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Overmolding in injection molding is a process that combines multiple materials together to form one single part. Typically, the first material (the substrate) is covered partially or fully by a second material (the overmold). The two most common applications for overmolding are:
- Combining threaded metal inserts with a plastic part
- Combining a harder plastic with a rubber-like material
This article will concentrate on the second application, elaborating on the considerations and guidelines necessary to ensure the success of your upcoming overmolding project.
Overmolding Overview
Here are the main reasons to overmold hard plastic with a rubber-like material:
- Aesthetics: Using two materials allows for different colors and textures, resulting in striking color contrasts.
- Grips: Rubber-like grips are commonly used in items like power-tools and kitchen utensils.
- Keypads and Buttons
- Seals: A rubber-like overmold can create air and water-tight seals.
Utilizing overmolding for these features reduces the number of individual parts and eliminates assembly and secondary operations, producing a seamlessly integrated product.
The two main types of overmolding are insert overmolding and two-shot overmolding. Factors such as part design, tooling lead time, and production quantities largely determine which method is most suitable.
Insert Overmolding
Insert overmolding (also known as "pick and place" overmolding) involves placing a substrate into a mold and injecting the overmolding material over it—this method is most commonly used by Fictiv. Insert overmolding requires two separate molds, one for the substrate and another for the overmold.
Two-Shot Overmolding
Two-shot overmolding, also known as multi-material or 2K overmolding, involves injection molding presses equipped with two barrels rather than the standard single barrel, allowing the molding of two different materials at once. This method requires a single, albeit complex and more costly, tool, making it best suited for higher production quantities where minimizing unit cost is crucial.
Though the two main types of overmolding differ, these guidelines apply to both.
Material Compatibility
Material choice is the first and most important consideration in any overmolding project because selecting compatible grades can create a chemical bond between the two materials. If they are compatible, the rubber-like material essentially fuses to the plastic, like glue, establishing a strong bond.
The most commonly used plastic grades for substrates include PC/ABS, ABS, Nylon and PBT. Typically, TPE, TPU or silicone rubber are most suitable for overmolding. Some plastics, such as POM, PP, HDPE, or PEEK, are not well-suited for overmolding due to their chemical composition, which prevents a bond from forming between the substrate and overmold. It is also crucial to choose a substrate material with a higher melt temperature than the overmolding material to avoid melting the substrate during the overmolding shot.
In recent years, TPE, TPU and material manufacturers have focused on specially formulated materials that bond with various grades of plastic—they are a great source of materials guidance.
Part Design Considerations
Uniform wall thickness is important for both substrate and overmolding features. Dimensional consistency is especially crucial for the substrate to minimize deformation and ensure it can be properly inserted into the overmolding tool. Generous draft is also preferable for both shots, as rubber-like materials have a higher tendency to stick in the tooling. It is also essential to have shutoff features on the substrate to avoid flash and create crisp lines for the overmolding. Mechanical features such as interlocks can further improve bond strength—several examples are shown below.
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Surface finish can influence overmolding positively or negatively. For instance, texturing the overmolding locations is preferable since a polished surface can cause the rubber to stick to the tool after molding. Texture applied to the substrate on the contact surfaces between the substrate and overmold can aid in material bonding, as these surfaces are hidden under the overmolding features.
Tooling Design Considerations
Gating, ejector pin locations, parting lines, and flow length are often an afterthought, but getting them wrong can reduce part performance or adversely affect overall aesthetics.
Here are some tips to ensure proper overmolding shot: Thicker gates, like edge gates, aid in the processing and bonding of the overmold. Centrally located gates are also important for complete filling and bond strength, taking into account the flow length. It is useful to consider the material flow path to ensure no substrate features inhibit flow. If such features exist, additional gates may be required. Thick and centrally located gates might leave a gate vestige in an undesirable part location, but there are ways and design features to hide the gating from view.
Additionally, ejector pin marks or parting lines located on sealing surfaces might prevent the seal from functioning correctly.
Fictiv DFM review support: Fictiv’s dedicated DFM engineers and Technical Applications Engineers (TAEs) are here to support you with your part and tooling design, so that your gates, ejector pins, parting lines, and flow length are dialed in—check out our new DFM feedback portal!
Production Processing
Part processing mainly falls under the molder's responsibility. However, even if material selection and design are properly managed, overmolding processing requires techniques and capabilities beyond standard injection molding. Not every molding factory is equipped or willing to accommodate overmolding products.
At Fictiv, a subset of our highly vetted network of manufacturing partners with proven overmolding success records are eligible for your overmolding projects. Additionally, Fictiv’s Supplier Quality Engineers (SQEs) provide oversight during sampling and production to ensure your overmolded parts are of the highest quality.
Here are the internal guidelines used by our manufacturing partners for processing:
- Print material manufacturer’s datasheet and recommended processing conditions
- Check the overmold and substrate raw materials' moisture levels to ensure they are properly dried.
- No mold release spray allowed (Mold release spray is a cardinal sin for overmolding)
- To achieve optimal bond strength, set melt temperatures to the upper recommended limits
- Use a high injection rate for the overmolding
- Use the highest possible injection pressure
- Dry and clean the substrate's surface
- Preheat the substrate before overmolding—the temperature depends on the material grade
- Typically use longer cooling times for the overmolding shot since the substrate restricts cooling
Fictiv is your operating system for custom manufacturing. We make sourcing quality, custom mechanical parts simple using our online platform, guided expertise, and highly-vetted partner network. For your next injection molding project, create an account and upload your designs to www.fictiv.com to discover the benefits of our design review process for yourself!
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