Sonitek / Application guide
Small inserts.
Better assemblies.
From the first printed prototype
to a repeatable thermal process.
A small metal insert can turn a plastic part into a serviceable assembly with reusable threads. The result depends on more than getting the insert into the hole: the part geometry, thermal tooling, installation motion, and cooling all have to work together.
This guide connects a 3D-printing demonstration from Markforged with two Sonitek thermal-insertion videos. Follow the progression from a single prototype to a repeatable production process, with practical design checks, illustrated examples, and a troubleshooting reference.
01 / Understand the joint
What a heat-set insert actually does
A heat-set insert has internal machine threads and an exterior designed to engage the surrounding plastic. During thermal insertion, heat passes through the metal into a localized region of thermoplastic. The material softens and flows around the insert's retention features, then solidifies as it cools. The metal provides the thread; the surrounding polymer carries the insert's loads.
Confirm that the substrate is suitable for heat insertion. Thermoset plastics do not remelt like thermoplastics; a cured resin print should not be treated as an FDM thermoplastic. Select a fastening method for the actual material and service environment. Technical reference: SPIROL.
02 / Design before you heat
Start with the exact insert and the finished hole
Specify the insert part number, thread, length, exterior geometry, material, and installation method. Two inserts with the same internal thread can require different receiving holes. For a printed part, measure the actual hole after printing; the CAD dimension alone does not establish fit.
| Design check | What to establish |
|---|---|
| Hole diameter & profile | Use the selected insert supplier's recommendation for that material and installation method. Match straight or tapered geometry. |
| Hole depth & screw travel | Provide the specified clearance beneath the insert and confirm that the assembly screw cannot bottom out. |
| Boss & surrounding material | Provide enough polymer to retain the insert without splitting or bulging. Include nearby edges, ribs, and local wall thickness in the review. |
| Printed structure | Review perimeters, local fill, layer direction, and reinforcement around the hole. Validate the actual print recipe on representative parts. |
| Load path & mating component | Check how the screw, insert face, mating part, and polymer share the load. Include tightening torque and repeated assembly. |
| Tool & fixture access | Allow a straight approach along the hole axis and support close to the installation site. Check tip clearance around ribs and recesses. |
The gripper demonstration uses M4 screws and a selected insert for which the presenter models a 5.74 mm cavity diameter and a 4.7 mm insert-length pocket. Those dimensions describe that example; they are not a universal M4 hole specification. The insert is installed from the rear of the gripper to suit the load path. Watch the CAD example at 1:43.
Blind holes need their own depth check. SPIROL's guidance for its heat/ultrasonic inserts recommends a minimum hole depth of insert length plus two thread pitches. For illustration, a 5.0 mm long insert with a 0.7 mm pitch gives 6.4 mm. Apply the selected supplier's specification and confirm screw clearance for your assembly. Read the hole-design reference.
03 / Build a repeatable installation
Align. Heat. Seat. Cool. Verify.
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Prepare the part and fixture
Confirm material, insert identity, receiving-hole geometry, and cleanliness. Locate the part securely in a nest that supports the installation area without distorting it. Follow the equipment instructions for hot surfaces, guarding, and safe loading.
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Match the tip to the insert
Use a tip that transfers heat through the intended contact surface while maintaining alignment and protecting the threads. Confirm that the tip and pilot can withdraw without catching the hardware. Sonitek offers thermal insertion tips for application review.
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Establish a material-specific starting recipe
Use supplier and equipment guidance, then trial representative parts. Stabilize tool temperature before testing. A controller setpoint, the actual tip temperature, and the temperature at the plastic interface are different measurements.
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Insert along the hole axis
Allow heat transfer while applying controlled motion and force. Avoid forcing a cold insert into place. Watch for tilt, displaced plastic, or part movement. Seat to the drawing's specified position, using a qualified stop or position control where available.
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Withdraw and allow the joint to stabilize
Keep the part supported during the validated withdrawal and cooling sequence. Do not disturb or torque the insert while the surrounding polymer is still soft. Establish cooling time through trials rather than appearance alone.
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Inspect and record
Check presence, position, alignment, thread access, and visible damage. Record the material lot, part revision, insert, tip, settings, and results. Use retention testing to establish whether an attractive installation also meets its functional requirement.
04 / Move from sample to production
Scale the controls along with the volume
Demo A: a serviceable printed gripper
Define the screw load and how often the jaw will be replaced. Verify the rear-access installation path, printed material around each hole, and screw engagement. Test the finished jaw under representative loads; a coupon is useful for development but cannot reproduce every feature of the final part.
