【Improvement Case Study】Countermeasures for Thin Burr Occurrence in Two-Colour Molded Parts

Introduction

In manufacturing environments, synchronising the molding and assembly processes is a critical challenge for simultaneously achieving improved production efficiency and quality. Particularly with injection-molded parts, even minor issues in mold design or molding conditions can significantly impact the assembly process, frequently leading to the release of defective products. This paper presents actual defect countermeasures implemented and their outcomes, using automotive exterior components as a case study.

Background and Recognition of Challenges

At our Gunma Plant, there had been a high incidence of moulding defects (inclusion of thin flash, flash formation) in two-colour moulded products (ASA + elastomer) for vehicle exterior components.

This product faced such significant quality issues that it was internally classified as one of the “worst three products”. Particularly concerning was the high risk of moulding defects being carried over into the assembly process, posing a major quality assurance concern on the shop floor.

Actual Defects and Root Cause Analysis

Primary Defects

Thin burr contamination: A major factor accounting for over half of product defects.

Rework due to burr occurrence: While not classified as defects, burr removal required dedicated personnel, wasting the man-hours of one worker UK.

▸ Thin Burr Inclusion

Defect Occurrence Mechanism

The primary cause of thin burr inclusion lay in the tunnel gate structure used for the second-colour TPE molding.

Thin burrs formed on the tab section, and in some cases the tab itself detached. This residue was forced onto the design surface during the next shot, creating a vicious cycle that produced defective products. It was also discovered that resin pressure caused resin to infiltrate the gap between the insert and the corner projection, gradually crushing the mould and forming an undercut condition.

Countermeasures and Trial History

Countermeasure 1: Insert Replacement

Instead of repairing the crushed surface, a new insert was manufactured. However, burrs reappeared after just one day (1200 shots). This did not provide a fundamental solution.

Countermeasure 2: Nitriding Treatment

Nitriding treatment was applied to enhance the insert's strength. While initially effective, burrs reoccurred after 1200 shots.

Countermeasure 3: Creating an Escape Route for Burrs

By shifting our approach, we deliberately implemented manual processing to ensure flash was ejected from the nested moulds. This eliminated residue within the nested cavities and drastically reduced gate flash contamination. Although tab jamming occurred, it was resolved through regular cleaning.

▸Burr Generation

Optimisation of Molding Conditions

This product's mold employs a four-cavity structure (multi-cavity molding). Consequently, subtle imbalances in gate pressure caused conflicting defects to occur simultaneously: ‘sink marks in one cavity and burrs in another’. Burrs were particularly pronounced during restart after short circuits.

Gate diameter adjustment: Effective but with a narrow operating window, insufficient for stable production.

Resin temperature reduction trial: Lowering the temperature within the manufacturer's recommended range of 190–230°C suppressed flash occurrence and secured an operating window where sink marks were less likely to appear even when pressure was increased. Consequently, the defect rate was reduced to

• 

  
  

Achievements

Improvements in Quality and Labour Hours

Completely eliminated the need for burr removal work, reducing labour hours equivalent to 1.5 positions.

This resulted in actual savings equivalent to 3.5 positions, amounting to ¥12.6 million in annual cost reductions.

Achievement of Process Synchronisation

By reducing the risk of defective parts escaping, we eliminated wasteful processes previously required, such as intermediate inventory, transport, and re-inspection. This established a system enabling direct linkage between molding and assembly. Consequently, we simultaneously achieved shorter process lead times and reduced inventory.

Future Developments

Consideration of Automated Equipment Introduction

An automatic clip insertion mechanism using a parts feeder. However, due to increased costs and the risk of equipment downtime, the investment effectiveness is under careful review.

Simple Synchronisation via Tray Method

Achievable at a lower cost, but preparation work remains in the preceding process.

Both initiatives aim for ‘process integration’ and ‘reducing the workload on the shop floor’. Improvement activities will continue.

Summary and Considerations

The key lesson from this case study is that

defect countermeasures are insufficient if they merely suppress symptoms; fundamental root cause analysis and optimisation across the entire process are essential.

Advancing improvements through both condition adjustment and mold countermeasures leads to production innovation, such as synchronisation with the assembly process.

The importance of determining optimal conditions not through desk-based analysis or historical data alone, but by deriving them through verification on the actual machine.

Through this improvement activity, the Gunma Plant achieved a ‘significant reduction in defect rates’ and ‘synchronisation of the molding and assembly processes’. While continuing efforts to prevent recurrence, new challenges are anticipated, with a view towards aut