1. Heating temperature of glass
The bottom temperature measured by the heating system is not the roller temperature, but the average temperature after the heating element at the bottom of the tempering furnace compensates the heat absorbed by the roller glass. For this reason. The measured temperature is generally higher than the measured upper temperature, so the temperature setting of upper heating is generally higher than that of lower heating.
Different thicknesses of glass require different heating temperatures. The thinner the glass to be tempered is, the higher the heating temperature will be, and the thicker the glass is, the lower the heating temperature will be. Operators should understand the relationship between electric furnace temperature and heating time and the influence of electric furnace temperature on different thicknesses of glass. Therefore, it is impossible to clearly point out which temperature is the best for tempering glass technology, and the choice of temperature depends to a great extent on the quality of the original glass and the process parameters adjusted by operators. Tempering temperature of 5mm-15mm thick glass is between 680-720℃. In actual production, the heating temperature is controlled mainly by adjusting the heating time. Every operator should master the basic calculation method of heating time. The basic calculation method is about 40 seconds to 50 seconds per mm thick glass. If there are holes, holes and slots in the glass, the heating time should be increased by 1.
Avoid the overload phenomenon of the electric furnace. The electric furnace overload here refers not to the area occupied by glass in the electric furnace, but to the relationship between glass thickness, heating temperature and heating time. Under normal operation, there is always glass absorbing heat in the heating area of the central heating element of the electric furnace, and there is always glass in this area of the electric furnace. If the heat consumption of a certain area in the electric furnace exceeds the heating effect, the temperature in this area will begin to drop until the temperature balance occurs, so once the electric furnace appears.
Influence of tempering temperature on product quality. Improper temperature during tempering will have a series of effects on the quality of tempered glass, such as excessive glass fragments, glass bending, glass roller marks, white spots on the lower surface of glass, pockmarks on the surface of glass, and glass breakage during cooling. How to make the glass reach the required temperature quickly, we should pay attention to the following aspects in the heating process:
2. Add uniform heating.
The success of glass tempering mainly depends on the lowest temperature of the glass plate. In order to toughen glass, it must first reach the tempering temperature, so it is necessary to achieve uniform heating of all parts of the glass surface. We must understand the heating mode of glass in the electric furnace. There are three ways to heat glass in electric furnace: conduction, radiation and convection. Conductive heating is mainly the process of transferring the heat from the ceramic roller in the horizontal tempering furnace to the glass, which only occurs on the lower surface of the glass. The lower surface of the glass has one more way to obtain heat than the upper surface, which will inevitably cause the temperature difference between the upper and lower surfaces of the glass in the horizontal tempering furnace. Convection heating is mainly a heat transfer process in which hot air is blown to the glass surface by the agitation of compressed air or circulating fan in the heat balance in the electric furnace. Radiation heating refers to the heating process in which the heat generated by the electric heating wire in the tempering furnace is directly radiated to the glass surface, or the heat is transferred to the radiation plate, and then radiated to the glass to be heated. After understanding the heating mode in the furnace, we can adopt different temperature control strategies in different stages of heating in actual production. How to heat the glass as evenly as possible, we should pay attention to the following aspects:
When the glass just enters the furnace body, the original thermal balance state in the furnace body is suddenly broken, which will cause the temperature of the furnace to drop sharply, with the drop range of 20 ~ 100℃. Therefore, in the first half of glass heating, the uneven temperature rise at all points is most likely to occur. At this time, if the convection heating method is added, the upper and lower thermal balance will be opened, and the high-temperature air in the furnace will be circulated by hot air through the high-temperature air circulation system in the furnace, which can effectively solve the problem of glass. In the second half of heating, with the increase of glass temperature, the furnace temperature gradually tends to the set value, and the temperature difference at each point of glass is no longer obvious. Therefore, the heating speed should be improved, and each heat balance can be closed until the glass temperature is consistent with the set furnace temperature, and the heating process will come to an end.
When the external environment temperature of glass is the same, the contact surface area between the glass edge and the thermal medium is large, but the reflection of radiant heat is small, resulting in a relatively fast heat absorption efficiency. That is, the temperature of the glass edge rises rapidly, especially for large glass, and the temperature difference between the glass edge and the middle is more obvious. If the temperature difference between the middle and the edge of the glass is too large, it will cause the convex and concave phenomenon of the glass edge. If we encounter this phenomenon, we can control it from two aspects. On the one hand, it can increase the heat balance pressure and heat convection in the furnace, so as to adjust the heat balance in the furnace to reduce the temperature difference between the middle and the edge of large area glass, on the other hand, it can appropriately extend the heating time of glass, so as to reduce the temperature difference between the middle and the edge of glass.
It is necessary to avoid the influence of the temperature difference between the upper and lower surfaces of glass on the toughening effect of glass. When the glass enters the quenching section from the heating furnace, if the upper temperature of the glass is higher than the lower temperature, the front and rear ends of the glass will bend upward; If the temperature above the glass is lower than the temperature below, it will cause an upward bulge in the middle of the glass. Influence of the arrangement of the upper glass sheet on the heating uniformity. The glass laying arrangement of each furnace and the interval time of each furnace should be uniform during the laying. If the glass plates keep moving forward with the same laying arrangement in the tempering furnace, the temperature difference of each roller will be relatively obvious, and as a result, the glass will be deformed or broken in the cooling section when the laying position changes. In order to ensure the uniformity and consistency of the roller temperature and avoid the excessive temperature of the rollers between the glass plates, if there is a gap in the longitudinal direction during the laying, this gap should be filled when laying down a furnace of glass.
At present, most of our manufacturers use the horizontal tempering furnace to produce tempered glass, so we focus on the horizontal tempering furnace to talk about the process method of temperature control. In order to achieve qualified and high-quality tempered products, there are two basic requirements in temperature control. First, the glass must be heated to the required temperature quickly; Second, the temperature of each part of the glass surface should not be too different, that is, it should be heated evenly; The temperature control in the tempering process is carried out around these two core aspects.
