How to Choose Heat Set Inserts for 3D Printed Parts

Heat set inserts for 3D printed parts are threaded metal inserts designed to create stronger, more durable screw threads inside plastic parts. In 3D printing, they are commonly used when a printed part needs to be assembled, disassembled, repaired, or fastened more than once.

Instead of driving a screw directly into plastic, a heat set insert is installed into a printed hole using controlled heat. The surrounding plastic softens, flows around the knurled or ribbed surface of the insert, and then solidifies as it cools. This creates a mechanical bond between the insert and the printed part. McMaster describes heat-set insert tips as transferring heat to the insert so surrounding plastic softens and later solidifies around the insert’s ridges to resist torque.

Choosing the right insert is not only about thread size. A reliable fastening design also depends on the insert shape, outer diameter, hole size, material, wall thickness, insertion depth, screw length, and how the load will act on the printed part.

For most 3D printed parts, the insert should be selected together with the part design, not after the model is finished.

This guide explains how to choose heat set inserts for 3D printed parts without relying only on screw size or generic hole recommendations.

In this guide, you will learn:

what mistakes cause inserts to fail

how to choose the right screw size

how insert geometry affects hole design

why material behavior matters

how boss design affects strength

heat set inserts for 3D printed parts — Cross-section diagram of a heat set insert installed in a 3D printed boss.

Why It Matters

Poor insert selection can cause several common failures:

the insert pulls out of the printed part
the insert spins when the screw is tightened
the plastic cracks around the hole
the insert sinks too deep during installation
molten plastic blocks the internal thread
the boss deforms or bulges
the screw bottoms out before clamping the assembly

These failures usually do not come from the insert alone. They come from a mismatch between the insert, the printed hole, the material, and the mechanical load.

A heat set insert works best when the printed plastic has enough volume to flow around the insert features and enough surrounding structure to resist torque and pull-out loads. SPIROL’s hole design guidance for heat and ultrasonic inserts also emphasizes that the hole must provide enough plastic displacement to fill the insert’s external retention features.

Step-by-Step Guide

1. Choose the Screw Size First

Start with the screw size required by the assembly.

Common screw sizes for 3D printed parts include:

Screw Size	Common Use Case
M2	small electronics, compact printed parts
M2.5	light-duty enclosures, small mechanisms
M3	general 3D printing, enclosures, brackets, robotics
M4	stronger brackets, frames, load-bearing covers
M5	larger mechanical assemblies

For many desktop 3D printing projects, M3 is the most common starting point because it balances strength, availability, and part size.

Do not choose a larger insert only because it looks stronger. A larger insert requires more surrounding plastic, more wall thickness, and more installation heat. If the printed boss is too small, a larger insert can weaken the part instead of strengthening it.

Common Heat Set Insert Selection Factors

Selection Factor	Why It Matters
Thread size	Determines screw compatibility and assembly load capacity
Insert outer diameter	Affects required hole size and surrounding plastic support
Insert length	Influences pull-out resistance and required boss depth
Knurl geometry	Affects grip, torque resistance, and plastic flow during installation
Printed material	Changes heat response, creep behavior, and long-term retention
Boss wall thickness	Determines how well the printed part supports the insert
Assembly load	Helps decide whether strength, repeatability, or serviceability matters most

Insert selection should be evaluated together with printed material, hole tolerance, boss geometry, and expected assembly load rather than by thread size alone.

2. Match the Insert Type to the Plastic Part

Heat set inserts come in different external shapes. The most common types for 3D printed parts are:

Insert Type	Best For
Tapered heat set inserts	easier alignment during installation
Straight heat set inserts	consistent hole geometry
Short inserts	thin walls or shallow bosses
Long inserts	deeper engagement and better pull-out resistance
Double-sided inserts	installations where orientation matters less

Tapered inserts are often easier to start because the narrower end helps guide the insert into the hole. McMaster also notes that tapered heat-set threaded inserts are easier to align in a hole during installation than straight inserts.

For beginner-friendly 3D printed parts, tapered brass heat set inserts are often a practical choice.

3. Check the Outer Diameter, Not Just the Thread Size

Thread size only tells you which screw fits inside the insert. It does not tell you how large the printed hole should be.

For example, two M3 inserts can have different:

outer diameters
knurl patterns
lengths
flange shapes
tapered profiles
recommended hole sizes

The pilot hole should be designed based on the insert’s outer geometry, not just the internal thread.

A common engineering approach is to design the hole slightly smaller than the insert’s outer retention features, so the heated insert displaces plastic and forms a mechanical lock after cooling. Hackaday explains that heat-set insert installation holes are undersized because the inserts are not meant to be pushed in by hand before heating.

Always check the insert supplier’s drawing before finalizing the CAD hole.

