How to design exhaust needles for producing cast iron parts using green sand technology?

We will discuss in detail the design of exhaust needles based on the characteristics of green sand technology and cast iron parts. Green sand cast iron has its unique process characteristics, so the design of exhaust needles also needs special consideration. 

The influence of the characteristics of green sand cast iron parts on the design of exhaust needles: 

1 Large and concentrated gas generation: Wet sand contains about 3.5% -5% moisture, which rapidly vaporizes under the impact of high-temperature molten iron. At the same time, coal powder, organic matter, and other substances in the sand also burn and produce gas, resulting in a large amount of gas being produced instantly during the initial pouring stage. 

2. Negative pressure effect: During the filling of molten iron, instantaneous negative pressure will be formed in certain parts of the mold cavity (especially in the area where the molten iron is finally filled), which will act like a "suction tube", strongly suction the gas in the sand pores and exhaust needle. If the exhaust needle is not designed properly, the metal liquid can easily be "sucked in". 

3. Good fluidity of molten metal: Cast iron (especially gray cast iron) has good fluidity and relatively lower surface tension compared to cast aluminum, making it easier to penetrate small gaps. 

4. Moderate pressure: Compared to cast steel, cast iron has slightly lower pouring temperature and static pressure, which provides a certain safety space for the design of exhaust needles.

Key points for targeted design of exhaust needle for green sand cast iron parts: Based on the above characteristics, the core of the design is to provide sufficient exhaust area in the early stage of casting, while effectively resisting the metal liquid backflow caused by instantaneous negative pressure. 

1. Exhaust groove shape: "Gradual expansion" or "throttling" design is preferred, and inclined groove (gradual expansion) is strongly recommended. Description: The exhaust groove is narrow on the inside of the cavity and wide on the outside of the atmosphere, forming a small taper. Advantages: Anti backflow: When the metal liquid is pushed towards the exhaust groove under negative pressure, it will flow from a wider area to a narrower area, reducing the cross-sectional area of the flow channel and increasing the resistance sharply, effectively preventing the metal liquid from penetrating deep. Smooth exhaust: Gas flows smoothly from narrow to wide areas, with minimal resistance. Processing: Although processing is slightly more difficult than straight grooves, it can be achieved through wire cutting or specialized grinding tools. Avoid using simple straight grooves: Straight grooves have poor flow resistance under negative pressure and carry higher risks. 

2. Exhaust groove size (key parameter), groove width (key among key): Recommended range: 0.15 mm~0.25 mm. Upper limit principle: For green sand cast iron, the groove width should never exceed 0.25mm. 0.20mm is a very commonly used and safe size. This width utilizes the surface tension of molten iron, making it difficult for it to automatically invade, while also providing sufficient resistance under negative pressure. If the casting is thick, the static pressure is high, or the position is critical (such as the highest point of the mold cavity), a narrower size should be selected, such as 0.15mm or 0.18mm. Groove depth: Recommended range: 0.4 mm~0.8 mm. To ensure sufficient exhaust cross-sectional area, the groove depth is usually 2-4 times the groove width. For example, a slot width of 0.20mm is matched with a slot depth of 0.5mm-0.8mm. Slot length (exhaust needle wall thickness): Recommended range: 20 mm~30 mm. A sufficient length can form a stable 'flow resistance section', increasing the frictional resistance of the molten metal flow and allowing it to solidify before reaching the atmospheric side. 

3. Number and layout of exhaust slots: usually 4 to 8, evenly distributed on the circumference. Layout: It is necessary to ensure that the exhaust duct is connected. For the gradual expansion design, it is necessary to clearly indicate which end is the cavity side (narrow end) and which end is the atmospheric side (wide end), and indicate on the drawing that "the narrow end faces the cavity during installation". 

4. Installation position and quantity of exhaust needles: The "dead zone" and highest point of the mold cavity are the places where gas is most likely to accumulate. The area opposite the pouring system or where the molten iron is last filled: This is where the gas is compressed the most and the negative pressure effect is most pronounced. Upper mold of large flat castings: helps to remove gas accumulated at the top of the mold cavity. Near the core: especially where the gas generated by the sand core needs to be discharged through the mold cavity. Quantity: More is better than less. Exhaust needles can be installed in areas where there is suspicion of holding breath. Its cost is much lower than the waste loss caused by pores.

 Design Summary and Recommended Specifications For a typical green sand cast iron component, the recommended specifications for the exhaust needle are as follows: Material: 45 # steel, quenched and tempered, with the working part (grooved part) surface quenched to HRC 40-45 to enhance wear resistance and resistance to molten iron erosion. Appearance: Cylindrical with shoulder, commonly used diameter of Φ 8mm or Φ 10mm. Exhaust groove: Shape: Gradual sloping groove (narrow cavity side, wide atmospheric side). Slot width: 0.20 mm (cavity side). Groove depth: 0.6 mm. Quantity: 6, evenly distributed. Slot length: 25 mm. 

Special tips for use and maintenance: 

1 Strictly prevent blockage: The sand particles and coatings of green sand are more likely to block narrow exhaust ducts. After each use, compressed air must be blown back from the atmospheric side and cleaned with extremely fine steel wire or needle. Regularly check if it is unobstructed. 

2. Correct installation: Ensure that the exhaust needle and the mold mounting hole have an interference fit or tight fit, without any looseness. The narrow end must face the cavity! After installation, it is best for its end face to be flush with the cavity wall or slightly recessed (<0.5mm), and avoid protrusion. 

3. Timely replacement: Once there are signs of erosion, rounding or enlargement of the exhaust groove edge by molten iron, it should be scrapped and replaced immediately. Conclusion: For green sand cast iron parts, a successful exhaust needle design is to use a gradually expanding groove and strictly control the groove width below 0.25mm. 

This design can not only cope with the huge gas generation and instantaneous negative pressure in the early stage of pouring, but also reliably prevent the intrusion of molten iron, which is a key small component to ensure the quality of castings and reduce the scrap rate.