There are hundreds of variations of molded-in inserts for the insert molding process. On the surface, it may seem simple enough to just specify a thread type and depth. While doing so may yield the desired results, it’s important to dig into the application to make sure that you are selecting the right insert for your job. Using the wrong insert could lead to long-term cost implications, production issues, and quality issues.
This guide is a brief overview of your options when selecting molded-in inserts. We’ll cover common types of molded-in inserts, design details, material options, and some examples of custom options.
This guide is an overview of your options when selecting molded-in inserts. We’ll cover common types of molded-in inserts, design details, material options, and custom options.
Types of Inserts
- Female Threads – Female threaded inserts are the most common type of molded-in inserts.
- Threaded Studs – External threads are used as an attachment point for other components.
- Compression Limiters – These are sleeve-like features that span the thickness of a plastic component so that when pressure is applied through the length, the plastic will not crush.
- Locating Features – Dowels, sleeves, and bushings can be insert molded to provide a precision location point for assemblies.
- Custom – Features like detents, custom mounting points, lock features, bumps stops, etc. can all be custom designed and insert molded.
Insert Design Details
- Flange – Flanges on threaded inserts create a surface that bottoms out against the assembled part surface. Flanges can also prevent pull-through conditions on thicker parts.
- Knurl & Grooves – There are several different designs for knurls and grooves, but they all serve the same general purpose. When using threaded inserts, the insert needs to resist rotational forces and pull forces. Knurls resist rotation, and grooves resist pulling.
- Bottom / Through Hole – Female threaded inserts can blind threaded or thru-threaded.
- Symmetry – During the loading process, asymmetric inserts may be more difficult to handle.
- Non-Knurled – Inserts like pins and compression limiters may have a smooth outer surface. These inserts cannot have rotational forces applied as they are likely to rotate in the plastic.
- Stainless Steel
- Coated Carbon Steel
Although insert molding is most commonly used for creating threads and locating features, there are mostly unlimited design options. The most important factor to consider when looking at custom inserts is cost. Custom designs may require upfront investment and minimum order quantities, which will drive up the piece price.
The image below shows a few different versions of molded-in inserts. Starting at the left, there is threaded stud. The lower half of the stud in knurled and is where the plastic would be molded to encapsulate the insert. The next insert is an example of compression fitting. The third is a basic knurled female thread and the last the a knurled female thread with a flange.
This example below is a cut-away of a technique for connecting two plastic components. The red and black are the plastic, yellow are the two inserts, and blue the bolt. You can see the the plastic is highly resistant to the compression and torque forces created by tightening the screw.