Problems related to the use of filling wax in precision casting

2020-08-21 17:27:50 1365

Some problems about wax pattern making in fine casting




All walks of life have different requirements for fine castings. In order to meet the needs, fine casting skills show its wide range of habits, such as the diversity of data, the complexity of shapes, the batch production, the accuracy, uniformity, stability and appearance accuracy of specifications. Among these characteristics, there are new developments and improvements today. For example, the material is stainless steel with a weight of 40g, and the outline specification is a fine casting with a length of 55mm, a width of 3216mm, and a thickness of 13mm. The requirements for it are: ① direct use as cast, free of machining; ② The roundness and surface roughness of the shaft hole on the casting must meet the rules of the Japanese Industrial Code (JIS); ③ The four holes around the shaft hole cooperate with another part. In addition to meeting the rules of JIS, the relevant specifications require that the casting cannot be flexed; ④ The minimum specification tolerance of the casting is ± 012mm ⑤ The depth of the cast words, symbols, etc. is 013mm, and the lines are required to be clear; ⑥ The minimum specification tolerance between planes is 011mm ⑦ The fins, fillets, surface roughness, shrinkage, deformation, etc. that are not specially specified shall be implemented according to the specifications of the manufacturer. As an example of fine castings, we can feel the requirements of today's industrial production for fine castings and the level that producers can reach. From the modern investment casting process, it is not easy to achieve the above requirements. Therefore, it is very difficult to reach the above intention if we do not strictly handle, operate correctly and cooperate with each other from each process, link and position.

Problems related to the use of filling wax in precision casting

1、 Wax




Wax is the first link in the production of precision casting. As we all know, due to the turning over of casting production, there are various requirements for wax. The simplest way for producers is to choose from the 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, which is required to have characteristics 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 mold, gating system, bonding, repairing, sealing, resin wax, etc. From the perspective of wax materials used for wax patterns, there are many kinds of wax materials, and they are still being opened. Filler wax is widely used. The commonly used fillers are 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 have 59 seconds, 62 seconds and 99 seconds respectively when the temperature drops from 104 ℃ to 49 ℃, which shows that the type 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.

Problems related to the use of filling wax in precision casting

1. Prevent wax from overheating during melting




(1) The direct heating oil bath furnace 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. It is intended to prevent wax containing filler from piling up in addition to uniform temperature. 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, semi transparent due to the separation of fillers, and smoke or evaporation in the wax bucket.




(2) During wax recycling, in order to remove impurities and water, it is necessary to heat the wax above its melting point, which is generally controlled below 120 ℃. In addition to ensuring that the wax material has certain activities to remove impurities, the water contained in the wax will also evaporate 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 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 for wax deterioration. Therefore, try to reduce the dewaxing temperature and pressure to maintain the wax function.




2. Adhere to ambient temperature




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




(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 wax materials have poor thermal conductivity, it is difficult to make wax materials uniform by relying on the heat of wax barrel. 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 mold




Different forming methods shall be 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.

Problems related to the use of filling wax in precision casting

2、 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 heated by electricity. 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 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 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 the wax cylinder uniform, both inside and outside, 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 is different due to different wax mold shapes. It has a lot to do with the wax mold shortening, and then affects the wax mold quality.




3、 Conclusion




The wax mold production is the same as the subsequent assembly, shell making, dewaxing, pouring and cleaning. It is necessary to emphasize the importance of the operation process, the cooperation between various links, and the implementation of the process cards of each process and the relevant skills handling documents. It is also necessary to constantly improve relevant documents in compliance with 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 prevention, data application, equipment repair, fixture production and inspection, waste product analysis, repair, scrapping, etc. Someone should be responsible for complying with the process principles in the entire production activity.


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