Different industries have different requirements for precision castings. In order to meet the needs, precision casting technology demonstrates its wide adaptability, such as the diversity of materials, the complexity of shapes, the batchness of production, the accuracy of dimensions, In terms of uniformity, stability and surface accuracy, there are new developments and improvements in these characteristics today. For example, a precision casting with a weight of 40g stainless steel and a length of 55mm, a width of 3216mm and a thickness of 13mm is required:

1 as-cast use, free machining;

2 The roundness and surface roughness of the shaft hole on the casting must meet the requirements of the Japanese Industrial Standards (J IS);

The 4 holes around the 3 shaft holes are matched with the other parts, and the relevant dimensions are not only required to meet the requirements of J IS, but also the castings are not allowed to flex and deform;

4 The minimum dimensional tolerance of the casting is ±012mm. 5 The depth of the cast characters, symbols, etc. is 013mm, and the lines are required to be clear;

The minimum dimensional tolerance between the 6 planes is 011mm

7 Specially specified at the end of the wing, fillet, surface roughness, shrinkage, deformation, etc., are all performed according to the production unit standard. As an example of precision castings, you can feel the demands of today's industrial production for precision castings and the level that producers can achieve. From the modern investment casting process, it is not easy to achieve the above requirements. Therefore, it is very difficult to achieve the above objectives without strict management, correct operation, and coordination between each process, each link, and each post. This article only discusses the issues related to wax mold manufacturing.

First, the wax

Wax is the first step in the production of precision casting. It is well known that due to the development of casting production, the requirements for wax materials are also diverse. The easiest way for producers is to choose from commercially available waxes, or they can be prepared on their own, or they can be commissioned to produce on demand. But no matter what kind of wax, the required properties are the same, namely: melting point, freezing point, softening point, thermal conductivity, fluidity, shrinkage, shrinkage, strength, hardness, toughness, release property, coating property, Recyclability, dimensional stability, and residual ash content after high temperature burning. On the basis of the specific casting production, we choose a kind of wax material, which requires the wax material to have certain characteristics in some aspects, and it is also important to consider it from the cost. In order to meet the specific requirements of various castings, waxes are classified into liquid, semi-liquid, semi-solid, solid; filled and unfilled; water-soluble and water-insoluble; regenerated and non-recycled; wax For molds, for casting systems, for bonding, for repairing, for sealing, for resin waxes, etc. From the wax mold wax, the variety is also extremely large, and continues to develop. Filled wax (FILLER WAX) is widely used. Commonly used fillers are powdered polyethylene, polystyrene, organic acids, fatty acids and starch. It is added in an amount of 30% to 45% of the total amount. This kind of wax has good heat preservation property, that is, it can be formed at a lower temperature, and its fluidity is good, and the wax mold detail can be clearly reproduced, for example, it is hardened. The styrene, the addition of isophthalic acid and the three waxes with the trade name P2 FILLER are reduced from 104 °C to 49 °C for 59 seconds, 62 seconds and 99 seconds, indicating the type of filler. The effect of wax properties is greater. The shrinkage of this type of wax is less than 5% less than that of the unfilled material. A problem closely related to the performance of the wax is the effect of the preparation, use and treatment of the wax on the properties of the wax.

1. Prevent the wax from overheating during the melting process

(1) Using an indirectly heated oil bath furnace and forcibly circulating the heat transfer oil, the melting temperature is controlled within the range of 84 to 98 ° C, and it is always in agitated state, in addition to uniform temperature, the wax containing the filler There is also the effect of preventing precipitation. Too high a temperature and local overheating may deteriorate the performance of the wax due to problems such as wax oxidation and carbonization. The phenomenon of overheating is that the color of the wax changes, and the black spots on the inner wall of the wax barrel are translucent due to the separation of the filler, and smoke or vapor is generated in the wax barrel.

(2) When the wax is reused, in order to remove impurities and moisture, it must be heated above the melting point of the wax, usually below 120 °C. In order to ensure that the wax has a certain flow capacity to remove impurities, the moisture content is also It will evaporate due to the agitation of the agitator.