Demo B: a housing with several inserts
Map each location's axis, seating height, hardware material, and support. Check that every tip can contact its insert without colliding with the housing. Validate each location independently before qualifying the complete cycle.
| Stage | Useful equipment approach | Control to add |
|---|---|---|
| Proof of concept | A suitable temperature-controlled hand tool and supported sample | A documented method, measured holes, and a clear acceptance test |
| Low-volume assembly | A guided thermal arbor press and dedicated nest | Consistent alignment, seating depth, and operator sequence |
| Repeated production | A controlled thermal press with application-specific tooling | Defined process limits, repeatable loading, and traceable checks |
| Multiple locations | A multi-tip platen or application-specific insertion arrangement | Location-by-location heat transfer, tolerance accommodation, and seating verification |
Sonitek's recent demonstration describes brass and stainless-steel hardware installed across multiple planes. That illustrates an application-specific tooling opportunity, not a promise that every insert can share one recipe. Review mixed metals, insert sizes, and part tolerances with the tooling design. See the Sonitek production demonstration.
The Sonitek TS-100 supports prototyping and low-volume work. For more complex layouts, explore custom heat-staking tooling and discuss the part, throughput, and inspection requirements before selecting a machine.
Compare processes at the assembly level. Heat and ultrasonic insertion each have suitable applications. Consider access, number of inserts, total handling and cooling time, part sensitivity, and measured joint performance. Avoid choosing solely from a single-insert cycle time. Read the process comparison.
05 / Diagnose the cause
Troubleshooting without chasing settings
Use the observations below as a diagnostic starting point. A symptom can have several causes; inspect the part and tooling before changing the recipe.
| Observation | Investigate | Next check |
|---|---|---|
| Insert tilts | Initial alignment, pilot fit, fixture support, or an off-axis approach | Check the unloaded tool path and part location; inspect perpendicularity after a trial. |
| Insert spins or pulls out | Hole geometry, inadequate material engagement, local printed structure, or premature loading | Measure the hole; section a sample and test retention after cooling. |
| Flash, sink, or a distorted boss | Hole/material volume, excessive heat exposure, overtravel, or insufficient support | Review dimensions and the temperature-motion-depth combination. |
| Cracks or layer separation | Undersized hole, excessive force, weak local geometry, print orientation, or insufficient heat transfer | Confirm the material and print recipe; evaluate a supported sample. |
| Inconsistent seating height | Part variation, unstable temperature, datum movement, tool length, or depth control | Measure the part before installation and the seated insert afterward. |
| Blocked or damaged threads | Tip interference, plastic ingress, insert damage, or incorrect screw engagement | Inspect the tip and thread with an appropriate gauge; check screw length. |
Change deliberately. Keep the material, insert, and fixture identified during every trial. For initial troubleshooting, change one factor at a time so the effect is understandable. Use a planned experiment when interactions need evaluation. The table is an engineering troubleshooting framework; it does not assign a confirmed cause to a particular part.
06 / Qualify the joint
A flush insert is only the first check
Set acceptance limits before production. Visual inspection can detect obvious defects, but the assembly must also resist its specified loads. Sectioned samples can reveal whether plastic engages the insert's external retention features. SPIROL describes this verification method.
- Dimensional: presence, position, seating height, axis, and surrounding-part condition.
- Functional: thread access, screw engagement, tightening torque, and mating-part fit.
- Mechanical: specified pull-out and torque-out resistance, with a documented fixture and method.
- Service conditions: repeated assembly, temperature, aging, or other exposures relevant to the product.
- Process capability: repeated samples across relevant part, material, operator, and warm-up conditions.
Part revision / material grade & lot / print or molding conditions / insert part number / measured hole / tip & nest / temperature setting / insertion motion or force / seating target / cooling time / inspection results / retention results / disposition.
Define the sample size and pass/fail values with the responsible engineering and quality teams. A single successful sample does not establish a qualified production window.
Common questions
Can I use a soldering iron?
Markforged demonstrates this for a 3D-printed prototype. Suitability depends on the tool, insert, part, and required consistency. A guided press and purpose-designed tip can help control alignment and seating as the process becomes repetitive.
Is there one temperature for all inserts in a polymer?
No. The insert and tip geometry, contact, installation motion, and material grade affect the process. Qualify a combination of settings for the actual assembly.
Does a metal insert make the whole joint metal-strength?
No. The polymer, local geometry, printed or molded structure, and mating-part load path still limit the assembly. Test the complete joint against its service requirements.
Watch / Apply / Compare
Three demonstrations, one development path
1. Design the printed prototype
Markforged walks through a gripper, insert-specific CAD cavities, tool alignment, installation, and cooling. Watch on YouTube.
2. Guide the insertion motion
Sonitek demonstrates threaded-insert installation with a precision arbor-style thermal press. Use the footage to consider tool access and the operator sequence. Watch on YouTube.
3. Plan the production tooling
Sonitek's multi-plane application demonstrates the move toward coordinated installation of several pieces of hardware. Watch on YouTube.
Bring your part into the conversation
Develop the process around your assembly.
Share your CAD or drawing, polymer grade, insert specification, sample parts, required retention, and production volume with Sonitek. Those details help define the tooling and evaluation needed to move from a promising sample to a repeatable process.
Discuss your insertion application