Mold Structure Improvement Design for Extruded Small-Radius Elbows

Literature Overview

This paper by Xu Shaohua, Zhang Ying, and Zeng Li (2015), published in Modern Manufacturing Engineering and funded by the Jiangsu Provincial Science and Technology Support Project (BE2010113), addresses the quality issues encountered in the extrusion forming of small-radius elbows for serpentine tube production lines. The authors describe the original mold structure, identify quality problems, and present an improved mold design based on extensive process trials.

Core Findings and Technical Approach

Background: Small-Radius Elbow Production

Small-radius elbows (typically R/D < 1.0) are used in serpentine tube assemblies for applications such as heat exchangers, chemical reactors, and cryogenic systems. The small radius requires significant material deformation, making the extrusion process challenging:

• High deformation strain at the inner arc surface.
• Risk of wall thinning, wrinkling, and cracking.
• Difficulty achieving dimensional accuracy.
• Mold wear and die life concerns.

Original Mold Structure and Quality Problems

The original mold design exhibited several quality issues:

Improved Mold Design

The improved mold design addressed these issues through several key modifications:

1. Expanded compression deformation zone: The original mold had a concentrated deformation zone that caused localized high strain. The improved design spreads the deformation over a larger area, reducing peak strain and promoting more uniform material flow.
2. Corrected deformation parameters: Through extensive process trials, the authors optimized:

• Compression ratio (the ratio of material compression to bending)
• Die opening angle
• Contact surface geometry
• Lubrication parameters

1. Enhanced constraint features: Additional constraint elements were added to the mold to guide material flow and prevent wrinkling and tearing.
2. Modified die geometry: The contact surfaces were redesigned to distribute contact stresses more uniformly, extending die life and improving surface finish.

Process Parameter Optimization

The process trials involved systematic variation of key parameters:

Engineering Practice Implications

FMEA Analysis of Extrusion Forming

Applying Failure Mode and Effects Analysis to the extrusion forming process:

5W2H Analysis of the Improvement

Process Control Considerations

For successful implementation of the improved mold design:

1. Material selection: The base material must have adequate formability for the required strain levels. Austenitic stainless steels (304, 316) and low-carbon steels are generally suitable; high-strength alloys may require warm forming.
2. Temperature control: Maintaining consistent forming temperature is critical for reproducibility. Even modest temperature variations (±10°C) can significantly affect formability and final properties.
3. Lubrication: Proper lubrication reduces friction, prevents galling, and promotes uniform material flow. The lubricant must be compatible with the material and withstand the forming temperatures.
4. Die maintenance: Regular inspection of die surfaces for wear, galling, and deformation is essential. A die maintenance schedule should be established based on production volume and wear rates observed during trials.

Key Reflections

This paper illustrates the iterative nature of manufacturing process development. The improvement was not achieved through a single insight but through systematic experimentation and parameter optimization. The authors' approach—starting with the original design, identifying specific quality problems, and then systematically modifying the mold structure and process parameters—is a model of good engineering practice.

The expansion of the compression deformation zone is particularly insightful. In many forming operations, engineers instinctively concentrate the deformation in a small area to minimize tooling complexity. However, this case demonstrates that spreading the deformation over a larger area can significantly improve quality by reducing peak strain and promoting more uniform material flow.

The emphasis on process trials is commendable. In an era of increasing reliance on simulation and virtual prototyping, this paper reminds us that physical trials remain essential for validating and refining process designs, particularly for complex forming operations where material behavior is difficult to predict accurately.

Study Insights and Recommendations

The practical implications of this study for manufacturing engineers are clear:

• Mold design is iterative: Initial designs should be considered prototypes, with expectations of refinement based on trial results.
• Deformation zone management: The geometry of the deformation zone is as important as the die geometry itself. Spreading deformation over larger areas generally improves quality.
• Process parameter optimization: Systematic parameter trials (using methods such as Taguchi or response surface methodology) can significantly improve forming quality and reproducibility.
• Quality metrics: Establish clear quality metrics (wall thickness variation, dimensional accuracy, surface finish, mechanical properties) and use these to guide process optimization.

This case study reinforces the principle that manufacturing quality is determined by the interaction of material, process, and equipment. Improvements to any one of these elements must be evaluated in the context of the entire system. The mold structure improvement described here achieved quality gains not by changing the material or the equipment, but by optimizing the process parameters and die geometry—a reminder that process knowledge and engineering judgment remain irreplaceable in manufacturing.