1. Field of the Invention
This invention relates to the bending of glass sheets, and in particular, to a pickup and shuttle system to transfer hot glass sheets directly from within a furnace to a shaping station with minimal conveyor roll marking.
2A. Technical Consideration
Shaped and tempered glass sheets are widely used as side or rear windows in vehicles, such as automobiles and the like. To be suitable for such applications, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the frames defining the window opening. It is also important that the windows meet strident optical requirements and that the windows be free of optical defects that would tend to interfere with the clear viewing therathrough.
During fabrication, glass sheets intended for use as shaped windows in vehicle are subject to thermal treatment, to temper the glass for strengthening and increase the resistance of the shaped window to damage resulting from impact. In addition to increasing the resistance of the glass sheets to breakage, tempering also causes the glass sheet to fracture in relatively small, smooth surfaced fragments that are less injurious than the relatively large jagged fragments that result from the breakage of untempered glass.
The commercial production of shaped glass sheets for such purposes commonly includes conveying a flat glass sheet along a substantially horizontal path that extends through a tunnel-type furnace to heat the sheet to its heat softening temperature, shaping the heated glass to a desired curvature, and cooling the bent sheets in a controlled manner to a temperature below the annealing range of the glass. In prior art apparatuses, the glass sheet is lifted off a conveying surface either by bottom segmented press surfaces situated between conveying rolls, by vertically reciprocating lifting fingers, by suction from a vacuum drawing arrangement, for example, an upper mold or a vacuum pickup, or some combination of these. The glass sheet is then either pressed directly against the complementing upper mold surface, dropped onto a forming ring to shape the sheet by gravity, or transferred to a shaping station where it is deposited between mold surfaces and pressed to shape.
In these arrangements, a common problem is the marking of the heat softened glass sheet due to the glass sheet slowing or stopping on the conveying rolls prior to it being lifted off the conveying rolls so as to ensure proper alignment between the glass and the lower mold or lifting ring or vacuum pickup. The resulting optical defects which may occur are commonly referred to as roll burn, which affects those portions of the glass sheet that are in extended contact with the hot conveying rolls, and roll ripple, wherein those portions between adjacent conveying rolls begin to sag prior to the shaping operation. In addition, if the glass sheet is engaged while still moving downstream, scuffing of the glass sheet surface may occur due to the engagement of the moving glass sheet with a stationery lifting and/or forming apparatus.
It would be advantageous to develop a system whereby heat softened glass sheets may be transferred directly from the furnace to a shaping station without stopping the glass sheet so as to reduce marking.
2B. Patents of Interest
U.S. Pat. Nos. 4,282,026; 4,361,432; 4,437,871; and 4,437,872 to McMaster et al. and 4,227,908; 4,229,199; 4,229,200; 4,233,049; and 4,280,828 to Seymour each teach a drop forming apparatus wherein a hot glass sheet is engaged by stationary upper vacuum pickup positioned above the conveying means and subsequently deposited onto a contoured shaping ring. The force generated by the impact of the glass sheet on the ring provides the bending force required to shape the sheet and conform it to the contours of the ring. The pickup may reciprocate vertically to engage the glass sheet or auxiliary lifters may be positioned between conveying rolls and beneath the hot glass sheet to lift the glass sheet for engagement with a vacuum pickup. Auxiliary shapers may be used to impart additional contours in the glass sheet. The ring subsequently shuttles from its pickup transfer station to a quench unit that rapidly cools the shaped glass. Throughout the operation, the vacuum pickup remains horizontally stationary within the furnace and the glass is transferred directly to a ring mold.
U.S. Pat. No. 3,607,187 to McMaster teaches a sheet shaping method and apparatus wherein flat glass sheets are removed from a conveyor by an upper vacuum mold. The mold draws the glass sheet thereagainst to shape it. The mold thereafter moves down stream to a cooling station where it deposit the sheet on a conveyor for movement through the cooling station.
U.S. Pat. Nos. 4,221,580; 4,285,715; and 4,433,993 to Frank and 4,430,110 to Frank et al. teach a horizontal press bending operation wherein heated glass sheets enter a shaping station and are lifted off the run-in conveyor rolls by a slotted lower mold. The glass sheet is pressed between the slotted lower mold and a shaped upper vacuum mold. After pressing, the lower mold is retracted to a position beneath the run-in rolls. A shuttling tempering ring is positioned below the vacuum mold and the vacuum is released so that the shaped glass sheet is deposited onto the tempering ring. The ring subsequently transfers the shaped glass sheet to a quenching station for tempering. The upper vacuum mold can reciprocate vertically but is horizontally stationary.
U.S. Pat. No. 4,297,118 to Kellar et al. teaches a shuttling, deformable vacuum mold that engages a heated glass sheet within a heating furnace. While still in the furnace, the mold deposits the shaped glass sheet on a shuttling tempering ring that is positioned beneath the mold. After depositing the glass sheet, the vacuum mold moves to a position outside of the furnace to cool prior to reentering the furnace to engage the next glass sheet. A tempering ring transfers the glass sheet from the furnace to a quenching station to temper the glass.
U.S. Pat. No. 4,517,001 to McMaster teaches the use of a travelling vacuum holder with a downwardly facing engaging surface to lift a heated glass sheet and transfer the glass sheet onto a carrier ring mold which moves into the heating furnace. The heated glass sheet is bent under the force of gravity on the mold as it is dropped thereon. The movement of the holder may be coordinated with the movement of the glass sheet so that there is no relative movement between the holder and glass sheet as the glass sheet is received by the holder. The bent glass sheet is subsequently removed from the furnace to a quench unit to temper the bent glass sheet.
U.S. Pat. No. 4,364,766 to Nitschke teaches a control system for monitoring and controlling pairs of hot glass sheets as they are conveyed through a heating, bending and tempering operation. The glass is conveyed through a heating furnace. As the glass pairs approach an overhead vacuum pickup in the furnace, a photoelectric sensor provides a glass sensing signal to a host computer that controls the rotational velocity of different sets of furnace conveyor rolls. By controlling the roll speeds, the distance between adjacent sheets of glass pairs can be established for engagement with the vacuum pickup.
U.S. Pat. Nos. 4,666,492 and 4,662,925 to Thimons et al. U.S. Pat. Nos. 4,666,493 to Frank et al. and 4,666,496 to Fecik et al. teach a horizontal press bending arrangement wherein heat softened glass sheets are engaged by a vacuum pickup within a heating furnace and transferred betwaen a pair of vertically aligned horizontally stationary upper and lower pressing molds. The glass sheet may be deposited onto an alignment device which positions the glass sheet between the upper and lower molds prior to the upper and lower molds sandwiching the glass sheet therebetween to shape it.