4. Design the Hole Depth Deeper Than the Insert

The insert hole should usually be deeper than the insert length.

This extra depth gives displaced plastic somewhere to move during installation. Without enough depth, molten plastic may be pushed into the internal thread or prevent the insert from sitting flush.

SPIROL’s design guidance states that holes for heat or ultrasonic inserts should always be deeper than the insert length.

A practical design rule is:

Hole depth = insert length + clearance for displaced plastic

The exact clearance depends on insert size, plastic type, and print accuracy.

5. Choose the Insert Length Based on Load

Insert length affects strength.

A longer insert usually provides more surface area for plastic engagement and can improve pull-out resistance. However, it also requires a deeper boss and more surrounding material.

Use shorter inserts for:

small covers
light-duty enclosures
thin printed walls
low-load parts

Use longer inserts for:

repeated assembly
brackets
robotics parts
parts under vibration
parts with pull-out load
parts where the screw will be tightened frequently

Do not use a long insert in a shallow boss just to gain strength. If there is not enough material under or around the insert, the boss may deform or crack.

6. Consider the Printed Material

The same insert can behave differently in different 3D printing materials.

Material	Insert Behavior
PLA	easy to install, but heat deformation is common
PETG	tougher than PLA, but can string or overflow during heating
ABS	more heat tolerant, often suitable for heat set inserts
ASA	similar to ABS, useful for outdoor parts
Nylon	tough and flexible, but requires careful hole control
Resin	brittle, may crack if the hole is too tight
Carbon fiber filled filament	stiffer, but layer bonding and brittleness still matter

PLA is easy to work with, but it softens quickly. The biggest risk is not getting the insert hot enough. The bigger risk is keeping heat in the part too long and deforming the boss.

PETG can hold up better mechanically, but it often produces more plastic flow around the insert.

Nylon can be strong, but its flexibility means the insert may behave differently under torque and pull-out load compared with rigid plastics.

Resin prints need extra caution because brittle materials may crack around the insert instead of flowing cleanly.

7. Design Enough Wall Thickness Around the Insert

The printed part must have enough plastic around the insert.

If the boss wall is too thin, the insert can crack the part during installation or split the boss when the screw is tightened.

A good insert design should consider:

boss outer diameter
distance from part edge
layer direction
screw load direction
wall thickness
infill and perimeter count
whether the screw will be removed repeatedly

For functional parts, the boss should not be treated as decoration. It is a load-bearing feature.

8. Think About Load Direction

Heat set inserts do not fail in only one way.

Different load directions create different failure risks:

Load Type	Common Failure
Pull-out load	insert pulls out vertically
Torque load	insert spins in place
Side load	boss cracks or bends
Repeated assembly	threads loosen over time
Vibration	insert gradually loses grip

If the screw will mainly clamp two parts together, the insert needs good torque resistance.

If the screw will pull upward on the insert, pull-out strength becomes more important.

If the part will see vibration, choose a deeper insert, enough boss material, and a screw length that provides full thread engagement without bottoming out.

9. Choose Installation Method Early

Most heat set inserts for 3D printed parts are installed with:

soldering iron
heat set insert tip
vertical press
modified drill press setup
dedicated insertion tool

A standard soldering iron can work, but a flat insert installation tip usually improves alignment. McMaster notes that installation tips transfer heat from a soldering iron to the insert, and Swartz Garage points out that a shoulder-style installation tip helps keep the insert straight during installation.

If straight alignment matters, do not rely only on hand pressure. Use a guide, press, or fixture when possible.

10. Test Before Final Production

Never assume the first CAD hole will be correct.

3D printed holes often print smaller than their modeled size. CNC Kitchen notes that printed holes commonly come out smaller than expected, so CAD hole size may need adjustment depending on printer calibration and material behavior.

Before using inserts in a final part, print a small test coupon with several hole sizes.

Test for:

insertion force
insert alignment
plastic overflow
flush seating
thread cleanliness
torque resistance
pull-out behavior
cracking around the boss

This small test can prevent a full printed assembly from failing later.

Common Mistakes

Mistake 1: Choosing Insert Size Only by Screw Size

An M3 screw does not automatically mean every M3 insert will fit the same hole. Different insert brands and geometries require different hole sizes.

Mistake 2: Making the Hole Too Large

If the hole is too large, there may not be enough plastic to flow into the insert knurls. The insert may install easily but later spin or pull out.

Mistake 3: Making the Hole Too Small

If the hole is too small, the insert may require too much heat or force. This can crack the boss, deform the part, or push molten plastic into the thread.

Mistake 4: Ignoring Hole Depth

A shallow hole can cause the insert to sit proud, block the internal thread, or trap displaced plastic under the insert.

Mistake 5: Using Thin Boss Walls

A thin boss may look fine in CAD but fail during installation or tightening. The plastic around the insert must carry the load.

