Insert molding is another plastic injection molding process that combines different materials into one object. Unlike overmolding, which involves a two-step process to create a similar outcome, insert molding is only one step. We put together this quick overview of insert molding to help you understand this important manufacturing process.
What is insert molding?
Insert molding is a type of plastic injection molding that involves joining separate parts into a single component. The result is a part assembly that is permanently joined together and fully encapsulated. Since it is a singular process, insert molding increases production efficiency and decreases manufacturing costs while producing a part that has improved mechanical and functional properties.
What is insert molding used for?
Insert molding allows an “insert” to be placed into the mold encapsulating it during the molding process, producing the final part. These inserts can be metal or plastic, with some common inserts being studs, contacts, and threads. Threaded inserts are most often used for parts that would benefit cost-wise by eliminating the need for a secondary operation to complete the assembly, such as adhering or soldering. Medical devices requiring sterilization are most often insert molded. The process creates tight seals around the instrument making them optimal for sterilization techniques. Some common examples of insert molded parts include medical syringes, waterproof cables, and screw-on knobs.
How does insert molding work?
Insert molding involves two different materials, such as threaded metal inserts that are “inserted” into a tool, and then thermoplastic resin is injected around it. In certain cases, both insert molding and overmolding can be combined. Plastic can be molded over plastic that has been molded over metal using insert molding. Another application involves using two materials, such as metal and rubber, that are insert molded into plastic at the same time.
Design considerations for insert molding
Given the complex nature of insert molded parts, several part design considerations need to be considered when designing for manufacturing. When choosing insert materials careful consideration must be taken to select materials that will not be damaged during the molding process. Additionally, for parts to properly bond, compatible materials such as ABS, PEEK, and acrylic thermoplastics should be considered. For parts that are exposed to increased mechanical friction during use, it is recommended to incorporate bushings, sleeves, and threaded inserts into the product design. Lastly, like overmolding, tool design needs to account for any shift to the substrate caused by the injection molding process. If the substrate were to shift inside the tool this could cause significant damage to the part and to the machinery. When designing tools for insert molding the designer needs to add in additional support features that hold the substrate, adequately seating it within the tool to prevent shifting.
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