Injection molding is a broad term used to describe one of the most important processes in manufacturing. This is a process that requires a mold, usually made of metal, with a cavity in the shape of the desired part. Molten plastic is injected into the mold and ejected. Repeating the process can produce thousands of identical parts. It's safe to assume that every high-volume plastic part on the market comes from an injection molding machine, as the benefits of using injection molding for production are numerous. These advantages include low cost per part, short cycle times, a wide range of materials and compatible, within-tolerance parts.
Various subprocesses add even more functionality to this already versatile technology. This article will discuss insert molding versus overmolding in detail and the benefits of each.
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1. What is Insert Molding?
Insert molding is a subset of injection molding technology, similar to overmolding, in which a metal part is placed into a mold cavity prior to the actual plastic injection. Precise positioning of the Insert in the mold manually or by robotic arm. The mold is then closed and the plastic is molded over the insert to form a single part.
One of the most common applications of insert molding is to create metal connection features for fasteners. Fasteners allow components to be assembled and disassembled safely without damaging the product. Heat-set threaded inserts are molded from plastic to reduce the risk of thread damage during installation.
Insert molding can also eliminate the need for fasteners by including the necessary metal parts in the mold, thereby firmly securing the parts into a single bonded part.
2. What are the Advantages of Insert Molding?
Insert molding is a versatile process with many advantages, some of which are listed below.
Reduce assembly costs: Injection molding machines can produce thousands of parts per day. This economy of scale significantly reduces individual part costs. Unlike CNC machined, sheet metal, or additively manufactured parts, which often require complex and costly assembly, insert molding integrates components directly into the plastic part, eliminating assembly steps and maximizing cost savings.
Enhance part performance: While plastics offer advantages like lower cost, design flexibility, and weight, they may lack the strength and durability of metals. Insert molding combines the best of both worlds by incorporating metal inserts into plastic parts. This enhances the part's strength, stiffness, and wear resistance, making it suitable for applications with demanding performance requirements.
Improved part consistency and reliability: By integrating inserts during the molding process, insert molding ensures precise alignment and secure bonding between the metal and plastic components. This eliminates potential assembly errors and weaknesses often associated with post-molding assembly methods, resulting in more consistent and reliable parts.
Design flexibility: Insert molding offers greater design freedom compared to traditional assembly methods. Complex shapes and features can be incorporated into the plastic part, while the metal insert provides specific functional elements. This allows for the creation of innovative and multifunctional components.
Weight reduction: Plastics are generally lighter than metals, and insert molding can help reduce overall part weight by using plastic for non-critical areas. This weight reduction can be beneficial in various applications, such as automotive and aerospace, where weight savings translate into improved fuel efficiency and performance.
3. What are the Disadvantages of Insert Molding?
Although insert molding has many advantages, there are some disadvantages to consider before choosing to use this sub-process. the
Multiple manufacturing techniques: Insert molding may involve a two-step manufacturing process. If the insert is a custom design rather than an off-the-shelf part, it will need to be fabricated using a metal forming process such as CNC machining. These metal forming technologies are typically much more expensive per part than a similar full injection molding process. In some cases, metal parts can be manufactured by die casting or MIM (Metal Injection Molding). This reduces the overall cost of the metal insert, but does not eliminate the added cost of the insert molded unit, as parts with metal inserts are generally more expensive than parts made of only plastic.
Adding part complexity: If custom metal inserts are required, designers must understand the design for manufacturability (DFM) principles of both technologies and how best to integrate them into a single usable part.
4. What is Overmolding?
Overmolding is a manufacturing process where a layer of plastic is molded onto an existing component, creating a single, integrated part. Unlike insert molding, which involves inserting a pre-made component into a mold, overmolding directly applies the second material to the surface of the first.
This technique allows for combining different materials to achieve desired properties, such as improved grip, durability, or aesthetics. For instance, soft rubber can be overmolded onto a hard plastic base for better handling, or different colored plastics can be used for branding purposes. Overmolded parts are commonly found in everyday products like tool handles, toothbrushes, and electronic devices.
5. What are the Advantages of Overmolding?
Overmolding is a versatile process with many advantages:
Increased material flexibility: Overmolding enables designers to take advantage of the benefits of multiple materials to create complex parts with different properties, adding visual complexity or adding tactile sensations.
No adhesive required: Overmolding allows dissimilar materials to fuse in the mold, eliminating the need for glue or other permanent bonding methods. This increases the overall durability of the part and reduces assembly costs.
Embedded seals: Overmolding offers the option of molding a soft seal into the part. An example is electronics enclosures that require an IP rating. Usually, this part will have a groove into which the o-ring can be installed later. However, it is more cost-effective and durable to permanently mold the seal as a one-piece component.
