What is molding process control?

When producing hot-bent glass, the temperature should be strictly controlled according to the product to be produced, and the temperature should be raised, kept constant and lowered. The general temperature control curve is as follows: 0℃ to 300℃ takes about 30min; 300℃ to 500℃ takes about 25min; 520℃ to 580℃ takes about 20min; the insulation time is 10-20min; the temperature is lowered from 580℃ to 520℃ for about 40min; 520℃ to 300℃ takes about 30min; 300℃ to room temperature takes about 50min. Strictly controlling the furnace temperature will not cause furnace explosion, or pitting or arc deformation due to excessive temperature.

(1) Initial temperature control Glass is a brittle material and will often break when heated by thermal stress. Glass is a poor conductor of heat and has a relatively slow heat transfer rate. In the initial heating process, due to the low temperature and low heat radiation, the glass surface is heated first, and then the heat is transferred downward by conduction. In this way, there is a large temperature difference in the thickness direction of the glass. In addition, the mold temperature in the furnace is low at the beginning, and the glass is often heated unevenly, causing inconsistent thermal expansion of the glass and generating stress. When the thermal stress exceeds the strength of the glass, the glass will burst. The size of the temperature difference is closely related to the initial heating speed. Within a certain range, the greater the heating speed, the greater the temperature difference. Therefore, the speed at the beginning of heating should be controlled within the limit speed at which the glass does not burst. Generally, the heating rate should be set at 180-280℃/h.

(2) Temperature control for hot bending The temperature of hot bending of glass is approximately between the softening transition point of glass (i.e., 560-620℃). The temperature of the hot bending furnace is between 650-750℃, and the furnace temperature and glass temperature differ by 90-130℃. Above the softening point of the glass, the temperature is adjusted according to the glass composition, thickness, and bending degree. The hot bending temperature of soda-lime glass is higher than that of lead glass, and the heating temperature for collapse forming is higher than that for self-weight softening bending. The holding time is 10-30 minutes, depending on the thickness of the glass and the complexity of the collapse forming. The holding time is longer if the glass is thicker or the collapsed shape is complex.
When the glass reaches the softening point, it begins to bend downward, and its shape is close to the mold’s. If the temperature is too high or the time is too long, the temperature of the glass will continue to rise and soften further. For solid molds, hot bending pitting may occur, and for hollow molds, the glass may have a drum surface, thus affecting the quality of the glass. If the heating temperature is not high enough, the glass may not be close enough to the mold, resulting in defects in the shape of the glass not conforming to the mold. In addition, during hot bending, the middle part bends quickly, which often causes the middle part to be over-bent and the two sides to be out of position. Therefore, heating must be strictly controlled.
The principle of controlling hot-bending heating is to turn off the heater corresponding to that position when the glass just sticks to the mold. For example, because the middle glass bends quickly, the mold sticking point moves from the middle to both sides, so the heaters corresponding to the mold sticking point positions should be turned off from the middle to both sides in sequence until all the glass sticks to the mold and all the heaters are turned off.

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