Introduction

Threaded inserts allow a great way to add threaded features to your product.  Screws can be repeatedly installed and removed with little fear of stripping the threads.  They can be installed into plastic parts in a number of ways.  Traditionally inserts can be molded in place, pressed fit, heat staked, or ultrasonically staked.  Plastic parts must be properly designed for each type of installation method.  Using adhesive to bond inserts into place is also possible, but will not be covered here.

Installation Methods

Mold in Place Inserts

An operator loads the inserts into the mold and the inserts are molded into place during the molding process.  Simple fixtures can be used to help install inserts more quickly and simultaneously.

Press Fit

Press fit inserts typically have a locking feature that engages when the insert is mechanically pressed into a correctly sized hole in the part after molding.  Adhesives can be used also.

Heat Staking

The insert is installed by heating it and then carefully pressing it into the molded part.  While a simple hand press can be used, a dedicated air press, with heat, speed, and time control is preferred.  One or more inserts can be installed at a time if they are all the same height..  For multiple inserts, a custom tool is required.

Ultrasonic Staking

Sophisticated ultrasonic equipment vibrates the inserts to generate heat to install the inserts after the part is molded.  While multiple inserts can be installed, the custom tooling required can be expensive.

Pros & Cons

Pros and Cons of Insert Installation Methods

	Mold In Place	Press Fit	Heat Stake	Ultrasonic
Custom Tooling Required	No, but simple fixtures can speed up installation into the mold.	Yes, for multiple inserts, simple	Yes, for multiple inserts, moderate	Yes, for multiple inserts, complex
Speed	Molding time is longer, entire process is faster	Quick	Slower	Fast
Mechanical Holding	High	Lower	High	High
Material Types	Thermoplastic and Thermoset	Thermoplastic and Thermoset	Thermoplastic	Thermoplastic
Automation	Yes, but press side automations can be complicated	Yes	Yes	Yes
Pros	High strength hold and lowest part stress. No secondary operation required Multiple types and sizes of inserts can be installed. Fixtures to load multiple inserts can be simple and greatly speed up install process	Simple tooling and equipment Only method suitable for thermoset materials	Self stress relieving if installed properly Inserts are not wasted if the part is rejected before installation.	Inserts are not wasted if the part is rejected before installation. Fast
Cons	Rejected parts waste all of the inserts. (Inserts can be reclaimed but it is seldom cost effective.)	Lower hold strength (adhesives can be employed)	Tooling required.	Install process can induce hoop stress and cause future failures. Expensive equipment and tooling.