Analysis and case of wax pattern defects in investment casting-porosity, shrinkage and crack

2020-04-08 10:55:12 1214

At that time, investment casting enterprises, especially small and medium-sized enterprises, were seriously short of technicians; There are few ideas, methods and experiences to reduce and prevent the shortcomings of investment casting; It has severely restricted the competitiveness, power and efficiency of enterprises and has become the bottleneck of enterprise development.




Wax pattern (also known as "investment pattern") manufacturing is the first and important process in production, and is the primary condition for obtaining high-quality castings. The wax mold has shortcomings, which wastes the labor cost, production time and useful resources of mold making; Unqualified wax molds flow into the shell making process and even the pouring process, and defective products occur. It will increase the production cost and delivery date of the factory, and affect the promise and economic benefits of the enterprise. Therefore, we should pay attention to and deal with the shortcomings of wax models.




Binding investment mold is composed of mold materials through equipment, binding, cooling, mold repair, assembly welding and other processes, as shown in Figure 1:

Analysis and case of wax pattern defects in investment casting-porosity, shrinkage and crack

At that time, wax based low temperature wax materials or medium temperature wax materials (such as K521) were widely used for binding wax models. In general, wax models have 13 kinds of defects, such as pores, shrinkage and cracks. It is described as follows:




one




1、 Blowhole




1. Disadvantages




Some surfaces of the wax mold are left with smooth air pockets, as shown in Figure 2.

Analysis and case of wax pattern defects in investment casting-porosity, shrinkage and crack

2. Causes




(1) Too much gas is entrained into the wax material of the equipment, without sufficient recovery treatment.




In production, four processes of wax melting, wax flake shaving, wax mixing and recycling are often used for low-temperature wax materials, and spiral blade mixer is often used to mix wax materials (or "wax pastes"). In the process of mixing wax, a lot of gas can not be prevented; And with the addition of mixer revolutions, the more gas is entrained. Especially after the wax material is mixed, there is no sufficient recovery treatment, so that many of the entrained gas overflows from the wax, but still remains in the wax.




(2) The orientation of wax injection hole is improper, and gas is entrained during wax injection.




The improper orientation of the wax injection hole caused the wax to swirl during injection into the mold cavity, and excessive gas was entrained.




(3) Improper molding planning is not conducive to cavity exhaust.




The distance of wax filling the cavity is too long, and the cavity has poor exhaust.






3. Prevention measures




(1) Strictly follow the wax equipment process and operating procedures.




When equipping white wax stearic acid wax, pay attention to the order of feeding, that is, add stearic acid first, and then take part in white wax or recover wax after it is fully melted, and heat it to the full melting condition, with the temperature ≤ 90 ℃. At the beginning of mixing, the rotation of the mixer can be higher (such as 400r/min), and the rotation of the mixer can be reduced (200~300r/min) after the wax temperature rises. After the wax material is properly mixed, it shall be recycled at least 0.5h at all times to make the gas caught in the wax fully overflow.




(2) Improve the orientation of the wax injection hole to prevent the wax from turbulent flow in the mold cavity.




The wax injection nozzle should be set on the inner runner or the surface with machining allowance, and the standard should match the wax injection nozzle of the wax press; The wax injection port shall ensure that the wax material is stably filled in the mold cavity with the shortest distance, without turbulence.




(3) Improve molding planning to facilitate the discharge of gas from the cavity.




Improve the molding planning, which is conducive to the discharge of gas from the cavity; If necessary, add vent holes.




two




2、 Shrinkage depression




1. Disadvantages




The thickened part of the wax mold shows a depression caused by partial shortening, as shown in Figure 3.

Analysis and case of wax pattern defects in investment casting-porosity, shrinkage and crack

2. Causes




(1) The thick part (depression) of the wax mold is short.




The injection pressure of wax mold is one of the main parameters of the molding process. A small injection pressure makes the shortening rate of the thick part (depression) of the wax mold large. When sufficient shrinkage is not available, the shrinkage occurs there.




Holding time is another process parameter of the molding process. After the wax is filled in the mold cavity, the time for holding the pressure is short, which increases the shortening rate of the thick part (depression) of the wax mold. When there is no sufficient filling, the wax mold shrinkage at this part is formed.




(2) The injection amount of wax mold is short and the feeding is not sufficient.




When the wax injection hole is small, or the orientation is improper, or the wax injection pressure is small, the amount of wax injected into the impression cavity is short. When the wax mold is too thick (concave) to be sufficiently supplemented, the wax mold collapse is formed.




(3) The wall thickness difference of wax mold is too large, which is not conducive to feeding.




The structural planning of wax mold is unreasonable, and the wall thickness difference is too large, which is not conducive to feeding.




(4) The wax material temperature is too high during wax injection, or the molding temperature is too high, or both.




The higher the wax material temperature, the higher the molding temperature, or both, the higher the wax material shortening rate; Shrinkage occurs when sufficient compensation is not available.




(5) The shortening rate of wax is large.




White wax stearic acid wax materials are used in production, usually 58~62 ℃ (i.e. 58~62 ℃) white wax and first level three pressure stearic acid. When new equipment materials are used, they account for 50% respectively. The shortening rate of No. 58 white wax is greater than that of No. 62 white wax. The shortening rate of white wax stearic acid wax is higher than that of white wax low molecular polyethylene wax.