Mistake 6: Installing Too Fast

If the insert is forced in before the plastic softens evenly, the hole may distort. If it is overheated, the boss may collapse.

Mistake 7: Tightening the Screw Before Cooling

The plastic needs time to solidify around the insert. Tightening too soon can weaken the bond or shift the insert.

Engineering Notes

Heat set inserts work by combining heat, plastic flow, and mechanical retention.
The insert does not become strong simply because it is metal. It becomes strong when the surrounding plastic locks into its external features.
Hole size should be based on insert geometry, not generic screw size.
Hole depth should allow space for displaced plastic.
Boss design is often more important than insert brand.
Printed hole accuracy depends on printer calibration, slicer settings, material, and orientation.
Inserts used in thin walls should be tested carefully before production.
For repeated assembly, screw length and thread engagement matter as much as insert size.
Stronger material does not automatically mean stronger insert retention. Layer bonding and boss geometry still matter.

Failure Examples

Example 1: Insert Spins During Tightening

Likely causes:

hole too large
insufficient plastic flow
weak knurl engagement
screw overtightened
boss wall too thin

Example 2: Insert Pulls Out

Likely causes:

insert too short
pull-out load too high
hole too large
weak layer orientation
not enough plastic below the insert

Example 3: Plastic Cracks Around the Insert

Likely causes:

hole too small
brittle material
boss too narrow
excessive insertion force
poor temperature control

Example 4: Insert Sinks Too Deep

Likely causes:

too much heat
too much pressure
soft material
no depth stop
hole deeper than needed without installation control

Example 5: Screw Will Not Thread In

Likely causes:

molten plastic entered the thread
hole too shallow
screw too long
insert misaligned
installation tip damaged the thread

Conclusion

Choosing heat set inserts for 3D printed parts is an engineering decision, not only a hardware selection task.

When choosing heat set inserts for 3D printed parts, the goal is not only to fit the insert into the hole, but to create a reliable fastening structure.

The best insert depends on screw size, insert geometry, hole design, material behavior, boss strength, installation method, and load direction. A good insert in a poorly designed hole will still fail. A properly designed hole, enough surrounding material, and controlled installation are what make the fastening system reliable.

For most functional 3D printed parts, start with the screw size, check the insert drawing, design the hole around the insert geometry, add enough depth for displaced plastic, and test before final production.

Heat set inserts are not just small brass parts. In 3D printed assemblies, they are structural connection points.

For a broader selection overview, see our guide on how to choose heat set inserts for 3D printed parts.

For more common engineering questions, see the 3D Printing Fastening FAQ.

Related Engineering Factors

Choosing the correct heat set insert depends on multiple engineering variables in 3D printed assemblies.

Important related engineering factors include:

insert outer diameter
hole size and printed tolerance
boss wall thickness
insert length and depth
pull-out strength
torque resistance
installation temperature
material heat resistance
layer adhesion and print orientation
long-term creep behavior

Different insert types may behave differently depending on the plastic material, surrounding geometry, and assembly load conditions.

Related engineering guides:

FAQ

What is a heat set insert in 3D printing?

A heat set insert is a metal threaded insert installed into a plastic 3D printed part using heat. It creates stronger and more durable internal threads than directly screwing into plastic.

How do I choose the right heat set insert?

Choose the screw size first, then select an insert based on outer diameter, length, geometry, material compatibility, hole depth, boss design, and load direction.

Are M3 heat set inserts the best choice for 3D printing?

M3 inserts are common for desktop 3D printing because they work well for enclosures, brackets, electronics housings, and general assemblies. However, smaller or larger sizes may be better depending on the part.

Should the hole be smaller than the heat set insert?

Usually, yes. The hole is commonly designed slightly smaller than the insert’s outer retention features so the heated insert can displace plastic and lock into place after cooling.

How deep should the insert hole be?

The hole should usually be deeper than the insert length to provide space for displaced plastic and reduce the risk of blocked threads.

Can heat set inserts be used in PLA?

Yes, heat set inserts can be used in PLA, but PLA softens easily. Use controlled heat, avoid long dwell time, and allow the insert to cool before tightening the screw.

Can heat set inserts be used in PETG?

Yes. PETG can work well with heat set inserts, but it may produce more plastic flow or stringing during installation. Hole size and cooling time are important.

Why do heat set inserts fail in 3D printed parts?

Most failures come from incorrect hole size, weak boss design, poor installation control, unsuitable material behavior, or load conditions that exceed the printed structure.

Do I need a special installation tip?

A special heat set insert tip is not always required, but it helps transfer heat evenly and keep the insert aligned during installation.

Should I test insert holes before printing the final part?

Yes. A small test print with several hole sizes is one of the best ways to verify fit, installation behavior, and strength before printing the final part.

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