Overflow downdraw glass tube forming apparatus

An apparatus for forming glass tubing comprising:

• an endless former and a molten glass hold,
• wherein the endless former is configured to be positioned substantially vertically and comprises a lower portion, a middle portion, an upper portion, an outer surface and an inner passage with an inner surface;
• wherein the molten glass hold has a first floor, the first floor comprises a first opening configured for positioning of the endless former therethrough;
• and a second floor spaced above the first floor, the second floor comprises a second opening configured to accommodate the endless former;
• wherein the molten glass hold is configured to engage the endless former such that the upper portion of the endless former engages the second opening of the second floor to fix the position of the endless former relative to the molten glass hold, the middle portion of the endless former extends between the first and second floors, and the lower portion of the endless former extends through and below the first floor;
• the endless former together with the first floor define a glass flow gap between the endless former and the first floor of the molten glass hold;
• wherein the first floor is configured to permit the flow of molten glass toward the endless former such that the molten glass may pass through the glass flow gap and down along the outer surface of the endless former;
• and
• wherein the second floor is in fluid communication with the inner passage, the second floor is configured to permit the flow of molten glass toward the endless former such that the molten glass may flow down along the inner surface of the inner passage to meet with the glass flow along the outer surface of the endless former to form glass tubing,
• wherein the first floor includes a first endless upward extension at the first opening, the first endless upward extension configured to impede the flow of molten glass and to cause the molten glass to flow over the first endless upper extension before passing into the glass flow gap, the first endless upward extension including a non-uniform height configured to impede the flow of molten glass to a greater extent from one direction relative to the flow of molten glass from a different direction.