 17a is defined at 
the center of the upper surface thereof with a step-form setting section 
17b for fitting the metal cap 4 upright in position being defined around 
the periphery of the protrusion 17a. The setting section 17b has a 
horizontal supporting surface 21 for supporting the bottom of the metal 
cap 4 and a vertical control surface 22 which engages with the internal 
circumferential surface of the lower opening of the metal cap 4 and 
controls horizontal shifting of the metal cap 4. 
An annular zinc collar molding cavity 17c opening upward is also defined on 
the upper surface of the mold body 17 around the periphery of the setting 
section 17b. 
For forming the zinc collar 6, the mold 16 is heated to about 50.degree. to 
300.degree. C., and the metal cap 4 is set upright on the setting section 
17b of the mold body 17, as shown in FIG. 1, wherein the bottom of the 
metal cap 4 is placed on the supporting surface 21 and the internal 
circumferential surface of the lower opening of the metal cap 4 engages 
with the control surface 22, and thus the entire metal cap 4 is placed in 
position. 
In this state, a predetermined amount of molten zinc 13 is poured from the 
upper opening of the mold 17 into the zinc collar molding cavity 17c. The 
process of molding the zinc collar 6 is completed simply by releasing the 
metal cap 4 from the mold 16 after the molten zinc 13 in the cavity 17c is 
solidified. Thus, the annular zinc collar 6 can integrally be formed on 
the metal cap 4 from the lower circumferential edge to the bottom thereof 
as shown in FIG. 2. 
In the first embodiment of forming the zinc collar 6, the zinc collar 
molding cavity 17c defined in the mold body 17 is opening upward, so that 
the molten zinc 13 poured into the cavity 17c starts to solidify from the 
portion on the bottom of the cavity 17c gradually upward and finally to 
the uppermost portion of the zinc collar 6. Accordingly, no voids will be 
formed in the internal portion of the zinc collar 6, and yield of products 
can be improved. Moreover, since the molten zinc 13 is poured onto the 
metal cap 4 when the molten zinc 11 layer formed on the external surface 
of the metal cap 4 is still in the molten state, the bond strength at the 
interface between the zinc collar 6 and the metal cap 4 can be improved. 
Further, since the metal cap 4 is heated to a temperature usually higher 
than that of the mold 16, the zinc collar 6 comes to have a smooth upper 
surface 6a corresponding to the temperature gradient therebetween. 
Besides, no burring occurs that the conventional method using a mold 
having a gate suffers, so that intricate procedures such as deburring and 
finish surface polishing are not necessary. Compared with the conventional 
method, the amount of the molten zinc 13 to be used for the molding can be 
reduced to greatly lower the production cost. 
Next, a second embodiment of the present method of forming a zinc collar on 
an insulator metal cap will be described referring to FIGS. 1 and 3. 
In the second embodiment, a metal cap 4 having been formed by casting and 
subjected to pretreatment in the same manner as in the first embodiment is 
first immersed in a molten zinc heated to about 440.degree. to 500.degree. 
C. to effect galvanizing. Subsequently, the thus treated metal cap 4 is 
removed from the molten zinc, and thus the surface of the metal cap 4 is 
entirely soaked with the molten zinc. Next, unlike in the first 
embodiment, the thus treated metal cap 4 is inverted and immersed in about 
10.degree. to 70.degree. C. water 18 in such a way that substantially the 
upper half including the socket 4a of the metal cap 4 may be immersed in 
water 18, followed by cooling and solidification of the molten zinc 
substantially on the upper half surface. With the molten zinc 
substantially on the lower half of the metal cap 4 being still in the 
molten state, the metal cap 4 is removed from the water 18. 
Then, in the same manner as in the first embodiment, the top pouring type 
mold 16 as shown in FIG. 1 is preheated and the metal cap 4 is set upright 
at the setting section 17b defined on the mold 16. A molten zinc 13 is 
poured from the upper opening of the mold 16 into the zinc collar molding 
cavity 17c to integrally form a zinc collar 6 on the metal cap 4 from the 
lower external circumferential edge to the bottom. Accordingly, in the 
second embodiment, like in the first embodiment, no voids will be formed 
in the internal portion of the zinc collar 6, and thus yield of products 
can be improved. Besides, intricate processing such as debur ring and 
finish surface polishing are not necessary, unlike the conventional method 
using a mold having a gate, and the amount of the molten zinc 13 to be 
used for the molding can be reduced to greatly lower the production cost. 
Further, to summarize the second embodiment of forming a zinc collar, a 
metal cap 4 is first galvanized, and then the molten zinc substantially on 
the upper half of the metal cap 4 is solidified. With the molten zinc on 
the lower half of the metal cap 4 being still in the molten state, and in 
this state a zinc collar 6 is formed on the lower circumferential portion 
of the metal cap 4. Accordingly, compared with the first embodiment of 
forming a zinc collar wherein a metal cap 4 is first galvanized; the 
molten zinc thus deposited on the entire surface is solidified by cooling; 
substantially the lower half of the thus treated metal cap 4 is again 
immersed in a molten zinc; and with the lower half being soaked with the 
molten zinc, a zinc collar 6 is formed along the lower circumferential 
edge of the metal cap 4, the second embodiment uses a simplified process 
for forming a zinc collar 6 and can further improve productivity. 
In the second embodiment, since the molten zinc is solidified using water, 
formation of alloy layer at the interface between the material of the 
metal cap and zinc can be inhibited, whereby not only the bond strength 
between the metal cap 4 and the zinc collar forming molten zinc can be 
enhanced but also the metal cap 4 can be handled with ease. The mold 16 
for forming a zinc collar used in the above embodiments have a very simple 
structure, since the setting section 17b for setting the metal cap 4 in 
position and the zinc collar molding cavity 17c are defined on the upper 
surface of the single mold 17, and the mold requires no high-accuracy 
approaching/separating mechanism unlike in the conventional method where a 
pair of die halves are used. Accordingly, the mold constitution can be 
simplified. 
While the invention has been particularly shown and described in reference 
to preferred embodiments thereof, it will be understood by those skilled 
in the art that changes in form and details may be made therein without 
departing from the spirit and scope of the invention.