Insert molding, widely used by the automotive, electronics, medical, industrial, and consumer goods industries, is a robust and efficient process for encapsulating a metal (or other material) component into a plastic injection molded part. Inserts vary widely depending on the application, but one common application is a female threaded insert that creates a strong attachment point for a part, such as a handle or a knob. Other inserts include hardware, components, like scalpels or contact plugs, or a custom stamped or machined component. Understanding the process and design considerations will ensure the injection molded component works as intended, no matter what type of insert is used.
Why Insert Molding?
Other methods for joining metal components with plastic include heat staking and ultrasonic welding. So why would you choose to use insert molding over these processes? Heat staking and ultrasonic welding are post-mold processes, meaning they are performed after the molding. Both involve pushing the component into remelted plastic – with heat staking, the insert is heated and pressed into the plastic, and ultrasonic welding uses a vibrational frequency to soften the plastic before pushing the insert into place.
Inserts have ridges and bumps called knurls or fins. With insert molding, melted plastic flows around these and secures the insert once cooled, preventing it from pulling out under strain. With heat staking and ultrasonic welding, there is a risk that the plastic won’t be melted enough to get into all the crevices to create that tight bond. Inert molding is preferred when pull-out strength is required. Also, the insert is not limited to metal; it can be made from any material that can withstand the temperature and pressures of the molding process. While all three processes benefit the right applications, insert molding provides improved product reliability, increased component strength, greater design flexibility, and increased efficiency as it doesn’t require a secondary operation.
Design Considerations for Insert Molding
If you have injection molded parts, you will likely understand the injection molding process. A mold contains two halves that the injection molding machine can open and close. Molten plastic gets injected under pressure into the closed mold. The process parameters (temperature, pressure, cycle times, etc.) are closely monitored as they significantly affect part quality. The part cools enough to hold its shape, the mold opens, and an ejection system removes the molded part or parts if there are multiple cavities. With insert molding, the insert is placed in the mold (by hand or automation) prior to molding, and the melted plastic forms around all or part of it, depending on the desired outcome.
To ensure the melted plastic properly encases the insert, part designers must adhere to some guidelines. Design for manufacturability (DFM) principles should be followed for the part regarding draft angles, wall thicknesses, undercuts, ribs and gussets, fillet and radii, bosses, etc. Below are a few general design considerations for insert molding. However, requirements may differ depending on the part and materials used. Consult with your injection molder for more specific design requirements.
- Inserts should be rounded with no sharp corners.
- Inserts should be flush with the top surface of the mold.
- The plastic beneath the insert should be 1/6 of the insert diameter to avoid sinking in areas that are too thick.
- If a boss is used, the outside diameter should be 1.5 times the insert diameter to ensure resin can flow around the insert to hold it in place.
- Radial ribs should be used on bosses with inserts to increase strength.
- The insert should be small relative to the plastic part.
- Shut off where cavity and core come together is needed for custom inserts to create a tight grip and prevent the insert from moving. Standard inserts generally are loaded onto a steel pin, but custom inserts require a contact point to be loaded.
- Thin metal inserts, electronic connectors, and parts that can be easily bent will bend under pressure during molding, so locators and pins will be required to hold them into place.
Ask The Sea-Lect Experts What is Right for Your Application
SEA-LECT Plastics has design engineers with vast experience in polymer selection and injection molding processes such as insert molding, heat staking, and ultrasonic welding. They can work with you to ensure the most beneficial process is used for your part. In addition, they can review your design and perform mold flow analysis to ensure your part can be molded in the most cost-efficient manner. Reach out to our team with questions or for a quote.