Which Process Fits Your Custom Manufacturing? Overmolding vs. Insert Molding

Injection molding is a wider field of manufacturing engineering. Based on the methods and technology employed, injection molding can be narrowed down to insert molding or overmolding as techniques for improving chemical and mechanical properties of the final product, as well as achieving highly recommended safety standards. Whether to choose between overmolding and insert molding, the user must precisely define their goals and intentions for the final product. For instance, we faced two common scenarios at our company while producing custom parts for our clients, and the decision to use overmolding or insert molding was based on the requirements in each case.

In the first scenario, we were tasked with manufacturing an industrial electrical terminal block. The block had to meet reliable electrical conductivity properties, secure terminal retention, and resist heat and vibrations. After analysis, we chose insert molding over overmolding. The team found that appearance and grip requirements could not supersede the functional requirement of integrating a conductive metal part into the plastic.

In the second scenario, we assisted a client in manufacturing a custom overmolded housing for a rechargeable electric toothbrush over insert molding. The fundamental requirements here were to provide a comfortable grip, water resistance, and an attractive appearance.

The two scenarios illustrate that both overmolding and insert molding are equally important methods of injection molding. However, they both have different applications. In this discussion, we briefly explain overmolding and insert molding, and find the differences between the two manufacturing methods.

Co to jest Overmolding?

In simplest terms, overmolding is a manufacturing process applied in injection molding where a part is encased or covered with a second material. The second part is typically rubber or plastic. The resulting bond leads to a single, integrated product. Overmolding enhances product durability, grip, aesthetics, and functionality.

The base component in overmolding is called a substrate. The substrate defines the structural properties of the final product. Therefore, the selection of the materials used to manufacture the substrate is essential. The choice of the materials for the substrate varies based on the application. The following table summarizes different materials and their applications.

The second material is called second-shot material and is normally melted and applied around the substrate. Second-shot material is selected because of its flexibility, durability, chemical resistance, or sealing capability. The table below summarizes some of these second-shot materials.

Overmolding serves primarily to improve product functionality. Through overmolding, manufacturers achieve better ergonomics, vibration isolation, improved grip, noise reduction, and enhanced cosmetics.

What is Insert Molding?

Insert molding is an advanced plastic manufacturing process where a pre-formed component (metal or another material) is placed inside the mold, and plastic is injected around it to create a single, strong, and integrated part. This process eliminates the need for secondary assembly operations. It improves product strength and ensures high precision.

In insert molding applications, components like threaded inserts, pins, bushings, electrical contacts, and metal shafts are commonly used. The molten plastic flows around the insert, forming a tight bond that enhances durability and performance. This method is widely used in industries such as automotive, electrical, electronics, medical devices, and consumer goods. Unlike overmolding, the primary functions of insert molding often revolve around providing structural reinforcement or integrating critical non-plastic functions (such as electrical conductivity or durable metal threading) directly into the product. Common inserts include brass threaded inserts, stainless steel shafts, magnets, electrical contacts, copper terminals, bearings, and bushings.

What Are the Major Differences between Overmolding vs. Insert Molding?

Both overmolding and insert-molding combine multiple materials into a single component. However, they serve different goals and have different production workflows. Understanding the differences between these methods can help the engineering team to choose the most suitable process based on the production volumes, product design, and performance requirements.  

Differences in Manufacturing Workflow

Insert molding is a single-injection molding process. The process starts by manufacturing the insert through machining, die casting, or other processes. Sometimes the inserts are not manufactured in the same factory, but sourced somewhere else. The insert is prepared and positioned in the mold, and molten plastic is injected around it. The final part is an integral of the insert and the plastic. Insert molding eliminates secondary assembly operations, thus simplifying production.

Overmolding on the other side often involves a two-stage process or a specialized two-shot machine. The first step is the production of the substrate. This rigid substrate can be manufactured through a primary injection molding cycle. From there, the substrate can be transferred to a separate mold (known as pick-and-place overmolding), or, if using a two-shot machine, it remains in the same machine and is rotated to the second cavity. Sometimes, the design can have one mold with two different cavities. In this case, the substrate is rotated from the first cavity to the second cavity using a rotary platen or indexing system. The second material is then melted, poured around the substrate, and allowed to cool. The resulting complete multi-material part is ejected with complete functional requirements integrated. Because of the two separate molding stages required in overmolding, the process is complex compared to insert molding.

Process Comparison

The following table summarizes the differences in the processes of insert molding and overmolding. Insert molding is a single-injection process, while overmolding is a two-stage injection molding process.

Differences in Manufacturing Complexity

Insert molding has a relatively simpler production flow than overmolding. The process only requires a single injection of materials once the insert is well set. Effectively, it has shorter cycle times, lower production costs, and less complex tooling. Overmolding is relatively complex with relatively higher labor costs. The substrate must precisely align with the second-shot mold. The process effectiveness depends on the compatibility between the substrate and the overmold. The process has longer development times and higher tooling complexity. However, on the positive side, overmolding delivers greater functional and aesthetic enhancements.

Differences in Materials selection

There is a significant difference in material combinations between insert molding and overmolding. For insert molding, plastic is normally used as a primary material to cover the insert. The insert can be formed from different materials like metal, ceramic, or an electrical component. On the other side, overmolding can include a combination of different materials between the substrate and the second material.

For instance, a rigid custom plastic substrate can be combined with a soft elastomer, or a metal with an elastomer, for multi-material designs. Achieving a robust chemical bond in these combinations requires precisely matching the surface energies and thermodynamic properties of both materials ^[1]..

