6 Key Measures to Improve Casting Surface Quality & Analysis of Core Influencing Factors

In modern manufacturing, the surface quality of metal castings has become a critical indicator of product quality. High-quality surface finish not only ensures stable key dimensions of castings but also lays the foundation for subsequent processes such as flexible production line machining and assembly precision. Controlling casting surface quality spans the entire production process. This article summarizes key measures to enhance casting surface quality and core influencing factors based on practical experience, with real-world cases to provide reference for production practice.

I. 6 Core Measures to Enhance Casting Surface Quality

1. Rational Use of Casting Filters for Molten Metal Purification

Incorporating casting filters into the gating system is an efficient way to improve both surface and internal quality of castings. Glass fiber filters are preferred for their smooth surface, low filtration resistance, low gas evolution, and ability to effectively trap slag and impurities in molten metal, reducing surface defects such as protrusions and pinholes. High-quality filters (e.g., Nissen series) simultaneously enhance mechanical properties of castings, achieving a "dual benefit"—optimizing surface smoothness while reducing internal shrinkage and porosity risks.

A large heavy machinery manufacturer previously faced frequent surface defects like sand holes and gas pores in large steel castings due to slag inclusions, resulting in a high scrap rate. After adopting Nissen glass fiber casting filters, these surface defects decreased significantly, with the scrap rate reduced by 20%. The castings’ surface finish improved noticeably, enhancing both appearance and internal quality, and boosting market competitiveness.

2. Precise Control of Pouring Temperature to Avoid Defects

Pouring temperature is a critical parameter affecting casting surface quality: too low a temperature easily causes defects like manganese sulfide gas pores, liquid slag inclusions, and gas pores from sand core gases, leading to surface pits and pitting; excessively high temperatures trigger mold expansion (especially for castings with complex sand cores). When the temperature reaches 1420°C or higher, the scrap rate rises, and at 1460°C, it can reach up to 50%. It is recommended to use induction furnaces for melting, with precise temperature control systems to stabilize molten metal temperature, balancing fluidity and mold stability.

A factory specializing in automotive engine block castings struggled with frequent surface defects like gas pores and cold shuts due to unstable pouring temperatures, resulting in a high defect rate. After introducing advanced induction furnace melting equipment, they precisely controlled pouring temperatures within an optimal range and adjusted temperatures in real time based on casting material and structure. Post-implementation, the surface quality of engine block castings improved significantly, with the defect rate reduced to below 5%, greatly enhancing production efficiency and economic benefits.

3. Enhance Sand Mold Compactness to Ensure Dimensional Stability

Insufficient compactness of sand molds or cores can cause surface sink marks and internal shrinkage in castings. Production requires strengthening mold rigidity through: optimizing molding processes to maximize sand mold compactness, reducing mold wall movement caused by molten metal static pressure or solidification expansion; structurally optimizing hollow sand cores (lighter and with lower heat capacity than solid cores, prone to collapse due to slow solidification) by filling them with green sand or sodium silicate sand for rigidity, or using a "two-piece shell core + internal reinforcing ribs" bonded structure to prevent molten metal infiltration into hollow sections.

An agricultural machinery parts foundry producing large gearbox housing castings frequently encountered surface sink marks and internal shrinkage due to insufficient sand mold compactness. The technical team optimized the molding process to increase sand mold compactness and filled hollow cores with green sand. Post-improvement, the surface quality of gearbox housings improved significantly, with sink marks and shrinkage almost eliminated. Product qualification rate rose from 60% to 90%, effectively reducing production costs.

4. Strict Control of Mold Quality to Minimize Surface Imperfections

Molds are the "master templates" for casting formation, directly determining surface precision: in material and structure selection, prioritize metal molds (instead of wooden molds) to reduce deformation and simplify split pattern molding; use aluminum alloys, wood, or aluminum-wood composites as needed; in precision and maintenance, strictly control mold manufacturing accuracy and surface finish, design gating systems to match patterns, and assign dedicated personnel for storage; establish mold repair standards—discard molds when deformation exceeds 2mm after repair to avoid defects like core misalignment and edge missing.

An aviation component foundry producing complex aero-engine blade castings initially used wooden molds prone to deformation, resulting in poor dimensional accuracy and rough surfaces. After switching to high-precision aluminum alloy molds with strict control over manufacturing accuracy and surface finish, coupled with enhanced maintenance, blade castings achieved significantly improved dimensional accuracy and reduced surface roughness. Surface quality met stringent aviation industry standards, with first-pass yield rising from 40% to 80%, securing more aviation orders for the enterprise.

