Research on Crack Prevention of Investment Casting Surface Layer

Abstract: The cracking and peeling of the surface layer in investment casting are long-standing problems that have not been solved in the precision casting process. Both silica sol process and water glass process exist in this issue. The solution to this problem in actual production. It's nothing more than strictly implementing the production process and strictly controlling the temperature and humidity of the dry environment of the surface layer reduce cracking and peeling by adjusting the drying time. There is currently no feasible process plan to truly eliminate this phenomenon. Based on this situation, I hereby I started conducting experimental research in this field in 2011 and selected a better process plan. After more than two years of use verification, the effect was good! Effectively solved cracking and peeling phenomenon.

The surface layer materials for investment casting include zirconium powder, white corundum powder, quartz powder, mullite powder, etc., and the binders include silica sol, water glass, etc. Over the years, precision casting has been carried out the silica sol process in the industry uses zirconium powder to prepare the surface layer slurry. In order to reduce costs, some companies choose white corundum powder as the surface layer slurry. In order to conduct experiments more challenging, silica sol and white corundum powder were specifically selected as the surface slurry for the study of surface crack prevention. Because the price of white corundum material is cheap, and the outer white corundum material has high water absorption and the surface layer slurry dries quickly and is prone to cracking.

1. Factors causing surface cracking and peeling

1.1 Material factors

1.1.1 Selection of Wax Materials

The inherent characteristics of wax material itself, such as a high coefficient of expansion and sensitivity to environmental temperature, make the module wax parts dry after being coated with the surface layer slurry

During the process, due to the expansion of wax components, the surface layer may crack, and in severe cases, soaking in pre wetting agents before performing secondary dipping operations may cause peeling.

1.1.2 Selection of binder silica sol

The surface layer silica sol should have a w (SiO2) content of 30% and a colloidal particle size of (8-9nm), which is a general normal indicator. Otherwise, it is difficult to maintain the wet strength of the slurry coating on the surface layer can lead to cracking and peeling of the surface layer during the production process.

In addition, qualified silica sol should be stored by enterprises to prevent freezing outdoors in winter and high temperature exposure to sunlight in summer. Storage time should be too long,all of these will cause the silica sol to deteriorate, weaken its performance, affect the wet strength of the surface coating, and lead to cracking.

1.1.3 Selection of refractory materials

Surface layer refractory powder: ① Good chemical stability: Because it comes into direct contact with high-temperature metals, in order to prevent reverse reaction with metal liquid and its oxides yes, the chemical stability of refractory powder should be good, and the powder should be pure Reasonable gradation: The formulation of the surface coating is composed of refractory materials and binders.The ratio between the two is the powder to liquid ratio of the coating. The powder to liquid ratio of the coating has a significant impact on the performance of the coating and the surface layer of the shell. A lower powder to liquid ratio the tendency for coatings to crack is greater, and coatings with high powder to liquid ratios have uneven coating and stress during drying, which can also cause cracking Refractory sand material: The sanding of the surface coating is to enhance the shell and fix the coating, preventing penetrating cracks caused by gelation when the coating dries. The type of sanding should be it should be consistent with the powder and have the same coefficient of expansion to enhance the adhesion between sanding and coating. Prevent different coefficients of expansion from causing stress during the drying process force leads to cracking.

1.2 Process factors

1.2.1 Process parameters for preparing surface slurry

1.2.2 Surface layer coating with slurry and sanding

The slurry should be evenly spread, and compressed air should be used to blow the concave corners of the module to prevent the coating from not being able to hang or uneven. Sandblasting should be done by pouring sand uniform, not easy to penetrate the surface layer.

1.2.3 Surface layer drying

To ensure good drying of the surface layer shell, it is necessary to strictly control the temperature, humidity, wind speed, and drying time of the surface layer drying chamber.

To ensure good drying of the surface layer shell, it is necessary to strictly control the temperature, humidity, wind speed, and drying time of the surface layer drying chamber.

(1) Temperature range of 22-25 ℃: If the temperature in the surface drying chamber is out of control, it may cause the wax mold to expand, resulting in surface cracking. Besides, the temperature is too high

If the surface layer dries too quickly, it will cause stress due to gel shrinkage, leading to cracking of the surface layer.

(2) Humidity 60%~70%: The surface drying room requires a humidity of 60%~70% to prevent the surface from drying too quickly! Generating stress causing opening

Crack.

(3) Wind speed is 0: The surface layer drying chamber does not have a fan on, which is also to prevent the surface layer from drying too quickly and to prevent stress caused by uneven wind blowing, leading to opening

Crack.

(4) Time 4-6 hours: If the drying time of the surface layer is too short, it will not dry thoroughly, affecting the strength, and even washing away when soaking the pre wetting agent in the transition layer.

If the time is too long, prevent the surface layer from drying out and causing stress cracking. So the drying process for the surface layer is in the forefront: under strict conditions of wind, temperature, and humidity, the drying time is 4-6 hours to ensure that the surface layer cracks less and does not crack.

The various factors that cause surface cracking have been analyzed above. The cracking of the surface layer is a subtle phenomenon. There is a factor that cannot be handled well cracking will occur in all cases, so in order to reduce cracking, it is necessary to strictly follow the various process parameters mentioned above.

The various factors that cause surface cracking have been analyzed above. The cracking of the surface layer is a subtle phenomenon. There is a factor that cannot be handled well cracking will occur in all cases, so in order to reduce cracking, it is necessary to strictly follow the various process parameters mentioned above.

2 Experimental study on crack prevention of surface layer

The author began researching crack prevention in the surface layer in 2011, as it was during the transition between summer and autumn and the climate was unstable. Module surface layer opening on the production line severe cracking and peeling phenomena. In this case, after multiple experiments, a crack prevention formula for the surface layer slurry and two drying methods were finally determined. through practical application, the occurrence of surface cracking phenomenon has been effectively solved.

3 conclusion

Through the crack prevention test of white corundum slurry sprinkled on the surface layer of white corundum sand, two dry environmental conditions were selected, and the effect was good. Draw the following conclusion:

(1) Wind blowing speed drying and crack prevention break the limitation of surface drying without wind blowing. The experimental formula used crack inhibitor HX-006 and inoculant HX-007。 The crack inhibitor improves the wet strength of the slurry coating and plays a role in crack prevention. Fertility agent HX-007: The surface layer of the fertility slurry is evenly distributed on both sides The layer coating is evenly hung and has consistent strength, so it dries quickly without cracking when blown by the wind.

(2) Ultra long delay drying and crack prevention. The module dried for more than 26 hours after being coated with slurry and sand, breaking the original process parameter of surface layer drying time of 4-6 hours,To verify the crack resistance of the formulated slurry under long-term drying conditions.

(3) The conclusion drawn from the previous two dry environments is that a ceiling fan can be used to blow the surface layer in a dry environment, the humidity can be relaxed a bit, and the time can be Control the temperature by yourself, but strictly adhere to 22-25 ℃, as uncontrolled environmental temperature can cause wax parts to expand and deform, affecting product dimensions.

(4) This crack prevention formula is suitable for various surface powders of precision casting silica sol process, such as zirconium powder, white corundum powder, mullite powder, etc; For water glass technology is also applicable. However, the parameters should be fine tuned according to different situations.