Sintering is an important link of the lost wax casting process, directly affect the quality and yield of casting jewelry. Sintering is mainly in order to remove the plaster in the water and wax, so that the binder. The heat physical and chemical reaction between the high-temperature refractory material, thereby improving the mechanical propertiesof casting, and
The plaster burn to a predetermined temperature, reduce the difference in temperature and liquid metal, improve the filling ability. If the firing temperature controlis undeserved, will not only affect the jewelry surface smoothness, will appear byplaster cracking caused by the flash
A main component of casting powder
Use wax casting material is 25%~30% gypsum and 70%~75% quartz powder, 1%additive. Quartz powder by ordinary quartz and cristobalite. Among them a semihydrated gypsum powder as cementitious material, quartz powder as refractory materials. The hydration condensation, a semi hydrated gypsum powder absorbent
A two water gypsum, anhydrite gypsum formation with the adhesive force and cohesion, so it has a certain strength. Additives for condensation control paste.Liquidity wetting and improve the ability of the vacuum. In burned, gypsumdehydration
And contraction, quartz powder in the process expands, can offset the gypsum contraction. Understand sintering process of physical and chemical changes can effectively control the sintering temperature, to ensure that the loss process casting
Two. And the transformation and physical and chemical changes in the burning
1 wax melting at 60~100 ℃. At 100 ℃, plaster mold of water into steam to escape, thetemperature should not be too fast, otherwise it will cause steam to escape theexpansion, cracking caused by plaster mold and reduce the plaster mold strength
2 gypsum in sintering shrinkage due to dehydration process. The dehydrationprocess:
CaSO4.2H2O120 ℃ CaSO4.1/2H2O
The monoclinic crystal orthorhombic crystals
200 ℃ CaSO4 Ⅲ (soluble) 400 ℃ CaSO4 Ⅱ (insoluble)
The three oblique crystal crystal
The expansion will be 3 quartz powder in burned, can counteract the gypsumcontraction.
Low temperature 180~270 ℃ high temperature quartz quartz, the volume of thevolume reached 5.6%
Tetragonal crystal equiaxed crystal
The high temperature of 573 ℃ low temperature quartz quartz, rate of 1.4% expansionat the line, and the phase transition in three to six party crystal crystal transformation temperature
Would be an instant reaction degree, volume change significantly, up to 24%
3 the wax, wax dipped into the plaster mold will be small. Burn out to overflow into theplaster of residual wax, to be burned at above 700 ℃
Three temperature control burned
In order to prevent gypsum in sintering process of cracking, the heating process to be slow, with particular attention to should be kept longer to dehydration and phase transition temperature of the time; both inside and outside the wall plaster moldtemperature of at least 30 to 60 minutes to reach consensus; in the physical and chemical reaction of 0~400 ℃ frequently, especially
Is 0~200 ℃. Considering this stage of the reaction. The transition, slow heating is very important for ensuring the quality of gypsum type. Be instantaneous reaction of allquartz to phase transition at the temperature of 573 ℃, the volume changesignificantly, it should be noted at this time at 480 ℃ or 630 ℃ heat. In addition, taking into account the large steel cup
Small, holding time can be adjusted, namely steel cup large holding time can be longer, the holding time of small steel cup can be shorter