Roasting is an important step of lost wax casting process, which directly affects the quality and yield of casting jewelry.
Roasting is mainly to remove the moisture and wax in the gypsum type, and to perform thermal physicochemical reaction between the binder and the refractory material, thereby improving the high temperature mechanical properties of the mold, burning the gypsum type to a predetermined temperature, reducing The temperature difference of the metal liquid and improveing the filling ability.
If roasting temperature is not properly controlled, it will not only affect the smoothness of the surface of the jewelry, but also the crevice caused by gypsum cracking.
1. The main components of the cast powder
The materials used in lost wax casting are 25%~30% gypsum and 70%~75% quartz powder, 1% additive. Quartz powder is composed of crystalline quartz powder and cristobalite. Among them, a semi-water gypsum powder is gelling material. The quartz powder is a refractory material. By hydration-condensation of gypsum, a semi-water gypsum powder absorbs water and becomes dihydrate gypsum, so that the gypsum type forms a gypsum hardened body having a cohesive force and cohesive force, thereby giving it a certain strength. The additive is added to control the coagulability of the slurry. The fluidity wettability and the ability to be vacuumed. During the burning process, the gypsum shrinks due to water loss, and the quartz powder expands during the process. To offset the shrinkage of gypsum. Understand the physical and chemical changes in the process of burning can effectively control the temperature of the burning, to ensure the process of lost wax casting.
2. Physical and chemical changes and phase transitions in the roasting.
(1) The wax melts at 60~100 °C. At 100 °C, the water in the plaster mold becomes steam, and the temperature cannot rise sharply, otherwise it will cause the water vapor to escape and expand, causing the plaster mold to crack and Reduce the strength of the plaster mold.
(2). Gypsum shrinks due to water loss during the burning process. The water loss process is:
Monoclinic crystal orthorhombic crystal
200°CCaSO4III (soluble) 400°CCaSO4II (insoluble)
Triclinic crystal orthorhombic crystal
(3). Quartz powder will swell during the burning process, which can offset the shrinkage of gypsum.
Low temperature cristobalite 180~270 °C high temperature cristobalite, the volume of the volume reached 5.6%.
Tetragonal crystal equiaxed crystal
Low temperature cristobalite 573 ° C high temperature cristobalite, the linear expansion rate at this time is 1.4%, and this phase change will react instantaneously when the phase transition temperature is reached, the volume change is significant, reaching 24%.
(3). When the wax melts, a small amount of wax will be immersed in the gypsum type. In order to burn the residual wax into the gypsum type, it must be burned to 700 ° C or higher.
3. Temperature control of the burning
In order to prevent the gypsum type from cracking during the burning process, the heating process should be slow, especially pay attention to maintain a long time when the dehydration and phase transition temperature is reached; the temperature of the inner and outer walls of the gypsum type should be at least 30 to 60 minutes to reach the same; The physicochemical reaction is frequent at 0~400 °C, especially 0~200 °C. Considering the reaction at this stage, the phase change, slow temperature rise is very important to ensure the quality of the gypsum type. Quartz reaches the phase transition temperature at 573 °C. At the moment, all reactions will be instantaneous, and the volume change will be significant. At this time, attention should be paid to the insulation at 480 ° C or 630 ° C. In addition, considering the size of the steel crucible, the holding time can be adjusted accordingly, that is, the holding time of the steel crucible can be longer. The small holding time of steel shovel can be shorter.