Insert Molding vs Overmolding: What Are the Differences?

Insert molding vs overmolding

Insert molding and overmolding are two distinct variations of injection molding technology, both of which are known for multi-material components. Insert molding involves placing a prefabricated insert into the cavity, whereas overmolding involves injecting two or more shots. 

When choosing between insert molding and overmolding, it is essential to understand their processes, capabilities, advantages and disadvantages, preferred applications, and other key aspects.

Therefore, this article will present a detailed comparative discussion between overmolding and insert molding technologies to help you make the best decision.

What is Insert Molding?

Insert molding 

Insert molding is a type of injection molding service specialized for the production of plastic parts with pre-fabricated metal or ceramic inserts—for instance, threaded fasteners and electrical connectors. 

Inserts are accurately positioned in the mold cavity, and when molten plastic is injected, it flows around those inserts and forms a strong bond on solidification. Meanwhile, the inserts are made with machining, casting, 3D printing, or any other suitable method.

Advantages of Insert Molding

Insert molding offers several advantages, including reduced assembly steps, greater structural robustness, optimized size, and durability. 

Let’s further break down the advantages.

• Structural Robustness: Inserts mechanically bond with molded plastics and provide high structural robustness. 
• Reduced Assembly Steps: Embedding metallic, ceramic, or other inserts into plastic reduces the assembly steps.
• Cost-effectiveness: Fewer assembly steps improve production speed and reduce labor costs. 
• Optimized Size: With insert molding, parts can be designed more compactly, helping reduce overall size and weight. 
• Durability: The metal insert molding parts are strong and durable; they can last up to 20+ years. 

Disadvantages of Insert Molding

Although it has many advantages, the insert molding process also has some limitations, such as increased tooling costs and limited insert compatibility with polymers. 

• Increased Tooling Cost: The introduction of inserts into the mold cavity requires a more complex mold tooling setup, increasing tooling costs. 
• Insert Compatibility: You cannot embed metallic inserts into any type of thermoplastic; they must be compatible with the bonding material. 
• High Initial Cost: Automated insert molding technologies are expensive to install & maintain. 

What is Overmolding?

Plastic overmolding 

What makes plastic overmolding different from insert molding is that two or more materials are injection-molded sequentially within the mold. Let’s take an example of a toothbrush handle: first, hard PP is injected into the mold; once it solidifies in the mold cavity, soft TPE is injected to overmold the grip onto the PP handle.

The overmolding process uses multi-shot injection molding machines, which sequentially inject two or more different materials into the mold cavity. Here, overmolding materials are mechanically and chemically bonded to form the final parts. 

They can use different barrels and automate the injection process. Additionally, the overmolding process allows shaping of the multi-color components. 

Advantages of Overmolding

Multi-material molding, excellent sealing, aesthetic appeal, reduced assembly requirements, and stronger parts are the key advantages of overmolding. 

Let’s further break down the advantages. 

• Multi-material Parts: Overmolding allows for the molding of multiple materials in a single part, enhancing material flexibility. 
• Soft TPE Grips: You can overmold soft TPE grips on thermosets and thermoplastics. 
• Sealing Parts: Overmolding is an effective method for embedding seals on the outer surfaces of stiff, rigid components. 
• Reduced Assembly: Overmolding parts eliminates or reduces the need for fasteners, adhesives, or other joining methods during assembly.
• Aesthetic Appeal: Overmolding can shape multicolor components, making it beneficial for items that require aesthetic appeal. 

Disadvantages of Overmolding

Multi-step molding, longer cycle time, risk of debonding, and higher tooling costs are the limitations of the overmolding process. 

• Multi-step Molding: The overmolding process requires multiple injection shots, increasing tooling & process complexity. 
• Longer Cycle Time: Plastic overmolding is time-consuming unless you are overmolding parts that would otherwise need to be produced separately and assembled.  
• Risk of Debonding: Using incorrect molding temperature or time between shots increases the risk of debonding. 
• Higher Tooling Cost: The complexity of the injection mold for overmolding requires a higher upfront cost 

Insert Molding Vs Overmolding: Key Differences

Insert and overmolding parts 

Insert and overmolding differ in process, production speed, material compatibility, cost, and application preferences. 

