Undercuts in plastic injection parts refer to features that prevent the part from being ejected directly from the mold. These features can add complexity to the design and manufacturing process, but they are often necessary to achieve the desired functionality or aesthetics of the part. As a supplier of Plastic Injection Parts, we understand the importance of carefully considering the design of parts with undercuts to ensure successful production. In this blog post, we will discuss the key design considerations for plastic injection parts with undercuts.
Understanding Undercuts
Before delving into the design considerations, it is essential to understand the different types of undercuts. There are two main types: internal undercuts and external undercuts. Internal undercuts are located inside the part, such as a groove or a boss on the inner surface of a hollow part. External undercuts, on the other hand, are on the outer surface of the part, like a lip or a protrusion.
The presence of undercuts requires special tooling and ejection mechanisms to remove the part from the mold. Without proper design and tooling, undercuts can lead to issues such as part damage, longer cycle times, and increased production costs.
Design Considerations
Mold Design
- Side Actions or Slides: One of the most common ways to handle undercuts is by using side actions or slides in the mold. Side actions are movable components in the mold that can be actuated to create space for the undercut during ejection. For example, if a part has an external undercut in the form of a lateral protrusion, a side action can be designed to move horizontally to clear the undercut before the part is ejected.
- Core Pulls: Core pulls are used for internal undercuts. They are mechanisms that can be retracted from the part before ejection. For instance, if a part has an internal groove, a core pull can be designed to withdraw from the groove, allowing the part to be removed from the mold.
- Mold Complexity: The use of side actions and core pulls increases the complexity of the mold. This complexity can lead to higher mold costs and longer lead times for mold fabrication. Therefore, it is crucial to minimize the number of undercuts and simplify the mold design as much as possible.
Material Selection
- Flexibility: The material used for the plastic injection part can significantly affect the handling of undercuts. Flexible materials, such as thermoplastic elastomers (TPEs), can often be deformed slightly during ejection to clear undercuts without the need for complex mold mechanisms. For example, a part with a small external undercut made of TPE can be bent slightly to release it from the mold.
- Shrinkage: Different materials have different shrinkage rates during the cooling process. This shrinkage can impact the fit and function of parts with undercuts. For example, if a part has an internal undercut and the material shrinks too much, it may cause the undercut to bind with the mold core, making ejection difficult. Therefore, it is important to select a material with a predictable shrinkage rate and consider the shrinkage in the design of the undercut.
Part Geometry
- Undercut Size and Shape: The size and shape of the undercut play a crucial role in the design. Smaller undercuts are generally easier to handle than larger ones. For example, a small external lip can be more easily released from the mold compared to a large, deep undercut. Additionally, the shape of the undercut can affect the ejection process. Smooth, rounded undercuts are often easier to release than sharp - edged ones.
- Draft Angles: Draft angles are slopes added to the vertical surfaces of the part to facilitate ejection. In parts with undercuts, draft angles are even more important. They help reduce the friction between the part and the mold during ejection, especially in areas near the undercut. For example, a part with an internal undercut should have sufficient draft angles on the inner walls to ensure smooth ejection.
Production Efficiency
- Cycle Time: The presence of undercuts can increase the cycle time of the injection molding process. This is because the additional mold mechanisms, such as side actions and core pulls, require time to actuate. To improve production efficiency, it is important to optimize the design of these mechanisms to reduce the time required for their operation.
- Parting Line Placement: The parting line is the line where the two halves of the mold meet. In parts with undercuts, the parting line should be carefully placed to minimize the impact on the undercut. A well - placed parting line can simplify the mold design and reduce the complexity of the ejection process.
Cost Considerations
- Mold Cost: As mentioned earlier, the complexity of the mold due to undercuts can significantly increase the mold cost. It is important to balance the design requirements with the cost of the mold. In some cases, it may be more cost - effective to modify the part design to reduce or eliminate undercuts.
- Production Cost: The increased cycle time and potential for part damage associated with undercuts can also increase the production cost. Therefore, it is essential to consider the overall production cost when designing parts with undercuts.
Case Studies
Let's take a look at a couple of case studies to illustrate the importance of these design considerations.
Case Study 1: A Consumer Product with an External Undercut
A client came to us with a design for a consumer product that had an external undercut in the form of a decorative lip. Initially, the design did not consider the moldability of the undercut. After a detailed analysis, we recommended using a side action in the mold. However, to reduce the mold complexity and cost, we worked with the client to modify the shape of the undercut to make it more rounded and less deep. This modification allowed us to simplify the side action design and reduce the overall production cost.
Case Study 2: An Automotive Component with an Internal Undercut
For an automotive component with an internal undercut, we faced challenges with ejection due to the high shrinkage rate of the initially selected material. We recommended switching to a material with a lower and more predictable shrinkage rate. Additionally, we designed a core pull mechanism with a smooth surface finish to minimize friction during ejection. These changes improved the part quality and reduced the production cycle time.
Conclusion
Designing plastic injection parts with undercuts requires careful consideration of mold design, material selection, part geometry, production efficiency, and cost. By understanding the challenges associated with undercuts and implementing the appropriate design strategies, we can ensure successful production of high - quality parts.
As a supplier of Plastic Injection Parts, we have the expertise and experience to help you navigate the complexities of designing parts with undercuts. Whether you need assistance with mold design, material selection, or part optimization, our team is ready to work with you. If you are interested in learning more about our Plastic Components or Plastic Injection Components, please reach out to us for a consultation. We look forward to partnering with you on your next project.
References
- Rosato, D. V., & Rosato, D. V. (2004). Injection Molding Handbook. Kluwer Academic Publishers.
- Throne, J. L. (1996). Thermoplastics Molding: Theory and Practice. Marcel Dekker.
