Key Design Considerations for Injection Mold Venting Success

Understanding Venting in Injection Mold

Venting in injection mold refers to the process of allowing trapped gases and air to escape from the mold cavity during the filling stage. Without adequate venting, these trapped gases can lead to a variety of issues, including incomplete fills, surface defects, and structural weaknesses in the finished part. The fundamental goal of venting is to create pathways for gases to escape, allowing the molten material to fill the mold cavity effectively.

Material Properties

The choice of material significantly impacts venting requirements. Materials like Texin and Desmopan resins, which are known for their free-flowing nature at their melt temperature, require careful venting design. Even very shallow venting can lead to part flash due to their high flowability. Therefore, it’s essential to design the part and tool to ensure uniform flow and inject the melt slowly to allow gradual escape of trapped gases or air.

Types of Venting in Injection Mold

There are several types of venting methods, each designed to address specific needs and challenges in mold design. Below are the main types of venting used in injection molding:

External Venting

This involves creating vent holes on the outer surfaces of the mold, allowing air to escape as the molten material enters. External vents can be placed strategically in non-critical areas of the mold to minimize their impact on part aesthetics.

Advantages:

• Simple to implement.
• Effective for many standard applications.
• Can be strategically placed to minimize visual impact on the final part.

Disadvantages:

• Limited in application for complex geometries.
• May require careful placement to avoid compromising part aesthetics.

Internal Venting

This method incorporates channels or grooves within the mold design to facilitate the escape of gases. Internal venting is often preferred for complex geometries, as it can be tailored to specific flow paths.

Advantages:

• Allows for greater control over gas removal.
• Can be tailored to complex part geometries, improving fill rates.

Disadvantages:

• More complex to design and manufacture.
• Requires precise engineering to avoid impacting part strength or integrity.

Automatic Venting Systems

Some advanced molds incorporate automatic venting systems that can adapt to varying pressures during the injection process. These systems utilize mechanical components that open and close vents based on the flow dynamics of the molten material.

Advantages:

• Can optimize venting in real time, adapting to changing conditions.
• Reduces the risk of gas entrapment and improves cycle times.

Disadvantages:

• Higher initial cost due to complexity.
• Maintenance may be required to ensure proper functioning.

Pin Venting

Pin venting utilizes small pins placed in the mold cavity to create tiny venting paths. These pins allow gases to escape while minimizing the impact on part surface finish.

• Advantages:
  • Ideal for parts with intricate designs.
  • Maintains a good aesthetic finish on the part.
• Disadvantages:
  • Requires careful positioning to be effective.
  • May not be suitable for all materials or part shapes.

Consequences of Inadequate Mold Venting

Proper venting ensures the seamless flow of molten material while allowing trapped air and gases to escape. Conversely, inadequate venting can lead to a myriad of problems that not only compromise product quality but also disrupt manufacturing efficiency.

1. Defects in Molded Parts

One of the most immediate consequences of inadequate venting is the emergence of defects in the molded parts. Common defects include:

• Incomplete Fill: When air is trapped within the mold cavity, it prevents the molten material from filling the entire space, leading to missing sections or voids in the finished product.
• Burn Marks: Trapped air can heat up and create localized hot spots, resulting in burn marks on the surface of the molded parts. This not only affects aesthetics but can also weaken the structural integrity of the product.
• Surface Imperfections: Poorly vented molds may lead to surface irregularities, such as blisters or uneven finishes, diminishing the overall quality of the part.

2. Increased Cycle Times

Inadequate venting can lead to prolonged cycle times due to the need for rework or adjustments. When defects occur, manufacturers may have to halt production to address the issues, which can include:

• Adjustments to Mold Design: Modifications may be necessary to improve vent placement or size, adding time and costs to the production process.
• Increased Inspection and Testing: More rigorous quality checks may be required to ensure that parts meet standards, consuming additional time and resources.

3. Increased Material Waste

The presence of defects due to poor venting often results in increased material waste. Defective parts may need to be scrapped or recycled, leading to higher operational costs and decreased profitability. This waste not only affects the bottom line but also has environmental implications, as more raw materials are required to compensate for production losses.

4. Equipment Strain

Inadequate venting can place additional strain on injection molding machinery. When trapped air creates pressure fluctuations within the mold, it can lead to:

• Increased Wear and Tear: The machinery may experience accelerated wear due to the additional stress caused by pressure imbalances, necessitating more frequent maintenance and repairs.
• Potential Equipment Damage: In severe cases, excessive pressure buildup can lead to equipment malfunctions or even catastrophic failures, resulting in costly downtime and repairs.

5. Quality Control Challenges

Quality control becomes increasingly challenging when venting issues are prevalent. The occurrence of defects can lead to:

• Inconsistent Product Quality: Variability in part quality can lead to customer dissatisfaction and affect brand reputation. Maintaining a consistent quality standard is essential for building trust and ensuring repeat business.
• Increased Inspection Costs: Companies may need to invest in more comprehensive quality control measures to catch defects early, which can increase operational costs and extend production timelines.