3. Prevention measures




(1) The feeding of wax is adjusted.




When binding the wax mold, the injection pressure is generally 0.2~0.5MPa. Proper travel injection pressure can not only add the density of wax in the molding, but also reduce the shortening rate of wax. Properly add pressure maintaining time, generally (3-10) s or longer; The same can reduce the shortening rate of wax, and the feeding of wax can only be adjusted.




(2) Increase the injection amount of wax, and adjust it only after adding feeding.




Properly increase the section of the wax injection hole, and add the wax injection amount per unit time; Or change the position of the wax injection hole, so that the wall thickness of the wax mold can be fully fed.




(3) Improve the wax pattern structure to make its wall thickness as uniform as possible.




If possible, try to make the wall thickness of the wax mold uniform; If necessary, the cold wax block (also called "wax core") prepared in advance is placed at the fat part of the wax mold, and then wax is injected to form the wax mold. According to the size and shape of the wax mold, the cold wax block can be placed partially or as a whole. The cold wax block should be positioned in the die with a tapered boss. The height of the boss should be recognized according to the size of the parts, and the control is generally 2~3mm. The wax used for the cold wax block shall be together with the wax of the wax mold.




(4) During wax injection, control the temperature of wax material and molding.




During wax injection, the wax temperature T=48~52 ℃ and the molding temperature T=18~25 ℃ are generally selected for low-temperature wax; For medium temperature wax, the wax temperature T=52~60 ℃ and the molding temperature T=20~24 ℃ are generally selected.




(5) Waxes with small shortening rate shall be selected.




For example, when 62 ℃ white wax is used instead of 58 ℃ white wax, its shortening rate is reduced. Or the white wax content in wade wax is 5-10%, that is, 55-60%, which can also reduce the shortening rate of wax. Or choose the wax material made of 60 ℃ white wax (95% content) and low molecular polyethylene (5% content), the shortening rate is about 1%, which is relatively low.




three




3、 Crack




1. Disadvantages




There are cracks in the wax mold, often showing the parting surface of the wax mold, as shown in Figure 4.

Analysis and case of wax pattern defects in investment casting-porosity, shrinkage and crack

2. Causes




(1) The shortening rate of wax is large and the plasticity is poor.




When 58 ℃ refined wax or white wax and the first level three pressure stearic acid are used to manufacture wax according to 50% each, the leisure shortening rate of wax pattern is about 2%. When the content of stearic acid in wax exceeds 80%, its resistance and plasticity are poor. It is easy to cause cracks in the wax mold.




(2) The temperature of molding, or molding room, or cooling water is too low.




The temperature of the molding is too low, or the temperature of the molding room is too low, or the temperature of the cooling water is too low, which causes the wax mold to cool too fast. When the wax mold shortens, it is easy to crack; Or the wax mold is cooled for too long in the molding process (that is, the mold lifting time is too long), which causes the wax mold to shorten and be blocked, and cracks are easy to occur.




(3) The wax mold structure/molding planning is unreasonable.




The wax mold structure/molding planning is unreasonable, the wall thickness of the wax mold is uneven, and the fillet of the thickness transition part is too small or sharp. When the wax mold is blocked from shortening, cracks will occur in its thin parts.




(4) Improper operation.




The wax pattern is cracked due to improper mold lifting method, or the time for drawing the mandrel is too late.






3. Prevention measures




(1) Waxes with small shortening rate shall be selected.




For low-temperature wax, when 62 ℃ white wax is used instead of 58 ℃ white wax, the shortening rate can be reduced. Or the white wax content in wade wax is 5-10%, that is, 55-60%, which can also reduce the shortening rate of wax. Or choose the wax material made of 60 ℃ white wax (95% content) and low molecular polyethylene (5% content), the shortening rate is about 1%, which is relatively low.




(2) Control the temperature of molding, molding room and cooling water.




The temperature of low temperature wax molding should be controlled at 18~25 ℃, and the temperature of medium temperature wax molding should be controlled at 20~24 ℃. The temperature of the molding room should be controlled together with the molding temperature, and the temperature of the cooling water should also be controlled together with the room temperature; If necessary, the mold making room should be equipped with air conditioner, and the temperature should be controlled at 20 ℃.




(3) Improve wax pattern planning (i.e. press planning).




Improve wax pattern planning to make its wall thickness as uniform as possible; If the wall thickness is uneven, the fillet shall be excessive (the excessive fillet shall be "1/5~1/3" of the sum of two adjacent wall thicknesses); If necessary, process reinforcement can be added to reduce the stress of wax mold shrinkage to the extent that no cracks occur.




(4) Strictly implement the operating procedures.




Improve the mold lifting method, and add mold lifting equipment if necessary to prevent cracks in the wax mold during mold lifting.




The cooling time of wax mold is generally 10~60min. After the wax mold is cooled, the mandrel cooled with the wax mold shall be taken out in time so that it cannot prevent the shortening of the wax mold.


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Qingdao Xinnuoke Holdings Group limited company.

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