In the late Neolithic age, more than 5000 years ago, ancient craftsmen widely used wax losing casting technology in the manufacture of bronzes. According to the plasticity and thermal volatility of beeswax, craftsmen at that time first carved the beeswax into a wax mold with the required shape, then wrapped the wax mold with clay and reserved a small hole, dried it, and roasted it to vaporize it. At the same time, the clay became a ceramic shell, and the inner wall of the shell left the negative mold of the wax mold. At this time, the molten metal is injected into the shell along the small hole, and the shell is broken after cooling, that is, the required metal billet is obtained.

The basic principles of modern lost wax casting technology are the same, but more complex and precise. This is mainly reflected in the stricter requirements for the accuracy of the wax mold. In modern technology, wax molds can be obtained not only by directly carving wax, but also by pressing the silica gel of the metal original mold (plate), and then injecting wax into the silica gel negative mold to obtain wax molds. The casting material is no longer clay, but cast gypsum. Such products are much more delicate than ancient castings.

The process flow of wax loss casting is: pressing the glue mold → opening the glue mold → injecting wax (mold) → trimming the wax mold → planting wax trees → pouring gypsum cylinder → gypsum vacuum → gypsum natural solidification → baking gypsum → melting gold, casting → frying gypsum → washing, pickling, cleaning → roughing. Each process is described below.

Pressing rubber mold

The glue used in making the glue mold is a raw rubber sheet with glue on both sides. The imported film is generally used. The price of one kind is relatively low. Because it contains less natural rubber components, the hardness after vulcanization is slightly higher, but the molding time is shorter, which is suitable for experienced mold openers to operate; The other one is slightly higher in price, contains more natural rubber, has a soft texture and full toughness, and is suitable for silver plate dies with complex patterns, small outline dimensions and obvious convex and concave. It is not easy to break the wax mold when taking out the wax mold.

The storage temperature of the raw film must be controlled. It can be stored for one year under 20 ℃ and 2-3 years under 0~5 ℃. In addition, there is also a rubber mold made in advance of a series of sizes and thicknesses. Its inner wall has convex and concave connecting feet, which is more convenient to use.

The equipment for pressing the rubber mold is the (vulcanization) die press. Its main components are composed of two heating plates with internal charged resistance wires and temperature sensing devices, thermostats, timers (some models do not have this device), etc. The die press is also equipped with lifting lead screws for die pressing and removal.

There is also a die frame, which can be divided into a single plate, double plate, four plate, and other models according to the number of openings. The material for manufacturing the die frame is usually aluminum alloy. Generally, the size of the die frame is 48mm × 73mm, sometimes thickened die frames are used to press larger originals, and the size is usually 64mm × 95mm.

The glue pressing die seems simple, but in fact, many details must be paid attention to. First of all, it is necessary to ensure that the die frame and raw film are clean. Before die pressing, the die frame should be cleaned as much as possible, and the operator should clean his hands and workbench; Secondly, we should ensure that there is no adhesion between the original and rubber. To do this, we should give priority to the use of silver plates. If it is a copper plate, we should silver plate the copper plate before pressing, because the copper plate is easy to adhere to rubber; Thirdly, we should pay attention to determine the appropriate vulcanization temperature and time according to the specific situation - these two not only basically conform to a certain functional relationship, but also are related to the thickness, length, and width of the rubber mold and the complexity of the original version. Generally, the molding temperature is set at about 150 ℃. If the thickness of the rubber mold is 3 layers (about 10mm), the general vulcanization time is 20-25 minutes, if it is 4 layers (about 13mm), the vulcanization time can be 30-35 minutes...

At the same time, the vulcanization temperature is also related to the complexity of the original version: if the original version is a complex and small style, the vulcanization temperature should be reduced and the vulcanization time should be extended (if some masters use the method of reducing the temperature by 10 ℃ and doubling the time). On the contrary, if the temperature is too high, it will affect the effect of the stamping.

The quality of the die also depends on the quality of the die plate. In general, first of all, the raw film should be firmly and tightly bonded. First of all, the cleanness of the raw film must be ensured. Do not directly contact the surface of the raw film with your hands. Instead, stick the raw film and then tear off the protective film on the surface of the raw film. Make sure there is no gap between the raw rubber and the original. When burying raw rubber, you should be careful, especially for some small patterns and minor pores such as the bottom hole of accessory stone inlay, you must fill them with broken small rubber particles and press them firmly with sharp objects (such as tweezers). To prevent the extruded rubber mold from being too hard, the raw rubber should be buried in the same direction.

To ensure that the rubber mold can be used for a considerable period, the rubber mold should have sufficient thickness, so a rubber mold should also be pressed with at least 3 layers of raw rubber. After the raw rubber is stacked on the original, the overall rubber mold should be slightly larger than the die frame, that is, the length and width should be slightly larger (it can be pressed into the die frame with force), and the thickness of the rubber mold should be slightly higher than the plane of the frame body by about 2mm after being pressed into the die frame.

The mold must also be preheated first, and then put into the mold frame that has been loaded with raw rubber. Tighten the handle to make the heating plate compress the mold frame. At the beginning of vulcanization, you can check whether the heating plate is compressed. After the vulcanization time is up, take out the mold quickly, and it is best to let it cool naturally (of course, you can also use cold water to shower). When it is not hot, you can use a sharp scalpel to open the mold while it is hot.

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