Two shot molding is a form of injection molding that allows for the insertion of different plastic resins into one product. The process is often used to add color and texture, for example in toothbrushes where a hard-wearing base plastic can be combined with softer seal surfaces or the creation of soft rubber handles for appliances. It is also a great way to make products that are dustproof and shockproof. In addition to the practical benefits, the process can save money on production costs by reducing the need for assembly and labor.
The basic process of 2 shot molding is very similar to that of traditional injection molding. However, during the injection of the initial resin into the mold, a separate injection unit fires another shot of a second material into the same cavity. The second material forms a molecular bond with the initial injection molded resin, thus joining the two parts into a single unified part. This is called two-shot molding, multi-shot injection molding, double shot injection molding, or overmolding.
To create the desired effect, the injection molding machine must be equipped with two separate feed hoppers and a rotating mold mechanism. The sprue and gate for the first shot must be closed off from the sprue of the second injection, so that the two shots are not mixed together or compressed under pressure from the second cavity as it is filled.
When the second shot is injected, the rotation of the mold and the reciprocation of the screw must be synchronized to prevent the new plastic from contaminating the first part. Additionally, the sprue of the second shot must be shut off during the molding step that is forming the substrate, and the runner of the second material must be closed off during the molding steps in which it is not active. This ensures that the initial injection molded resin is not being compressed by the new plastic of the overmold.
With the proper design and engineering, a good molecular bond can be formed between the substrate and the overmold to create a strong and durable product that has good mechanical properties. This is a significant improvement over conventional molding and assembly, as well as other more advanced injection molding processes such as insert molding or overmolding.
In addition to a more durable, more appealing product, the two-shot process provides greater accuracy in manufacturing. Because the product is never removed, shifted or repositioned in the mold between cycles, and provided thermal effects are controlled through good mold design, the final product can be as accurate as the mold itself.
From a cost perspective, the process is very attractive as it allows for multiple components to be made in a single cycle rather than requiring two separate injection molding processes. It also enables designers to use the best features of different materials; for example, using the load-bearing structural properties of one plastic with the aesthetically pleasing or functional qualities of another.