The Benefits of Insert Molding for Medtech Manufacturing—A Medtech Makers Q&A

By Sean Fenske, Editor-in-Chief

Molding is the most common component fabrication method in medical device manufacturing. There is a range of processes that address virtually every possible type of component one might need for an innovation. While there are commonalities that run throughout each, they are unique and address a unique need for a specific type of part.

One option is insert molding. This technique involves the encapsulation of a part (such as a metal or electronic piece) within a molded plastic shell. It’s a complex procedure and is typically best left to experts familiar with the variables involved with such a method. The right partner can help determine the best way to proceed, address design questions, identify the ideal material, and more.

To help further illustrate the value the right supply partner for insert molding can bring, Jim Jelfo, Tool Engineer at Roechling Medical, responded to questions in the following interview. In this Q&A, he explains what insert molding is, where it’s used in medtech, what materials can be used, and what other design considerations are important.

Sean Fenske: What is insert molding, and where is it commonly used for medical device manufacturing?

Jim Jelfo: Insert molding is a manufacturing process that integrates dissimilar materials by encapsulating plastic around prefabricated components. The insert, often metal or another preformed component, is precisely positioned within the mold before medical-grade resin is injected around it, creating a secure mechanical or molecular bond between the materials.

For medical device manufacturers, insert molding can provide several advantages, including reducing secondary assembly steps, lowering labor requirements, consolidating components and inventory, and improving overall part reliability. Depending on the application, it can also enhance structural strength, protect sensitive components, isolate electrical elements, create leak-resistant interfaces, support biocompatibility requirements, and improve ergonomics.

Insert molding is widely used across the medical device industry. Common applications include surgical instruments, diagnostic devices, implantable components, and fluid-handling systems. The process is particularly valuable for applications where multiple materials or components must function together reliably within a compact, highly engineered device.

Fenske: Are there limitations in the materials that can be used with insert molding, either the insert material or the plastic?

Jelfo: Yes. Material selection is an important consideration in insert molding because the insert and plastic must be compatible with each other and the molding process. Chemical incompatibility can result in poor adhesion or negatively affect the integrity of the finished component.

Temperature is another key factor. Inserts that are sensitive to the elevated temperatures used during injection molding may experience changes to their inherent properties. Differences in thermal expansion and contraction between the insert and the resin can also introduce residual stress into the finished part, potentially affecting dimensional stability or long-term performance.

Evaluating material compatibility, processing temperatures, and thermal behavior early in the design process can help prevent these issues and ensure a reliable, finished component.

Fenske: How does designing an insert-molded component differ from the design of a traditionally molded (e.g., injection molded) part? Are the tolerances different for the final component?

Jelfo: Designing an insert-molded component requires additional considerations beyond those of a traditional injection-molded part because the design must account for the interaction between the insert, plastic material, and molding process.

One of the most important considerations is insert positioning and retention. Inserts must remain securely positioned during molding and may require preload forces or mechanical locking features in the mold to prevent movement during processing or in the final application. Gate location and material flow are also important; when possible, directing plastic flow parallel to the insert’s longitudinal axis can reduce the risk of shifting and promote more consistent encapsulation.

Designers must also account for material behavior and geometry. Maintaining appropriate plastic wall thickness around the insert supports part strength and proper mold filling, while rounded insert corners can help minimize stress concentrations and interruptions in material flow. Plated or coated inserts may introduce additional considerations, including the potential for coating materials to interact with or leach into the surrounding resin.

Tolerances can also be more complex than with a traditionally molded component. The final tolerance stack must account for variation in the insert itself, plastic shrinkage, thermal expansion, and tooling allowances. Designers could also incorporate slight mismatch where the insert interfaces with adjacent molded surfaces. Addressing variables early in the design and DFM [design for manufacturability] process is critical to achieving a consistent, reliable final component.

Fenske: How is the mold for an insert molding application developed? How is the insert kept in place while allowing plastic to fully encapsulate it? Does this create limitations for the process?

Jelfo: Insert molding requires specialized mold design to securely position the insert while allowing the plastic to flow around and encapsulate it. Because metal inserts are repeatedly loaded into the mold, they can create additional wear on tooling surfaces. Hardened tool steel replaceable components are often incorporated to protect the mold from damage and simplify long-term maintenance.

Keeping the insert properly positioned throughout the molding cycle is critical. Hardened mold surfaces can be designed with precise clearances or preload to restrict movement during resin injection. When the product geometry does not allow for a dedicated tooling restraint feature, the molding process may be designed so that plastic flow and injection pressure move the insert against a positive stop, allowing it to remain in a consistent and repeatable position as the material cools. Sensors or camera systems can also be incorporated to verify the presence and correct orientation of the insert before molding begins.

These requirements introduce additional process considerations compared with traditional injection molding. Process engineers must carefully optimize injection speeds and pressure profiles to achieve complete encapsulation while minimizing insert movement or damage. As a result, the acceptable processing window may be narrower, making robust mold design, process development, and validation especially important for achieving consistent results.

Fenske: Are there minimums/maximums to keep in mind with insert molding where cost becomes more or less of a concern? Does the use of automation in insert molding processes affect price at certain volumes?

Jelfo: Production volume is an important factor when determining the most cost-effective approach to insert molding. Automation can reduce cycle times, improve repeatability, and increase process efficiency, but the additional upfront investment can be difficult to justify for lower-volume programs.

For lower volumes, manual insert placement often provides greater flexibility with significantly less initial investment. It can better accommodate design changes and may also be well suited for complex applications that are difficult to automate.

For mid-to-high-volume programs, customized automation becomes increasingly attractive. Automated cells may incorporate multi-axis robots, end-of-arm tooling, bowl feeders, conveyors, vision systems, and automated part handling. These systems can significantly increase the initial tooling and equipment investment, but the gains in efficiency and repeatability can offset that investment as production volumes increase.

Every program is different, and factors such as part complexity, cycle time, quality requirements, labor needs, and anticipated program life should be evaluated when comparing manual and automated approaches.

Fenske: What are the most important considerations medical device developers should keep in mind when specifying an insert molded part or selecting an insert molding partner? What is commonly overlooked, or what mistakes do you see most often?

Jelfo: Understanding how the insert, resin, part design, and molding process will interact is critical. Material compatibility should be evaluated early. Plastic shrinkage rates can be affected by the insert design, size, and features. Insert tolerances must be developed with the injection mold and finished component in mind because the tooling features used to locate and retain the insert can create dependent tolerances.

Manufacturability and operator safety should also be considered early in the design process. Difficult insert locations and complex geometry can increase manufacturing complexity and cost. High-heat resins may be required but can drive cost and create operator safety concerns. Where appropriate, robotic loading can improve safety, positioning, and process repeatability.

Ultimately, developers should engage an experienced insert molding partner early in the design process. Early collaboration and DFM can help identify material, tolerance, retention, automation, and processing challenges before tooling is built, reducing costly design changes and creating a more robust manufacturing solution.

Fenske: Do you have any additional comments you’d like to share based on any of the topics we discussed or something you’d like to tell medical device manufacturers?

Jelfo: Early collaboration between the medical device manufacturer and molding partner can have a significant impact on the success of an insert molding program. The earlier an experienced molding partner is involved, the more opportunities there are to optimize DFM and quality and ultimately provide a repeatable production solution. Choosing a partner with an extensive and reliable supply chain can also positively impact the design and launch of the program.

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