SAND CASTING EQUIPMENT.

In order to optimize the integrity of the casting, the liquid metal must be solidified under positive pressure throughout the casting process.
1. Most alloys exhibit consistent and predictable behavior during the temperature cooling process from liquid to solid. There are two different stages of contraction.
1.1 When the alloy casting temperature cools to the liquidus, this is usually called liquid shrinkage or superheat shrinkage. Secondly, when the alloy cools from liquid to solid, it is usually called solidification shrinkage.
1.2 Graphite iron castings (including gray cast iron, ductile iron and malleable cast iron) are accompanied by an unusual phenomenon during the cooling and solidification process, that is, the metal begins to expand. This expansion is usually attributed to the precipitation of the lower density graphite phase, which overcomes and exceeds the shrinkage associated with the solidification of the cooling fluid and austenite.

2. Xinyuanzhu Group suggests that the most important aspect of designing the riser and gating system of cast iron is to maintain the positive pressure of the liquid throughout the solidification process. Initially, atmospheric pressure must be allowed to act on the liquid in the riser. For this to happen, the riser must (feeding). Once the expansion begins, a well-designed riser system controls the expansion pressure and ensures that the casting automatically feeds during the rest of the solidification process. This is in contrast to steel, aluminum, copper, etc., because there is no expansion involved, which must supplement the casting with molten metal during the solidification process.
Control pressure
2.1 The riser neck is the most critical component in the design of the riser system, because it usually determines the size of the residual pressure on the liquid. The contact surface of the riser neck must be large enough to transfer the molten metal in the riser from the riser to the casting in a long enough time, and release the excessive pressure in the cavity if necessary, but it should be appropriate so that at the end of solidification Maintain the positive pressure of the liquid and facilitate the removal of the riser from the casting.
2.2 The riser neck can be regarded as a "safety valve" on the pressure vessel, and its design should ensure that the pressure in the casting is maintained at a manageable level. The molding material, or more specifically, the sand mold that bears the expansion pressure without expanding, usually determines the degree of control. If the mold material is weak, such as when using a clay sand mold, the riser neck should be designed to release some expansion pressure to avoid mold expansion. This is achieved by designing the riser neck to solidify at a relatively late stage, allowing some pressure to be released to the riser through the riser neck.
2.3 The use of stronger and harder model bonding materials (such as resin systems) can make the riser neck smaller when designing the riser neck, so as to solidify in advance during the expansion stage, thereby maintaining a higher liquid residual pressure. A riser neck that is too small will result in excessively high residual pressure in the casting, resulting in pores related to mold expansion. An excessively large riser neck usually results in a positive pressure loss on the liquid before the solidification is completed, resulting in shrinkage holes and gas discharge from the molten metal related to solidification.
2.4 Design rules
The size of the riser neck is usually based on the geometric modulus (Mc) of the casting. Typical values ??for iron castings produced in clay sand are between 0.6 (Mc) and 0.9 (Mc). The exact value depends on the hardness of the sand mold material, the chemical composition and degree of inoculation of the iron, and the cooling rate of the casting. If the riser moves closer to the casting, the heating effect of the sand between the casting and the riser neck will reduce the geometric modulus of contact while maintaining the equivalent thermal modulus. If the neck is short enough to make its length equal to or less than the smaller contact section size, the geometric modulus can be safely reduced by 0.6 times, that is, the modulus of the longer neck (Mn (short) = 0.6 mn (long) ). This means that the contact area is reduced by approximately 65%.
The successful feeding of graphite cast iron involves maintaining and controlling the positive pressure of liquid iron during the entire solidification process, correctly designing the riser and pouring system, and well controlling the metallurgy and pouring time. For graphite cast iron parts, no shrinkage will occur. Important.

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