Heat-set inserts
Metal threads installed into thermoplastics: retention mechanisms, insert and hole geometry, material limits and interpretation of performance.
Tags: components, fasteners, plastics, assembly
On this page
SPIROL distinguishes the insert's mounting hole in the plastic from the clearance hole in the mating part. An oversize mounting hole can reduce retention. An undersize hole can cause stress, cracks or flash. If the mating part fails to bear on the insert, or the screw bottoms, tightening can cause jack-out. Check resin, filler, hole depth, boss support and installation method against the exact insert. SPIROL hole design, SPIROL hole design, SPIROL hole design.
Identity and retention
PennEngineering calls thermal installation into a thermoplastic heat staking. The insert’s internal thread receives the screw. PennEngineering SI catalog.
Check that plastic has flowed around the knurls, grooves or undercuts beneath the seated face. SPIROL process comparison.
Installation families
| Family | Mechanism and scope |
|---|---|
| Thermal installation | A heated tip transfers heat through the insert, or the insert is preheated before insertion. Controlled force advances it as the thermoplastic softens and flows. SPIROL |
| Ultrasonic installation | A contacting horn delivers vibration, generating local heat at the metal/plastic interface. PennEngineering describes simultaneous pressure and vibration, followed by solidification when vibration stops. PennEngineering |
| Press-in | A distinct post-mold family. SPIROL's Series 50/51 use a helical knurl; this is not a thermal installation specification. SPIROL families |
| Self-tapping | External threads engage the host. SPIROL's Series 10 is specifically thread-forming for soft, flexible thermoplastics; that example does not cover every resin or every self-tapping design. SPIROL families |
| Molded-in | The insert occupies the mold before injection and plastic forms around it. It is not a post-mold heat-set operation. SPIROL families |
SPIROL identifies thermal/ultrasonic methods with remeltable thermoplastics; thermosets cannot be remelted. Its table includes press-in, self-tapping and molded-in methods for both classes. Synthesis: family-level applicability does not establish compatibility with every polymer. SPIROL process comparison
Thread geometry and mounting geometry
The following dimensions describe different interfaces. Explanatory synthesis: their symbols and tolerances belong to the exact supplier drawing; an internal thread designation cannot substitute for the surrounding mounting geometry. SPIROL separately catalogs metric/inch threads and straight/tapered mounting holes. SPIROL heat/ultrasonic catalog
| Term | Meaning |
|---|---|
| Internal thread | Nominal diameter, pitch or threads per inch, thread system and tolerance/class describe the screw interface. A metric pitch is a length per turn; threads per inch is a count per inch. |
| Insert outside diameter | The external envelope, potentially measured over knurls. It is not automatically the recommended hole diameter. |
| Pilot | A locating portion at the insertion end; it is distinct from the retaining knurl and from the internal thread. |
| Insert length | Axial body length; it differs from usable thread engagement and from required blind-hole depth. |
| Mounting hole | Host opening with its own diameter, tolerance, depth and taper. A tapered hole additionally needs a defined measurement location. |
| Boss and head | The boss is surrounding plastic; a head is an insert flange. Boss diameter, wall thickness, head diameter and head thickness are separate dimensions. |
SPIROL distinguishes Series 14 for tapered holes from Series 19/20 and 29/30 for straight holes. Its catalog also separates long and short variants. These are supplier families, not interchangeable descriptions of every insert sold under the same nominal thread. SPIROL catalog
SPIROL's hole-design article links hole size to insert style, plastic composition and installation method. Oversize holes reduce retention; undersize holes can produce stress, cracks or flash. It discusses filler-dependent changes, blind-hole clearance, boss support and screw bottoming. Synthesis: nominal insert OD, screw diameter and a hole recommendation for another resin are insufficient to derive a mounting-hole specification. SPIROL hole design
The same article distinguishes the host hole from the clearance hole in the mating component. Bearing against the insert helps prevent jack-out, in which tightening draws the insert out of its host. Screw bottoming creates another jack-out risk. Headed designs change that bearing arrangement. These relationships do not supply a universal counterbore or clearance dimension. SPIROL hole design
Polymer and insert material
Thermal conduction matters during heat installation. SPIROL identifies brass and aluminum as common choices for transferring heat through the insert. Synthesis: metal identity is one input to the process, not evidence that two inserts with equal threads require equal heating conditions. SPIROL process comparison
Fillers also matter: SPIROL's hole article treats glass/mineral-filled plastics separately from its baseline hole recommendations. A label such as “thermoplastic” therefore leaves resin formulation and filler content unresolved. SPIROL hole design
SPIROL describes its published comparison data as inserts installed in unfilled Nylon 6/6, with drilled and molded holes distinguished. It explicitly limits those data to comparisons between insert styles, rather than performance predictions for another assembly. No numerical performance table is reproduced here. SPIROL product FAQ
Process settings and performance
Explanatory synthesis: temperature in °C or °F, force in N, and time in s describe process inputs. Pull-out force in N describes axial retention; torque-out in N·m describes resistance to rotation. Screw-tightening torque, although also expressed in N·m, serves a different purpose. Equal units do not make these quantities interchangeable.
SPIROL's FAQ attributes performance to insert design, plastic specification, hole/component quality and installation quality. It calls for testing in the actual assembly. Synthesis: a catalog comparison is not an allowable service load, minimum acceptance value or tightening instruction. Temperature, insertion time, hole dimensions, torque and pull-out values remain specific to the product, polymer, geometry and test/process conditions. SPIROL product FAQ
Sources and coverage limits
Reviewed 30 October 2026 PDT. The cited public HTML pages expose no verified publication, edition or substantive update date. Review/retrieval dates do not supply those missing dates; site copyright years and HTTP response dates are not edition evidence.
| Source | Version and use |
|---|---|
| S1 — PennEngineering, SI Brand Insert for Plastics | Undated catalog landing page; installation terminology and mechanism. |
| S2 — SPIROL, Heat vs Ultrasonic Installation of Inserts in Plastic Assemblies | Undated HTML article. Header credits Christie Jones; closing text credits the original article to Christopher G. Jeznach. Used for mechanisms/material applicability, not comparative cost or performance rankings. |
| S3 — SPIROL, Inserts for Plastics | Undated catalog family descriptions; named alternatives retain their product/material scope. |
| S4 — SPIROL, Heat/Ultrasonic Inserts catalog | Undated category page; thread and mounting-hole families. No part drawing dimensions reproduced. |
| S5 — SPIROL, How to Design the Proper Hole for Heat / Ultrasonic Inserts | Undated HTML article by Christie Jones; qualitative geometry relationships only. Its numerical guidance is not adopted as a universal recipe. |
| S6 — SPIROL, Heat / Ultrasonic Inserts product page | Undated page and FAQ; comparative-test context and application limits. |
Direct public HTML downloads succeeded at review. A separate web-tool request for S6 returned 403 and a repeat returned 429; availability differs by access path. Fresh retrieval does not prove origin freshness. Linked PDFs and full standards were not used or inspected.
This reference does not establish additive-manufactured boss equivalence to molded parts, grade-specific compatibility, long-term creep/fatigue life, environmental resistance, a qualified installation window or customer capability. Those subjects remain outside the evidence reviewed. Related reference: Hardware and component references.