Tips to simplify designs
As a product design enhancement, the ability to composite two plastics properties into one component—or to merge an advanced composite, rubber or metal part into an injection molding to make a multi-material part with hybrid properties.
What’s not to love in the precision integration of hard or soft parts with a plastic molding, to make a smart-part that reduces BOM and assembly complexity and enhances product performance.
In the world of product manufacturing, producing parts with multiple materials or distinct layers is an increasingly common demand in an array of markets, from consumer electronics and medical devices to automotive components and industrial tools. Overmolding and insert molding are two versatile and design-empowering approaches, serving to deliver increased product functionality, reduced manufacture costs, high grade aesthetic details, and to simplify complex designs.
These two overlapping and complementary methods allow designers to combine divergent materials into a single, typically multi-functional component. The techniques allow the creation of parts that benefit from the properties of multiple materials, delivering highly enhanced functional capability.
These techniques provide valuable solutions for combining hard and soft materials, adding grip to tools, elevating aesthetics, and reducing the part count and assembly labor in a product.
Below we dive deep into both overmolding and insert molding, comparing their differences and overlapping processes, benefits, challenges, and their applications in a series of clear examples. By the end, we aim for you to have a clear understanding of the methods, design opportunities and when to select one or the other. We will focus closely on how they impact the design and production of complex, and highly capable composite parts that solve intricate and significant issues.
Alan B.
Mechanical Engineer
"Hands-Down the Best Platform and Partner for Fast, Quality Parts"
Jiga is the best way to get the parts you need, when you need them.
What is Overmolding?
Overmolding is a multi-step process where one material (typically a softer, elastomeric, or color/surface enhancing material) is molded over another substrate material which can be a rigid plastic, itself a composite such as carbon fiber, or a metal part. This technique allows for the creation of parts with divergent material properties in distinct areas, often for the purpose of adding ergonomic features/grips, seals, flexible hinges or color/text/image appliqués.
Process flow
It’s valuable to outline some variations and options that use similar and related processes.
Dual thermoplastic process
- Initial substrate molding: First, the substrate is created. This is formed by injection molding in a first-stage mold tool, typically from a rigid thermoplastic. This substrate forms the core of the eventual multi-material part.
- Secondary molding: The substrate is placed into a second mold cavity into which additional cavity features are cut to form the overmold features. These cavities are typically connected to firm a single flow path, with runner features that are formed on the substrate’s inner surfaces. The overmold is commonly (but not always) a softer, often elastomeric material. The new material bonds to the substrate, or mechanically keys to it, often both, creating a finished component with multiple material properties.
Non-Thermoplastic substrates
- The substrate can alternatively be formed from a variety of non-thermoplastic materials. This is commonly pressed, machined, or cast metal; or thermoset polymer; a composite material part of CFRP (Carbon fiber reinforced plastic) or GRP (glass reinforced plastic).
- The secondary molding stage is essentially identical, with the substrate placed in a mold tool into which the overmold material is then injected to form chemically iron mechanically keyed features to make a compounded final part.
One key difference in this process is that it is somewhat more common to mold rigid polymer features onto these harder substrates. Additionally, it is very common to mold true rubber features onto metal substrates, as they can tolerate the elevated processing temperatures involved.
Co-Molding
Co-molding is a variant of the overmolding process that offers considerable advantages, when the product volumes are sufficient to justify the higher setup costs.
This process has two variants, each offering unique advantages in some applications.
- In the older and more common form, a typically rotating toolset undergoes a mid-process reconfiguration. The substrate is molded, the tool is mode-switched by moving cores or half cavity switched to provide additional cavity for the overmold, then it is presented to the second barrel of a dual injection setup.
- Alternatively, and less often, a dual (or multi) barrel molding machine can shoot two or more materials into the same cavity and form progressive co-injected features in an essentially multi-material, single-stage molding.
Benefits of Overmolding:
Ergonomics: Overmolding is widely employed to add soft, traction surfaces to tools, electronic devices, and handles, improving user comfort and grip.
