How to set up an internal gate and design a pouring system for ductile iron small castings with multiple hot spots to ensure quality and improve yield?

For nodular cast iron parts with irregular shapes and multiple independent hot nodes, the selection of the sprue opening position should be closely coordinated with the solidification principle (simultaneous solidification or sequential solidification), in order to control shrinkage and porosity defects and improve yield as the primary goal.

1、 Design core idea: Adopting the principle of "simultaneous solidification" for the design of the sprue opening position. Core goal: rapid and balanced cooling, reducing the temperature difference between hot nodes, suitable for scenarios where hot nodes are dispersed, difficult to replenish or have high appearance quality requirements.  

Key measures:

1. Inner gate position: Priority should be given to opening in the thinnest wall area (around 4mm) or geometric center of the casting to ensure rapid dispersion and filling of molten iron. Distributed introduction: Two flat internal gates are arranged at the center of the casting, directly connected to the thin-walled area to avoid direct impact of hot spots: the gates must not face the thick hot spots (10 and 20mm hot spot areas), otherwise it will exacerbate local overheating.

2. Key points of pouring system design: The cross-sectional area of the sprue should be controlled within 1.2-1.5 cm ², and the flow rate of the iron water should be controlled within 0.8-1.2 meters per second to quickly cover the mold cavity and promote rapid solidification in the thin-walled area. Used in conjunction with cold iron: Place an external cold iron (such as graphite or quenched sand) at the thick walled hot joint (20mm area) to accelerate cooling. Cold iron size: The thickness of graphite cold iron is 1-1.5 times the thickness of the hot zone, and the coverage area is greater than 1.2 times the hot zone area. The placement of the cold iron should be closely attached to the back of the hot joint, and generally avoid opening the working surface to ensure that the chilling effect reaches the center of the hot joint. Install internal cooling iron on the back of isolated hot joints (material and fusion must be strictly controlled).  

3. Accurately control the pouring temperature field. The pouring temperature should be controlled between 1390-1440 ℃ for rapid filling, which is beneficial for thin-walled areas and thick walled areas. Shrinkage and concavity may occur due to high temperature, and defects need to be avoided through cold iron and riser.

2、 The riser design of this casting and the casting company is limited by the sand box. The riser needs to be designed as a dark riser module (calculated at a height of 10 centimeters). The module of the hot zone is about 12-15 mm. Our riser module calculation: the riser module should be 1.2 times greater than the module of the hot zone of the casting. 1.2X1.2=14.4 mm. The riser design is trapezoidal, and the final dimensions are lower width of 80 mm, upper width of 60 mm, and height of 50 mm. In addition, it is recommended to design a 5mm diameter exhaust hole at the top of the riser to prevent air resistance from affecting shrinkage.

3、 Preventive measures for other defects

1. Preventive measures for air holes (caused by improper coordination with the riser/cold iron): Install a φ 5-8mm exhaust needle at the top of the concealed riser (to avoid back pressure caused by holding air and affecting shrinkage). Spray alcohol zircon powder coating on the surface of the cold iron (to prevent the rapid cooling of the cold iron and the generation of water vapor). Control the continuity of the flow during pouring to avoid turbulent entrainment (bottom injection pouring system optimization can be used).  

2. Ceramic filters (with a pore size of 5-10mm) are installed at the root of the riser to filter the slag entering the shrinkage channel. Before pouring, let the molten iron stand for at least 3 minutes to ensure that the slag floats up. 4、 Collaborative optimization of process system 1 The total cross-sectional area of the matching sprue between the pouring system and the riser is: riser neck cross-sectional area=1:0.8-1.2 (if the original sprue is 30mm ² × 2, then a single riser neck needs 24-36mm ²). Adopting the "slow fast slow" pouring curve: A [initial slow pouring 20%] -->B [mid-term fast pouring 60%] -->C [final slow pouring 20%+riser overflow] 2 Control the performance of molding sand by adding 7-9% bentonite and ensuring a permeability of ≥ 120 (to prevent sand inclusion caused by negative pressure suction of sand particles). Sand mold hardness is 85-90 (too soft can cause wall migration and affect the shrinkage path).

Finally, it is important to note the control of residual magnesium content after spheroidization. For small items like this, residual magnesium should not exceed the standard and should be controlled at 0.04% or above as much as possible.