1. Field of the Invention
The present invention pertains generally to the production of curved or bent sheets of glass, and more particularly to a novel continuous shaping element for a press bending mold member.
2. Description of the Prior Art
Curved or bent sheets of glass are commonly employed as glazing closures or windows in present day automotive vehicles. In order to meet the rigid quality standards of the automotive industry, it is imperative that the sheets be bent to precisely defined curvatures which are dictated by the size and shape of the opening in which they are to be mounted. The configuration of the bent sheets is determined by styling considerations in designing the vehicles. While the sheets must be bent to precisely defined curvatures, it is also imperative that the bent sheets meet stringent optical requirements. Thus, the viewing area of the unit as installed must be free from optical defects which would tend to interfere with viewing therethrough. It is therefore necessary that the apparatus employed in bending the glass sheets not only precisely shape the sheets, but also that it do so without adversely affecting their optical quality.
Over the years as automotive styling has evolved, the shapes to which the sheets must be bent have become increasingly complex and difficult to efficiently produce on a mass-production basis. Such curved or bent sheets are now generally produced by a so-called "press bending" technique wherein flat glass sheets are heated to a temperature corresponding substantially to the softening point of the glass. The heated sheets are then pressed or shaped to the desired curvature between male and female mold members having complementary shaping surfaces. The bent sheets are cooled in a controlled manner to either anneal or temper the glass as dictated by their intended end use. Such press bending may suitably be carried out with the sheets oriented vertically, horizontally or obliquely.
In order to consistently achieve a high degree of accuracy in the configuration of the curved sheets while minimizing the application of pressure to the glass surfaces in their viewing area by the press members, the male or convex mold member is generally constructed with a substantially solid or continuous shaping surface which contacts the entire surface of the sheet. The cooperating female or concave mold member is constructed with an outline or ring-type shaping surface which engages only the peripheral edge portions of the sheet.
Various materials including wood, metal and refractories have heretofore been employed in fabricating the solid or continuous shaping surface of such male mold members. Due to a number of factors including deterioration of the material and distortion of the bending surface due to repeated heating and cooling of the mold, over extended periods of use, none has proven entirely satisfactory. Thus, while metal molds are durable, they tend to become distorted and, as warping or distortion occurs, it is necessary to frequently rework the mold and restore its surface to the prescribed configuration.
Adjustable surface solid metal molds were developed in response to the distortion problem. In such molds the shaping surface comprises a continuous, relatively flexible metal plate affixed to a rigid backing structure at strategic locations over its area by adjustable length posts or adjustment screws. The contour of the metal plate can thus be readily adjusted, within limits, to maintain the prescribed configuration by appropriately manipulating the adjustment screws. In order to produce the compound curvature found in many automobile windows, that is, sheets bent about mutually perpendicular axes of bending, it is necessary that the metal sheet be preshaped or stamped to the approximate required curvature since the metal is relatively inelastic and not adapted to being formed to compound curvatures by pure bending. The surface configuration can be modified slightly by manipulation of the adjustment screws, but excessive manipulation will cause creases or ripples to form in the surface of the metal sheet.
More recently, molds having so-called flexible continuous shaping surfaces have been proposed. As disclosed in U.S. Pat. No. 4,522,641, issued June 11, 1985, one such mold utilizes a shaping element comprising a resilient body having a latticework of flexible metal bands encapsulated therein. The shaping element, in turn, is affixed to a base member by a plurality of adjusting devices located at spaced intervals along the flexible bands. The encapsulating material comprising the resilient body and providing the design surface for the shaping element may be a material such as silicone rubber or other resilient material which is flexible and able to withstand the high temperatures to which it is subjected. The glass-facing surface of the shaping element is covered by a layer of material such as fiberglass cloth to present a smooth resilient surface to the heat-softened sheet. It is disclosed that one or more layers of untreated Fiberfrax.RTM. paper may be disposed between the shaping element and fiberglass cloth to insulate the silicone rubber from the heat and smooth out any irregularities which may appear on the surface of the shaping element.
Such molds, while they represent a distinct improvement over earlier solid male molds, have certain limitations which may restrict their use under some circumstances. Thus, it has been found that different degrees of rigidity of the shaping surface may be desirable for bending different glass parts, particularly parts which are of different thickneses such as the relatively thin sheets used in laminated units and the thicker sheets used for monolithic tempered units. The resilient body forming the design surface of the shaping element may not be sufficiently rigid to impart the desired curvature to the glass sheet being pressed thereagainst by the female press ring. On the other hand, if the resilient body itself is made sufficiently rigid for some purposes, it may be unduly rigid for others. There is no provision for adapting the mold to accommodate parts requiring different degrees of rigidity.