How to prevent porosity defects caused by increased humidity during the production of castings using coated sand technology in summer?

During summer, there is a significant increase in porosity defects when using coated sand to produce castings, which is related to high air humidity and increased rainfall in summer. In order to cope with the increase of porosity in summer coated sand, both coated sand supply enterprises and foundries need to take measures to eliminate the phenomenon of porosity in coated sand production castings.

1. Measures to be taken by the coated sand supplier: Raw material control and sand drying treatment: Strictly dry the coated sand raw materials (such as quartz sand) to ensure a moisture content of less than 0.2%. They can be heated to 100-150 ℃ using drying equipment (such as boiling drying furnace) for 2-4 hours to reduce the moisture adsorbed by the sand particles. Resin and curing agent moisture-proof: The environment for storing phenolic resin and curing agent (such as Urotropin) should be kept dry, with humidity controlled below 40% -50%. After opening, use as soon as possible to avoid prolonged exposure to humid air. In addition, the supply of coated sand should be tracked in a timely manner to monitor the sand usage of the foundry, and guidance and reminders should be provided to the foundry on moisture prevention and removal of coated sand. Adjusting process parameters to reduce sand mixing temperature and time: Control the temperature at 80-100 ℃ during sand mixing, shorten the mixing time to 5-8 minutes, reduce the possibility of resin deterioration due to high temperature moisture absorption, and avoid excessive friction of sand particles to produce electrostatic adsorption of water vapor.

2. Precautions for Foundry

a. Precautions for optimizing curing conditions during shell making in foundries: increase the curing temperature appropriately (such as from 210 ℃ to 230 ℃), shorten the curing time (from 180-210 seconds to 150-160 seconds), enable the resin to quickly cure and form a dense shell shape, and prevent moisture from infiltrating the interior of the molding sand. Add anti moisture additives: When mixing sand, add calcium stearate or silicone oil moisture-proof agents at a ratio of 0.5% -1% to form a hydrophobic film on the surface of the sand particles, inhibiting water adsorption. Local environmental humidity control: Install a layer flow drying hood above the core making machine and introduce dry air with a humidity of less than 50% and a temperature of 30-40 ℃ to ensure rapid surface drying during sand core molding; The overall humidity in the workshop is controlled below 60% using a dehumidifier (with one high-power dehumidifier installed every 100 square meters). Demolding and cooling control: Immediately place the sand core in a hot air circulation furnace at 50-60 ℃ for 1-2 hours after demolding, or irradiate the surface with an infrared lamp to accelerate the evaporation of residual water vapor; When cooling, avoid direct exposure to air and place them in layers on a drying rack. Optimize the pouring system by embedding an exhaust rope (diameter 35mm) inside the sand core, opening an exhaust groove (depth 0.3mm, width 20mm) on the parting surface sand shell, designing an open riser exhaust needle, and inserting an 8mm diameter steel pipe into the riser. The sand mold testing parameters should refer to the tensile strength of the sand shell or core, which should be equal to or greater than 2.8 megapascals. In summer, 10% of the tensile strength is provided compared to other seasons. Gas generation detection: It should be less than or equal to 16ml/g (tested at 950 ℃). When the proportion of pores increases, the dehumidification measures should be taken. When pores occur frequently, brush the sand shell with zinc oxide alcohol coating (ratio 1:3). Before molding, use gas spray to loosen the surface of the barbecue sand shell or sand core (temperature at 150 ℃, time greater than or equal to 30 seconds)

b. Optimization of pouring process and pouring system design: Optimize the layout of pouring and riser, increase exhaust channels (such as opening exhaust grooves or placing exhaust ropes at the top of the sand mold), control the pouring temperature at the middle and lower limits of the optimal pouring temperature of the liquid phase line of the casting material while ensuring fluidity. The liquid phase lines of castings made of different materials also have different phases, which requires calculation. For example, for gray cast iron parts, the carbon content is 3.2-3.5%, the silicon content is 1.8-2.2%, and the liquidus temperature is around 1180 ℃. Carbon is between 2.9-3.2%, silicon is between 1.6-1.8%, and the liquidus temperature is around 1230 ℃. The optimal pouring temperature for gray cast iron is the liquidus line+superheating temperature of 120-150 ° C, followed by adjusting the wall thickness of the joint casting. For complex geometric thin-walled parts, the superheating temperature is taken as the upper limit, and then added at around 30-50 ° C. For example, carbon content is 3.2%, silicon content is 1.8%, wall thickness is 4-8 mm, and pouring temperature is 1180+150+50=1390 ℃.

3. Real time monitoring of humidity through process inspection and feedback: Humidity sensors are installed in key processes such as sand mixing, molding, and pouring to monitor changes in environmental humidity in real time. When the humidity exceeds 65%, emergency dehumidification measures are immediately activated. Strength testing of sand molds (cores): After each batch of sand molds (cores) is produced, samples are taken for room temperature bending strength testing. If the strength decreases by more than 10%, the process parameters need to be readjusted or damp raw materials need to be replaced.

The above measures can effectively reduce the impact of summer air humidity on film coated sand casting and reduce porosity defects. In actual production, parameters need to be adjusted according to specific equipment and raw material characteristics. If necessary, a small amount of moisture-proof agent (such as calcium stearate) can be added to improve the moisture absorption capacity of sand molds.