5. Optimize Casting Process Design to Reduce Surface Defects

A rational process scheme is crucial for improving surface quality. Take generator end covers as an example: The original two-box molding process relied on clay cores for inner surfaces. Due to the complex shape, thin walls, and significant height of the castings, defects such as chill (a hard, brittle microstructure in cast iron), cracks, and flash burrs frequently occurred. After improvement, a "large flat face downward + self-contained sand core split three-box molding + bottom pouring method" was adopted, with multiple dispersed ingates to balance heat distribution. This stabilized molten metal filling, eliminated sand adhesion, burrs, and flash, and avoided surface cracks from shrinkage stress.

A motor manufacturer long struggled with surface quality issues in generator end covers. After adopting the new self-contained sand core split three-box molding process and bottom pouring method, end cover surface quality improved significantly, with chill, cracks, and flash burrs nearly eliminated. The products featured smooth, flat surfaces, meeting high-end customers’ strict appearance requirements and expanding the enterprise’s share in the motor market.

6. Improve Worker Operation Skills and Standardize Full-Process Operations

Worker operational standards directly affect casting surface quality: train new workers through "theoretical training + hands-on practice" in stages, enforce strict adherence to process regulations, and cultivate habits of working to drawings; experienced workers should pass down precise operation skills (e.g., properly placing asbestos cords during mold closing to prevent molten metal leakage, avoiding "meat dragging" when removing flash) to reduce surface roughness and pits from operational errors.

Zhang Ning, a senior foundry worker with 22 years of experience, has mastered exceptional brushing techniques. Brushing is a key process in molding to prevent mechanical sand adhesion from molten metal infiltration into cores and improve casting surface finish. His refined brushing methods have resulted in castings with surface finish far exceeding industry standards. Under his guidance, new workers quickly mastered brushing essentials, enhancing overall casting surface quality in the workshop.

II. 5 Core Factors Influencing Casting Surface Quality

Casting surface quality is affected by multiple factors across processes. Identifying key factors enables targeted optimization:

1. Rationality of Process Design

Neglecting coordination between machining datums and assembly dimensions in process design can cause surface defects: mismatched draft angles in conventional sand casting, or improper pouring positions and shrinkage rates, may lead to dimensional misalignment in mating castings, affecting overall appearance consistency.

2. Accuracy of Process Equipment

Poor sand box positioning accuracy or pattern installation precision on templates can cause casting misalignment (especially obvious in multi-box molding). Misalignment exceeding 5mm is irreparable, directly resulting in appearance-related scrap; damaged or unmaintained patterns lead to uneven surfaces, missing edges/corners, and irregular fillets, compromising geometric integrity.

3. Performance of Molding Materials

Insufficient mold coating thickness or excessively coarse sand grain size can cause sand adhesion, leading to rough surfaces and difficult cleaning; improper binder/curing agent dosage reduces mold strength, causing sand inclusion and deformation; high clay or water content in molding sand can cause large-area gas holes, forming dense pitting.

4. Furnace Charge Quality and Melting Control

Unqualified furnace charge composition (e.g., excessive phosphorus) or poor molten iron inoculation can cause hot cracks—minor cracks may affect appearance after welding, while severe ones result in scrap; inaccurate batching, material mixing, or use of transitional molten iron can cause excessively high or low casting hardness, indirectly reducing post-machining surface finish.

5. Standardization of Worker Operations

Accumulated flash during mold closing or failure to place sealing materials (e.g., asbestos cords) as required can cause molten metal leakage at core prints and excessive flash, increasing cleaning difficulty; loose sand in local mold areas from inadequate ramming can cause sand inclusion or iron nodules; unremoved accumulated ash during brushing leaves grooves and rough surfaces on blanks. Operational skills and equipment precision are the "dual cores" influencing surface quality.

III. Conclusion

Enhancing casting surface quality requires "full-process control + precise measures": reduce defects from the source through filter-based molten metal purification, pouring temperature control, sand mold compactness enhancement, mold/process optimization, and standardized operations; address factors like process design, equipment accuracy, and material performance to form a "prevention + control" closed-loop management. In practice, prioritizing efficient measures like casting filters can improve both appearance and internal quality, safeguarding casting quality.

Blog Author Profile

DAWN | Pig Iron & Castings Procurement Advisor
18 years in the foundry trenches give me an edge: I know how pig iron’s chemistry impacts casting quality and can troubleshoot defects like cracks and porosity. With a 1M MT/year pig iron and 60k MT/year casting output from our in-house factory, plus 200+ verified suppliers on our platform, we offer fast price comparisons. Expect a 24-hour inquiry response—my goal? Not just closing deals, but being your go-to partner in the foundry world.

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