Is it appropriate to maintain negative pressure in the production of ductile iron large parts using the lost foam process? How long is it appropriate for castings to be insulated in a sand box?

1、 How long is it appropriate to maintain negative pressure during the production of ductile iron large parts using the lost foam process?

1. Appearance and surface quality of castings

Suitable situation: The surface of the casting is smooth, without obvious sand holes, pores, shrinkage or bulges, with clear edges and corners, and no surface roughness caused by sand mold collapse. Short time: There may be surface "bulges" (sand molds lose negative pressure support when the metal liquid is not completely solidified and are pushed up by internal air pressure) and sand holes (sand particles enter the uncured metal liquid). Long duration: If the sand mold has poor air permeability, it may cause small pores to appear on the surface of the casting (prolonged negative pressure allows gas to seep into the sand).

2. Internal quality of castings

Suitable situation: Through non-destructive testing or anatomical examination, there are no shrinkage holes or looseness inside the casting, especially in thick and large parts (such as flanges and bearing seats) with dense tissue. Time too short: When the metal liquid solidifies and shrinks, the sand mold collapses due to the loss of negative pressure support, unable to provide sufficient supplementary shrinkage pressure, and is prone to shrinkage and loosening in thick areas. Long duration: Generally, it has little impact on the internal quality, but may increase energy consumption or cause local sand compaction due to prolonged negative pressure compression of the sand mold, making subsequent sand cleaning difficult.

3. Sand mold and coating status

Suitable situation: After pouring, the sand mold remains intact as a whole, without obvious swelling or collapse, and the coating does not show large-scale peeling or cracking. Short time: Sand molds are prone to expansion (negative pressure is released in advance, metal liquid gravity and gas pressure cause sand molds to expand), and the coating may be damaged due to sand mold collapse, leading to sand sticking in castings. Long duration: If the sealing of the sand box is poor, prolonged negative pressure may cause local sand particles to be sucked into the surface of the casting, or the coating may develop fine cracks due to prolonged negative pressure pulling.

4. Post pouring riser and runner status

Suitable situation: The solidification time of the riser matches the main body of the casting, and there are no obvious voids or looseness in the molten metal in the sprue after solidification. Short time: The riser may solidify prematurely, losing its shrinkage compensating effect, resulting in shrinkage defects in the casting body; The sprue may show "broken pouring" marks due to sand mold collapse.

When making practical judgments, comprehensive adjustments should be made based on the casting structure (such as wall thickness, complexity), pouring temperature, sand mold permeability, etc. The core principle should be "the sand mold can still maintain stable support after the casting solidifies" to ensure that the metal liquid is completely solidified and fully replenished before releasing the negative pressure.

Case study shows that when using the lost foam process to produce large parts of 500-7 ductile iron machine tools (such as boxes), the negative pressure holding time after pouring needs to be determined comprehensively based on the weight, wall thickness, and structural complexity of the casting. The core is to ensure that the casting is completely solidified and the sand mold can provide stable support to avoid defects such as deformation and shrinkage. For castings weighing 5 tons, 7 tons, 10 tons, and 15 tons, the reference range is as follows:

5-ton castings: This type of box usually has a maximum wall thickness of about 80-120mm, and it is recommended to maintain negative pressure for 15-20 minutes. 7-ton castings: with a wall thickness ranging from 100-150mm, the structure may contain many ribs or protrusions, and the negative pressure holding time requires 20-25 minutes.

10 ton casting: It belongs to a large box body, with a maximum wall thickness of 150-200mm, and a complex internal structure (such as bearing holes and cavities). It is recommended to maintain negative pressure for 25-30 minutes.

15 ton castings: Thick parts (such as box bases and flanges) may have a wall thickness exceeding 200mm, a long solidification time, and a negative pressure holding time of 30-40 minutes. In actual production, it is necessary to flexibly adjust according to the pouring temperature (which can be appropriately extended when the temperature is too high), sand mold permeability (shortened when the permeability is poor to avoid holding air), and casting solidification state (such as extending for 5-10 minutes after the riser is completely solidified), with "complete solidification of thick parts of the casting" as the core judgment criterion.

