The shift to electric vehicles has quietly become one of the more demanding proving grounds for fastener and insert technology. EV platforms impose requirements that don’t often coexist in conventional automotive design: lightweight plastic housings that must meet structural load requirements, thermal cycling ranges far exceeding traditional underhood environments, vibration profiles from both road inputs and high-frequency electrical systems, and increasingly stringent ingress protection standards for high-voltage enclosures. Add in the service and repairability considerations driven by battery cost, and you have a design environment that pushes insert selection well past the defaults.
This post covers the insert types most commonly specified in plastic battery housings and EV structural components, the engineering rationale behind those choices, and the application-specific factors that should drive your selection process.
Why Inserts in EV Applications Are Different
Before getting into insert types, it’s worth framing what makes EV battery enclosures a distinct design problem relative to conventional automotive plastics.
Structural load requirements are high. Battery enclosures are load-bearing structures in most vehicle architectures. The housing must handle static mounting loads, dynamic road inputs, and in some architectures, contribute to the vehicle’s torsional stiffness. Fastener joints in these structures see real sustained loads (not just assembly loads), which puts greater demands on pull-out and fatigue performance than typical interior trim or underhood covers.
Thermal cycling is severe and asymmetric. Battery packs generate heat during charge and discharge cycles, and many include active thermal management systems. The result is a housing that experiences cyclical thermal loading across a wide range, often with different rates of expansion in the metal fasteners, the insert body, and the surrounding thermoplastic. Inserts that perform well in static thermal testing can fail progressively under cyclic loading through a mechanism that doesn’t show up in standard pull-out testing.
Dielectric performance matters. In high-voltage enclosures, the insert’s placement relative to live conductors and the interface between the metal insert and the surrounding plastic are both relevant to electrical isolation. This is rarely the binding constraint in most insert selections, but it shapes which materials are acceptable in close proximity to high-voltage architecture.
Sealing interfaces must be maintained. Many battery enclosures are rated to IP67 or IP69K. Fastener joints in these housings are part of the sealing system, and the joint must maintain consistent clamp load through thermal and mechanical cycling without relaxing to the point of compromising the gasket or seal. Inserts that creep or allow the boss to relax over time are a sealing liability.
Repairability has economic weight. A battery pack is the highest-cost component in an EV. Inserts in serviceability interfaces (covers, thermal management components, module retention features) will be cycled in service. Single-use behavior is not acceptable at these joints.
Heat-Set (Ultrasonic and Thermal) Inserts
Heat-set inserts installed via ultrasonic or thermal methods are the baseline for most thermoplastic battery housing applications. They’re well-understood, available in a wide range of geometries and sizes, and compatible with the resins commonly used in EV housings, primarily glass-filled polyamide (PA6-GF and PA66-GF), polyphthalamide (PPA), glass-filled polypropylene, and various grades of PBT.
The retention mechanism (molten plastic flowing into knurls, undercuts, and retention features before re-solidifying) creates a strong, low-profile installation that doesn’t require access to the back face of the part. For housings with complex geometry, consolidation of multiple housing sections into a single molded structure, or areas where back access is impossible, this is often the only viable approach.
What to watch in EV applications: Thermal cycling performance. Standard knurled inserts perform well under static load but can exhibit progressive loosening under cyclic thermal loading as differential expansion between the brass insert body and the thermoplastic matrix generates repeated interfacial shear. For applications with significant thermal cycling exposure, specify inserts with multi-directional retention geometry (double-lead knurls, diamond knurls, or combinations of axial and radial retention features) that resist both axial and rotational loosening under cycling.
Brass is the default insert material, but stainless steel inserts are increasingly specified in battery applications where galvanic corrosion from condensation or coolant exposure is a concern, or where the application sits near high-voltage architecture and a non-magnetic material is preferred.
Molded-In Inserts
Molded-in inserts are placed in the mold prior to injection and become encapsulated by the plastic as the part is formed. The result is the highest possible retention strength for a given insert geometry: the plastic encapsulates the insert completely, and there’s no heat-affected zone or melt interface to limit performance.
In EV battery housing applications, molded-in inserts are most commonly found at primary structural mounting points: battery pack-to-vehicle frame attachments, module-to-housing retention features, and high-load thermal management interface joints. These are joints where the pull-out and push-out loads are high enough that post-mold insertion methods leave too much performance on the table.
