Process for redrawing silica glass rods

Claim 2

In a process for precision redrawing of a silica glass rod having a diameter of 0.3 to 1 inch comprising the steps of supporting the rod in a vertical position between upper and lower tension-applying feed members, applying heat to the rod in a heating zone located between said feed members to heat the glass to a drawing temperature above 3,000* F., fEeding the rod downwardly past said upper feed member toward said heating zone, and pulling the drawn portion of the rod past said lower feed member at a greater speed to draw the rod and reduce its perimeter at least 70 percent while simultaneously forming a tapered neck with a length of several inches extending downwardly from said heating zone to a horizontal plane of constant size whose location fluctuates somewhat, said glass being allowed to cool and harden as it moves down from said heating zone to said last-named plane, the improvement which comprises directing a cooling gas against lower portions of said tapered neck near said last-named plane and spaced 1 to 2 1/2 inches below the center of said heating zone to effect localized cooling of said lower portions and thereby minimize changes in the cross-sectional dimensions of the rod during the drawing operation while drawing the rod to a diameter between about 0.01 inch and about 0.1 inch.

Claim 3

A process as defined in claim 1 wherein said neck has a length up to 3 inches and the cooling gas is directed against portions of the neck located within about one-half inch of said plane of constant size.

Claim 4

A process as defined in claim 2 wherein the cooling gas engages peripheral portions of the glass rod which are spaced vertically about 1 to about 2 inches from said heating zone.

Claim 5

A process as defined in claim 1 wherein said localized cooling maintains the length of the tapered neck between about 2 inches and about 3 inches throughout the drawing operation.

Claim 6

A process as defined in claim 1 wherein said localized cooling reduces the glass temperature at least 50* F. and reduces the length of said tapered neck at least 0.3 inch.

Claim 7

A process as defined in claim 1 wherein said neck comprises a soft upper portion in which the diameter is reduced more than 70 percent and a lower congealing portion of greater axial length in which the diameter is further reduced to the final diameter at said plane of constant size and wherein the lower part of said congealing portion is subjected to said localized cooling to effect shortening of said neck.

Claim 8

A process as defined in claim 1 wherein said glass rod is hollow and said tapered neck has a length of about 1 1/2 to about 2 1/2 inches.

Claim 9

A precision redrawing process of the character described for forming small diameter glass tubing to close tolerances comprising supporting in an upright position between upper and lower tension-applying feed members a rod formed of a glass consisting essentially of vitreous silica and having a diameter of 0.3 to 1 inch, directing flames radially inwardly against the periphery of the rod in a heating zone located between said feed members to heat the glass to a drawing temperature above 3,000* F. at which it glows and is highly plastic, feeding the rod downwardly past said upper feed member toward said heating zone, and pulling the drawn portion of the rod past said lower feed member at a greater speed to draw the rod and reduce its perimeter at least 70 percent while simultaneously forming a tapered neck with a length of several inches extending downwardly from said heating zone to a horizontal plane of constant size at which the hardened glass resists further elongation, said rod being allowed to cool and harden as it moves down from said heating zone to said last-named plane and being reduced to a diameter of about 0.01 to about 0.1 inch, characterized in that a cooling gas is directed against lower peripheral portions of said tapered neck located within about half an inch of said last-named plane to effect localized cooling of said neck and to shorten the neck and maintain its length between about 2 inches and about 3 inches throughout the drawing operation.

Claim 10

A precision redrawing process of the character described for forming small diameter glass tubing to close tolerances comprising supporting in an upright position between upper and lower tension-applying feed members a rod formed of a glass consisting essentially of vitreous silica and having a diameter of 0.3 to 1 inch, directing flames radially inwardly against the periphery of the rod in a heating zone located between said feed members to heat the glass to a drawing temperature above 3,000* F. at which it glows and is highly plastic, feeding the rod downwardly past said upper feed member toward said heating zone, and pulling the drawn portion of the rod past said lower feed member at a greater speed to draw the rod and reduce its perimeter at least 70 percent while simultaneously forming a tapered neck with a length of several inches extending downwardly from said heating zone to a horizontal plane of constant size at which the hardened glass resists further elongation, said rod being allowed to cool and harden as it moves down from said heating zone to said last-named plane and being reduced to a diameter of about 0.01 to about 0.1 inch, characterized in that a cooling gas is directed against lower peripheral portions of said tapered neck located within about half an inch of said last-named plane to effect localized cooling of said neck and to shorten the neck and maintain its length between about 2 inches and about 3 inches throughout the drawing operation.

Claim 11

A precision glass redrawing process as defined in claim 8 characterized in that circumferentially spaced Jets of said cooling gas are directed radially inwardly against the periphery of said rod to shorten the neck at least 0.3 inch.