1. Pattern making
The quality of pattern is one of the keys to the success or failure of lost foam casting. Whether the bucket tooth castings of excavator produced by lost foam casting are of high quality depends on the quality of pattern 50%. The EPS beads used for packing were selected. Finally, beads 4 and 5 were selected. The amount of foaming agent before the preparation of the beads was guaranteed to be over 5.5%. The foaming agent was made from a foam factory in Luoyang with a density of less than 0.018g / cm3.
2. Coating and coating process
2.1 Coating
Coating performance and coating quality are the key to lost foam casting. The function of the coating is not only to prevent sand sticking and make the bucket tooth casting of the excavator obtain a smooth surface, but also to protect the surface of the mold, improve the rigidity of the pattern, ensure the stability of the mold when the molten steel is replaced with the pattern during the pouring process, and make a large amount of gas generated by the gasification of the pattern escape quickly. Therefore, in addition to the traditional casting properties, lost foam casting coatings require higher strength and permeability. Therefore, we have developed a water-based forsterite powder coating, and the effect is very ideal after use. See Table for coating composition.
| Component | Forsterite powder (270 mesh) | Anhydrous sodium carbonate | Bentonite | Cellulose | Milk white gum | Binder | Water |
| Dosage / kg | 100 | 1-3 | 6-10 | 1-2 | 3-4 | 2-3 | Appropriate amount |
2.2 Coating process
The combination of dip coating and brush coating is adopted. Dip and paint first, and then brush at the exposed place, and then place it in the drying room stably for baking, and the temperature of the drying room shall be ≤ 50 ℃. After drying the first coating, brush the second and third times, and finally make the coating thickness between 1.5 ~ 2.5mm. Add an appropriate amount of foaming agent for the second and third times to improve the air permeability of the coating.
1.3 design of gating system
In order to give full play to the advantages of lost foam casting and improve the process yield, we use group casting to cast excavator bucket teeth. The so-called group casting, as the name suggests, is multi casting in one box, that is, the same sand box is put into multiple shapes and cast at one time. In this way, as long as there is enough molten steel, multiple bucket tooth castings of excavator can be poured at one time by controlling the cross-sectional area of inner sprue. With this method, we have successfully poured 24 hammers, 46 magnetic liners (8kg each) and 24 hammers (8lb each). For the self-made pouring bucket with teeth on the bottom of the box (see Fig. 5), we need to use the suction bucket with teeth on both sides of the box to place the sand in parallel with each other. For the pouring bucket with teeth on both sides of the box, we need to place the sand in the bottom of the box (see Fig. ).
So far, there is no fixed formula for the design of lost foam casting gating system. Generally, the calculation formula of ordinary sand mold casting is used and appropriately enlarged to determine. There is no final conclusion on how much to enlarge. Moreover, this method is only suitable for single excavator bucket tooth casting, and it is not necessarily suitable for group casting closed gating system. Therefore, after many tests, we finally determined the section size of each sprue, f straight = 50 mm × 50 mm, f = 40 mm × 40mm, within f = 35mm × 35mm。
1.4 buried box and vibration
Forsterite sand of 0.75 ~ 1.0mm is selected as the molding material, and the self-made bottom pumping negative pressure sand box with the size of 2.1m is used as the sand box × 1．0m × 1．5m。 During molding, lay 150mm thick molding sand at the bottom of the sand box, scrape and vibrate, and then put 10 bucket tooth patterns into the sprue, assemble the sprue, and use the two pouring sand adding method to add sand while vibrating. Vibration can reduce the internal friction of dry sand, improve the fluidity of sand and increase the filling density of sand. The higher the density of dry sand, the better the stability of the casting mold, and the higher the quality of bucket tooth castings of excavator. In order to improve the quality of bucket tooth casting of excavator, we select three-dimensional vibrating table and obtain satisfactory results.
1.5 negative pressure casting
We use sk-20 water ring vacuum pump, the pouring temperature is controlled in the range of 1480 ~ 1530 ℃, and the negative pressure value is ≤ 0.015MPa. When pouring, first point through the sprue, and then pour smoothly and quickly, so that the molten steel always fills the gate cup until the end of pouring. In this way, the EPS in the sprue vaporizes rapidly and is filled with molten steel in a short time, which can effectively prevent the negative pressure suction gas from entering the mold cavity. At the same time, a certain static pressure can be established in the gating system. Combined with the bottom injection, it is conducive to maintain the equilibrium state of molten steel EPS decomposition gas pattern and the layer by layer filling of molten steel in the mold cavity, so that the molten steel is not easy to be involved in gas and residue in the filling process, and can effectively avoid pores At the same time, it can also prevent waste products caused by local box collapse caused by bucket tooth casting of excavator.
1.6 water toughness treatment
Stop the pump within 3min after pouring, turn over the box and pour out the bucket tooth casting of the excavator within 25min, then quickly enter the water and stay in the water for 30min. The water temperature when entering the water is ≤ 45 ℃, and the water temperature after entering the water is ≤ 60 ℃. In this way, the carbide and residual pearlite structure in the as cast state are eliminated, and a relatively single austenite structure can be obtained, which greatly improves its strength and toughness and achieves the purpose of work hardening.
2 production and use effect
(1) The casting of bucket teeth of excavator is well formed, and the geometric dimensions fully meet the requirements.
(2) The work load of the excavator is reduced by 50% without burr and shrinkage on the surface of the bucket, and there is no burr on the surface of the bucket and riser.
(3) The yield of bucket tooth casting of excavator is more than 40% higher than that of traditional sand casting.
(4) Four sets of bucket teeth have been excavated and tested in the mine, and the service performance is stable without tooth breakage. The average mining capacity of each set is 200000 ~ 220000 T, which is better than the bucket teeth produced by sand casting. At present, it has been popularized and used in an open-pit mine with good results.