Abstract
This paper presents an analysis of the surface slag defects observed in heavy-section machine tool tray castings produced by our company. The study focuses on identifying the causes of these defects and proposes effective measures to mitigate them. By optimizing the gating system design and employing appropriate coatings, the slag defects were significantly reduced, thereby enhancing the aesthetic quality of the castings and meeting customer specifications.
Keywords: heavy-section, machine tool tray casting, anti-sulfur infiltration coating, slag defect
1. Introduction
Machine tool tray castings serve as crucial components in CNC machines, significantly impacting the dimensional stability and machining accuracy of the overall equipment. Due to their exposure during operation, these castings necessitate stringent surface quality standards. However, the production of heavy-section machine tool tray castings, typically with wall thicknesses exceeding 100 mm, presents unique challenges, including slow solidification rates and susceptibility to surface slag defects. This paper delves into the analysis of slag defects encountered during production and outlines the improvements implemented to address these issues.
2. Product Structure Analysis and Technical Requirements
This section examines a specific machine tool tray casting produced by our company, highlighting its dimensions, weight, material, and technical specifications.
2.1 Product Dimensions and Weight
The machine tool tray casting under study measures approximately 800 mm × 800 mm × 200 mm in outline dimensions and weighs 700 kg .
2.2 Material Specification
The casting is manufactured from HT300 gray iron, a material chosen for its suitable mechanical properties and casting characteristics.
2.3 Technical Requirements
The product must meet stringent technical requirements, ensuring that the machined surface is free from any casting defects, including pores, sand holes, and slag defects, to guarantee excellent aesthetic quality and functional performance.
3. Casting Defects Analysis
During the production process, surface slag defects and inclusions were consistently observed on the upper surface of the castings , adversely affecting both the aesthetic quality and the customer’s installation experience.
4. Casting Process Analysis
To better understand and address the slag defect issue, a comprehensive analysis of the casting process was conducted.
4.1 Casting Process Overview
Our company employs a furan self-hardening resin sand process, utilizing pattern plate molding with one mold for each casting. The gating system comprises a φ60 mm ceramic sprue, trapezoidal runner with dimensions of 40 mm (top) × 50 mm (bottom) × 50 mm (height), and six ingates measuring 60 mm in width and 7 mm in thickness. The cross-sectional ratios of the gating system are 1:1.8:1.2 (ingate: runner: sprue), configuring a semi-closed system. Additionally, two 100 mm × 100 mm × 22 mm, 20 PPI filters are placed in the runner.
4.2 Numerical Simulation Analysis
To identify the root causes of the observed defects, numerical simulation using AnyCasting software was performed. The simulation analyzed the mold filling, gas entrapment, and slag inclusion processes.
The simulation revealed that the semi-closed gating system resulted in turbulent flow and significant gas entrainment during mold filling. This led to extensive iron oxide formation, which aggregated into larger slag particles or dispersed as slag pores on the casting surface, potentially causing casting scrap.
5. Process Optimization and Improvement Measures
Based on the simulation results and defect analysis, several optimization measures were implemented to mitigate the slag defect issue.
5.1 Optimization of Gating System
The original semi-closed gating system was modified to an open system to reduce the pouring velocity and pressure, enabling a smoother and more controlled iron flow during mold filling. This minimized splashing and turbulence, thereby reducing secondary slag formation. Additionally, the slag trap structure of the filter was optimized, with the runner positioned in the lower mold box and the ingate dimensions adjusted to 100 mm × 10 mm, while maintaining the number of ingates. The cross-sectional ratios of the optimized gating system are 1:0.75:0.47 (ingate: runner: sprue).
Another AnyCasting simulation was conducted to assess the improved gating system . The results demonstrated a smoother iron flow with minimal turbulence and gas entrainment, leading to fewer and smaller secondary oxide slag particles.
5.2 Casting Trial with Optimized Process
Following the gating system optimization, casting trials were conducted using the same melting process. Post-shot blasting inspection revealed a reduction in the number and size of slag defects, indicating partial improvement.
5.3 Further Analysis and Additional Measures
Despite the partial improvement, slag aggregation persisted. Further discussion with foundry experts revealed that the slag defects primarily comprised oxides and sulfides of Ba and Ca, originating from the silicon-calcium-barium inoculant used. The sulfur content in the reclaimed sand was detected at around 0.2%, slightly elevated.
To address this, the following additional measures were taken:
- Low-Acidity Curing Agent: A low-acidity curing agent was selected, with its addition controlled at approximately 45% to reduce the sulfur content in the reclaimed sand.
- Modification of Inoculant: The silicon-calcium-barium inoculant was replaced with ferrosilicon inoculant, maintaining the same addition amount to control the introduction of slag-forming elements such as Ba, Ca, and S into the iron melt.
- Antioxidant and Anti-Sulfur Infiltration Coating: Given the large section size and slow solidification of the castings, combined with prolonged high-temperature baking of the mold, an antioxidant and anti-sulfur infiltration coating was applied to shield against sulfur infiltration from the mold and reduce iron oxidation, thus minimizing slag and oxide formation.
5.4 Casting Trial with Further Optimizations
After implementing the additional measures, another casting trial was conducted. Post-shot blasting inspection showed a significant reduction in slag defects, with a clean surface free from large slag pores .
The effectiveness of the improvements was further validated through multiple small-batch production runs of similar castings, resulting in a notable decrease in slag defects and scrap rates. Currently, the optimized process is in full-scale production, successfully addressing the slag defect issue.
6. Conclusion
6.1 Importance of Gating System Design
The design of the gating system plays a crucial role in reducing casting defects. For products with different structures, an appropriate gating system can effectively prevent slag inclusion and ensure smooth iron flow during mold filling.
6.2 Application of Numerical Simulation
Numerical simulation techniques offer valuable insights into the feasibility of casting process designs. By analyzing simulation results, process rationality can be assessed, and improvements can be made to preemptively address potential issues and defects, ultimately enhancing product quality.
6.3 Selection of Coatings
The choice of coating is equally important, particularly for products with specific structures and production processes. The right coating can significantly reduce casting defects, improve product quality, and decrease scrap rates.