The lost wax casting process commonly used in the modern jewelry manufacturing industry is developed from the ancient casting process. In the late Neolithic period more than 5,000 years ago, ancient Chinese craftsmen widely used the lost wax casting process in the manufacture of bronzes.
The development of modern lost wax casting
The basic principle of modern lost wax casting technology is no different from ancient casting technology, but it is more complicated and precise. This is mainly reflected in the stricter requirements for the precise position of the wax mold. The wax model obtained in modern technology is not only the direct engraving of the wax, but also the female mold can be obtained by molding the metal original mold (plate) with silicone, and then the wax model can be obtained after wax injection from the silicone female mold. The casting material is no longer clay, but cast plaster. Such products are much finer than ancient castings.
The essence of lost wax casting is precision casting in machining methods. The application of precision casting to the mass production of jewelry is a prominent feature of modern jewelry manufacturing. Lost wax casting in jewelry manufacturing can meet the needs of mass production, and can also take into account changes in styles or varieties, so it occupies an important position in the production methods of jewelry manufacturing. The casting methods of lost wax casting include vacuum suction casting, centrifugal casting, vacuum pressure casting and vacuum centrifugal casting, etc., which are currently the main means of mass production in the jewelry manufacturing industry.
Introduction to the Lost Wax Casting Process
The process flow of lost wax casting is: pressing the rubber mold-opening the rubber mould-injecting wax (mould)-trimming the wax mold (welding wax mould)-planting wax trees (-weighing)-pouring plaster tube- Gypsum vacuum-gypsum natural solidification-baking plaster-melting gold, casting-fried plaster-washing, pickling, cleaning (- weighing)-cutting rough (- rolling). The following describes each process separately.
Pressed rubber mold
The glue used to make the rubber mold is a raw rubber sheet with glue on both sides. The storage temperature of raw film must be controlled. Generally, it can be stored for one year under 20℃, and it can be stored for 2 to 3 years at 0~5℃.
In addition, there is also a rubber mold pre-made with a series of sizes and thicknesses. The inner wall has convex and concave connecting feet, which is more convenient to use.
The equipment for pressing the rubber mold is a (vulcanization) molding machine, and its main components are composed of two heating plates with resistance wires and temperature sensing devices, a temperature controller, and a timer (some models do not have this device). The press moulding machine is also equipped with a lifting screw for pressing and taking out.
The rubber mold may seem simple, but there are many details that must be paid attention to. First of all, it is necessary to ensure that the stamper frame and the raw film are clean. The stamper frame should be cleaned as much as possible before the stamping, and the operator should wash his hands and workbench; secondly, it is necessary to ensure that the original plate and the rubber will not stick. At one point, the silver version should be used first.
The press mold must also be preheated first, and then put into the press mold frame that has been filled with raw rubber, tighten the handle to make the heating plate press the press mold frame, check whether the heating plate is pressed tightly at the beginning of vulcanization and after the vulcanization time is up Take out the plastic mold quickly, and let it cool naturally (of course, you can also use cold water to wash it) until it is not hot, then you can open the plastic mold with a sharp scalpel while it is hot.
Injection Wax
After the rubber mold is opened, the wax injection operation can be carried out. The wax injection operation should pay attention to factors such as wax temperature, pressure and compression of the rubber mold.
The heater and temperature sensor in the wax injection machine can make the wax liquid reach and maintain a certain temperature. Generally, the temperature of the wax in the wax injection machine should be kept between 70~75°C. This temperature can ensure the fluidity of the wax liquid. If the temperature is too low, the wax liquid is not easy to fill the wax mold, causing the wax mold to be incomplete; on the contrary, if the temperature of the wax liquid is too high, it will cause the wax liquid to overflow from the gap of the rubber mold or overflow from the wax injection port, which is easy to form flash or burn a finger.
When injecting wax, you should use both hands to clamp the rubber mold in the splint (which can be a plexiglass plate or a wooden board, an aluminium plate, etc.). Pay attention to the distribution of your fingers to make the rubber mold evenly pressed; align the nozzle of the rubber mold with the wax injection nozzle parallel Push in, hold the wax injection nozzle firmly and hold your hands still, use your foot to gently press the wax injection switch and release it immediately. After holding your hands for 1 to 2 seconds, place the rubber mold for a while, and then open the rubber mold (if the rubber mold has a bottom , The bottom of the mold should be pulled out first) and take out the wax mold. After the wax mold is taken out, check it carefully. If there are serious problems such as missing edges or broken feet, such a wax mold is a waste. If there are some relatively small defects, the wax mold should be trimmed.
Trimming Wax Model
Generally speaking, the wax mold taken out after wax injection will have some problems more or less, such as flashing, multiple sides, broken claws, visible blisters, partial or overall structural deformation, small holes, and unclear flower headlines, Flowerhead and so on. Surgery blades can be used to repair defects such as flashing, multiple sides, unclear flower heads, and flower heads. For sand holes and broken claws, you can use a wax soldering device to repair; small holes can be penetrated with soldering needles; The deformation of the mold can be corrected in hot water at 40~50℃.
In addition, for rings with different hand sizes, if you wait until the model is held and then change the ring, it will undoubtedly be laborious and expensive. Therefore, the general production companies directly change the finger circle when repairing the wax mold. It is very convenient to use a wax welder to change the finger ring. After welding, use a blade to trim the weld.
Types of Wax Trees
After the wax model is trimmed, wax trees need to be planted before further operations can be carried out.
