Heat-Set Inserts for 3D Printing: The Complete Guide to Finding the Perfect Hole Size

Why Use Heat-Set Inserts?

3D printed plastic threads are notoriously weak. Print a bolt hole in PLA and repeatedly thread a screw, and you'll quickly strip the threads. Heat-set inserts solve this problem by embedding a brass threaded sleeve into your 3D printed part, giving you metal-thread durability in a plastic part.

They're essential for any 3D printed enclosure that needs repeated assembly and disassembly, functional prototypes, and cases for electronics where you'll be screwingpcb mounts in and out multiple times.

Choosing the Right Hole Size

This is the part that trips up most people. The general rule: measure the narrowest diameter of your insert (usually the base) and model your pilot hole to match that diameter.

Here's a quick reference for common M3 brass inserts:

• M3 brass insert, PLA/PETG: ~4.0mm pilot hole
• M4 brass insert, PLA/PETG: ~5.2mm pilot hole
• M2.5 brass insert, PLA/PETG: ~3.3mm pilot hole

Always check your specific insert manufacturer's specifications — tolerances vary between suppliers.

Depth Matters as Much as Diameter

Your hole needs to be deep enough to allow material to squeeze out as the insert embeds. The common guideline: make your depth at least 1.5x the insert height to allow extra material to flow below the insert.

For a 5mm tall insert, that means a minimum hole depth of 7.5mm. Go shallower and you'll get plastic bubbling up around the insert, weakening the bond.

Add a Chamfer

Always add a small chamfer to the top of your hole. This guides the insert in during installation and prevents the molten plastic from flowing into the threads during insertion. A 0.5–1mm chamfer is usually sufficient.

Installation Temperature

Use the lowest temperature that achieves insertion. For most soldering irons, 220–250°C works well for PLA. Going hotter risks melting the surrounding plastic. Here's the process:

1. Preheat your soldering iron to working temperature
2. Insert the brass insert into the hole
3. Press the heated tip into the insert until it bottoms out
4. Hold steady for 2–3 seconds to let plastic flow around the threads
5. Remove the iron and let the part cool with the insert in place

Minimum Wall Thickness

To ensure insert pull-out strength, maintain adequate wall thickness around the insert. General guidelines:

• At least 1.5mm wall thickness around the insert body
• Avoid placing inserts near part edges — minimum 2mm from any surface
• Through holes only need to be as deep as the insert — no need to go deeper

Material Compatibility

Heat-set inserts work in most thermoplastics:

• PLA: Easiest — low melting point makes insertion straightforward
• PETG: Works well — slightly higher temp needed
• ABS: Requires higher temperature — warping risk during insertion
• Nylon/PC: High temperature required, but inserts hold very well in these materials
• Resin: Not recommended — different insertion method (press-fit or ultrasonic) is more appropriate

Troubleshooting Common Issues

Insert spins in the hole: The hole is too big. Use a smaller pilot hole or switch to a slightly larger insert.

Plastic melts around the insert: Soldering iron is too hot. Reduce temperature and work faster.

Insert not flush with surface: The chamfer is too small or the iron isn't going deep enough. Increase chamfer size and check insert seating.

Insert pulls out under load: Hole depth is insufficient, or wall thickness is too thin. Increase depth and wall thickness.

Where to Buy Quality Inserts

Quality matters with inserts. Cheap brass from unknown suppliers often has surface imperfections that prevent plastic from gripping properly. Stick to reputable suppliers like:

• Accu: Comprehensive charts and specifications
• McMaster-Carr: Industrial quality, fast shipping
• Misumi: Precision inserts for critical applications

For most maker projects, Accu's M3/M4 brass insert packs offer the best balance of price and quality.