Overmolding is a manufacturing process that bonds two or more materials together. It allows designers to create products that can’t be made using one material, and it provides several benefits such as improved grip, ease of use, and aesthetics. In addition, it can also help manufacturers increase durability, shock and vibration absorption, and chemical resistance in their products.
Overmolded parts can be made with a wide variety of materials, including plastics, silicone, rubber, and even glass. They can be molded directly onto existing parts, or inserted into the base part of an injection-molded product. The overmold can be colored or textured to provide a distinctive look. For example, a toothbrush handle can be overmolded with rubber to improve the user experience.
To successfully overmold, manufacturers need to understand the physics behind the process. They must consider factors such as shrinkage, adhesion, and temperature sensitivity. They should also evaluate the tensile properties of both the overmold and substrate, which can affect how well the component can stretch and return to its original shape. Finally, they must understand how to avoid defects that can occur during the molding process, such as thinning, feathering, delamination, and air entrapment.
The most common type of overmolding is insert molding. In this process, hard plastic components are molded first, and then soft elastomers are injected into the molds to create an adhesion bond. The soft elastomers are usually heated to the same temperature as the plastic, which allows them to adhere to the hard plastic surface. This type of overmolding is best for products that require a high level of flexibility, such as rubber handles on plastic toothbrushes.
Another method of overmolding is two-shot molding, which uses a single injection machine to produce both the substrate and the overmold. This method is ideal for large quantities of production, and it’s typically cheaper than co-injection overmolding. However, it can be challenging to get the right results. The shutoff between the substrate and overmold needs to be sharp, and the gate needs to be sized correctly to prevent thinning or feathering. In addition, the ratio of flow length to wall thickness should be analyzed, and vents must be properly located to avoid flash.
A third overmolding technique is pick-n-place molding, which involves producing the substrate parts and then placing them in a second mold for the overmolding. This method is less expensive than the two-shot and co-injection methods, but it requires more complex equipment and may not be suitable for small batches of production.
Injection overmolding is used in a wide range of applications, from consumer electronics to automotive components. It can enhance a product’s usability, safety, and aesthetics, while saving time and money by eliminating assembly steps.
When selecting overmolding materials, smart manufacturers follow a standard evaluation process. New materials are constantly being developed, and it’s helpful to have a general evaluation framework that can be applied across all types of materials. The right inspection techniques can help ensure that a finished overmolded part meets all of the necessary criteria, such as non-destructive testing (NDT) techniques like X-ray inspection, ultrasonic inspection, and dye penetrant inspection.