Tips to simplify designs
To recap for clarity, this is a second stage discussion about the similarities, differences, applications, relative benefits of overmolding, and insert molding. (Here’s the first article)
- Overmolding – molding polymer or rubber features onto an existing part that may be made from thermoplastic or thermoset polymer, or metal, or composite material that has been produced in a separate process.
This part is typically placed into a tooled cavity that allows space for it to precisely seat AND allows additional volume that represents the negative space into which the overmolded polymer or elastomer can be molded.
The cavity is then closed and the tool – including the to-be-overmolded substrate – is presented to a molding machine to have the overmold volumes filled with either rigid thermoplastic, or elastomeric thermoplastic, or a thermoset material.
- Insert molding is a closely related process. It typically involves placing one or more ready-made parts into a tooled molding cavity in where they are supported, but in which they fill a minority of the interior volume.
These inserted parts are commonly small metal or molded plastic components used for a variety of functions—such as threaded hard-points, windows, reinforcing hardware, etc.
The closed tool is then presented to a molding machine in which a typically rigid thermoplastic is injected into the unfilled portion of the cavity, to form a complete part that encapsulates the overmolded elements in a finished sub-assembly.
Although overmolding and insert molding have significantly overlapping characteristics, as both involve combining multiple materials into one part, they differ considerably in process details and in application/design intent.
Materials used
- Overmolding: This process most commonly combines a rigid substrate with a soft or elastomeric overmold material. The two materials must bond well to ensure a durable part.
The elastomer can be a thermoplastic, typically injection molded in a second stage injection mold tool. However, it can also be a thermoset rubber (silicone, EPDM, nitrile, etc.) applied to a more thermally resilient substrate such as a metal component.
- Insert molding: Involves placing a pre-made metal or plastic insert into the mold, which is then surrounded by plastic. The insert does not need to bond with the plastic chemically; the plastic merely encapsulates the insert.
Process complexity
- Overmolding: Requires two separate molding steps and distinct mold tools, OR very complex additional features for moving cores in a single tool, which increases the complexity and cost of production.
- Insert molding: While a single molding step is employed, the manual or automated placement of the insert can introduce complexity and risk, as a displaced insert can cause severe damage to a tool when pinched in the closure.
Applications
- Overmolding: Widely employed to add soft grips, flexible hinges, cosmetic appliqués or seals to rigid plastic parts. It’s frequently seen in waterproof products, power-tools, electronics, clothing, and general consumer products.
- Insert Molding: Optimal for integrating metal components like bushings, threaded inserts, or brackets into plastic or rubber parts, often used in automotive, electronics, and medical applications.
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Lead Engineer
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Applications of Overmolding and Insert Molding
Both overmolding and insert molding are used across most sectors. Understanding the specific applications of each process informs designers and manufacturers as to which process to choose for their parts.
Applications of Overmolding:
- Handheld devices: Many consumer electronics, such as smartphones and remote controls, use overmolding to add tactile surfaces or shock-resistant materials around the rigid plastic or metal body.
- Medical devices: Overmolding is widely used to create ergonomic, soft-touch surfaces for medical instruments, and for fluid seals in sample handling disposables, ensuring leak-free function, comfort, and ease of use.
- Power tools: Adding rubberized grips to power tools improves user comfort and safety while reducing the effects of vibration during operation. Motor mountings are often internally overmolded for lower user impacts.
- Sealing components: Overmolding elastomeric materials onto rigid parts is used to integrate seals in valves, pumps, and fluid-handling components as well as waterproof or dustproof features in consumer products.
Applications of Insert Molding:
- Automotive components: Insert molding is extensively used to incorporate metal components, such as bushings and threaded inserts, into plastic parts for increased durability and performance in automotive applications. It is common to transfer or compression mold highly durable rubber buffers to metal mounting components for engine mounts and suspension bushes/bump stops.
- Medical devices: Syringes, implants, blood sampling disposables and surgical instruments often use insert molding to combine the biocompatibility and durability of metal with the flexibility of plastic.
- Electrical connectors: Insert molding is widely used to encapsulate metal terminals, pins, or other electrically conductive elements into rugged plastic housings for robust and reliable connectors.
- Consumer goods: Items like appliance housings, tools, and furniture can benefit from insert molding, integrating metal components for hard-point purposes into low-cost molded parts, for improved strength and longevity.
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Purchasing Manager
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Material considerations for Overmolding and Insert Molding
Selecting the optimal materials for each aspect of a multimaterial component is crucial for the success of both overmolding and insert molding processes. The materials must cooperate and closely couple to meet the typical diversity of a design’s functional and performance specifications.
For Overmolding
- Substrate materials: These are typically rigid thermoplastics such as ABS, polycarbonate, or PBT. Where overmolding is required on a metal part, there are few restrictions, as chemical coupling is not an option, and mechanical coupling works equally well with all metals.
- Overmold materials: Softer thermoplastic elastomers (TPEs), such as Santoprenes, silicones or TPUs are commonly used for overmolding. Where thermoset rubber is used in transfer/compression overmolding, a wide range of options are open – silicones, butadiene rubbers, nitrile rubbers, and EPDM being common choices.
- Bonding: The materials must bond well, either through chemical adhesion or mechanical interlocking, to ensure the part’s integrity. In many cases, careful selection of a well-evaluated and tuned thermoplastic material pairing and tight control over molding parameters will deliver excellent adhesion that is stronger than the elastomer.
For Insert Molding
- Insert materials: Commonly inserts are made from metals (such as brass, steel, or Aluminum) or rigid plastics able to tolerate the elevated temperatures in the overmolding process.
- Molding materials: Various thermoplastics, including nylon, polycarbonate, and PEEK, are often used in insert molding to encapsulate the metal insert. There are few restrictions, as the metal parts rely on encapsulation and mechanical coupling to the molding, so virtually any thermoplastic is suitable for receiving inserts.
Choosing between Overmolding and Insert Molding
When deciding between overmolding and insert molding, several factors come into play. Here’s how to make the right choice based on design requirements, material compatibility, and intended application:
When to choose Overmolding
- When you need to combine soft and hard materials, such as adding comfort grips to rigid components.
- If you’re aiming to improve the aesthetic, branding or tactile finishes of a product.
- When functionality requires different material properties in specific areas of the part, for example adding seals or shock absorbers.
When to choose Insert Molding
- When you need to combine durable metal components like threaded inserts or conductive elements with a plastic part that must meet generally lower stress or but highly localized and specific functional tasks.
- When structural integrity or load-bearing strength is essential in part of a molded component, such as in automotive and aerospace applications.
- When highly compliant rubber material must be hard mounted to fixed hard-points, such as in automotive suspension components.When the target is to reduce assembly time and cost (in high volume products) by eliminating the need for post-molding operations, such as fitting inserts post-molding by ultrasonic insertion, the process delivers high value..
Conclusion: Overmolding vs Insert Molding
Overmolding and insert molding offer huge operational and strategic advantages in manufacturing complex and multifunctional components. Overmolding excels when your need is for a soft, flexible material bonded to a rigid part for improved ergonomics, aesthetics, or localized functionality. On the other hand, insert molding serves best when you need to integrate metal or pre-formed components into plastic parts, providing localized strength, durability, hard mounting, and reducing assembly time.
A deep understanding of the specific needs of components you are specifying, including material compatibility, integrated functionality, and complex performance requirements, is crucial in choosing the most appropriate technique.
Both processes serve as critical design-enabling tools in the increasingly demanding and complex process of designing and manufacturing products, function-enabling, innovative and cost-saving solutions for complex, multi-material designs.
