Causes and Solutions for Splay Marks in Injection Molded Products

1. Characteristics of Splay Marks Caused by Moisture

• They are irregularly distributed and extend along the injection flow direction; in severe cases, they cover the entire plastic part surface with willow-like slender streaks.

• A key distinction: fixed-position silver streaks are not always caused by insufficient drying. If the material is properly dried, silver streaks should appear randomly. In custom plastic injection molding, recognizing this difference is critical for accurate troubleshooting, as it directly impacts the efficiency of defect correction for tailored parts.

2. Root Causes of Moisture-Induced Splay Marks

1. Inadequate Drying: Pellets are not fully dried before molding, or dried pellets absorb moisture again after prolonged storage in the hopper, resulting in water content exceeding the allowable limit.

2. Moisture Vaporization and Resin Degradation: In the high-temperature barrel, moisture in the pellets vaporizes, causing the resin to degrade and produce carbon dioxide gas.

3. Gas Entrapment and Streak Formation: These gas bubbles become trapped in the molten resin and are pushed into the mold cavity during filling, forming silver streaks on the part surface. These streaks not only mar appearance but also reduce the part’s mechanical strength and structural integrity.

   In custom plastic injection molding, where materials and part geometries are often customized, the risk of moisture absorption and subsequent splay marks is heightened, making strict moisture control even more essential.

3. Inspection Methods for Material Dryness

3.1 Glass Compression Test

1. Clamp a single pellet between two glass slides and place it on a heating plate set to 280–300°C.

2. As the pellet melts, apply steady pressure to flatten the molten material.

3. Check for bubble formation: no bubbles indicate adequate drying (suitable for molding); bubbles mean further drying is required.

   This method is widely used in custom plastic injection molding workshops due to its simplicity and real-time feedback, making it ideal for validating material readiness for custom part production.

3.2 Empty Injection Test

1. If splay marks appear in thick-wall sections of the part, perform an empty injection test (eject molten material directly without filling the mold).

2. For materials like ABS, inspect the surface of the ejected molten plastic: a smooth, bubble-free surface indicates proper drying; a dull surface with bubbles signals insufficient drying.

4. Solutions to Eliminate Moisture-Induced Splay Marks

4.1 Optimize Material Drying Processes

• Strict Moisture Control: Ensure pellets are dried to a water content below 0.3%, following the material supplier’s recommended drying parameters.

• Prevent Reabsorption: If dried pellets absorb moisture in the hopper, install or upgrade heating and insulation devices (e.g., infrared bulbs on the hopper top, with a power density of 25–30W per square meter). Control batch feeding to limit pellet residence time in the hopper to 30–60 minutes. Alternative drying methods include steam jackets or resistance wire heating.

4.2 Adjust Mold Design

• Enlarge Runner and Gate Sizes: Narrow sprue, runner, or gate designs cause excessive shear heat, leading to resin degradation. Increasing their dimensions reduces shear stress.

• Optimize Venting: Ensure proper vent size (0.25mm for crystalline plastics, 0.38mm for amorphous plastics) to allow air and vapor to escape.

• Enlarge Cold Slug Wells: Capture cold material at the start of injection to prevent it from entering the mold cavity and causing defects.

4.3 Optimize Molding Process Parameters

• Increase Back Pressure: Reduces air content in the molten plastic, minimizing gas entrapment.

• Lower Melt Temperature/Injection Pressure/Speed: Reduces resin degradation and shear heat, preventing excessive gas formation.

• Ensure Effective Venting: Verify the vent system is unobstructed to allow smooth escape of air and steam.

4.4 Check Injection Molding Machine Specifications

Conclusion