How to Avoid Voids in Injection Molding

author: CHEN

2025-04-14

Voids are internal air pockets or cavities that form inside molded plastic parts during the injection molding process. These voids can weaken the integrity of the part, lead to material wastage, and affect the overall quality of the product. Voids are particularly problematic in parts that require high strength, transparency, or a smooth surface finish. For injection molding manufacturers, avoiding voids is crucial for producing durable, functional, and high-quality components.

Next HOPO MOULD will explore the causes of voids in injection molding, their consequences, and provide practical tips and strategies for preventing them to ensure your production runs smoothly.

TAIZHOU HOPO MOULD & PLASTIC TECHNOLOGY CO., LTD is a professional plastic injection mould maker and relevant plastic products manufacturer

What Are Voids in Injection Molding?

Voids refer to small, unwanted air pockets or empty spaces that form within a molded part. These pockets can occur in various shapes and sizes, often caused by improper filling or cooling within the mold cavity. Voids can occur at any stage of the molding process but are most often seen in thicker parts where the material flow is disrupted, or in complex geometries where air can become trapped.

What Common Issues Caused by Voids

- Weakened Part Structure: Voids reduce the overall strength of the molded part.
- Appearance Defects: Voids can lead to cosmetic issues like surface blemishes or inconsistent finishes.
- Poor Functionality: Voids can impair the part's performance, especially in pressure-sensitive applications.
- Material Waste: Voids often necessitate remanufacturing or scrapping parts, leading to material wastage and increased costs.

Common Causes of Voids in Injection Molding

To effectively avoid voids, it’s essential to understand the root causes behind their formation. Here are some of the primary reasons voids appear in injection molded parts:

1. Insufficient Injection Pressure - Low injection pressure can prevent the mold cavity from fully filling, leaving air pockets trapped inside the part.
2. Inadequate Injection Speed - If the injection speed is too slow, the molten plastic may not flow efficiently through the mold, leading to incomplete filling and voids.

3. Incorrect Cooling Time - If cooling occurs too quickly or unevenly, it can cause parts to solidify too soon, leading to internal voids.

4. Mold Design Issues - Thin walls, complex shapes, or poorly designed runners can create areas where air is trapped, leading to void formation.

5. Improper Material Flow - If the material flow is not optimized, it may leave certain areas underfilled, creating voids in the part.

6. Air Entrapment - Air can become trapped in the mold cavity due to poor venting or improper filling, leading to voids.

How to Avoid Voids in Injection Molding?

1. Optimize Injection Pressure

One of the most effective ways to prevent voids is by ensuring that the injection pressure is high enough to completely fill the mold cavity. If the pressure is too low, the material won't flow efficiently, leaving air pockets behind.

- Tip: Fine-tune the injection pressure to match the specific material being used and the complexity of the part design.

2. Control Injection Speed

Maintaining an optimal injection speed is essential to ensure smooth and uniform material flow. If the injection speed is too slow, it may result in incomplete filling and the formation of voids. Conversely, if the speed is too high, it can lead to material degradation or flash.

- Tip: Use a controlled injection profile that starts slow and increases gradually to prevent air from being trapped.

3. Adjust Cooling Time

Cooling time plays a critical role in preventing voids. If the material cools too quickly, it may solidify before completely filling the mold cavity, resulting in voids. Ensuring an adequate cooling time allows the material to solidify uniformly and completely.

- Tip: Use cooling channels in the mold design to promote even and controlled cooling. Adjust cooling times based on the material's characteristics and the part's geometry.

4. Improve Mold Design

A well-designed mold helps ensure uniform material flow and prevents void formation. Design modifications to reduce voids may include:

- Use of thicker part walls: Thin sections can result in uneven cooling and material flow, leading to voids.
- Runners and gating systems: Optimize runners and gates to ensure uniform filling.
- Ventilation: Proper venting allows trapped air to escape, preventing voids.

- Tip: Work with experienced mold designers to create molds that are optimized for uniform material flow and proper venting.

5. Select the Right Material

Choosing the right material for injection molding is crucial for preventing voids. Materials with high flowability, good thermal properties, and low viscosity typically fill mold cavities more efficiently, reducing the likelihood of voids.

- Tip: Test different materials and choose one that suits the design and flow requirements of your specific application.

6. Proper Venting

Ensuring adequate venting is crucial for allowing trapped air to escape during the injection process. Poor or insufficient venting can lead to air entrapment, which results in voids. Make sure that the mold is designed with appropriate vents that allow air to escape as the mold fills.

- Tip: Inspect vent locations regularly and ensure they are clear of debris to maintain proper air escape.

7. Use of Hot Runner Systems

Hot runner systems maintain a consistent temperature throughout the injection molding process, which helps improve material flow and reduce the chances of void formation. Hot runners also reduce cycle times and minimize material waste by keeping the molten material at an optimal temperature.

- Tip: If you're dealing with complex parts or high-performance materials, consider implementing a hot runner system to enhance flow consistency and reduce voids.

8. Monitor Process Parameters

Regular monitoring of injection molding machine settings such as injection speed, pressure, temperature, and cooling times can help identify potential issues that might lead to voids. Use automated monitoring systems that can adjust parameters in real-time to optimize the molding process.

- Tip: Invest in process control software that allows for real-time adjustments and tracking of molding conditions to ensure consistent quality.

Preventing Voids in a Complex Plastic Part

Problem: A manufacturer of complex plastic components for medical devices experienced issues with voids inside the molded parts. These voids compromised the structural integrity of the parts, leading to increased rework and scrap.

Solution: The manufacturer implemented several changes to reduce voids:
1. Increased injection pressure and optimized the injection speed for smoother flow.
2. Improved mold design by increasing wall thickness and adding more venting to allow air to escape.
3. Implemented hot runner systems to ensure uniform temperature distribution and reduce material degradation.
4. Monitored cooling time more carefully to avoid premature solidification.

By adopting these best practices, the manufacturer was able to significantly reduce voids, improving the overall quality of the parts and reducing production costs.

Voids in injection molding can lead to serious quality issues that affect both the performance and appearance of your molded parts. By optimizing injection pressure, controlling injection speed, improving mold design, selecting the right materials, and ensuring proper venting and cooling, you can significantly reduce the risk of voids in your molding process.

Implementing these strategies will help ensure a more efficient, cost-effective, and high-quality injection molding process. By preventing voids, manufacturers can produce stronger, more reliable parts that meet their customers' needs, leading to greater satisfaction and reduced waste.

TAIZHOU HOPO MOULD & PLASTIC TECHNOLOGY CO., LTD is a professional plastic injection mould maker and relevant plastic products manufacturer established in 2012. Our Factory is in Zhejiang, China. We enjoy convenient land, water and air transportation, so we could provide short delivery time once the production finished. HOPO offer many kinds of high quality moulds, including daily use commodity moulds, home appliance moulds, automotive moulds, pipe fitting moulds and industrial products moulds, also could directy offer you those relevant plastic parts prodcution.