The thread is the weakest part of the joint
A screw thread cut directly into a polymer relies on the material's own shear strength, and most engineering polymers have far less of it than a metal. The thread holds well enough the first time and progressively less each time the fastener is removed, until the screw spins. On a part that will be serviced, that is a failure waiting for a maintenance schedule.
Sheet metal has a related but different limit. A thread in thin sheet has only the sheet thickness to engage, so a thread formed or cut into 1 mm of steel carries a fraction of what the same screw carries in a thick section. Inserts exist to solve both problems, and choosing the right type is a joint-design decision rather than a purchasing one.
Heat-set inserts
A heat-set insert is pressed into a moulded hole with a heated tip. The plastic around it melts locally, flows into the knurl or the ring grooves on the insert, and locks it in place as it cools. The result is a metal thread in a plastic part, installed after moulding with a bench tool, which makes it the usual choice for small and medium volumes where modifying the mould is not worth it.
The installation is what decides the outcome. The hole diameter has to match the insert and the polymer, the heat has to be enough to melt without burning, and the insert has to be driven in square. A heat-set insert installed into an undersized hole cracks the boss; one installed with too little heat tears the plastic and pulls out under load.
Moulded-in inserts
A moulded-in insert is placed on the tool pin before the mould closes, so the polymer flows around it and the part comes out with the metal thread already embedded. It is the strongest of the common options because the material surrounds the insert completely and there is no secondary operation.
The cost is that it has to be designed in before the tool is cut, and it slows the cycle because an operator or a robot has to place each insert. It also loads the tool: an insert that shifts under injection pressure damages the mould, so the pin that locates it matters as much as the insert itself. Where a joint has to hold under vibration or repeated assembly, the extra cost is usually justified.
Press-in and self-clinching inserts for sheet metal
In sheet metal the insert is swaged or pressed into the hole so the sheet material itself flows into a groove on the insert. The result is a permanent threaded boss in a thin panel, strong in pull-out and in torque because the joint is formed from the sheet rather than threaded into it.
The trade is preparation: the hole has to be the exact size the insert calls for, the panel has to be flat around it, and the insert has to be set with the correct force. An oversized hole or a pressed insert in a distorted panel holds a fraction of its rated load, and the failure usually appears when the joint is torqued rather than when it is assembled.
Designing the boss, not just the insert
Most insert failures are boss failures. A boss with too little wall around the insert splits when the insert goes in or when the screw is tightened. A boss with too little material beneath the insert bottoms out and the insert sits proud. And a boss standing on a thick section of a moulded part shrinks differently from the surrounding wall, which sinks the surface opposite.
The usual rules are a boss outside diameter of roughly twice the insert diameter, a wall of at least half the nominal wall thickness around it, and a cored hole that leaves enough material under the insert to take the screw's engagement length. None of that is exotic, and all of it is cheaper to design than to correct after the tool is cut - which is exactly the kind of review a moulder runs before releasing a tool. A production partner such as MadeInDayIn, which builds its own moulds and runs the press floor, is in a position to move a boss or a pin before the steel is cut rather than after.
Choosing the type
Choose a heat-set insert for small and medium volumes, for a design already in production, or where the tool cannot be modified. Choose a moulded-in insert where the joint is critical, the volume is high, or the part will be assembled and disassembled repeatedly. Choose a self-clinching insert for sheet metal panels, brackets and enclosures where a permanent threaded boss is required.
Whichever is chosen, the specification is the same four numbers: the screw size, the pull-out and torque the joint must survive, the material it goes into, and how many assembly cycles it has to withstand. An insert quoted without those is a part number rather than a solution.
References
The insert family described here is set out under threaded insert, and the thread form it carries under screw thread. The material behaviour that decides how well an insert holds is covered under plastic, and the process that embeds an insert during forming under injection moulding. Fastener and material standards are published by ASTM International, and the dimensional traceability behind a torque specification by the US National Institute of Standards and Technology.
Frequently asked
Should I use a heat-set or a moulded-in insert?
Heat-set if the tool is already cut, the volume is modest, or the design may still change - it is installed after moulding with a bench tool. Moulded-in if the joint is critical or the part will be assembled repeatedly, because the material surrounds the insert and the joint is stronger. Moulded-in has to be designed in before the tool is cut and adds cycle time.
Why did my insert pull out of the plastic?
Usually the boss, not the insert. Too little wall thickness around the insert, too little material beneath it, or a hole that is the wrong size for the polymer. Heat-set inserts also pull out if the installation was too cold, because the plastic never flowed into the knurl. Check the boss dimensions against the insert drawing first.
Can I thread directly into a 3D-printed part instead?
For a joint that is assembled once, sometimes. For a joint that is serviced, no - the same shear-strength limit applies, and printed parts are usually weaker between layers than in the plane. An insert, or a captive nut in a designed pocket, is the more reliable answer.