What is Insert Molding for Plastic Components?

An in-depth look at the process with design considerations

If you have typed “what is insert molding” into a search engine and found your way here, you are in the right place. 

Maybe you want to move a part or component to plastic from metal to reduce costs while making the product lighter, corrosion-resistant, or watertight. However, fasteners, hardware, and other components may still be required for assembly, strength, or functionality, and you are wondering if insert molding is the best method. Insert molding is one method for combining these preformed components with plastic. In this post, we will walk you through insert molding as well as some design considerations.

Ready to Optimize Your Part Quality? Request a quote today and discover how LMC Industries get your parts program running like clockwork. 

What is Insert Molding?

Insert molding allows manufacturers to encase part of a preformed component (usually metal) in plastic. This method is used for a multitude of products, such as inserting threaded metal hardware, embedding electrical components inside protective housing, or creating handles for metal tools. Any component that can withstand the pressure and temperature of injection molding is a candidate for insert molding.

Prior to molding, the insert may be prepped (e.g., open ends sealed, cleaned, roughened). The insert is placed in the mold, which is overmolded with thermoplastic during the injection molding process. Once cooled, the part can move on to any other procedures that may be required.

While heat staking and ultrasonic welding can be used to insert metal components, insert molding is often the better choice. Both heat staking and ultrasonic welding are post-molding processes that require remelting a portion of the thermoplastic material. With heat staking, a metal tip transfers heat to the insert, and the machine presses the insert into the plastic. Ultrasonic welding requires a pre-drilled or molded hole the insert is put into. Ultrasonic vibrations are used to create heat and melt the plastic around the insert. 

Issues with both methods can result if there is insufficient melt, which can damage the part and the insert. Also, there is the risk of inadequate plastic flowing into the insert’s fins and knurls to achieve desired holding strength. Additionally, these two methods are limited to metal components, unlike insert molding. Generally, better and more reproducible encapsulation is achieved with insert molding than these other techniques.

When is Insert Molding Appropriate?

While this technique adds costs to the injection molding process, it also saves on secondary processes downstream. Insert molding should be used when there is a functional need for it and the value (i.e., improved product performance) justifies the additional cost it brings. Reasons for using inserts include:

• To allow a part to be serviceable or frequently disassembled
• To meet close tolerances on female threads
• To permanently attach two high load-bearing parts 
• To provide electrical conductance

Insert Molding Design Considerations

With any injection molding, proper design is critical. The engineer should incorporate standard design for manufacturability (DFM) practices, such as having an adequate draft angle, uniform wall thickness, and proper design of structural features. In addition, some other considerations are applicable to ensure the best results. 

1. The insert and resin are not chemically bonded with insert molding, so the part must be designed so the two parts mechanically bond. Often, the inserts have knurls, threads, or undercuts to grip the plastic. Contact pins used in electronics may have flat, serrated, collar, or star retention features to hold them in place. It is crucial to ensure the component won’t be pulled out or rotated. Performance depends on the material’s shear strength and the retention feature or knurl pattern on the insert.
2. Corners of knurls or inserts should be rounded. Sharp corners or edges can produce notches or areas of stress concentration in the plastic material. 
3. For inserts used for joining, the load placed on an insert is distributed over its entire outer surface, so they are often attached directly to the nominal wall without the use of a boss. The insert’s top should be flush with the surface or extend beyond it so the insert is not pulled out when the screw is tightened – even when a boss is used. To prevent sink marks, the molding beneath the insert should be one-sixth of the diameter of the insert.
4. The coefficients of linear thermal expansions differ significantly between metals and plastics. The materials expand and shrink in different amounts. Since plastic tends to have greater shrinkage, it can lead to areas of high mold-in stress around the inserts. Because of this, thicker boss walls are needed to compensate for the higher stress and prevent part failure over time. 
5. If the part is exposed to thermal cycling during use, molded inserts may not be suitable. If the part is exposed to high or low temperatures during use, brass or aluminum inserts are preferred as the difference between the coefficient of linear thermal expansion is not as great as it is with steel.

Go Around Instead of Into: Overmolding is used in almost every industry, including automotive, consumer products, electronics, agriculture, and medicine. Learn more.  

Insert Molding Without the Worry 

One of the many benefits of working with LMC Industries is that we offer metal stamping as well as plastic injection molding so that we can create the complete part in-house. This gives us greater control over the quality of the final product. 

If you are new to insert molding, don’t worry; we perform design for manufacturability (DFM) analysis on your parts to ensure it is optimized to be defect-free and most cost-effective. Your product is in good hands with us. 

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