An Introduction to Overmolding

Overmolding can produce products by combining two different types or colors of polymers. This technology can improve the functionality and aesthetics of products, and is widely used in electronics, automotive parts, medical devices, and daily necessities. Overmolding has two stages of development. The first stage of development uses two injection molding machines and two sets of molds. In this stage, the first part of the product undergoes traditional single-color injection molding on the first injection molding machine, and then it is demolded and transferred to the second injection molding machine for the second part of the product. This production method is gradually eliminated due to its low efficiency, complex processing steps, and inability to guarantee the quality of its products. This production method is gradually being phased out due to its low efficiency, complex processing steps, and inability to guarantee product quality. The second stage of development is to use overmolding machines, where the first and second parts of the product are both injection molded in the same machine. The second stage of development involves using overmolding machines, where the first and second parts of the product are both injection molded in the same machine. This production method produces high-quality products and improves production efficiency. It has replaced the former method and become the current mainstream production method of overmolding.
 

Overmolding improves product quality, saves production costs, allows for flexible design, and enhances product functions. By using two different materials, various hardness levels, colors, and functions can be achieved, improving the overall performance and quality of the product. Overmolding is completed in a production process, reducing the need for assembly and secondary processing, thereby reducing production costs. Overmolding allows designers to combine different colors and materials in a single product to achieve more complex and diverse designs. For example, using overmolding in areas such as handles and buttons that require different touches and functions can enhance the product's user experience and durability.
 

Overmolding can be used to manufacture electronic products, automotive parts, medical devices, and daily necessities. The wide application and continuous development of overmolding have secured its important position in modern manufacturing, helping manufacturers improve product quality, design flexibility, and production efficiency.
 

Mold design: Design the overmolding mold, including two cavities, for injection molding two different materials.
Material preparation: Prepare two different plastics, usually with one as the substrate and the other as the coating material.
Injection molding: During the injection molding process, the substrate is first injected into the first cavity of the mold, then the mold is rotated or shifted to transfer the molded substrate to the second cavity, where the coating material is injected.
Cooling and demolding: After the product cools and solidifies in the mold, the mold is opened, and the product is demolded.
 

The overmolding process requires combining the first shot part with the second shot part. Different combination methods affect the appearance and mechanical properties of the product in various ways. There are four main ways to combine overmolding products: simple fitting, covering, splicing, and complete wrapping. Most two-color headlights use the covering or splicing method and share the same bonding principle based on molecular chain diffusion theory. When heated to a high temperature, the material converts from a solid to a molten state, causing the thermal motion of its molecular chains to become active, diffuse, and entangle, forming an interface layer that increases bonding strength.
 

The parallel overmolding machine, also known as the core rotation overmolding machine (see Figure 1), is the most widely used overmolding equipment. After the mold is closed, the first shot material is injected into the first shot mold, and the filling, pressure holding, and cooling steps are performed to form the first shot part A, but the mold is opened without ejection. The movable mold is then rotated 180° to the second shot mold position, the mold is closed again, and the second shot material is injected into the second shot mold cavity, followed by the filling, pressure holding, and cooling steps. The second shot part B and the first shot part A form a two-color product. Finally, the mold is opened and the product is ejected.
 
 
Figure 1 Working principles of overmolding machines