Planting wax trees is to make the wax molds in a certain order and weld them on a wax stick in layers along the circumferential direction with a wax welder, so that a wax tree with a shape similar to a big tree is finally obtained, and then the wax The tree is filled with plaster and other processes. The basic requirement for planting wax trees is that the wax patterns should be arranged in an orderly manner. The key is that the wax patterns cannot be in contact with each other. It can maintain a certain gap and weld the wax patterns to the wax tree as much as possible. The wax tree should be "planted" as many wax molds as possible to meet the needs of mass production.
The wax tree must be "planted" on a circular rubber chassis. The diameter of the rubber chassis is matched with the inner diameter of the stainless steel cylinder. Generally, the diameter of the rubber chassis is 3 inches, 3 1/2 inches and 4 inches. There is a protruding circular concave hole in the center of the chassis, the diameter of the concave hole is equal to the diameter of the wax stick of the wax tree. The first step in "planting wax trees" is to dip the head of the wax stick in some melted wax liquid, and insert it into the recessed hole of the chassis while it is hot to make the wax stick and the recessed hole firmly bond. The second step is to weld the wax pattern onto the wax stick layer by layer, starting from the bottom of the wax stick (from bottom to top) or from the head of the wax stick (from top to bottom). If the technique of "planting trees" is more proficient, there is little difference between the two methods; but generally, the method of starting from the head of the wax stick (from top to bottom) is more used, because the biggest advantage of this method is to prevent The melted wax drops onto the welded wax pattern, which can avoid unnecessary rework caused by the wax drops.
The operation process of planting wax trees should pay attention to the following problems.
1) There is generally an angle of 45° between the wax mold channel of the wax tree and the wax stick, that is, the direction of the wax mold is inclined upward. Only in this way can the golden water be smoothly injected into the plaster mold. This included angle can be adjusted appropriately according to the size and complexity of the wax mold. Small and complex wax molds can reduce the included angle; conversely, a larger wax mold can increase the included angle.
2) Before planting wax trees, the rubber chassis should be weighed first. After planting the wax tree, weigh it again. The weight of the wax tree can be obtained by subtracting the results of these two weighings. The weight of the wax tree is converted into the weight of the metal according to the ratio of the density of the paraffin wax to the cast metal, and you can estimate how much metal is needed for casting.
3) After planting the wax tree, it is necessary to check whether the wax patterns have been welded firmly. If the welding is not strong, the wax mold will fall off during plaster casting, which will affect the casting process. Finally, check whether there is enough gap between the wax patterns. If the wax patterns are stuck together, they should be separated; if there are drops of wax on the wax tree, they should be repaired with a blade.
Pour gypsum and vacuum
Put the wax tree and the chassis together with a stainless steel tube (steel bell), and wrap the single-sided tape on the outside of the steel bell (the tape should be about 20cm higher than the upper edge of the steel bell) for use.
1) Mix gypsum slurry: prepare the corresponding weight of gypsum powder and water according to the specific volume of the steel bell (if distilled water is not available, tap water is also available), the general ratio of gypsum powder and water is 2~2.5:1 (the gypsum slurry at this time) The specific gravity is about 2), which can be adjusted according to the dryness, wetness, coldness and warmth of the climate, as well as the size and complexity of the mold. The water temperature is more suitable for 20~25℃. Put the water in the mixing container, start the mixer, gradually put in the gypsum powder, stir for about 10 minutes, and then vacuum for the first time (1~2 minutes).
2) Gypsum filling: The gypsum slurry after vacuuming is poured slowly along the inner wall of the steel bell. Do not pour the gypsum slurry directly on the wax tree until the gypsum slurry is about 1cm below the wax tree, and immediately perform a second vacuum. After vacuum for 2 minutes, leave it naturally for 6-12 hours to ensure the solidification of gypsum.
Baked plaster
The baking of plaster molds is an important process to ensure normal casting. Generally speaking, the baking time of 18K gold molds is 6-12 hours, and the baking time of platinum molds is 12-20 hours. The main functions of baking are: dewaxing, drying and casting heat preservation. Take the baking of 18K gold as an example, the dewaxing temperature is 0~350℃, and the holding time is 2 hours; the drying temperature is 350~700℃, and the holding time is 2~3 hours; the casting holding temperature is 600~500℃, and the holding time is 1~ 2 hours.
The general baking process is: first preheat the resistance furnace to the starting temperature, put the gypsum mold nozzle down into the furnace so that the wax liquid flows out and evaporates; after 1 hour of constant temperature in the starting temperature zone, use 1~2 Gradually increase/decrease temperature and constant temperature at hour intervals. Note that the heating (or cooling) rate should be kept at 100~200℃/hour, otherwise the heating up too fast will easily cause cracks in the plaster mold, which may cause damage or scrapping of the plaster mold. If the temperature is too slow, it will easily cause wax or dryness Incompleteness affects the quality of castings. The baking time of the plaster mold mainly depends on the size and complexity of the golden tree, which can be adjusted according to the specific situation.
Melting gold, casting
Vacuum suction casting machine casting: When the plaster mold baking is nearing the end, it starts to melt the prepared gold and keep it in a molten state. After the plaster mold is heated, place an asbestos washer on the mouth of the suction casting machine, quickly take the steel bell out of the electric furnace, and put the nozzle upwards into the mouth of the vacuum suction casting machine to be cast, and lightly step on the suction casting plate while injecting gold water. The casting can be completed. After casting is completed, pay attention to turn on the bleed switch to deflate.
Conclusion
The lost wax casting method can be applied to the production of various styles of wax molds, especially when the decorative floral ornaments are more complicated and slender, the lines are rounded and smooth, and the original version that requires engraving and decoration can be more effective.