Improved aesthetics: Overmolding can dramatically enhance a product's appearance by combining different colors, textures, and finishes. For instance, a soft-touch overmold can be applied to a hard plastic base, providing a comfortable and visually appealing product.
Enhanced product performance: By combining materials with different properties, overmolding can improve a product's functionality. For example, overmolding a rubber grip onto a metal tool can provide better grip and shock absorption.
Reduced assembly time and costs: Overmolding can simplify the manufacturing process by combining multiple components into a single piece, reducing labor and assembly time. This can lead to significant cost savings.
Improved product durability: The seamless integration of materials achieved through overmolding creates a stronger and more durable part compared to those assembled using adhesives or other mechanical fasteners.
6. What are the Disadvantages of Overmolding?
Although overmolding has many benefits, there are some disadvantages to consider before deciding to use this process.
Multi-step process: Overmolded parts are made in a two-step process. This increases part cycle time and is therefore more expensive than molding a single part without overmolding. This also requires two tools, or complex two-shot molds, adding to the upfront cost. However, when the alternative is to create two separate injection molded parts and then assemble them after the fact, overmolding becomes a value-added solution.
Debonding: Bonding two dissimilar materials together in an injection mold presents a risk of delamination. This usually occurs if the temperature is not within the optimum range for a particular combination of materials. In some cases, mechanical interlocks may be required when materials cannot be reliably bonded together by heat.
7. Overmolding vs. Insert Molding: A Comparative Overview
Overmolding is a process that involves applying a layer of material, typically a softer plastic or rubber, over an existing part (the substrate). This technique is beneficial for creating ergonomic grips, reducing vibrations, and providing seals for moisture-sensitive products. Common applications include tool handles, medical devices, personal care products, and consumer electronics. Overmolding enhances user comfort, improves product appearance, and offers additional protection for the substrate.
Insert molding, on the other hand, involves placing a pre-made part, usually metal or another material, into a mold and then injecting plastic around it to form a single integrated component. This method is particularly useful for creating parts that require a combination of materials for structural reinforcement or functionality, such as threaded fasteners in plastic parts, electrical components, and automotive parts. Insert molding improves structural strength, reduces assembly time and costs, and enhances design flexibility.
Feature |
Insert Molding |
Overmolding |
Process |
Pre-made insert placed in mold, then plastic injected |
Layer of plastic or rubber applied over existing part |
Material of Insert |
Metal, ceramic, or other rigid materials |
Softer plastics or rubber |
Material of Molding |
Thermoplastics |
Thermoplastics |
Applications |
Threaded fasteners, electrical components, automotive parts |
Tool handles, medical devices, consumer electronics |
Advantages |
Increased strength, reduced assembly time, design flexibility |
Improved ergonomics, aesthetics, protection, seals |
8. Overmolding vs. Insert Molding: Material Use
Insert molding
Insert molding involves embedding pre-made components into plastic to form a unified product. This technique often employs various materials for the inserts and the encapsulating resin:
Inserts:
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Metals: Common metals like brass, steel, aluminum, and copper are used due to their strength, wear resistance, and conductive properties. Metal inserts often feature threaded holes or complex geometries for added functionality.
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Electronic components: Pre-wired connectors, circuits, and sensors are embedded into plastic housings, providing electrical functionality and protection.
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Ceramics and glass: These materials are selected for their hardness, insulation properties, and thermal resistance, making them suitable for high-performance applications.
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Fabrics and foams: These are used for their cushioning, aesthetic, and functional properties in various consumer goods.
Encapsulating resins:
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Thermoplastics: These include ABS, polycarbonate, nylon, HDPE, and PBT, chosen for their strength, stiffness, and moldability.
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Thermosets: Epoxy and phenolic resins are used for their high-temperature resistance and dimensional stability.
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Elastomers: Materials like TPE, silicone, and TPV provide flexibility and a soft touch, ideal for products needing durability and elasticity.
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Composites: Filled resins with glass or carbon fibers offer enhanced strength for demanding applications.
Overmolding
Overmolding involves molding an additional layer of material over an existing part, creating a strong bond between the two materials. The materials used in this process include:
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Substrate (Base layer): Typically made from a rigid plastic such as polycarbonate, ABS, or nylon.
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Overmold material: Often a softer material like TPE, TPU, or silicone to provide a tactile feel, improved grip, or cushioning.
9. Overmolding vs. Insert Molding: Applications
Insert Molding
Insert molding involves placing a preformed component, such as a metal part or electronic component, into a mold and injecting plastic around it. This creates a single, integrated part with enhanced features. Applications include:
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Automotive industry: Used for parts like gears, electrical sensors, and fasteners, combining metal and plastic to enhance strength and functionality.
