Time to read: 8 min
Injection molding is an exceptional manufacturing method for producing parts at scale, but there are times when a single plastic material doesn’t cut it. Fortunately, specialized solutions like insert molding offer additional options for enhanced performance.
Insert molding is a manufacturing process that integrates pre-formed metal or plastic components into a complete part during the injection molding cycle, providing combinations of various materials and/or functions within a single, fabricated piece.
This article details the insert molding process, its practical applications, and important injection molding design principles that provide engineers and product designers with the right tools to employ this fabrication method effectively.
What Is Insert Molding?
The insert molding process involves placing subcomponents into the mold cavity before molten plastic is injected around them. When plastic enters the mold and flows around the inserts, a strong bond forms between them, resulting in a one-piece part with a secure bond.
Insert injection molding integrates dissimilar materials into a single part. Inserts, such as metal threads or plastic pins, are embedded directly into the mold, where encapsulating plastic forms a robust, integrated component in a single cycle. This eliminates secondary assembly, enhancing efficiency and part durability.
The Insert Molding Process
Proper design for the insert molding process ensures the inserts bond securely with the encapsulating plastic, creating durable, functional parts in one cycle.
Step 1: Source and Prepare the Insert
Operators or automated systems source injection molding inserts, such as metal screws or plastic pins, from suppliers or in-house fabrication, then clean and apply surface treatments to ensure strong bonding with the encapsulating plastic. These treatments may include chemical priming for adhesion, coatings for corrosion resistance, or texturing to enhance mechanical interlocking with the plastic insert injection molding process.
Step 2: Place the Insert
Operators manually position injection molding inserts in the mold for low to medium production volumes. For high-volume manufacturing, robotic systems position injection molding inserts with high speed and precision. The molds include features such as locator pins or grooves to hold injection molding inserts securely in place while the encapsulating plastic is injected. In advanced setups, vision systems inspect insert placement before plastic injection to prevent costly misalignments.
Step 3: Inject the Plastic into the Mold
The injection molding machine forces molten thermoplastic into the mold, ensuring that the resin flows smoothly and evenly around the insert. Proper flow control prevents shifting or damaging the insert during this critical stage.
Step 4: Cool and Solidify the Injected Material
As the mold cools, the injected plastic hardens around the insert. Cooling time varies depending on the material, part geometry, and wall thickness.
Step 5: Eject the Molded Part
Once the part solidifies, ejector pins push it out of the mold. Mold designs incorporate retention features, such as undercuts or grooves on injection molding inserts, to prevent shifting during ejection, ensuring the insert remains securely bonded within the encapsulating plastic. Strategic ejector pin placement and rigid mold support prevent deformation, ensuring the integrated component retains its intended shape under ejection forces.
Common Insert Types for Injection Molding
Insert molding supports a wide range of injection molding inserts, each selected to meet the final product’s functional and environmental requirements.
Metal Inserts
Metal inserts are widely used in metal insert injection molding for mechanical reinforcement or electrical conductivity. Common metals include brass inserts (enable precise threading for secure fastening), stainless steel inserts (provide excellent corrosion resistance and high strength), and aluminum inserts (offer a high strength-to-weight ratio and efficient heat transfer).
Plastic Inserts
Plastic insert injection molding uses high-performance engineered plastics when the base molding material lacks specific properties, such as chemical resistance or thermal stability. For example, a nylon base may need a PEEK insert for high-temperature performance. Common plastic inserts include glass-filled composite inserts (plastics reinforced with glass fibers, enhancing dimensional stability and mechanical strength while retaining electrical insulation), PEEK (Polyetheretherketone), or PPS (Polyphenylene Sulfide) inserts (withstand extreme temperatures and chemical exposure).
Specialty Inserts
In addition to metals and plastics, insert molding can accommodate components made from other specialized materials such as ceramic inserts (offer exceptional heat resistance and electrical insulation), magnetic components (can be embedded to create integrated sensing or actuation mechanisms within plastic housings), and electronic modules, including sensors, circuit boards, and RFID tags, that can be encapsulated to create smart, connected products with protected electronics.
Advantages of Insert Molding
- Improved Mechanical Strength: Embedded injection molding inserts, such as metal threads, increase load-bearing capacity and durability in stressed areas like fasteners. Inserting metal or more rigid plastic can increase part strength.
- Eliminated Secondary Assembly: By embedding inserts during molding, insert injection molding removes the need for screws, adhesives, or clips, reducing assembly labor and costs. It also eliminates the need for post-processes such as tapping, heat-staking, or ultrasonic welding.
- Miniaturized Components: Plastic molding with inserts enables highly compact designs, integrating functional elements like connectors or sensors into small, precise parts for electronics or medical devices without additional components.
- Enhanced Reliability: A single molding cycle creates a seamless bond between the insert and encapsulating plastic, reducing failure points from mechanical joints or fasteners compared to traditional assemblies. However, proper design improves the retention of the substrate and helps mitigate delamination of the overmold, which is more likely with softer materials.
- Streamlined Production: Combining multiple functions into one integrated component lowers tooling needs, assembly time, and material costs, especially for high-volume manufacturing.
Challenges and Limitations of Insert Molding
Insert molding offers significant benefits but presents engineering and production challenges. Addressing these limitations early in design and planning prevents costly errors.
- Insert Shifting: Unsecured injection molding inserts may shift under high-pressure plastic injection, causing misalignment or poor bonding. Mechanical retention features or automated placement with vision systems ensure precise positioning.