(3) For the wax rod used in the insulation of the insulation tube, the heating temperature is only required to be 2 to 3 °C higher than the injection temperature.

(4) When the wax is removed in the autoclave (pressure tank), the steam temperature can be as high as 150 °C or more. This condition is the main link that causes the wax performance to deteriorate. Therefore, try to reduce the dewaxing temperature and reduce the dewaxing pressure. To maintain the performance of the wax.

2, maintain the ambient temperature

In the wax mold manufacturing room, whether in winter or summer, the indoor temperature should be kept within 20 to 25, so that the size of the wax mold is in a stable environment, and at the same time, it is matched with the environmental conditions of the slurry shelling process to ensure The quality of the shell.

(1), heating and insulation of wax

The wax enters the compact from the wax drum under pressure and cools and solidifies in the mold. In this process, the temperature difference is a major factor affecting the shape, size and stability of the wax pattern. Therefore, when the wax is heated by the wax barrel, the uniformity of the temperature of the wax barrel is extremely important, and the thermal conductivity of the wax is poor. It is difficult to make the wax uniform by the heat of the wax barrel, and the temperature is uneven. The wax barrel is produced without a sound wax pattern. For this reason, the wax drum is usually heated and insulated by a heat preservation tank, and the heat preservation tank can be set at 58 to 62 ° C, which is slightly higher than the injection temperature. It can be used after heating for a long time and at least 8 hours. The heat preservation tank is an oil bath heating device. The heat transfer oil circulates under the pressure of the oil pump to heat and keep the wax barrel at a uniform temperature. The temperature controller of the heat preservation tank can set the required temperature as needed. When the wax barrel with uneven temperature is pressed, the wax mold will have defects such as filling, cold separation, granular surface, flow pattern and mesh pattern.

(2), the forming method of the wax mold

Different forming methods are produced depending on the size, shape and technical requirements of the casting. Different forming methods have different wax materials. From the power of the wax, there are manual, electric, pneumatic, hydraulic, etc.; from the pressure points, there are low pressure, medium pressure, high pressure, etc.; from the wax state, there are liquid, semi-liquid, semi-solid, solid, etc.; Types, metal, rubber, gypsum, etc., from castings, industrial products, art and so on.

Second, the wax molding machine (pressure wax machine)

The wax is pressed into a wax mold in the waxing machine, that is, the properties of the wax mold are largely restricted by the parameters of the waxing machine, which is directly related to the design, structure and performance of the waxing machine. For example, for an upright high-pressure semi-solid waxing machine, the wax temperature, injection pressure, shot time, cooling time, and molding temperature must be checked before the waxing operation. Then proceed according to the regulations of the process card. The performance of the wax forming machine is mainly reflected in the process parameters that it can adjust, including manual and automatic; wax barrel temperature adjustment, wax inlet temperature adjustment; combined force adjustment, combined stroke adjustment; injection pressure Adjustment, injection speed adjustment, injection time adjustment; return speed adjustment, etc. For example, some castings have two hole rings at each end of the wax mold. Since the connection between the hole ring and the body is relatively weak, micro cracks visible to the naked eye are liable to occur at the joint, and then the injection speed and the holding time are adjusted. The defect is eliminated. It can be seen that the process parameters that can be adjusted by the waxing machine, the adjustment range, the adjustment method and the correct control are extremely important for the manufacture of qualified wax patterns. The central part of the waxing machine is a wax cylinder. Its size is <100mm × 400mm, used to place the wax stick, the fully preheated wax stick is placed in the wax cylinder, and the wax cylinder is electrically heated, the temperature is kept at the injection temperature, and the injection port of the pressure type is placed on the wax cylinder After the wax injection port (shot outlet) (the wax inlet has an independent heating and insulation system), the oil pump can be started to drive the upper piston downward to push the wax into the mold; after holding the pressure for a certain period of time, fold the pressure Type, remove the wax pattern. The main parameters of the whole process are as follows: 1 The temperature of the semi-solid wax is controlled to be slightly higher than the injection temperature, usually 55-60 °C. In order to make the temperature of the wax cylinder uniform, the surface of the wax cylinder must be in the incubator for 8 hours. Above; 2 injection pressure is adjustable, usually 214 ~ 218MPa 3 pressure wax machine to press the clamping force is adjustable, usually 310 ~ 315MPa 4 injection speed is carried out with injection time, it is wax The shape of the mold is different, and it has a great relationship with the shrinkage of the wax mold, thereby affecting the quality of the wax mold.

