Page 496 - 04. Subyek Engineering Materials - Manufacturing, Engineering and Technology SI 6th Edition - Serope Kalpakjian, Stephen Schmid (2009)
P. 496

'            ,_::f;f;          t hermoforming of thermoplastics (Section 19.6), but no pressure or


          476                   Ceramics, Glasses, and Superconductors: Processing and Equipment
                    Chapter 18
             z                                 vacuum is involved. Typical applications are dishes, sunglass lenses,
                    _‘;x:;§1

                                               mirrors for telescopes, and lighting panels.
                                                                          Glass ceramics (trade names: Pyroceram,
                                               Glass Ceramics Manufacture.
                                  Mold (Steel)
                                               Corningu/are) contain large proportions of several oxides, as noted in
                                               Section 8.5. thus, their manufacture involves a combination of the
                                               methods used for ceramics and glasses. Glass ceramics are shaped into
                  1__.»                        discrete products (such as dishes and baking pans) and then heat treated,
                                               whereby glass becomes dei/itri/‘ted (recrystallized).

          FIGURE l8.l3  Centrifugal casting of glass.  l8.3.4 Glass Fibers
          Large telescope lenses and television-tube
          funnels are made by this process. Source:  Continuous glass fibers are drawn through multiple orifices (200 to
          Courtesy of Corning Glass Works.     400 holes) in heated platinum plates at speeds as high as 500 m/s.
                                               Fibers as small as 2 /.tm in diameter can be produced by this method. In
                                    order to protect their surfaces, fibers subsequently are coated with chemicals. Short
                                   fibers (chopped) are produced by subjecting long fibers to compressed air or steam
                                    as they leave the orifice.
                                        Glass wool (short glass fibers)-used as a thermal insulating material or for
                                    acoustic insulation-is made by a centrifugal spraying process in which molten glass
                                   is ejected (spun) from a rotating head. The diameter of the fibers is typically in the
                                   range from 20 to 30 /sum.


                                    l8.4   Techniques for Strengthening and Annealing Glass

                                    Glass can be strengthened by a number of processes, and discrete glass products
                                   may be subjected to annealing and to other finishing operations to impart desired
                                   properties and surface characteristics.

                                   '!'hermal Tempering.  In this process (also called physical tempering or c/oil] temper-
                                   ing), the surfaces of the hot glass are cooled rapidly by a blast of air (Fig. 18.14).
                                   As a result, the surfaces shrink and (at first) tensile stresses develop on the surfaces.
                                                                    As the bulk of the glass begins to cool, it con-
             STQP1            Step 2                Step 3          tracts. The already solidified surfaces of the
              -------                                               glass are then forced to contract, and conse-
                                        Thickness                   quently, they develop residual compressive sur-
                         iiiiii ii'                                 glass in the same Way that they do in metals
                      T  ' ef
           ‘i`=  "i‘:‘   Surface CODIS rapiduz""“' Center Coots     face stresses, while the _interior develops tensile
                          surface contracts,
           no stresseis  Centeradjusts, 0n|y  Surfaceis Compressed’  stresses (See alSO_ Secuon 2'11)` Compress”/6
                                                centercontracls,
                                                                    surface stresses improve the strength of the
                                                center in tension
                           minor SIVGSSGS
                                                                    and other materials.
                                                                         The higher the coefficient of thermal ex-
                                                                    pansion of the glass and the lower its thermal
                        Compression
                                          Tension
                                                                    conductivity, the higher will be the level of resid-
                          W  i&FNT
                                                                    the glass becomes. Thermal tempering takes a
                                          ggir
                                                                    relatively short time (minutes) and can be applied
                                                                    to most glasses. Because of the high amount of
                              Residual stresses                     ual stresses developed, and hence, the stronger
                                                                    energy stored in residual stresses, tempered glass
          FIGURE l8.l4  (a) The stages involved in inducing compressive  shatters into a large number of pieces when bro-
          surface residual stresses for improved strength. (b) Residual stresses in  ken. The broken pieces are not as sharp and haz-
          a tempered glass plate.                                   ardous as those from ordinary window glass.
   491   492   493   494   495   496   497   498   499   500   501