Venting in injection molding is an important aspect of the injection molding process that ensures high-quality plastic parts and protects the mold. Without proper venting, this trapped air can cause various defects in the final product. The design of the venting system is critical and should be considered during the mold design phase. Factors such as vent placement, dimensions, and the number of vents all play a role in effective venting. In this post, we will explore the injection molding venting types and design considerations in injection molding.
What is the Purpose of Venting in Injection Molding?
The venting in injection molding is channels or vent holes. The purpose of the injection molding venting system is to remove air and gases from the mold cavity during the injection process. These vents are essential for preventing the accumulation of air and gases, which can cause various molding defects.
Expel trapped air: Venting allows air in the mold cavity and feeding system to escape as molten plastic is injected. This prevents air traps that can cause defects.
Release decomposition gases: It helps remove gases generated from the decomposition of plastic and additives at high temperatures.
Prevent defects: Proper venting prevents various defects like burn marks, short shots, surface imperfections, and internal stresses in the molded parts.
Improve part quality: Effective venting minimizes surface defects like flow lines and weld lines, enhancing the overall quality of the product.
Optimize molding process: It reduces injection pressure, injection time, and pressure holding time, improving production efficiency and lowering energy consumption.
Protect the mold: Venting helps prevent gas accumulation that can erode mold surfaces and parting lines.
Assist in part ejection: Venting allows air to enter the mold during part ejection, preventing vacuum formation between the product and cavity wall.
Types of Venting in Injection Molding Methods
Venting in injection molding methods are essential for ensuring the quality of molded parts by allowing trapped air and gases to escape the mold cavity. Here are the primary types of venting methods used in injection molding:
1. Parting Line Vents
Parting line vents are vents created at the parting line where the two halves of the injection mold meet. They are typically placed directly opposite the gate and at all runner ends to allow air to escape as plastic material flows into the mold cavity. They are simple to implement and easy to clean while the mold is still in the machine. However, they may not be sufficient for complex part geometries or deep cavities.
2. Vent Grooves
Vent grooves are shallow channels or grooves machined into the mold to allow trapped air and gases to escape from the mold cavity as molten plastic is injected. These grooves are typically located at the end of the melt flow path to ensure efficient venting without allowing the molten plastic to leak out. Vent grooves are particularly useful for molds that produce large and medium-sized plastic parts where a significant amount of gas needs to be removed.
3. Ejector Pin Vents
Ejector pin vents are grooves or channels incorporated along the length of ejector pins. These vents allow air to escape from the mold cavity during the injection of molten plastic and also during the ejection of the molded part. Ejector pin vents are particularly useful in molds with deep or intricate features where traditional venting methods may not be effective. Ejector pins serve the dual purpose of ejecting the molded part and providing venting, making them efficient in design and function. However, the presence of vent grooves along ejector pins may leave marks on the finished part.
4. Insert Piece Venting
Insert-piece venting involves the use of venting channels or grooves that are integrated into the mold inserts. These vents allow trapped air and gases to escape during the injection of molten plastic, ensuring that the mold cavity is filled completely and effectively. Insert piece venting is used in molds that feature ribs lattice structures, or multi-cavity molds. The vent placement is flexible without affecting the mold’s structural integrity. However, the mold design will be more complex.
5. Clearance Venting
Clearance venting utilizes the gaps or clearances between mold components, such as the space around ejector pins, core pins, and other moving parts, to allow air and gases to escape from the mold cavity. Clearance venting is particularly used for molds with deep cavities, complex shapes, or small precision parts. Clearance venting can be easily incorporated into the mold design. However, the venting capacity is limited.
6. Porous Inserts Venting
Porous inserts are made from breathable materials like sintered metals, typically stainless steel. They consist of fine particle spherical powdered stainless steel sintered at high temperature and evenly covered with tiny air venting holes. These inserts have a continuous fine porous structure that allows gases to escape while preventing molten plastic from leaking. Porous inserts venting are used in molds where traditional venting methods are insufficient. This venting method allows for continuous gas flow during the injection process. However, it may have lower strength compared to solid inserts.
7. Vacuum Venting Systems
Vacuum venting systems use vacuum devices to actively remove air from the mold cavity before or during injection. They typically consist of a vacuum pump connected to strategically placed vents in the mold. The vacuum venting system is effective for high-precision parts where air traps are a significant issue. This venting system provides thorough air removal but adds complexity and cost to the molding process.
8. Dynamic Gas Vents
Dynamic gas vents utilize specialized venting valves that actively manage the escape of air and gases from the mold cavity. Unlike traditional static vents, dynamic vents can open and close based on the pressure conditions within the mold, allowing for more efficient venting. These valves are typically placed inside the mold cavity and enable gases to dissipate through a venting channel, ensuring that the mold is adequately vented throughout the injection process. It enhances venting efficiency and can be tailored to specific molding conditions.
Injection Molding Vent Design Guideline
Vent Locations
Place vents directly opposite the gate and at all runner ends/cold well slugs. Set vents at the end of the material flow and in thicker sections of the plastic part. For complex geometries, determine optimal vent locations after several trial molds.
Vent Dimensions
Standard vent land width: 0.06 inches.
Vent clearance: 0.12 – 0.50 inches wide x 0.02 inches deep.
Perimeter vent land: 0.125 – 0.250 inches.
Typical vent groove dimensions:
Vent groove width: 3-5 mm
Vent groove depth: Less than 0.05 mm
Vent groove length: 0.7-1.0 mm
Material-Specific Venting Requirements
Vent depths vary based on material viscosity:
Low viscosity materials: 0.01-0.03 mm
Medium viscosity materials: 0.03-0.05 mm
High viscosity materials: 0.05-0.08 mm
Specific vent depths for common materials (in inches):
ABS: 0.001 – 0.0015
Acetal: 0.0005 – 0.001
Nylon: 0.0003 – 0.0005
Polycarbonate: 0.0015 – 0.0025
Polyethylene: 0.0005 – 0.0012
Polypropylene: 0.0005 – 0.0012
Additional Considerations
Design vents on the cavity side of the parting surface for easier manufacturing and cleaning. Vent direction should not face the operator; use curves or bends to prevent operator burns. For thermosetting plastics, vent all shunts in front of the gate.
Consider using ejector pin vents when necessary. Ensure vents are always routed to the atmosphere.
Common Injection Molding Venting System Issues and Solutions
Here are some common injection molding defects caused by improper venting issues and their solutions:
Burn Marks
The inadequate venting will result in the trapped air in the mold cavity being compressed and heated rapidly, and the molded part will have burn marks or discoloration on the surface.
Solutions:
Improve venting at the burn mark locations. Add or enlarge existing vents, especially at the end of flow paths. Consider using vented ejector pins or porous inserts in problem areas and adjust processing conditions (e.g., lower injection speed) to allow more time for air to escape.
Short Shots
Short shots are caused by inadequate venting, the trapped air preventing the complete filling of the mold.
Solutions:
Add vent grooves or vent holes at short-shot locations. On the parting surface, create vent slots with a depth of 0.020-0.04 mm and a width of 5-10 mm. Position vent holes at the last filling point of the cavity. Use vented ejector pins in areas below the parting line. Consider vacuum venting systems for complex parts or large moldings. Increase mold temperature to improve material flow and allow more time for air escape.
Conclusion
Proper venting in injection molding is not just a technical requirement but a critical factor in achieving high-quality injection molded parts and maintaining efficient, cost-effective production processes. By understanding and implementing effective venting strategies, manufacturers can significantly enhance their injection molding operations, leading to improved product quality, customer satisfaction, and overall competitiveness in the market.
