Why Do Nylon Injection Molded Parts Turn Yellow? Causes and Solutions

Introduction

Nylon (PA6 and PA66) is one of the most widely used engineering plastics due to its excellent mechanical strength, wear resistance, heat resistance, and good processing performance.

It is commonly used in:

• Ergonomic chair structural parts
• Automotive components
• Electrical connectors
• Gears and mechanical parts
• Industrial equipment
• Consumer products

However, during injection molding production, many customers encounter a common quality issue:

Why do nylon injection molded parts turn yellow after production?

Nylon yellowing not only affects the appearance of products but may also indicate material degradation, especially when parts are exposed to high temperatures, UV radiation, or long-term use environments.

This article explains the main causes of nylon yellowing and provides practical solutions.

1. Thermal Oxidation and Material Degradation

One of the most common reasons for nylon yellowing is thermal degradation during injection molding.

PA6 and PA66 require relatively high processing temperatures. If the material experiences excessive heat history, such as:

• Excessive melt temperature
• Long residence time inside the barrel
• Excessive screw shear
• Improper injection molding parameters

the molecular chains of nylon may break down and react with oxygen.

This oxidation process can cause:

• Yellow or brown discoloration
• Reduced mechanical properties
• Surface quality problems

Typical situation:

The part looks normal immediately after molding but gradually becomes yellow after storage or use.

2. Yellowing of Glass Fiber Reinforced Nylon

Glass fiber reinforced nylon is widely used for high-strength structural applications, including:

• PA6 GF30
• PA66 GF30
• PA66 GF40

These materials provide:

• Higher strength
• Better stiffness
• Improved dimensional stability

However, glass fiber reinforced nylon may experience:

• Surface whitening
• Floating fibers
• Uneven color
• Visible yellowing

Main reasons:

1) Poor compatibility between glass fiber and nylon resin

If the bonding between glass fiber and nylon matrix is insufficient, glass fibers may become exposed on the surface, causing uneven appearance.

2) Glass fiber surface treatment influence

The sizing agent used on glass fibers needs to match the nylon system.

Poor compatibility may result in:

• Color changes after high-temperature processing
• Reduced surface appearance

3. Yellowing Caused by Flame Retardant Additives

Flame-retardant nylon is commonly used in:

• Electrical connectors
• New energy components
• Electronic products

Examples include:

• PA66 FR V0
• PA6 FR materials

During processing or long-term use, some flame retardant systems may cause:

• Additive migration
• Surface deposits
• Color changes

This may appear as:

• Yellow spots
• Surface discoloration
• Reduced gloss

Therefore, flame-retardant nylon requires careful formulation design to achieve both fire safety and color stability.

4. UV Exposure and Environmental Aging

For outdoor applications, nylon parts may be exposed to:

• Sunlight
• UV radiation
• High temperature
• Humidity

Long-term exposure can cause photo-oxidation.

Typical affected products include:

• Outdoor equipment
• Automotive parts
• Sports products
• Bicycle components

Possible results:

• Surface yellowing
• Material embrittlement
• Reduced service life

5. Raw Material Storage and Processing Contamination

Nylon is a moisture-sensitive engineering plastic.

Improper storage or processing conditions can also cause yellowing.

Raw material issues:

• Excessive moisture absorption
• Long-term exposure to air
• Contaminated materials

Production issues:

• Poor barrel cleaning
• Mixing with other plastics
• Uneven colorant dispersion

These factors can affect the final appearance of injection molded parts.

Solutions to Prevent Nylon Yellowing

1. Optimize Injection Molding Parameters

Proper processing conditions are critical.

Control melt temperature

Avoid:

• Excessive processing temperature
• Long material residence time

Typical processing ranges:

• PA6: around 250–280°C
• PA66: around 270–300°C

(The exact parameters depend on material grade and injection molding equipment.)

Reduce shear heat

Optimize:

• Screw speed
• Back pressure
• Injection speed

to minimize thermal degradation.

2. Select High-Performance Anti-Yellowing Nylon Materials

For applications requiring excellent appearance and long-term stability, choose modified nylon materials with:

• Anti-yellowing additives
• Improved antioxidant systems
• UV resistance
• High surface quality formulation

A professional material supplier can optimize:

• Nylon resin system
• Additive package
• Color formulation
• Glass fiber compatibility

3. Improve Glass Fiber Reinforced Nylon Surface Quality

For PA6 GF30 and PA66 GF30 applications, surface appearance can be improved through:

• Optimized glass fiber content
• Better fiber-resin bonding
• Surface modification technology
• Improved mold flow design

This helps reduce:

• Floating fibers
• Whitening
• Uneven color

For products such as ergonomic chair bases and chair back frames, the material needs to balance:

• High strength
• Dimensional stability
• Good surface appearance

4. Add UV Stabilizers and Antioxidants

For outdoor and long-life applications, nylon compounds can include:

• UV stabilizers
• Antioxidants
• Light stabilizers

Benefits include:

• Better weather resistance
• Improved color stability
• Longer service life

5. Improve Nylon Drying Process

Nylon absorbs moisture easily.

Insufficient drying may cause:

• Silver streaks
• Bubbles
• Surface discoloration
• Reduced mechanical performance

Recommended practices:

• Proper material drying before molding
• Control moisture content
• Avoid long exposure after drying

Recommended Nylon Materials for Different Applications