Abstract: This paper systematically analyzes the causes, common processes and locations of sand hole defects in the sand casting process of BB3 type multistage pump bodies in cast steel parts, and proposes targeted solutions. Through literature review and case analysis, it was found that the defects of sand holes mainly result from insufficient strength of the molding sand, design flaws of the gating system and improper operation, and are prone to occur in the molding, core setting, closing and pouring processes. The specific locations include gates, risers, thick parts of castings and inner cavities. By optimizing the proportion of molding sand, improving the design of the gating system and strengthening process control, the occurrence rate of sand holes can be effectively reduced and the quality of castings can be improved.

Chapter 1 Introduction

With the development of industrial technology, stainless steel multistage pumps are increasingly widely used in fields such as chemical engineering, power, and water conservancy. Cast steel BB3 type multistage pump body as the core component, its quality directly affects the performance and service life of the pump. However, in the sand casting process, sand hole defects are one of the common casting defects, seriously affecting the strength and appearance quality of the castings. This paper aims to conduct an in-depth analysis of the causes, common processes and locations of sand hole defects, and propose effective solutions to provide reference for foundry enterprises.

Chapter  2  Analysis  of  the  Correlation  between  Structural Characteristics of Castings and Sand Hole Defects

2.1 Structural characteristics of Cast steel BB3 pump body

The BB3 pump body adopts an axial split structure, consisting of the pump body, the pump cover and the static friction pair to form a single shell double volute structure. Its typical features include:

1. The impellers are symmetrically arranged back to back to achieve

self-balancing of axial force;

2. Double volute flow channel design reduces radial force impact on the pump body;

3 The pump body/cover components are horizontally split for easy disassembly, assembly and maintenance;

4. The first-stage impeller adopts a double-suction structure to enhance cavitation resistance.

These structural features, while enhancing the performance of the pump body, also impose higher requirements on the casting process. For example, the complex curved surface of the double volute flow channel can cause uneven filling of the molten metal, increasing the risk of sand hole formation.

2.2 The impact of stainless steel split-case pump structures on sand holes

The flow channel structure of the stainless steel split-case pump directly affects the flow characteristics of the molten metal. Research shows that when the cross-section of the flow channel changes abruptly or the wall thickness varies greatly, turbulence and entrainment are likely to occur, causing the molding sand to fall off and form sand holes. For example, in the diffusion section of a multistage pump volute, if the wall thickness gradient is not properly controlled, local differences in cooling rates will trigger thermal stress, causing the mold to crack.

Chapter 3 Analysis of the Causes of Sand Hole Defects

Sand hole defects are holes with sand grains embedded on the surface or inside the casting. Their causes are complex and diverse, and can mainly be attributed to the following aspects:

3.1 Insufficient strength of molding sand

Insufficient strength of the molding sand or sand core is one of the main causes of sand hole defects. During the pouring process, the high temperature of the molten metal and the scouring effect may cause the mold or core to fall off, resulting in sand holes. If the refractoriness of the molding sand is insufficient, under the effect of the high-temperature molten metal, the surface of the molding sand will soften and fall off, further intensifying the formation of sand hole defects.

3.2 The gating system is poorly designed

An unreasonable design of the gating system, such as an improper gate position or an overly fast pouring speed, can cause excessive scouring force of the molten metal on the mold, resulting in mold cracking, mold sand shedding and the formation of sand holes. In addition, too short or too long pouring time may also affect the occurrence of sand holes, and the pouring time should be tailored to the size, structure and weight of the casting.

3.3 Improper operation

Improper operation of processes such as molding, core setting, closing and pouring is also an important cause of sand hole defects. For example, the loose sand in the cavity before the box is closed is not blown clean, the sand core is damaged and not repaired, and foreign objects fall into the cavity during the pouring process, all of which can lead to the formation of sand holes.

Chapter 4 Processes and Locations Prone to Sand hole Defects

The common processes and locations of sand hole defects in the sand casting process mainly include the following aspects:

4.1 Molding Process

In the molding process, uneven compactness of the mold, unremoved sharp sand, and insufficient strength of the cavity surface can all lead to sand hole defects. Problems such as the mold not being properly repaired, the corners of the casting not being compacted, and the large parting drop of the casting can also cause sand to drop during pouring, resulting in sand holes.

4.2 Core setting and mold closing procedures

During the core-setting and closing process, damage to the sand core, failure to clean the loose sand, and the shedding of sand grains due to collisions during box closing can all result in sand hole defects. In addition, uncleaned burrs at the parting surface of the sand core and loose sand falling into the cavity during box closing are also common causes of sand holes.

4.3 Pouring Process

During the pouring process, too fast pouring speed, unreasonable gate design

leading to sand flushing, and foreign objects falling into the cavity during the pouring process can all cause sand holes. In addition, excessively high or low pouring temperatures may also affect the occurrence of sand holes.

4.4 Locations prone to sand holes

The common locations of sand hole defects in castings mainly include gates, risers, thick and large parts of the casting, and inner cavities. These locations are prone to mold shedding or sand grain embedding in the casting due to the large scouring force of the molten metal or slow heat dissipation.

Chapter 5 Solutions to Sand hole Defects

In view of the causes and common processes of sand hole defects, the following solutions are proposed in this paper:

5.1 Optimize the molding sand ratio

Improving the strength and refractoriness of the molding sand is the key to solving the sand hole defect. By adjusting the proportion of the molding sand and increasing the content of binders and additives, the wet compressive strength and refractoriness of the molding sand can be enhanced, and the formation of mold shedding and sand hole defects can be reduced.

5.2 Improve the gating system design

Reasonable design of the gating system, including gate position, size and pouring speed, can effectively reduce the scouring force of the molten metal on the mold and lower the occurrence rate of sand hole defects. In addition, tailoring the pouring time to the size, structure and weight of the casting is also an important measure to reduce sand hole defects.

5.3 Strengthen process control

Strengthening process control in processes such as molding, core setting,  closing, and pouring to ensure proper operation, thorough cleaning, and  avoidance of collisions can effectively reduce the formation of sand hole defects. For example, thoroughly purge the cavity before closing the box, leaving no blind  spots; Check whether the sprue cup and the runner are clean before pouring; Avoid  foreign objects falling into the cavity during the pouring process, etc.

5.4 Use a casting filter

Using a casting filter in the runner section can effectively remove inclusions in the molten metal and fundamentally eliminate sand hole defects in the casting. The selection of casting filters should be based on the type of molten metal and the pouring conditions.

Chapter 6 Conclusions

Sand hole defect is one of the common casting defects in the sand casting process of BB3 type multistage pump body cast steel parts. Its causes are complex and diverse, and the prone process and location are clear. By optimizing the molding sand ratio, improving the gating system design, strengthening process control and other measures, the occurrence rate of sand hole defects can be effectively reduced and the quality of castings can be improved. In the future, with the continuous development of casting technology, the prevention and control of sand hole defects will be more refined and intelligent, providing a strong guarantee for the sustainable development of casting enterprises.