Gas-Fusion

 

The term Gas-Fusion™ refers to the process we use at Hextek to manufacture our glass light weight mirror blanks. Simply stated, the fabrication starts with several individual components which are fused together and subsequently pressurized to form the final structure. The glass components used to make a blank consist of tubing and plates. We use only Schott Borofloat 33® plates and Duran® tubes. The quality and consistency of this material is unmatched for borosilicate glass in these forms. A basic blank consists of an assembly of tubes (Duran) sandwiched between two face sheets (Borofloat 33). These components are loaded into a furnace and surrounded by tooling. The glass / tooling combination is heated to the point at which the glass starts to reach the plastic state. Concurrently, the tube ends create a seal by fusing to the face plates (figure 1). With additional heating, pressurized gas is introduced into all the fused tubes simultaneously via holes centered on each tube through the back plate. The net effect is like expanding soap bubbles (figure 2). When the tubes continue to expand they bump into each other and form a vertical wall that becomes a rib (figure 3). The process lends itself to forming fillets between the rib and face plate eliminating the possibility of stress-risers at that interface. The pressurization has a secondary advantage by pushing on the upper plate preventing collapse of the entire blank. This is key to the success of process is the ability to heat the glass hot enough to accomplish complete fusion. We guarantee that every Hextek blank is 100% fused and that the fusion bonds will not come apart unless under extreme force.

 

Fusion-Inflation Diagrams Large
Historically, there have been issues with poorly made sandwich style mirror blanks that “delaminate”. We have never had this problem because we use the proper combination of high temperature and pressure to accomplish the fusion. In our 25 year history, with over 600 mirrors in service there has never been any evidence of this problem. For this reason, NASA selected our 1.6 meter substrates for use to test the James Webb Space Telescope mirror segments at cryogenic temperatures (32K) suspended inverted and supported from the back plate.