Two-shot molding vs. overmolding: What you need to know

Injection molding is a popular manufacturing process that’s ideal for quickly creating precise parts with intricate shapes, all without leaving behind a lot of material waste. Common applications include packaging, automotive dashboards, mechanical parts like gears, and even popular kids’ toys.

 

Many different processes fall under the umbrella of injection molding, including two-shot molding and overmolding. These two processes are similar but have a few key distinctions — here’s what engineers and designers need to know:

 

What is two-shot molding?

Two-shot molding, also known as dual-shot, multi-shot, or double-shot molding is a subcategory of injection molding that allows engineers to create multi-material or multi-colored parts without adding extra assembly steps.

 

The two-shot injection molding process is best understood in terms of the different layers of materials or colors that are created by the injection molding machine. The first material is injected into a mold to create the substrate, around which the other material or materials will be molded. The substrate solidifies and cools before being transferred — by hand, robot arm, or rotary plane — to the other chamber of the mold.

 

From there, the mold opens and the side with the substrate rotates 180° to meet the other mold chamber and injection molding nozzle. Once the substrate is in place, the second material is injected and bonds with the substrate to form a firm hold.  Once the second layer cools, the final part is ejected.

 

Engineers should know that two-shot injection molding can be sped up or slowed down based on how the substrate is transferred to the other chamber of the mold. Hand and robot arm transfers take longer than a rotary plane, but rotary platen molding is more expensive and generally only an efficient option for high volume production runs.

 

Additionally, it’s critical that molds are made out of materials that will easily bond together and that the molds align correctly to prevent deformities in the part.

 

Advantages and disadvantages of two-shot molding

Two-shot plastic injection molding is an excellent technique for efficient and cost-effective manufacturing. This process also produces highly durable end parts and components.

 

From a design standpoint, two-shot molding offers designers a lot of flexibility because this process can create complex geometries and accommodate multiple colors, making for more aesthetically pleasing parts.

 

Further, since one machine makes the whole part and no post-processing is required, engineers can dramatically reduce manufacturing time, which in turn keeps costs low. However, it’s worth noting that the initial two-shot mold costs can be high and the two-shot molding machine is more expensive than a standard injection molding machine. Luckily, these costs are often offset by labor savings and assembly costs on large production runs.

 

What is overmolding?

Overmolding, like two-shot molding, is a multi-shot injection molding process that produces a single end product from two or more different thermoplastics. This process is ideal for engineers who want to build strong, functional, aesthetically pleasing parts that won’t separate over time.

 

To start the overmolding process, an engineer injection molds a substrate out of the more rigid overmold material. Then, the substrate is placed in an overmold tool or overmold cavity within the same tool. The molten overmolding material is then ejected into, onto, or around the substrate. After the molten material cools, the substrate and overmold are chemically or mechanically bonded. The entire overmolding process can take as little as 30 seconds.

 

Product teams must keep in mind that all thermoplastics used in the overmolding process must be chemically or thermally compatible with one another. Compatibility generally is not an issue with metal substrates because they can be used with any plastic overmold, but product teams can encounter compatibility issues when overmolding plastic with plastic. If the substrate and overmold aren’t compatible, the end product might be deformed or poorly bound.

 

However, if two plastics with less-than-ideal compatibility must be used, teams can design mechanical bonding features into the part after the fact, though this is likely to result in higher costs.

 

Advantages and disadvantages of overmolding

Overmolding and two-shot injection molding share many of the same advantages. They’re both ideal for quickly creating durable, reliable, and vibration-resistant parts with complex geometries, but overmolding is best suited for low-volume production runs.

 

Compared to two-shot molding, overmold designs are also easier to make because engineers can use any standard injection molding machine to conduct this process.

 

In terms of disadvantages, the tolerances of parts made via overmolding are often inferior to those that can be achieved with two-shot injection molding. It’s also important to remember that plastic compatibility requirements can constrain designers.

 

Choosing between two-shot molding and overmolding

Two-shot molding and overmolding are both simple and effective processes for creating durable parts made of two or more materials or two or more colors. To choose between the two, engineers should consider the size of their production run.

 

Two-shot molding usually only makes sense for larger production runs, whereas overmolding is better for low volume production runs. Still, teams must do their due diligence and evaluate all critical considerations of each potential manufacturing process against their specific project requirements to ensure they’re making the right choice.