How to prevent the deformation and distortion of wax mold in precision casting

2020-08-05 16:53:44 1126

01. Select excellent casting wax;




Wax is the first link in the production of precision casting. As we all know, due to the development of casting production, there are various requirements for wax. The simplest way for producers is to choose from the medium temperature wax sold in the market. They can also make their own wax and entrust professional factories to produce as required. However, no matter what kind of wax, the required properties are the same, namely: melting point, condensation point, softening point, thermal conductivity, activity, shortening, shrinkage, strength, hardness, resistance, release, coating, reusability, specification stability and residual ash weight after high temperature ignition. On the basis of the production of specific castings, we choose a wax material, which requires the characteristics of medium temperature wax material in some aspects; It is also an important factor to consider the cost together. In order to meet the specific requirements of various castings, wax is divided into liquid, semi liquid, semi solid and solid; With or without filler; Water-soluble and water-insoluble; Regenerative and non regenerative; Used for wax pattern, casting system, bonding, repairing, sealing, resin wax, etc. From the perspective of wax materials used for wax patterns alone, there are also many kinds of wax materials, and they are constantly turning over. Waxes with fillers are widely used, such as powdered polyethylene, polystyrene, organic acid, fatty acid and starch. Its participation amount is 30%~45% of the total amount. This kind of wax has good thermal insulation, that is, it can be formed at a lower temperature, and its mobility is good. The wax mold details can be clearly reproduced. For example, three kinds of wax added with thermosetting styrene, isophthalic acid and P2 FILLER, the time to drop from 104 ℃ to 49 ℃ is 59 seconds, 62 seconds and 99 seconds respectively, which shows that the variety of filler has a great impact on the function of wax. The shortening rate of this kind of wax is 5% less than that without filler. The function of wax is closely related to the preparation, application and treatment of wax.

How to prevent the deformation and distortion of wax mold in precision casting

02. Prevent wax from overheating during melting;




(1) The oil bath furnace with direct heating is used and the heat transfer oil is forced to circulate. The melting temperature is controlled within the planning range of 84~98 ℃, and it is always in the state of agitation. In addition to uniform temperature, it is intended to avoid the accumulation of wax containing fillers. Excessive temperature and partial overheating will deteriorate the function of wax due to oxidation, carbonization and other problems. Overheating is characterized by the change of wax color, black spots on the inner wall of the wax bucket, translucent due to the separation of fillers, and smoke or transpiration in the wax bucket.




(2) In order to remove impurities and moisture, it is necessary to heat the medium temperature wax to a temperature above the melting point of the wax, which is generally controlled below 120 ℃. In addition to ensuring that the wax has a certain mobility to remove impurities, the moisture contained in the wax will also be transpiration due to the stirring of the agitator.




(3) As for the wax pressing rod used under the insulation of the insulating cylinder, its heating temperature only needs to be 2~3 ℃ higher than the injection temperature.




(4) When the medium temperature wax is removed in the autoclave (pressure tank), the steam temperature can be as high as 150 ℃ or above. This condition is the primary link that causes the wax function to deteriorate. Therefore, try to reduce the dewaxing temperature and pressure to adhere to the wax function.




03. Balance the ambient temperature;




In the wax pattern making room, no matter in winter or summer, the indoor temperature should be kept within 20~25, so that the wax pattern specifications are in a stable environment, and cooperate with the environmental conditions of the slurry coating process to ensure the quality of the shell

How to prevent the deformation and distortion of wax mold in precision casting

1. Wax heating and insulation




The wax enters the mold from the wax holding barrel under pressure, and is cooled and condensed in the mold. In this process, the temperature difference is the primary factor affecting the shape, specification and stability of the wax pattern. Therefore, the uniformity of wax barrel temperature is extremely important when wax is pressed with wax barrel for heat preservation of wax materials. As the poor thermal conductivity of wax materials, it is difficult to rely on the heat of wax barrel to make wax materials uniform. A wax barrel with uneven temperature cannot produce sound wax molds. For this reason, the wax barrel is generally heated and insulated with a thermal insulation tank, which can be set at 58~62 ℃, slightly higher than the injection temperature. In a long time, at least 8 hours of heating homogenization will be used later. The thermal insulation tank is an oil bath heating equipment. The heat transfer oil circulates under the pressure of the oil pump to heat the wax barrel at a uniform temperature. The temperature controller of the thermal insulation tank can set the required temperature as required. When the wax bucket with uneven temperature is pressed, the wax mold will have shortcomings such as insufficient filling, cold shut, granular appearance, flow pattern and reticular pattern.




