Production of car cylinder, cylinder head, the inlet and outlet, water jacket cavity and hot section of the internal corner of the Department of frequent thinning that is the veins defects, because the burr position in the cavity is difficult to clean, Defects will hinder the normal flow of cooling water, serious cause the engine to burn the cylinder; into the exhaust channel will affect the airway vortex characteristics, seriously affect the performance of the engine, so to solve the veins defects for cylinder head and other car casting It is vital.
First, the causes of veins defects
At 573 ℃, the quartz sand undergoes a crystal transition from β-quartz to α-quartz. The thermal stress caused by phase-change expansion leads to cracking of the surface of the sand core, and the molten iron penetrates into the crack to form a veins. The basic conditions of the veins are generated, one is the thermal stress produced by the phase change expansion is greater than the sand core surface strength; the second is the molten iron liquid and has sufficient fluidity. The veins defect is a kind of expansion type of casting defects, the production example shown in Figure 1. It is generally believed that: ① the higher the content of the original sand SiO2, the more serious the veins defects. (2) The higher the concentration of the original sand, the more severe the plaque defects. ③ the higher the pouring temperature, the more severe the veins defects. But the casting temperature is determined according to the casting structure, wall thickness, can not be arbitrarily changed.
Where can reduce the thermal stress factors will reduce the occurrence of moire, where can improve the core strength and toughness of the core factors will reduce the occurrence of veins, where the factors can accelerate the solidification of iron can also reduce the pulse generation. Practice has proved that the tendency of resin sand to produce veins defects in order of order: cold box resin sand → PEPSET resin sand → coated sand → furan resin sand → alkaline phenolic resin sand.
Second, to solve the problem of veins defects
The measures taken against veins defects can be attributed to the following:
(1) to reduce the expansion of the sand core expansion of the original sand is the root cause of the occurrence of veins, so to reduce the expansion of the original sand is to solve the problem of veins defects first consider the original sand at high temperature there is crystal transition, accompanied by volume expansion, If the quartz sand after 900 ℃ above high temperature calcination after cooling crystal phase transition will be irreversible, so the original sand regeneration or roasting after use will significantly improve the veins defects; the production of cast iron pieces do not need the original sand has too high SiO2 content, reaching 92 % Can be, because the higher the SiO2 content of the greater the degree of thermal expansion, the general production can try to use recycled sand.
(2) to adjust the original sand particle size distribution If the original sand particle size is too concentrated, the overall expansion will be more obvious manifestation, the general use of three or four sieve sand, car cylinder head recommended particle size 0.355 ~ 0.154mm (50/100 Head) scrub sand, try to avoid the use of particle size of 0.224 ~ 0.106mm (70/140 mesh) and more fine sand.
(3) to improve the sand core at room temperature, high temperature strength in the same sand under the same circumstances, the sand core strength depends on the type of adhesive and sand core compactness, because the cold box is high pressure shot sand, sand core tightness, The expansion space of the sand at high temperature is small, so it is easy to produce veins defects.
(4) the use of special sand ① chromite sand mainly by the cold effect to prevent the veins, quartz sand can be mixed with a certain percentage. ② zirconia refractory high coefficient of expansion is low, but the price is high, rarely used. ③ roasting sand is the scrub sand heated to 870 ℃ above, and insulation for a certain time, so that the full expansion of silica sand to achieve phase change, so roasting sand, also known as phase change sand. After the phase change treatment, not only the expansion rate dropped significantly, and the sand in the crystal water, organic pollutants, carbonates and oxides by high temperature calcination was a lot of removal, so roasting sand and scrub sand compared to not only acid consumption Low value, burning amount and low gas volume, less mud content, more important is the low expansion rate. Not only can reduce the amount of adhesive added, more effective to prevent the occurrence of Mold grain, hot cracking, sticky sand and other defects, improve casting accuracy. ④ Baozhu sand spherical shape is smooth, no convex veins. Its mobility and filling is good, can get a good shape and mold strength, collapsibility is good, easy to sand operations, the use of binder than other similar types of sand has a greater savings, low thermal expansion and chrome ore And other special sand the same, so the production of castings of high dimensional accuracy, rupture and surface defects, casting high yield. Baozhu sand compactness, high strength, even if the use of repeated regeneration is also very broken, can reduce the casting process of dust on the production environment of pollution, good regeneration, reduce industrial waste emissions, conducive to environmental protection. High refractoriness is the main component of alumina, refractoriness ≥ 1790 ℃, to meet the casting of various metals and alloys, easy to recover, so to solve the cavity suture and veins flaw is a good choice.
(5) the role of special coatings chilled paint can speed up the solidification of molten iron, generally suitable for automotive castings, simple and easy; for thick pieces of the effect is not obvious.
(6) anti-veins additives At present, the market a wide range of anti-vegan additives, according to the ingredients are mainly organic and inorganic, the first is a mixture of silicate and metal oxide, its low melting point, at high temperatures with Silica sand to form a glass body to fill the sand core crack, to prevent the molten iron into the cracks, this additive can be made into a round grain, the sand core strength of small, adding the amount of 1.0% to 5.0%; the second is organic additives, Adding less 0.3% to 1.5%, the intensity of the impact than the inorganic obvious and large amount of gas.