Abstract: The correlation between an injection stage and an injection speed initially set in an injection condition file is graphically displayed on a display screen. A modification section is set by assigning a starting point and an end point on the graph. Then, the shape of a segment which connects the starting point and the end point is selected and assigned, out of those of a straight line and a circular arc. Thus, a section between the two points constitutes one new injection stage. In this new injection stage, moreover, the injection speed is not always a constant value, and can be optionally set so that it linearly increases or decreases or changes along a circular arc in this section.

Abstract: An alkali-resistant glass composition which comprises 40 - 55% by weight of iO.sub.2, 6 - 12% by weight of Al.sub.2 O.sub.3, 8 - 30% by weight of CaO, 3 - 6% by weight of an alkali metal oxide of Na.sub.2 O and/or K.sub.2 O, 3 - 10%, by weight of ZrO.sub.2 and 2 - 20% by weight of ZnO together with or without 2 - 7% by weight of Fe.sub.2 O.sub.3. This glass composition has especially remarkable alkali-resistance and is excellent in fusibility, workability, spinnability and devitrification-resistance.