Surfacing Process and Economic Analysis for Large Covering Parts Insert Molds

Literature Overview

The paper by He Bolin, Yu Yingxia, Zhang Jianxin, and Wei Xingbao, published in Machinery Design and Manufacture (2009, No. 11, pp. 238-240), presents a comprehensive study of surfacing technology applied to large covering parts insert molds in automotive manufacturing. The research was conducted by East China Jiaotong University's Key Laboratory of Road Vehicle Engineering and the Luoyang First Tractor and Engineering Machinery Company, supported by the former Ministry of Machine Building's Education Department Science and Technology Fund. This work bridges the gap between welding technology and tool engineering economics.

Technical Problem Statement

Large covering parts molds in automotive manufacturing require frequent replacement of insert blocks due to severe wear during production. Traditional approaches using tool steel inserts require extensive machining, heat treatment, and grinding operations, resulting in high costs and long lead times. The authors propose an alternative approach: using ordinary cast iron as the base material and applying surfacing layers to create functional mold surfaces.

Comparison of Approaches

Surfacing Process Design

The surfacing process for mold insert blocks requires careful consideration of the following factors:

1. Surface quality requirements - Mold surfaces require smooth finishes for high-quality part surfaces
2. Hardness requirements - Adequate hardness for resistance to abrasion and impact during stamping
3. Dimensional accuracy - Surfacing buildup must accommodate subsequent grinding to final dimensions
4. Bond strength - Critical for preventing delamination during high-pressure stamping operations
5. Thermal stability - The surfacing layer must maintain properties under repeated thermal cycling

Process Parameters

Economic Analysis

The economic advantages of the surfacing approach are substantial and quantifiable:

Cost Structure Analysis

The traditional approach to mold insert manufacturing involves:

• Selection of expensive tool steel materials
• Extensive CNC machining to achieve precise geometry
• Full hardening and tempering heat treatment cycles
• Precision grinding of working surfaces
• Regular replacement when worn

The surfacing approach eliminates most of these steps:

• Use of inexpensive cast iron blanks
• Minimal machining of the cast iron base
• Surfacing deposition of the functional layer
• Grinding of the surfacing layer to final dimensions
• Easy repair by additional surfacing when worn

Technical Challenges and Solutions

Challenges in Cast Iron Surfacing

Cast iron presents unique challenges for surfacing due to:

• High carbon content leading to hard, brittle carbide formation in the HAZ
• Graphite flakes creating stress concentration points
• Low ductility making the material prone to cracking
• Poor machinability of the base material

Solutions Implemented

The authors addressed these challenges through:

1. Proper preheating of the cast iron base to 250-300°C to reduce thermal stresses
2. Selection of appropriate surfacing electrodes with suitable alloy composition
3. Controlled welding parameters to minimize heat input and reduce HAZ hardness
4. Multi-pass deposition to manage dilution and achieve proper composition
5. Post-weld stress relief treatment to minimize residual stresses

Microstructural Analysis of Surfaced Edge

The surfacing layer on the mold edge must achieve adequate hardness while maintaining sufficient toughness to resist impact during stamping. The microstructure of the deposited layer typically includes:

• Pearlite and ferrite matrix providing the base toughness
• Carbide particles (Fe3C, alloy carbides) providing wear resistance
• Possible martensite formation depending on cooling rates and alloy content
• Graphite inclusions from the base material at the fusion boundary

The hardness profile from the base material through the fusion zone to the surface of the deposit shows a characteristic gradient, with the highest hardness at the surface and decreasing hardness toward the base material. This gradient is beneficial as it provides wear resistance at the working surface while maintaining toughness in the supporting structure.

Engineering Practice and Implementation

For successful implementation in production environments, the following practices are recommended:

• Establish qualified welding procedures with documented parameters
• Train operators in surfacing techniques specific to mold repair
• Implement regular inspection of surfacing layer condition during production
• Maintain records of surfacing cycles and service life for each mold insert
• Develop standardized repair procedures for worn surfacing layers
• Conduct periodic hardness and surface roughness measurements

Study Insights and Implications

This paper demonstrates that surfacing technology can fundamentally transform mold manufacturing economics. The 80% reduction in design workload and 90% savings in precious metal materials represent transformative improvements for automotive manufacturing operations. The approach also enables rapid mold repair, significantly reducing downtime. The use of ordinary cast iron as a base material with a functional surfacing layer represents a paradigm shift from the traditional approach of using expensive materials throughout the entire component. This philosophy of functional gradient design, where only the critical surface receives expensive material treatment, has broad applicability across tool and die manufacturing. The demonstrated service life of 25,000 pieces confirms that the surfacing approach is not merely a cost-cutting measure but a technically sound solution that meets production requirements.