Overcoming Injection Molding Challenges in Medical Design

Injection molding is an essential manufacturing process for producing high-quality, precision medical components where device reliability and patient safety are paramount. However, even with the most advanced injection molding tooling techniques, the process has inherent limitations that engineers must consider when designing parts. These limitations can impact everything from part geometry and feature resolution to production costs. In this post, we’ll explore the challenges associated with complex geometries and how designers can strategically overcome these obstacles while balancing complexity, cost, and manufacturability, particularly with the help of Design for Injection Molding (DfIM) and DFM for medical devices.

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1. Complex Geometries & Feature Resolution: Understanding the Challenges

Limitations of Traditional Tooling:

Injection molding is highly effective for producing parts with consistent shapes. However, there are limitations when it comes to feature resolution (the minimum size of a feature that can be reliably formed) and certain complex geometries. Features such as sharp internal corners, deep undercuts, and intricate freeform surfaces can be particularly challenging to produce with standard machining and Electrical Discharge Machining (EDM) techniques.

Challenges:

• Sharp Internal Corners: In theory, a sharp internal corner has a radius of zero, which creates a point of high stress concentration in a finished part, making it prone to failure. From a molding perspective, these corners can also impede smooth polymer flow, leading to incomplete filling or surface defects.
• Deep Undercuts: Undercuts are features that prevent the part from being ejected in a straight line from the mold, effectively locking it in place. These features require more complex mold components like slides, lifters, or side actions—all of which increase mold complexity, tooling lead time, and cost. This is especially significant in medical injection molding, where precision is critical.
• Complex Freeform Surfaces: Organic, curving surfaces that do not follow a simple mold release direction can be difficult to machine into the mold and challenging to replicate with high fidelity, especially for plastic injection molding manufacturers working within tight tolerances.

Best Practice:

Engineers should carefully evaluate whether a complex feature is critical to the part's function. In many cases, simplifying part geometry—for instance, by designing a minimum internal radius of 0.5x the wall thickness—can reduce tooling costs and cycle times while still achieving the desired performance. These approaches fall under plastic part design optimization, a key aspect of modern design for manufacturing services.

2. Strategies to Overcome Complexity: Advanced Tooling Solutions

To address the challenges of deep undercuts and complex geometries, advanced injection mold design services and tooling techniques like slides, lifters, and collapsible cores are often employed. These mechanisms add motion to the mold, allowing for the creation of intricate part designs without compromising function.

• Slides: Movable mold components that shift during the molding cycle to form or release undercuts.
• Lifters: Components that move at an angle during ejection to pull undercut areas away from the mold wall, ideal for creating internal features or holes.
  
• Collapsible Cores: Intricate core inserts that mechanically collapse inward after molding, permitting the formation of significant internal undercuts or threads—vital in insert molding for medical devices.
  

Pros:

• Enables complex custom injection molding solutions for geometries previously considered un-moldable.
  
• Facilitates undercut release, preventing part damage and improving consistency, which is essential for ISO 13485 injection molding and FDA-compliant manufacturing processes.
  

Cons:

• These mechanisms add moving parts to the mold, significantly increasing the initial tooling services cost, lead time, and design complexity.
• Molds with more moving components require more frequent inspection and upkeep, demanding a rigorous quality management system for the manufacturing industry.

Best Practice:

Where possible, incorporate design elements that facilitate the molding process, such as draft angles and features that reduce stress concentration points. Working with a tooling company or a provider of precise tooling solutions helps minimize the need for advanced mechanisms while maintaining part functionality.

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3. Alternative Approaches: Secondary Operations and Advanced Techniques

In some cases, features that are difficult to mold can be created through secondary manufacturing processes or addressed with advanced molding techniques such as insert molding and overmolding services.

Secondary Operations:

• Machining: For features that demand extreme precision, machining can be used post-mold to add threads, holes, or sharp edges.
• Laser Cutting: Offers an efficient method for adding fine details or precise openings to molded parts, especially where traditional tooling would be too costly.

These operations are often supported by a robust quality certification for the manufacturing process to ensure each modification maintains dimensional integrity and compliance with medical regulations.

Insert Molding & Overmolding:

• Insert Molding: This technique involves placing pre-formed components, such as metal threaded inserts or electrical contacts, into the mold before injecting plastic around them—perfect for medical device prototypes.
  
• Overmolding: The process of molding one material over another, often used to combine a hard thermoplastic substrate with a soft elastomer for ergonomic grips, seals, or gaskets. These techniques are common in low-volume injection molding or rapid prototyping services.
  

Pros:

• Allow for component consolidation and eliminate downstream assembly steps, reducing labor and total part cost.
  
• Enable the creation of multi-material or multi-functional parts that would otherwise be impractical with traditional tooling.
  

Cons:

• Secondary operations and multi-material techniques add process complexity and cost.
  
• They may have design constraints such as material compatibility, adhesion strength, and part size that could restrict design flexibility.
  

Best Practice:

If your design requires complex features that are hard to mold directly, consider using 3D printed prototypes to validate design feasibility. Partnering with a reliable additive manufacturing company or custom manufacturing services provider can streamline development through iterative testing and additive manufacturing solutions.

4. Balancing Design, Cost, and Manufacturability with DFM

When designing medical components, it is essential to strike a balance between part complexity, tooling costs, and manufacturability. This is best achieved through a philosophy of Design for Manufacturability (DFM), which includes early collaboration with design and manufacturing services partners.

Best Practice:

Work closely with your tooling supplier to review design concepts early. Incorporating DFM principles like analyzing part geometry, material flow, draft angles, and ejection strategy will lead to a smoother development cycle and a more robust and economical manufacturing process. Teams offering DFM development services often also support carbon DLS prototyping, additive manufacturing tooling, and digital manufacturing solutions for fast turnaround and precision.

Conclusion

Injection molding presents unique challenges when designing complex medical components. However, by understanding these limitations and applying strategies like advanced tooling, secondary manufacturing process options, and a robust DFM process, engineers can create high-quality, functional parts while optimizing for cost and manufacturability.

At Aprios, we specialize in helping clients navigate these challenges to create optimized injection-molded parts for the medical industry. From plastic injection molding services and tooling solutions to ISO-certified manufacturing, we offer end-to-end expertise to ensure precision, compliance, and long-term performance in every part we deliver.