Aesthetics: The technique allows for secondary colors and textures in a single part, providing options in color and texture change to alter the look and feel.
Reduced assembly processes: By combining multiple functional elements into a single part in the molding process, designers can reduce the net partcount, simplifying assembly.
Enhanced functionality: Overmolding can add functionality, such as seals, shock mounts, sealed button actions, windows, flexures, or insulating features to extend individual component capabilities.
Challenges in Overmolding:
Material compatibility: Many substrate and overmold candidate materials pose bonding issues with each other. The substrate and overmold materials must be carefully selected for adhesion and performance and be processed according to manufacturer specifications to optimize the result.
Complex/costly tooling: The need for multiple molds and/or more complex tools and equipment can restrict the utility/application/financial viability of selecting the process. The need for precision substrates and tools and the coordination of parts/cavities adds complexity and cost to the tooling.
Cycle times: Given that overmolding is a multi-step process, it requires more machine time than single-stage injection molding, which is particularly challenging for low-to-moderate-volume production.
Javier L
Principal Systems R&D Mechanical Engineer
"Game changing in the online manufacturing space"
Jiga is the best way to get the parts you need, when you need them.
What is Insert Molding?
Insert molding is essentially the reverse of overmolding. The generic difference between insert molding and overmolding is the relative volumes of the ‘substrate’ component. In an overmold, a large substrate has smaller features added to it by appliqué molding onto its surface or into holes within it.
In an insert mold, inserts, typically smaller than the finished part, are placed in a tool and fully or partially encapsulated by a polymer molding.
In an insert molding, one or more pre-made components (typically a metal, or sometimes a thermoplastic part, or an electronic component, or a ceramic/cermet part) are positioned and supported within a mold cavity. This method allows the creation of parts that integrate the characteristics of the typically hard material of the insert for strength, wear resistance, conductivity etc—with the ease of complex detail, aesthetics, and simple manufacture, and cost-effectiveness of injection-molded parts.
Process
- Insert placement: The premade insert(s) (such as a threaded metal nut, a sheet metal bracket/reinforcer, or a clear molded window) are manually or automatically placed into the mold, and the tool is then closed and prepared for injection of the encapsulating polymer.
- Injection molding: Thermoplastic is injected into the mold, filling the cavity around the insert and coupling to it either chemically, mechanically, or both. As the plastic cools and solidifies, it creates a single, unified part.
Advantages of Insert Molding:
- Material integration: Insert molding couples the advantages of two or more materials—for example, the strength/toughness of metal with the flexibility and formability of plastic.
- Strong bonding: The plastic fully encapsulates the insert, by design, creating a secure and durable bond without the need for adhesives or fasteners.
- Reduced weight: By using a plastic body around a metal core, the overall weight of a part can be reduced without compromising strength or functionality at the critical points.
- Consolidated parts: Insert molding eliminates the need for secondary operations, reducing the number of components in a BOM and transferring precision aspects to automated and highly repeatable processes.
Challenges of Insert Molding:
- Insert placement: Inserts must be accurately placed within the mold. This can be a delicate process, but it is well-suited to automation to avoid adding the labor costs.
- Tooling wear: Repeated insertion and removal of metal inserts can lead to faster wear in support areas of molds, increasing maintenance and replacement costs. Careful automated handling and optimized tool hardness can mitigate this to a large degree.
- Cycle time: Similar to overmolding, insert molding can take longer than basic injection molding due to the need to place and secure the inserts. However, this added time will typically be less than the alternative manual assembly route.
Conclusion
Both of these processes represent significantly empowering tools that designers widely employ in solving complex component problems and achieve intrinsically precise sub-assemblies of two or more materials to deliver multiple functions in a single part.
They involve costs in tooling and additional processing but often achieve extremely improved properties in the resulting components and typically remove labor and imprecise or higher-risk methods that more than compensate for the added costs, when production volumes (or product value) are sufficient.
