Overmolding vs. insert molding is a comparison between two molding processes that have things in common but also key differences.
Overmolding molds one part from one material and then molds another material over the first molded part.
That’s why it’s sometimes called multi-material molding or, when injection molding is used, two-shot molding.
Insert molding begins when a pre-fabricated, pre-formed, or previously molded part is placed in a mold and then molded over with a different material.
Sometimes, the insert is made of metal. However, non-metal inserts can also be used.
With overmolding and insert molding, parts are bonded together seamlessly. Whether it’s to improve comfort, provide sealing, increase safety, or reduce noise and vibration, these two processes are used in a range of applications.
But which process do you need? And how do they compare?
What is Insert Molding?
Insert molding is an injection molding process that involves adding inserts, particularly metal, into an injection-molded part. Placing the insert within the mold cavity occurs before molten plastic injection. Consequently, on cooling, the insert becomes a permanent plastic part.
One of the most common applications for insert molding is the creation of metal attachment features for fasteners. Fasteners enable assemblies to be securely assembled and disassembled without product damage. Heat-set threaded inserts are molded into plastic to reduce the risks of thread damage during installation.
Insert molding can also eliminate the need for fasteners by including the necessary metal parts in the mold, thus firmly securing the parts into a single bonded component.
Insert Molding Benefits
- More robust part when compared to two independently molded parts which are assembled in a secondary assembly process.
- Elimination of glue, which can be a high failure mode for these types of parts.
- Lower total assembled part cost.
- Less part to part variability.
Disadvantages of Insert Molding
- Insert molding is suitable for several products. However, it is not suitable for others. Use the following. Below are a few disadvantages of the process.
- Resin can shrink around or enter the insert leading to a functional problem.
- It requires a high level of technical expertise for complex parts.
- It requires multiple manufacturing technologies, i.e., metal and plastic.
What is overmolding?
Overmolding is a multi-step injection molding process where two or more components are molded over top of one another. Overmolding is sometimes referred to as two-shot molding because it is a two-step process.
First, a base component (otherwise known as a substrate) is molded and allowed to cure. Overmolded substrates are often made of plastic.
Then, a second layer is molded directly on top of the first to create a single solid piece. Overmolding is commonly used to manufacture plastic parts that feature a rubber handle.
The two-shot process of overmolding a toothbrush, for example, consists of forming a base layer for the plastic handle and a top layer of rubber (to make the toothbrush less slippery to hold).
Overmolding Benefits
- Better ProduCt Performance. A primary reason to design and manufacture a plastic overmolded product or part is improved product performance.
- Increased Shelf Appeal. Overmolding helps a product stand out from the competition.
- Lower Production Costs. Overmolding is a rare process that adds advantages in several ways and also lowers production costs.
Disadvantages of Overmolding
The overmolding process, though it offers several advantages, it also has some limitations or potential drawbacks that should be considered before choosing this method of manufacturing.
A few disadvantages of the overmolding process are,
- Overmolding requires specialized equipment and tooling, which can be more expensive than traditional assembly methods.
- There are a few material compatibility issues. Not all materials are compatible with overmolding. Some materials do not bond properly, leading to delamination or failure of the overmolded part.
- Overmolding has limitations on part design, particularly for complex geometries or components with intricate features.
- The two-step nature of overmolding can result in longer cycle times compared to single-shot injection molding.
- Warping can occur due to the differential shrinkage of the two materials during cooling.
- Overall, overmolding is an efficient manufacturing method that offers several benefits. However, it is important to carefully consider the potential drawbacks and assess their impact on the specific application before making a decision.
Comparing insert molding to overmolding
Overmolding and insert molding enable manufacturers to create multi-material products without using adhesives, help eliminate secondary assembly steps, and improve the final product.
However, it’s important to note the differences between insert molding and overmolding, including:
Process
Overmolding involves injecting two shots of materials to form the substrate and the overmold.
Insert molding only involves injecting one shot of molten plastic, though the metal insert needs to be purchased or created separately.
Speed
Insert molding involves one shot while overmolding involves multiple, which means the insert molding cycle time is generally faster than the overmolding cycle time.
However, that doesn’t necessarily mean that insert molding is always the fastest option for production.
In some cases, manufacturers may be unable to find pre-made metal inserts that meet their requirements, meaning they’ll need to create a custom metal insert, which can lengthen production timelines.
Cost
Insert molding and overmolding can reduce assembly costs and accelerate production, helping companies simultaneously save on production costs and generate greater profits when producing large quantities of parts.
However, overmolding is more expensive than insert molding, as it involves two steps. This is particularly true when it comes to prototype or small production runs, as overmolding requires manufacturing two tools one for the substrate and one for the overmold.
Applications
Overmolding is often used to make toothbrushes, medical instruments, disposable razors, and phone cases, or to enclose electronic circuit boards
Similarities between insert molding and overmolding
Overmolding and insert molding both:
- Support the use of plastic and rubber, including thermoplastic elastomers (TPE)
- Join together multiple materials without adhesives or fasteners
- Leverage the individual properties of the constituent materials
- Support the use of either injection molding or compression molding
- Follow the design rules of injection molding or compression molding
- Have some special design considerations of their own
Sometimes, overmolding and insert molding can be used for the same applications. For example, a plastic toothbrush can have a hard plastic handle with a softer silicone grip.
Both parts are molded, but not necessarily in the same tool, in the same machine, and in a continuous cycle.
Choosing Between Insert Molding, Overmolding, or Injection Molding
Injection molding, which includes the sub-processes insert molding and overmolding, is a versatile and low-cost manufacturing production process that is used in the large majority of consumer products.
Injection molding often results in the lowest cost per part when compared to other manufacturing techniques like CNC machining and even 3D printing.
Once injection molding is chosen for a specific 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 the processes, it is important to accurately define the product application.
Each of these processes has specific use cases that are suited to different product types. It can be difficult to gauge which process will best suit your particular product, so it’s good to get expert advice early on.