6. Limitations in Design Flexibility

Inadequate venting can also impose limitations on mold design and complexity. Engineers may have to simplify designs or avoid intricate geometries to mitigate venting issues. This can result in:

• Reduced Innovation: Constraints on mold design can stifle innovation and limit the ability to create complex or customized parts that meet specific customer needs.
• Competitiveness: In a highly competitive market, the inability to produce innovative and high-quality parts can hinder a company’s market position.

Strategic Vent Placement in Injection Molds

To optimize your injection molding process, determining the correct placement of vents is crucial. Effective venting allows trapped air and gases to escape, ensuring a smooth flow of molten material and reducing the risk of defects. Here are key considerations for vent placement in injection molds:

• Parting Line Vents: Placed along the mold’s parting line to allow air escape as the molten material fills the cavity, preventing air entrapment.
• Edge Vents: Located along the edges or corners of the mold cavity to facilitate air release in complex geometries or thick sections.
• Through Vents: Designed to pass entirely through the mold, ideal for thick-walled parts, allowing quick air escape while preventing material leakage.
• Pressure Relief Vents: Strategically positioned within the mold to relieve excessive pressure buildup during injection, preventing mold or machine damage.
• Vents in Deep Sections: Positioned at the deepest points or bends of the mold to ensure effective air escape in areas prone to entrapment.
• Vents Near Gates: Placed close to the injection gate to minimize air entrapment at the start of injection, improving fill rates and reducing burn marks.
• Multiple Vent Locations: Distributed throughout larger or intricate molds to enhance airflow, reduce pressure buildup, and improve overall part quality.

Injection Molds Vent Design Principles

Shallow Venting

When venting is necessary, it should be as shallow as possible to minimize the risk of flash. Start with a depth of 1/2 mil and increase the depth as needed. This approach ensures that the vent is effective while minimizing the potential for material leakage.

Uniform Flow

Design the part and mold to ensure uniform flow of the molten plastic. This can be achieved by optimizing the gate location, runner design, and part geometry. Uniform flow minimizes the formation of trapped gases and air pockets, making venting more effective.

Mold Shrinkage

Shrinkage values vary depending on the material and part design. For Texin and Desmopan TPU resins, typical shrinkage values range from 0.007 to 0.020 in./in. (mm/mm), depending on the part thickness. Complex part designs and flow patterns can make shrinkage more challenging to predict and control. Therefore, it’s essential to consider shrinkage when designing vents to ensure that they do not contribute to dimensional inaccuracies.

Vent Land Dimensions

The term “vent land” describes the area between the mold cavity and the surface of the mold. The standard width for vent land is 0.06 inches, with vent clearances typically ranging from 0.12 to 0.50 inches wide and a depth of 0.02 inches. These specifications are vital for effective venting. While most vents are ground, any milled vents should be polished to achieve a smooth finish.

Common Vent Depths for Various Plastics

Different plastic materials necessitate specific vent depths to ensure proper air evacuation and prevent entrapment. The following table serves as a quick reference for common vent depths for various plastics:

Perimeter Vent Land Specifications

Perimeter vent land surrounds the mold cavity and should maintain a minimum width of 0.125 inches and a maximum width of 0.250 inches. Adequate perimeter venting is essential for releasing air trapped within the cavity during the injection cycle, thereby enhancing material flow and overall part quality.

Mold Steels

Mold steels such as AISI P-20, S-7, and H-13 are commonly used for Texin and Desmopan resins. These materials offer good wear resistance and machinability, essential for long-term mold performance. Aluminum (Type 6061 T-6) can be used for short-run or prototype molds due to its lower cost and ease of machining.

Surface Finish

Texin and Desmopan TPU resins can stick to highly polished surfaces, making a rougher mold surface finish preferable. An SPID-2 finish (formerly SPE/SPI #5 or vapor hone) is an excellent choice for TPU resins. Extending the surface treatment to sprue bushings, runners, and other mold components helps ensure easy ejection of the entire shot.

Draft and Taper

A generous draft and taper help avoid problems removing the part from the mold. Incorporating a 2° or greater taper on all part walls in the direction of the draw is recommended. A lesser taper may require frequent use of a mold release agent to aid part removal.

Undercuts

Parts with undercuts that are pushed from the mold without side action may not hold tight tolerances. In such cases, alternative ejection methods or mold designs should be considered to ensure proper part removal and dimensional accuracy.

Conclusion

Successful injection mold venting requires careful consideration of material properties, mold design, and part geometry. By following the principles outlined in this article, manufacturers can minimize defects and produce high-quality molded parts. Effective venting is essential for achieving optimal performance and reducing costs in the injection molding process.

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Fuquan Chen

This article was written by engineers from the BOYI team. Fuquan Chen is a professional engineer and technical expert with 20 years of experience in rapid prototyping, mold manufacturing, and plastic injection molding.