Wax shrinkage in precision lost wax casting
With the continuous development of economy and industry, people’s demand and requirements for products are becoming higher and higher. At the same time, the ability of precision lost wax casting technology to solve the complex structure of products has been recognized by the industry. Common precision dewaxing casting processes include water glass process, silica sol process, composite process, etc. Among them, the silica sol process has achieved the highest casting quality and the lowest processing speed in many processes. It is also widely used in casting products because of its high density, such as small and medium-sized parts and ultra-small parts.
The process steps of silica sol lost wax casting are divided into:
Wax pressing (wax injection molding) – wax repair – wax mold inspection – group tree (wax module tree) – shell making (first impregnation, grinding, then second impregnation, and finally air drying mold shell) – dewaxing (steam dewaxing) – mold shell roasting – chemical analysis – Casting (pouring molten steel into mold shell) – Vibration shelling – casting and cutting and separation of casting rod – grinding gate – shot blasting
It can be seen that half of the process is related to wax molds. Because the precision tolerance of CT6 silica sol dewaxing casting can be guaranteed, it means that the minimum tolerance accuracy is only plus or minus 20 filaments. Therefore, the requirements for the shrinkage of wax molds used in the process are very high. In all steps, the shrinkage of wax mold is closely related to the size of subsequent products. The results show that the quality of wax mold is one of the important factors that directly affect the quality of castings. The size of the wax mold directly determines the size of the finished product after casting the mold shell in the later stage. Therefore, green medium temperature wax with high stability is usually used in silica sol dewaxing casting process. Water glass and composite dewaxing casting process usually use white low-temperature wax, which has more advantages in terms of cost.
After being used at high temperature for a long time, some components of medium temperature wax materials will be oxidized and deteriorated. In addition, metal ions will also enter the wax material, making the wax worse and containing more impurities. Worse, the shrinkage of wax is very different from that of new wax. Because the shrinkage is different from solid impurities, it is difficult to completely solve it only by precipitation, dehydration or filtration. The factory must regularly remove some old wax and supplement new materials to ensure the overall stability of wax performance. When the shrinkage fluctuates greatly, it is necessary to separate a part of wax and increase the input proportion of new wax to neutralize the shrinkage of wax. At present, there are professional medium temperature wax manufacturers that can regenerate waste wax. Due to the special impurity removal process, the ash content in the regenerated wax can be lower than that in the original new wax, and other properties can be adjusted at will. Companies with general capacity will also equip their factories with a set of systematic medium temperature wax degradation and dehydration unit, which will greatly stabilize the shrinkage of medium temperature wax. It can be seen that different factories have different treatment methods and different wax shrinkage. Therefore, sometimes in order to save costs, when customers change suppliers, they directly send the molds of the original suppliers to us for use. Samples must be tested to ensure that the dimensions of the final product meet the drawing requirements. In some cases, due to the difference in wax shrinkage, it is necessary to modify the size of some molds.
In order to strictly and stably control the shrinkage of medium temperature wax, the quality of medium temperature wax is very important. We need to control the wax material from the source, reduce the impurity content, pay attention to the wax treatment and use process, and do not allow high-temperature treatment to prolong the service life of the wax material. In order to control the shrinkage rate of medium temperature wax in real time, the factory will customize the mold dedicated to wax part size measurement, and set the specified wax injection machine. In addition, professionals are assigned to operate the machine in strict accordance with the specified parameter settings. At the same time, the specialist will measure the wax molds and record the data every day, and compare the latest data to confirm whether it is necessary to adjust the current operation.
In addition to the above control and supervision, in daily production, in order to control the size of finished wax film, the setting parameters of wax injection machine must be set in strict accordance with the values given by technicians. Department. Due to the high temperature of medium temperature wax during injection, it will shrink slightly after cooling and molding, which means that its size will also change slightly. If the values of temperature, pressure and time do not match during wax injection, it will directly lead to the loss of wax size control.