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Medical devices: Essential for creating components in medical equipment such as defibrillators and implanted devices, ensuring durability and sterility.
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Electronics: Encasing electronic parts and wires in plastic to improve safety and performance, such as in chargers and other consumer electronics.
Overmolding
Overmolding involves molding an additional layer of material over an existing part, enhancing its appearance, grip, or functionality. It is widely used in:
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Consumer goods: Manufacturing items like toothbrushes, cell phone cases, and kitchen utensils that benefit from ergonomic grips and aesthetic enhancements.
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Automotive components: Creating two-tone interior parts such as dashboard panels, handles, and knobs for improved aesthetics and tactile feel.
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Medical products: Producing comfortable and safe-to-use medical devices with softer outer layers for better patient interaction.
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Beauty industry: Designing attractive and functional packaging for cosmetics, including makeup brushes and perfume bottles.
Combined applications
Some products benefit from both insert molding and overmolding. For instance, a device might start with insert molding to encapsulate electronic components and then undergo overmolding to add an ergonomic outer layer. This combination is often used in advanced consumer electronics and medical devices.
10. Overmolding vs. Insert Molding: A Design Guide
In this section, we will focus on the points between overmolding design and insert molding design.
Overmolding design guide
Keep in mind that overmolding is rarely used to cover the entire substrate. Instead, overmolding should be applied in sections. For each of these, keep these helpful design tips in mind:
Work with your manufacturing partner to ensure compatibility between substrate and TPE or TPU.
Use TPE or TPU with a lower melting temperature than the base plastic.
Apply an overmold that is slightly thinner than the substrate supporting it.
Design the overmold to sit directly below the surface of the substrate.
If you need more holding power, design undercuts, keyways, and other mechanical features to lock materials together.
Insert molding design guide
Designers may face some unique engineering challenges when using insert molding. The advantages of increased strength and versatility must be weighed against the need for more careful manufacturing review of the design. Here are some helpful guidelines to consider:
Of most concern is the shrinkage of the resin. This creates hoop stress around the fitting which, over time, can lead to cracking, especially when the part is under mechanical tension. You can counteract this by:
Use resin materials with low shrinkage;
Use filler-reinforced resin materials;
Surround the insert with a larger area of plastic;
Support the blade with bosses and ribs;
Preheat inserts before molding. This allows the resin and insert to cool and shrink at the same time, relieving some of the stress between the materials.
Using a precast insert with a knurled surface helps lock it in place.
Avoid sharp corners and use rounded contours to reduce stress.
Design the insert so that it is slightly recessed. This helps avoid damaging the tool.
11. Overmolding vs. Insert Molding: How to Choose
Which process is right for you depends entirely on your application. Keep in mind that they are not mutually exclusive molding methods, as they serve different purposes. In fact, it's not uncommon to use both approaches on one product. For example, consider a power tool that has an overmolded grip on the handle, but also uses a nut to hold the housing together.
When to use insert molding
There are several main reasons why you should consider insert molding. First, your product application requires a strong mechanical fitting to hold two or more components together, typically for a case or housing. Threaded nuts are great for this, but snap connectors come in a variety of styles as well. the
Insert molding is used to place rubber or plastic handles on metal parts, such as handles on hand tools or kitchen knives. Insert molding is ideal for sealing wires and electrical connectors into permanent plastic housings that keep out dust and moisture.
Inserts should be used when the plastic enclosure needs to be opened occasionally for servicing and then locked again, such as when performing routine maintenance or changing batteries. They cost a little more, but that cost can be offset by making a more durable end product.
When to use overmolding
Product developers should consider overmolding to improve the grip and feel of parts that need to be held in the hand, or to protect the end user from vibration, heat or electricity. Overmolding is also a great way to permanently bond rubber to metal, as is the case on wheels and casters.
Overmolding also offers more opportunities to be creative, applying colorful designs to improve the appearance of a part as well as its performance.
Overmolding adds cushioning and shock absorption to many common household items, protecting users from accidental injuries.
12. Conclusion
Injection molding, which includes insert molding and overmolding sub-processes, is a versatile and low-cost manufacturing production process used for most consumer products. Injection molding often achieves the lowest cost per part compared to other manufacturing techniques such as CNC machining or even 3D printing.
Once injection molding has been selected for a particular application, the next step is often whether to use insert molding, overmolding or just stick with plain injection molding. When trying to weigh the advantages of a process, it is important to accurately define the product application. Each process has specific use cases for different product types. It can be difficult to tell which process is best for your particular product, so it's best to get expert advice early on. Contact a HingTung representative to leverage decades of extensive manufacturing expertise. We'll help steer your design decisions in the right direction so you can choose between insert molding, overmolding, or just injection molding.