Third, wax mold defects

For various reasons, the wax molds produced by the wax forming machine will have some defects, mainly the following: 1.

The main reason is that the temperature of the wax is low, the injection speed is slow, and the molding temperature is low, which causes the wax to cool quickly during the flow, and the portion of the corner and the side or the thin wall portion of the wax mold is not full, and the corners are Rounded corners appear similar to the underfill of metal castings.

2, bubbles

One is a surface bubble, that is, a subcutaneous bubble visible to the naked eye, which has an individual distribution,

Densely distributed; the other is a deep bubble in the wax pattern, usually a concentrated, larger bubble that is invisible to the naked eye, but from the local part of the wax pattern, appears in the central portion of the wax pattern and the last cooled portion. This is the result of the release of internal pressure and gas expansion.

3, surface wrinkles

Due to insufficient wax temperature, low injection speed, improper matching of wax motion and molding temperature; or due to damaged or unclean inner surface of the molding; improper placement of chilled metal blocks, leaving motion on the surface of the wax mold Mark of. Deeper texture, similar to cold casting defects in metal castings; slight wrinkles can be removed by sanding. There is also around the core, around the hole, sometimes visible seams, slightly concave, the actual meeting of the two strands of wax flow is not well fused, this is the result of insufficient temperature and insufficient pressure of the wax.

4, the surface is rough

Since the injection pressure is too small or the injection speed is insufficient, the contact density between the wax and the inner surface of the pressed type is insufficient, and the surface state of the pressed type cannot be faithfully reflected, and wrinkles are severely accompanied.

5, shrinkage

The surface of the wax mold is concave, mainly because the injection pressure and time are not enough, or the wax temperature is high, the cooling time is not enough, and sometimes the release agent is too much, and the wax property may be lowered. Surface shrinkage involves a large area and repair is difficult.

6, deformation

After the wax mold is taken out from the press type, in addition to the shrinkage of the size, the deformation is artificially caused by the improper handling of the wax mold; it is common that the wax mold is still cooled to change the temperature field and the deflection is common. Therefore, the wax mold that has just been taken out from the press type should be carefully placed, usually flattened on the basis of a larger plane, and if necessary, a shape suitable for the deformed portion is used, and the metal block is embedded in the wax mold to prevent deformation, even if it is sufficient. cool down. It is also possible that the wax is too soft, the components of the pressed type are not suitable, and the like.

7, hairy wings (slit)

This is the most common type of defect in which a thin wax sheet escapes at the junction of the profiled block, the junction of the core and the core seat, and the like. The reason for this is mainly that the molding type precision is insufficient, the bonding surface of the pressing type or the inner part of the molding is damaged or attached with uncleanness, or the clamping force is insufficient, and the injection pressure is too high. Or the temperature of the wax is too high. The hairs must be thoroughly removed and the wax pattern can be used.

8, sticking

The wax mold and the pressure type are adhered because the release agent is not used, or the temperature of the wax and the molding is too high, or the pressure type is damaged and the pressure type is not clean.

Fourth, the conclusion

Wax mold manufacturing is the same as subsequent assembly, shell making, dewaxing, casting, and cleaning. It is important to emphasize the importance of the operation process. The cooperation between the various links must be carried out in the process card of each process and related technical management documents. . And in the implementation of the implementation of the relevant documents to meet the needs of normal production. The staff in each position must have a full understanding and practical implementation of the content and requirements of the relevant process cards. Otherwise, despite the various technical documents, the results will be ineffective. In the prevention of defects, material utilization, equipment maintenance, fixture manufacturing and inspection, waste analysis, repair, scrapping, etc., there must be rules to follow, someone is responsible, the process system is effectively implemented in the entire production activities.