- Tooling Complexity: Insert injection molding requires custom molds with tight tolerances (e.g., 0.005–0.01 mm) for multi-cavity or multi-insert parts, increasing costs and lead times. Precision tooling improves consistency and reduces scrap.
- Bonding Problems: Poor adhesion or mismatched thermal expansion between inserts and thermoplastics compatible with insert properties can weaken bonds and can lead to delamination. Selecting suitable plastics and applying primers or surface treatments enhances bonding.
- Mold Wear: Hard metal inserts, like stainless steel, abrade mold surfaces, risking dimensional inaccuracies. Hardened steel molds, protective mold coatings, or wear-resistant mold inserts mitigate wear and extend tool life.
- Increased Cycle Time: Insertion of the substrate is an additional step in the molding process to account for. Automation will reduce this impact compared to manual placement of the insert.
Design Guidelines for Insert Injection Molding
Insert molding yields optimal results with intentional design, anticipating how the injection molding insert and encapsulating plastic interact during fabrication. Some key best practices for designing successful insert-molded parts include:
- Retention Features: Incorporate undercuts, knurls, grooves, holes, or ribs on inserts to mechanically lock them into the encapsulating plastic after molding, preventing shifts under injection pressure or during use.
- Wall Thickness: Ensure uniform wall thickness of the encapsulating plastic around inserts to avoid warping, sink marks, internal voids, or weak spots.
- Plastic Flow Paths: Position gates and runners to allow molten plastic to flow smoothly around the insert, preventing turbulent flow or trapped air that causes incomplete filling, poor bonding, or cosmetic defects. Ensure enough space for material flow around the inserts.
- Ejection Considerations: Design molds with ejector pins placed away from the insert to prevent dislodging it from the encapsulating plastic during demolding, avoiding part damage.
- Tolerance Planning: Account for thermal expansion and shrinkage of both the encapsulating plastic and insert, ensuring a snug fit for adhesion while allowing slight clearance to prevent plastic cracking or misalignment, using simulation software like Moldflow or SolidWorks Plastics.
- Tooling Precision: Employ high-precision tooling, such as CNC-machined molds with tolerances below 0.01 mm, to align inserts accurately in the mold cavity, minimizing performance issues, cosmetic flaws, or tool wear.
- Tooling Placement: Consider features that may be used to hold the insert in place during the molding process, such as standoffs or pins for threaded inserts, and position them in the design accordingly.
Compatible Overmold Materials for Insert Molding
Insert molding works best when the encapsulating plastic is a thermoplastic that melts at a suitable temperature and flows smoothly under injection molding conditions to bond securely with the injection molding insert. Common thermoplastics used in insert injection molding include:
- Acrylonitrile Butadiene Styrene (ABS): Offers high dimensional stability and ease of molding, ideal for consumer electronics.
- Nylon (Polyamide, PA): Strong and flexible, often paired with molded-in metal parts for load-bearing applications like automotive brackets.
- Polycarbonate (PC): Tough and impact-resistant, used in electronics enclosures and medical devices.
- Polyetheretherketone (PEEK): Provides superior heat and chemical resistance, suited for aerospace and medical components.
- Polypropylene (PP): Lightweight and chemical-resistant, common in automotive and consumer products.
- Polyethylene (PE): Flexible and cost-effective, used for low-load applications like packaging.
- Thermoplastic Polyurethane (TPU) / Thermoplastic Elastomer (TPE): Flexible and rubber-like, ideal for overmolding grips, seals, and impact-absorbing features.
Applications and Industries Using Insert Molding Solutions
Insert molding integrates dissimilar materials into durable integrated components, reducing assembly time and enabling compact designs across major manufacturing sectors.
Automotive
Automotive manufacturers use insert molding for high-performance components that endure heat, vibration, and stress. Threaded brass inserts in engine covers and under-the-hood parts ensure secure fastening, as seen in a tier-1 supplier’s single molded part that replaced a bolted assembly, cutting costs by 20% and improving vibration resistance. Electrical housings and sensor assemblies embed metal contacts for reliable function in harsh environments, while interior fasteners reduce space and assembly time.
Medical Devices
Syringe needle hubs integrate plastic bodies with stainless steel shafts for sterile, single-use components. A medical device company embedded a stainless-steel sensor into a plastic housing, reducing production time by 35% and eliminating alignment errors. Sensor housings with ceramic injection molding inserts support electronics with chemical resistance, and reinforced handles ensure durability through sterilization.
Consumer Electronics
Insert injection molding enables compact, reliable electronic components. USB, HDMI, and power connectors mold metal inserts into plastic bodies for electrical contact and strain relief. Device enclosures incorporate grounding and shielding to meet regulatory standards. Tactile buttons and switches combine flexible plastics with inserts for responsive, durable inputs, streamlining production.
Industrial Equipment
In harsh industrial settings, insert-molded components withstand high loads and wear. Housings with embedded metal bushings ensure stable mechanical movement, while heavy-duty switches integrate electrical and structural elements via plastic molding with inserts. Control knobs and panel mounts with inserts resist vibration, maintaining alignment for reliable machinery interfaces.
How Fictiv Supports Injection Molding Projects
Whether you’re embedding electrical contacts into a housing or adding structural support to a compact part, we offer AI-driven DFM and expert advice from humans for scalable insert molding services. Talk to our team of experts to help you reduce risk, accelerate the product development process, and deliver quality parts, fast.
If you’re designing a part that requires encapsulated components or multiple advanced materials, insert molding could save you time and money while improving performance.
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