2、 How to calculate the solidification time after casting when producing ductile iron parts using the lost foam process?

1. Empirical formula estimation (applicable to simple structural components)

The solidification time (t, unit: min) of ductile iron, with the maximum wall thickness (δ, unit: mm) of the casting as the main parameter, can refer to the empirical formula: t=K × δ ²

Among them, K is the solidification coefficient. In the lost foam process, the K value of ductile iron is usually taken as 0.015-0.025 min/mm ² (affected by pouring temperature and sand mold thermal conductivity: the higher the pouring temperature, the larger the K value; Sand molds have good breathability, fast thermal conductivity, and a relatively low K value.

Example: If the maximum wall thickness of the casting is 100mm and K=0.02, the solidification time t=0.02 × 100 ²=20 minutes.

2. Consider the correction of process specificity

In EPC process, due to the influence of gas production from combustion of foam pattern and negative pressure state of sand mold, the setting time needs to be properly corrected:

Complex casting structure (such as thick protrusions and internal cavities): solidification time increases by 10% -20%; High pouring temperature (such as 1400-1450 ℃): slow cooling of the metal liquid, increasing solidification time by 5% -15%; Poor air permeability of sand molds (such as fine quartz sand particles): slow heat dissipation, slightly increased solidification time.

3. Auxiliary judgment in actual production

Observing the riser: When the surface of the riser solidifies and there is no flow of molten metal, the main body of the casting is basically solidified; Temperature measurement method: A thermocouple is embedded in the thick part of the casting. When the temperature drops below the solidus line (about 1150 ℃ for ductile iron), it is considered to have solidified.

Summary: The core is to estimate the basic time through the empirical formula of maximum wall thickness, and then modify it based on process details. Finally, the actual solidification state (such as riser and temperature measurement) is verified to ensure that the negative pressure is released after the casting is completely solidified. 3、 How long is it appropriate to keep large castings of ductile iron in a sand box after pouring using the disappearing mold process?

When producing large parts (box type) of 500-7 ductile iron machine tools using the lost foam process, the insulation and unboxing time in the sand box after pouring needs to be determined based on the weight, wall thickness, and internal stress control requirements of the casting. The core is to avoid cracking and deformation of the casting due to rapid cooling, while also considering production efficiency. The following are reference ranges for 5-ton, 7-ton, 10 ton, and 15 ton castings:

5-ton casting: The wall thickness of the box is medium (80-120mm), the structure is relatively simple, and the recommended insulation and unboxing time is 8-10 hours. At this point, the temperature of the casting drops to around 600-700 ℃, and the internal stress is basically released, making it less likely to deform due to rapid cooling when unpacking. 7-ton castings: increased wall thickness (100-150mm), may contain complex inner cavities or thick flanges, insulation time needs to be extended to 12-14 hours to ensure slow cooling of thick areas and reduce tissue stress. 10 ton castings: The thick parts of the large box body (such as the base and bearing seat) have a wall thickness of 150-200mm. It is recommended to open the insulation box for 16-24 hours to avoid cracks caused by excessive temperature differences between the inside and outside. 15 ton castings: The wall thickness of the key load-bearing parts of the ultra large box may exceed 200mm, requiring longer slow cooling. The insulation and unpacking time is generally 25-30 hours, and if necessary, insulation materials can be covered to extend the slow cooling time and ensure that the overall temperature of the casting drops uniformly. In actual production, the surface temperature of castings can be monitored by a temperature gun (it is more secure to open the box below 500 ℃), or combined with the adjustment of casting structure: castings with slender ribs and severe transition between thin and thick walls need to extend the insulation time appropriately; If there is an annealing process in the future, it can be shortened appropriately, but it is necessary to ensure that there is no significant risk of deformation of the casting when opening the box.