2. Forming method of wax pattern




Different forming methods are adopted according to the size, shape and technical requirements of castings. Different forming methods have different wax materials. The power of wax pressing includes manual, electric, pneumatic, hydraulic, etc; From pressure, there are low pressure, medium pressure, high pressure, etc; According to wax materials, there are liquid, semi liquid, semi solid, solid, etc; There are metal type, rubber type, gypsum type, etc. from the point of molding, and there are industrial products, artworks, etc. from the point of casting.

How to prevent the deformation and distortion of wax mold in precision casting

3. Wax pressing forming machine (wax pressing machine)




Wax is pressed into wax mold in the wax press, that is, the nature of wax mold is largely constrained by the parameters of the wax press, which directly relates to the planning, structure and function of the wax press. For example, for a vertical high pressure semi-solid wax pressing machine, it is necessary to check the wax temperature, injection pressure, injection time, cooling time and molding temperature before wax pressing. Then operate according to the rules of the process card. The function of wax pressing forming machine is mainly shown in the process parameters it can schedule, including manual and automatic; Wax barrel temperature scheduling, wax injection port temperature scheduling; Combined force scheduling and combined travel scheduling; Injection pressure scheduling, injection speed scheduling, injection time scheduling; Return speed scheduling, etc. For example, for some castings, there are two hole rings at both ends of the wax mold. Because the connection between the hole ring and the body is relatively thin, it is easy to have visible microcracks at the connection. Later, after scheduling the injection speed and persistence time, this disadvantage was eliminated. It can be seen that the process parameters, scheduling planning, scheduling methods and correct control that can be scheduled by the wax pressing machine are extremely important for making qualified wax molds. The central part of the wax press is the wax cylinder. The specification is<100mm × 400mm, which is used to place wax rods. The fully preheated wax rods are put into the wax cylinder, and the wax cylinder is electrically heated. The temperature is kept at the injection temperature. When the injection inlet of the molding is placed at the wax injection port (injection port) of the wax cylinder (the wax injection port has an independent heating and thermal insulation system), the oil pump can be started to instigate the upper piston to move downward to press wax into the molding; After maintaining the pressure for a certain period of time, fold the mold and take out the wax mold. The main parameters of the whole process are: ① The temperature of semi-solid wax is controlled slightly higher than the injection temperature, which is generally 55~60 ℃. In order to make the temperature of wax cylinder uniform and consistent, it is necessary for the wax cylinder to stay in the insulation tank for more than 8 hours; ② The injection pressure is adjustable, generally 214~218MPa ③ The tightening force of the wax press feed type is adjustable, generally 310~315MPa ④ The injection speed is implemented by the injection time, which varies with the wax mold shape, and it has a great relationship with the wax mold shortening, and then affects the wax mold quality.




The wax mold production is the same as the subsequent assembly, shell making, dewaxing, pouring and cleaning. It should focus on the importance of the operation process, and the cooperation between various links. It is necessary to implement the process cards of each process and the relevant skills management documents. In addition, relevant documents shall be constantly improved during the implementation to get used to the requirements of normal production. It is necessary for the staff on each post to fully understand and implement the contents and requirements in the relevant process cards, otherwise, although there are various skills documents, their functions are useless. There should be rules to follow in terms of defect avoidance, material application, equipment repair, fixture production and inspection, waste product analysis, repair, scrapping, etc., and someone should be assigned to follow the process principles in the entire production activity.


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