How to deal with sand holes in the production of small wall thickness ductile iron parts on a horizontal molding line?

Regarding the sand hole defects that occur in the production of ductile iron small castings (wall thickness ≤ 3mm) on the horizontal molding line, it is necessary to analyze and take improvement measures from multiple dimensions such as sand properties, molding process, pouring system, and operation control. The specific treatment methods are as follows:

1. Analyze the causes of sand holes. The essence of sand holes is the formation of holes on the surface or inside of castings due to the entrapment or erosion of sand particles during the filling process of molten metal. Small castings are more sensitive to the compactness, strength, and casting stability of molding sand due to their thin wall thickness, fast solidification, and high metal flow rate. Common causes include insufficient molding sand performance, low compactness, poor wet strength, insufficient refractoriness (such as low SiO ₂ content or excessive recycled sand), and improper moisture/coal powder ratio. Modeling process issues: uneven local compaction of sand molds (such as corners and parting surfaces), sand leakage caused by template/sand box wear, and poor quality of mold repair (such as low strength of repair materials). Unreasonable design of the pouring system: high flow rate in the internal pouring channel eroding the wall, weak slag blocking ability of the pouring system (slag entrapment), and large fluctuations in pouring temperature/speed.

2. Targeted handling measures

a. Optimize the performance of molding sand to improve compactness and uniformity: Adjust the parameters of the molding machine (such as sand injection pressure and time) to ensure that the compactness of the sand mold surface and edges meets the standard (generally, the compactness of green molding sand is ≥ 1.6g/cm ³), and avoid local softening. Improving sand composition: clay sand: adding high-quality sodium based bentonite (replacing calcium based) to 4-6% (accounting for the original sand) to enhance bonding strength; Control the effective coal powder content by 3-5% (forming carbon film anti sticking sand), and the amount of regenerated sand should be ≤ 50% (reducing dust and brittleness). Control moisture: The moisture content of the green sand should be maintained at 3.5-4.0% to avoid being too high (reducing strength) or too low (increasing brittleness), and evenly mixed by a sand mixer.

b. Improve the modeling process template and sand box maintenance: check the flatness of the template parting surface (to avoid protrusions or depressions that may cause local thinning of the sand mold), repair the wear on the edges of the sand box (to prevent sand leakage during box closure). Quality control of mold repair: Avoid making the mold too thin. Use high-strength molding sand (such as adding a small amount of clay) and compact the repair area. It is forbidden to fill it with mud at will.

c. Optimize the design of the pouring system to reduce flow rate and erosion: adopt a closed pouring system (F straight: F horizontal: F inner=1:1.5:2~3), with a flat and porous inner runner design (reducing single stream flow rate), and control the flow rate at 0.5-1.2m/s (slightly lower for thin-walled parts). Add buffering and filtering: Add buffering bags (such as ceramic rings) at the bottom of the sprue, and place ceramic filters (with a pore size of 0.5-1.0mm) at the front end of the inner sprue to slow down the flow rate and filter out slag. Matching pouring parameters: pouring temperature: ductile iron liquidus line is about 1150 ℃, and it is recommended to pour ultra-thin wall castings at a temperature of 1390-1450 ℃ (for thin-walled parts, the upper limit should be taken to avoid excessive overheating and erosion). Pouring speed: Fast and continuous pouring to avoid interruption causing cold insulation or local sand holes.

d. Strengthen the surface protection of sand molds and apply fire-resistant coatings: Apply high alumina alumina based coatings (particle size 80-120 mesh) to the surface of the mold cavity, with a thickness of 0.2-0.3mm. After drying, form an isolation layer to reduce the penetration of molten metal. The coating should be uniform and avoid accumulation (otherwise poor local breathability may cause pores).

e. Control the quality and process stability of molten iron: Ensure the temperature and composition of molten iron at 1550-1560 ℃ (to avoid poor low-temperature fluidity), control the composition (C: 3.6-3.8%, Si: 2.4-2.8%), and avoid abnormal solidification shrinkage caused by compositional deviation. Slag treatment: Before pouring, clean the surface slag of the molten iron or use slag blocking cotton/skimmer to prevent slag from being rolled into the mold cavity and forming sand hole defects.

f. Data analysis and verification of defect characteristics: Count the location (such as near the gate, thickness transition zone), quantity, and morphology (single/dense) of sand holes, and determine whether they are related to specific process steps (such as a certain pattern area). Comparative experiment: After adjusting the sand ratio or pouring parameters, small-scale trial production is carried out to compare the sand hole waste rate and verify the improvement effect.

3. Summary

The sand holes in small castings need to be comprehensively controlled through "sand strengthening+casting optimization+process stability". The key is to improve the strength and fire resistance of the molding sand, design a reasonable pouring system to reduce the flow rate, and strictly control the quality and operation details of the molten iron. If it is resin sand molding, its high strength advantage can be given priority consideration; Clay sand requires fine adjustment of bentonite, coal powder, and moisture, combined with coating protection, to effectively reduce sand hole defects.