How to Avoid Voids in Injection Molding: The Secret Weapon for "Mine-Clearing" in Plastics

Imagine taking a bite of a freshly baked cake only to find hollow cavities inside—disappointing, right? Voids in injection molding are like those hidden cavities in a cake. They not only ruin a part’s appearance and dimensional accuracy but also pose a "time bomb" to its structural strength. Where do these hidden air bubbles or cavities in plastics come from, and how can we defuse them using a "mine-clearing manual"?

1. Where Do Voids Come From? The "Breathing Crisis" in Plastics

1.1 Air "Hiding"—Troublesome Trapped Air

• Scenario: When molten plastic rushes into the mold like a flood, air in corners and narrow gaps doesn’t have time to escape and gets trapped.

  • Data: When the depth of a mold cavity exceeds three times its width, the risk of trapped air increases by 50%!

  
  

• Case Study: When injection molding wheels for toy cars, tiny voids often form between the spokes due to trapped air.

1.2 Plastics "Blowing Bubbles"—Moisture in Raw Materials

• Mechanism: Hygroscopic plastics (e.g., nylon, PC) act like sponges. If not dried properly, they "blow bubbles"—water vaporizes at high temperatures and gets trapped in the part.

  • Data: When the moisture content in nylon exceeds 0.2%, the void occurrence rate soars to 80%!

  
  

• Experiment: Injection molding with undried ABS raw materials results in cross-sections showing dense "pearly white" bubble layers.

1.3 Cooling "Emergency Brake"—Shrinkage Crisis

• Phenomenon: The outer layer of plastic cools and hardens rapidly, while the inner layer shrinks and "hollows out," creating vacuum voids (especially in thick-walled parts).

  • Data: The void rate in thick-walled PP parts (wall thickness >4mm) is three times higher than in thin-walled parts!

  
  

2. Mine-Clearing Solutions: Three Magic Tools for "Ventilating" Plastics

2.1 Mold Design: Building an "Escape Route" for Air

• Tool 1: Adding Venting Grooves

  • Operation: Cut venting grooves 0.02–0.05mm deep and 5–10mm wide at the last-filled positions of the mold cavity (e.g., tops, ribs).

  • Effect: After adding venting grooves to a gear mold, the void rate plummeted from 25% to 3%!

  
  

• Analogy: It’s like opening an emergency lane on a congested highway, allowing air to evacuate quickly.

2.2 Raw Material Handling: Letting Plastics "Breathe Dry Air"

• Tool 2: Precise Raw Material Drying

  • Data Guide:

    • Nylon (PA66): Dry at 120°C for 4–6 hours, moisture content ≤0.1%

    • Polycarbonate (PC): Dry at 110°C for 8–10 hours, moisture content ≤0.02%

    
    

  • Tool: Use hot-air dryers with dew-point monitoring to avoid "false drying."

  
  

2.3 Process Adjustment: Gentle Filling for Stable Plastic Flow

• Tool 3: Stage Injection for Gentle Filling

  • Two-Stage Injection Method:

    1. Fast Filling: Fill the first 80% of the cavity at high speed (e.g., 100mm/s) to push air out quickly.

    2. Slow Packing: Fill the last 20% at low speed (e.g., 30mm/s) to avoid turbulent flow and air entrapment.

    
    

  • Case Study: After adopting stage injection for a medical catheter, internal voids changed from a "honeycomb structure" to completely bubble-free.

  
  

• Key Data: Packing pressure should be 60–80% of injection pressure, and extending packing time by 5–10 seconds can reduce shrinkage voids by 50%.

3. Practical Tips: Diagnosing Voids with "Look, Listen, Ask, and Test"

4. Advanced Skills: "Debloating" Thick-Walled Parts

• Tool 4: Using Blowing Agents

  • Add 0.5–1% chemical blowing agents (e.g., azodicarbonamide) to create micro-foaming inside the plastic, filling shrinkage spaces and reducing voids by 90%!

  
  

• Tool 5: Gradient Cooling

  • Design the mold to cool thick-walled areas 20–30% slower than thin-walled areas. For example:

    • Thin-wall cooling water temperature: 15°C

    • Thick-wall cooling water temperature: 25°C

    
    

  
  

5. Ultimate Mantra: The Void-Free Magic Song