Methods of inserting threads for plastic injection molding

Some overmolding for injection molded parts require that assembled plastic parts be disassembled and reassembled. In these cases, the best fastening method is a metal threaded insert. What is the best way to get these inserts into a part? Fastener manufacturers and industry experts provide insights into considerations and explain four methods of insertion.
The Basics
When choosing the best insert type and insertion process for your application, keep these considerations in mind:

Strength requirements – The key strength factors are the resistance of the insert to being pulled out of the part (pull-out force) and the resistance of the insert to twisting in the part when the mating fastener is twisted (twist-out force). The longer the insert, the greater the pull-out resistance; the larger the diameter of the insert, the greater the torque capacity. The knurl pattern is also critical; more on that below.
Materials – Both the plastic material and the insert material are important, depending on the application. Two insertion processes – ultrasonic and heat staking – involve remelting previously molded plastic, so they are only suitable for thermoplastics. For thermosets, there is the option of molding in the insert, or cold pressing the insert later, at which point the elasticity of the resin material becomes critical.

The most common material for the threaded insert itself is brass. However, as sustainability demands increase, lead-free alternatives such as stainless steel or aluminum are becoming more popular. Stainless steel tools
Cost – Because threaded inserts can be molded into the part at the outset or pressed into the part later, consider the total cost of the operation. This includes molding time and cost, component handling, and assembly. To better understand the tradeoffs, here’s a quick look at the four main insertion processes:
1. Heat-staking threaded inserts
Great for:
Thermoplastics
High performance combined with low installed cost
Hot-staking and ultrasonic insertion start with a preformed hole (size specified by the insert manufacturer) that is slightly smaller than the insert’s outer diameter and can be formed during the molding process or drilled afterwards. The outer wall of the insert can be straight or tapered. Both straight and tapered inserts align well to the fastener (as long as the hole is formed correctly), with self-aligning tapered inserts being easier and quicker to press in.
With heat-staking, a heat press is used to heat the insert to the point where the insert melts a small portion of the hole’s perimeter as it is pushed into the part. The softened resin flows into the knurl pattern and then hardens to form a strong bond with the insert. Hot staking is the more common process for thermoplastics because the process is easy to control, relatively low cost, and well suited to automation (multiple inserts can be pressed in at once).

2. Ultrasonic-Assisted Threaded Inserts
Great for:
Thermoplastics
High overall performance

Similarly, ultrasonics can be applied to melt the border area in the hole. Instead of applying heat, the ultrasonic process creates a vibration frequency to melt the plastic. Its speed is comparable to hot staking, but requires precise control and can create a larger process impact zone, which can cause material fragmentation. The ultrasonic process can also be loud and more difficult to automate, which is why it is less likely to be used, especially by shops that don’t already do ultrasonic welding.

3. Molded Threaded Inserts
Great for:
Thermosets and Thermoplastics
Best pull-out and torque performance

With molded-in inserts, a pin is machined or inserted into the mold core. The insert is then placed over the pin with each cycle of the molding process. With this process, the resin completely encapsulates the insert, which generally results in the best overall bond and eliminates the need for any subsequent insertion process. However, this process requires a more complex core with tight tolerances between the pin and the insert. It also takes time to place the insert between each cycle of the injection molding machine.
In-mold inserts can be “through-threaded,” meaning both ends of the insert are open, so solid contact with the core and cavity surfaces is required to prevent resin from flowing inside the insert. Or they can be “blind-threaded,” meaning one end of the insert is closed, so no contact with the cavity surface is required—for example, in cases with thicker walls, or where fasteners should not be present on one side of the cavity.
4. Cold-pressed threaded inserts
Great for:
Thermosets
Easy installation at minimal cost
Simply a simple press
Although they may not perform as well as the above types, cold-pressed inserts are a very economical alternative because they are easy to install and generally do not require special auxiliary equipment.
Sometimes called “expansion inserts,” one type of press-in insert has slots machined into the sides that allow them to flex when inserted and can be easily pressed in with just finger pressure. When installing a mating screw, the sides of these inserts are forced outward to create a “biting” contact with the ID wall of the hole. They essentially work like a sheathed lag bolt you find at the hardware store.

Higher performance press-in inserts have solid bodies and require a press for insertion. While not meeting the strength standards of processes that melt plastic around the insert, these inserts still offer a rugged alternative to the insertion process with great cost effectiveness.
About Knurling

Knurling is a metalworking process where a pattern is cut or rolled onto the outside of the workpiece. For threaded inserts, the knurl pattern is critical and directly affects pullout and torque resistance.
Straight knurls (parallel to the length of the insert) offer the most torque resistance, but less pullout resistance. Grooves between the knurl bands increase pullout resistance.
Diagonal or spiral knurls balance resistance to forces in both directions. Hexagonal or diamond-shaped knurls are probably the most common and offer resistance in all directions.
Wide Range of Sizes

Standard threaded inserts for plastics range in diameter from approximately 1/8″ – 9/16″ to lengths of 1/8″ – 5/8″, with thread sizes ranging from #0-80 to 3/8-16 (M2 to M10, depending on type). This size range accommodates a wide range of applications in industries such as electronics, automotive, aerospace, defense, medical, industrial, and entertainment equipment.

For physically smaller applications, such as handheld devices, microPEM inserts are also available in diameters as small as 1 mm (0.039″) and lengths as small as 1.75 mm (0.069″) . These micro inserts can accommodate M1 fasteners, the smallest ISO-specified M-type thread. They can be installed in straight or tapered holes using either heat staking or ultrasonic processes.

All four types of insertion processes – heat staking, ultrasonic, swaging or cold swaging – offer unique benefits and capabilities to meet your design requirements. To achieve maximum insert performance, it is important to consider the insert type along with the application, cost, and other components.