Prevention of sand sticking casting defects of excavator bucket teeth

(1) There should not be too much recycled sand in the core sand, because there are too many fine sand particles, dust, dead clay and iron coated sand in the recycled sand, which leads to the low sintering temperature of the molding sand. Therefore, new sand should be added in time.

(2) There shall be no problem with the compactness of the sand mold. The sand mold at the sand sticking part of the casting must not be tight. Each sand mold must be inspected before pouring to enhance and ensure the compactness and compactness uniformity of the mold. Special compaction shall be carried out on the parts that are difficult to compact and the corners. The compactness of the sand mold should be high (usually ≥ 85), good uniformity is required, and the sand particle gap should be reduced. In particular, pay attention to the repair in place after the core is damaged, and there should be no local looseness due to the repair.

(3) Pay attention to the setting of gating system and riser, and avoid local overheating of castings and molds. When designing the gating system, the direct punching of the inner sprue into the mold wall should be avoided.

(4) Properly reduce the pouring temperature, pouring speed and pouring height. The so-called “appropriate reduction” means to reduce the thermodynamic and physicochemical effects of liquid metal on the sand mold on the premise of ensuring that it can be poured into castings. Pay attention to reducing the pouring height, upper mold height and gate cup height to reduce the metal hydrodynamic pressure, static pressure and thermal impact on the mold. For bucket tooth castings, although the wall is not thick, the pouring temperature is high, which correspondingly prolongs the thermal action time between the liquid steel and the cavity surface. During pouring, the dynamic and static pressure of the liquid steel on the cavity is large, creating conditions for mechanical penetration and sand sticking.

(5) Anti sticking sand coating can be used. The coating is evenly applied to the parts prone to sand sticking, and the thickness of the coating can be appropriately increased in the parts with serious sand sticking (the coating thickness is 0.75 ~ 1.0mm, and 0.3mm for each application. Multiple applications are required to ensure the thickness in dry state). The practical experience is to try not to solve the problem of sand sticking by using the coating or surface sand that can form a peelable glass sand sticking layer at the casting mold interface through reaction (such as adding hematite powder to molding sand and core sand, etc.). The coating without cracking and sintering at high temperature is adopted. Coating can change the surface tension between mold and molten metal and reduce the mechanical penetration of molten metal to the mold surface, so as to effectively solve the sand sticking defect. Of course, it is also one of the ways to select high alumina gate brick pipe with good material and eliminate clay refractory products with black dot metal oxide impurities on the inner wall.

If the problem of sand sticking is still not completely solved by taking the above measures, we have to consider the fundamental treatment of molding sand. The silica sand originally used is temporarily stored and not used. According to the principle of improving the anti permeability and sand sticking ability of molding materials to eliminate the sand sticking defects of castings, a batch of fine-grained raw sand with high fire resistance is adopted. First, zirconium sand with particle size of 0.20 ~ 0.10mm (75 ~ 150 mesh) is used; Second, only zirconium sand can be used as core sand and surface sand. Formula of surface sand: ① zirconium sand 100% + bentonite 2.5% + water 3.75% (based on zirconium sand); The wet compressive strength is 20 ~ 30KPa, and the dry compressive strength is more than 1MPa. ② Zirconium sand 100% + water glass 6% + water 3.1%; Wet compressive strength 12 ~ 20KPa, dry tensile strength ≥ 800kpa). If all magnesia is used, the cost increase of sand mold is about 1:10. This scheme is generally not accepted by small and medium-sized enterprises. If silica sand is used persistently, it is recommended to use the formula of anti sticking sand coating: the refractory aggregate consists of zirconium sand powder, quartz powder and graphite, which is 48%, sodium bentonite 4.5%, binder sodium silicate 4.5%, aluminum sulfate 0.65%, and then mixed with an appropriate amount of water and stirred evenly before use. This way can solve the problem of sticking sand.

In addition, it should be noted that the box should be opened in advance to speed up the cooling of castings, so as to prevent solid hot sand sticking; For the patterns used and the inner surface of the core box, keep it as smooth as possible.