Let’s break down these differences one by one. 

Molding Process

Both are subtypes of injection molding and follow the same basic principles: injecting liquid plastic into a mold cavity designed for the required shape. The only difference is that the overmolding process uses two- or multi-shot injection (either automated or manual), whereas the insert molding process uses single-shot injection. 

Cycle Time

The cycle time for molding each piece is longer for overmolding, as it requires a fixed time interval between injection shots. 

Material Compatibility

Insert molding is utilized to produce plastic parts with embedded metal, ceramic, or thermoset inserts. Primarily, metal insert molding is popular, in which plastics are molded around machined metal pieces. 

On the other hand, you need to consider whether the selected overmolding materials can bond together (via thermal or chemical bonds). 

Injection Molding Cost

Comparing the cost of insert molding vs overmolding, injection molding tooling is more expensive than insert molds due to higher tooling costs, longer cycle times, and the need for automation. However, it might not be true for mass-produced items.

Application Preferences

Metal insert molding is preferred for applications where structural strength is required, and minimal assembly steps are a priority—for instance, electrical connectors, fasteners, etc. 

On the other hand, the overmolding process is preferred for grips, seals, handles, etc. 

A Quick Overview of Insert vs Overmolding 

How to Choose between Insert Molding Vs Overmolding?

insert vs overmolding

The following are the five key factors to consider while choosing between insert molding vs overmolding for your project: desired functionality, part complexity, production volume, material compatibility, and assembly needs. 

1. Desired functionality

Do your plastic parts need to be embedded with metal for mechanical, electrical, or other functionality? If yes, choose insert molding. 

2. Part Complexity 

Choose overmolding for complex parts with multiple materials, especially thermoplastics, thermosets, elastomers, and rubbers; otherwise, choose insert molding or regular injection molding. 

3. Production Volume 

Consider the production volume you need: which one is suitable for higher volumes, between insert molding vs overmolding, depends on the material type and complexity of the part. 

4. Material Compatibility

Consider which different materials you are using for molding. For instance, the insert molding process is preferred for molding plastic-to-non-plastic components, whereas soft rubbers and silicone are the most used overmolding materials. 

5. Assembly Needs 

Assess the assembly requirements of components by analyzing their designs. Insert molding reduces assembly requirements, whereas overmolding excels at sealing & aesthetics.

What is meant by Silicone Insert Molding?

Silicone insert molding refers to molding Liquid Silicone Volume(LSR) parts with metal or ceramic inserts. Inserts are placed inside the mold before injecting LSR, and they forms strong bond after injection & solidification. 

Common examples of silicone insert molding parts include medical seals, electrical connectors, and automotive grommets.

Summing Up

To sum up, selecting between insert molding vVs overmolding mainly revolves around one question: do your parts need metallic, ceramic, or other pre-fabricated inserts or not? If yes, choose insert molding. Otherwise, choose overmolding services to make parts by injecting two or more materials, rather than embedding machined or cast pieces.

At ProleanTech, we offer extensive plastic molding capabilities, including precise yet affordable overmolding services for a variety of applications, from consumer goods to automotive parts & medical components. Additionally, we also have an in-house mold-making facility.

For further information or cost-estimation, upload and design on our online portal today. 

FAQs

What are the disadvantages of insert molding? 

Increased tooling costs, material compatibility issues, higher automation requirements, and the need for accurate positioning of inserts in the mold cavity are disadvantages of insert molding. 

Which moulding material is best? 

It is difficult to point out a single best material for molding; it depends on the application requirements and design characteristics. However, ABS, PE, Nylon, and TPE are some popular choices in industries. 

What are the 5 types of molding?

The 5 types of molding are injection molding, compression molding, transfer molding, blow molding, and rotational molding.