Differences in typical applications

Both insert molding and overmolding have special application areas. The following table highlights some of the application areas of these methods.  

How Does Insert Molding Improve Mechanical Strength?

Insert molding is not just a manufacturing technology. The main reason behind the choice of this technology is to enhance the mechanical and structural strength. A product made of plastic alone cannot offer the load transfer capacity. This requires supporting them with high-strength metallic inserts during injection molding. The resulting product, even though it may have low weight, possesses high mechanical strength and corrosion resistance properties. The insert molding directs loads that would be subjected to the polymer to the metallic reinforcement. Consequently, the product achieves increased structural reliability. The mechanical properties of the insert molding are enhanced through incorporating diverse design elements like knurling, undercuts, through holes, and surface roughness.

Role of Knurling

Diamond, straight, or helical groove patterns are normally machined on the surface of the inserts as surface deformation procedures. This rough texture creates micromechanical engagement between the insert and the molten polymer. During injection, the molten polymer flows and settles around the knurls. When it solidifies, it locks into these grooves, significantly improving pull-out strength and torque resistance.

Role of Undercuts

Another way to enhance mechanical properties in insert molding is to design reverse-angled features into the insert geometry. Molten polymer fills these cavities, forming a physical lock. Consequently, the axial or rotational movement is prevented, especially for vibration-prone or high-load applications. Undercuts inserts are mostly used in industrial or automotive equipment.  

Role of Through-Holes

Through-holes are open pathways through the inserts. Molten plastic flows through the holes, solidifying to form a solid anchor. This anchor enhances the retention strength between the insert and polymer, either vertically or horizontally. While designing for Through-Holes, you need to carry out flow analysis to ensure there are no voids.

What are the DFM Guidelines for Metal Inserts?

Molding quality requires proper design of the inserts. For metal inserts, the design must consider effective knurling recommendations, insert positioning, draft angles, and gate locations. Knurling Recommendations

Knurling offers stronger mechanical retention compared to adhesives. Different knurling patterns have different applications. For example, in cases of high loads, you are recommended to choose diamond knurl. The crisscross patterns of these knurls provide multidirectional mechanical locking essential for torque and pull-out resistance. This knurling is applicable in automotive sensor housings, electronic connectors, industrial machinery, and medical device assemblies.

Another type of knurling is helical knurl that is excellent for dynamic loading. They contain continuous spiral grooves around the insert, which resist both rotational forces and axial pull-out. Apart from high fatigue performance, helical knurls have reduced stress concentration, making it applicable in robotics, power tools, and aerospace assemblies. You can choose these knurls for inserts that are subjected to repeated loading and vibrations.

The following table is a summary of the different types of knurls and their applications.

Insert Positioning

Insert positioning influences bond quality, dimensional accuracy, and mechanical strength. A slight movement or displacement of the insert in the mold during injection can lead to weak bonding, flash, and poor part quality, leading to rejection. To ensure proper insert positioning, follow the guidelines below:

1. Maintain Uniform Wall Thickness Around the Insert.

A uniform wall thickness around the insert promotes uniform melt flow, cooling, and stress distribution. Thick sections contribute to differential shrinkage because of slow cooling, while thin sections lead to voids and weak spots because of incomplete filling. To avoid thickness variations, you can use gradual transitions with tapers or fillets. For many custom thermoplastics, the recommended wall thickness is 2-4 mm. Additionally, engineers must account for the mismatch in the Coefficient of Linear Thermal Expansion (CLTE) between the metal insert and the plastic to prevent post-molding radial stress ^[2].

2. Support Long Inserts to Prevent Movement during Injection.

Inserts that are slender or long can easily move because of the high forces of the molten polymer, which enters the cavity at high speed. If not supported, these inserts can tilt or rotate and vibrate during filling. This can cause uneven plastic coverage, affecting product quality. Thus, you should support the inserts at both ends if possible. For hollow inserts, you can use core pins to ensure stability.

3. Use Locating Features or Dedicated Nests for Repeatable Placement

In cases of high-volume production, repeatability is a critical design factor. It requires special attention to avoid errors. You therefore need locating features to constrain the inserts in all critical directions. Common locating features include locating pins, V-grooves, precision pockets, and shoulders.

4. Design for Robotic Loading in High-Volume Production.

Sometimes the demand for the insert-molding products may double or triple the expected volumes. In such cases, manual insert loading may not be practical due to strict cycle time constraints and the need for absolute placement consistency. To maintain consistency, fast production cycles and reduced labor costs, you are advised to adopt robotic loading.

Cost Comparison

The following table summarizes the costs of insert-molding compared to overmolding.

Wnioski

There is no straight formula for choosing between insert-molding over overmolding. Both techniques have their unique applications in different industries. The choice between the two should be guided by product function, but not by the available manufacturing options. Where structural integrity matters most, you can choose insert molding. It serves to ensure the product has long-term mechanical durability. On the other side, overmolding serves well in cases where product functionality matters most, especially in vibration environments, sealing, and tool handling.

Odniesienie

[1] Awaja, F., Gilbert, M., Kelly, G., Fox, B., & Pigram, P. J. (2009). Adhesion of polymers. Progress in Polymer Science, 34(9), 948-968. https://doi.org/10.1016/j.progpolymsci.2009.04.007

[2] Elsevier B.V. (n.d.). Współczynnik rozszerzalności cieplnej. ScienceDirect Topics. Retrieved from https://www.sciencedirect.com/topics/engineering/coefficient-of-thermal-expansion