The practical trade-offs are significant. Molded-in inserts add cycle time and labor to the molding operation, require precise fixture design to locate inserts accurately within the mold, and introduce risk of mold damage from misplaced inserts or insert movement during injection. Flash on the insert threads is a common quality issue that requires secondary cleaning. For complex housings where only a subset of bosses see high structural loads, a hybrid approach using molded-in inserts at the high-load locations and ultrasonic inserts at lower-load interfaces is often the most practical solution.
Weld line management is an underappreciated design consideration for molded-in inserts. The insert disrupts polymer flow during injection, and weld lines that form downstream of the insert can significantly reduce the effective strength of the surrounding boss. Gating strategy and insert placement should be co-designed, not treated as independent decisions.
Self-Tapping and Thread-Forming Inserts
Self-tapping inserts, along with the related category of thread-forming screws that don’t use a separate insert, are common in lower-load interfaces in battery housings: cover panels, wire management features, secondary electronics brackets, and access panels for diagnostics or service.
For insert applications specifically, thread-forming (interference-fit) inserts are pressed or driven into a pre-formed boss and develop retention through mechanical interference with the boss wall. They’re fast to install, require no heat source, and work reasonably well in short-term or low-cycle service. The limitations in EV applications are around sustained load and thermal cycling performance: the interference fit that creates retention also creates residual hoop stress in the boss, and that stress can relax over time, particularly in semi-crystalline resins at elevated temperature.
For service interfaces that will be cycled repeatedly (covers removed for inspection, battery module access panels, cell interconnect covers) thread-forming inserts are generally not the right choice unless the design can accommodate the torque-out and pull-out reduction that comes with cycling.
Boss cracking is the primary failure mode to watch in press-in applications with glass-filled materials. The interference required for adequate retention in unfilled resins may be enough to crack a glass-filled boss immediately or after a small number of installation cycles. Boss wall thickness requirements for press-in inserts in GF materials are significantly higher than for heat-set equivalents.
Expansion Inserts
Expansion inserts are installed into a pre-drilled or molded hole and expanded mechanically (typically by driving a mandrel through the insert body) to create a radial interference fit with the surrounding material. They’re one of the few insert types that can be installed post-mold without heat, and they’re particularly useful in applications where the insert must be installed in a finished part, where access to the back face is impossible, and where the material or geometry makes ultrasonic installation impractical.
In EV battery applications, expansion inserts appear most frequently in composite and hybrid material housings, where glass or carbon fiber-reinforced thermoset panels or SMC (sheet molding compound) components are joined to thermoplastic structures, or where inserts must be retrofit into existing assemblies during service. Ultrasonic installation is not applicable to thermosets; expansion inserts fill this gap.
The retention characteristics of expansion inserts are highly geometry-dependent and typically lower than equivalent heat-set or molded-in inserts in thermoplastics. They are appropriate for low-to-moderate loads and should not be the default choice at structural interfaces.
Clinch and Press-Fit Inserts for Hybrid Structures
EV battery enclosures increasingly use hybrid structural approaches: thermoplastic housings with metal reinforcement inserts, aluminum extrusion frames with overmolded plastic features, or multi-material assemblies that join dissimilar substrates at fastener interfaces. In these structures, clinch nuts, clinch studs, and pressed-in fastener inserts installed into sheet metal members are common at the metal-to-plastic interface joints.
These aren’t inserts in the traditional sense; they’re fastener elements installed into the metal members of the assembly. But they’re part of the joint design at many of the highest-load interfaces in the battery system, and they interact directly with the plastic housing geometry. The clamp load distribution from a clinch nut in an aluminum extrusion, transmitted through a bolted joint into a glass-filled PA boss, determines how that boss is loaded in service. Getting that interface right requires thinking about the full load path from the fastener head through both substrates.
Key Selection Parameters for EV Battery Insert Applications
Regardless of insert type, several parameters should drive the selection and validation process for any battery housing application:
Pull-out and torque-out after thermal cycling. Static pull-out data is a starting point, not a validation. Develop a thermal cycling protocol that reflects the actual temperature range and cycle count of the application (active thermal management systems can produce hundreds of charge cycles per year) and validate retention after cycling, not just at baseline.
Clamp load retention. For sealing-critical joints, measure actual clamp load through the gasket or seal as a function of thermal cycling and time. Loss of clamp load is the failure mode; pull-out strength is only a proxy.
Corrosion compatibility. Define the chemical environment the insert will see in service: condensation, battery coolant chemistry, any cleaning agents used in assembly or service. Brass is adequate for many environments but is not universal. Stainless steel, nickel-plated steel, and aluminum inserts are all available and appropriate to specific environments.
Electrical isolation requirements. For inserts in close proximity to high-voltage architecture, confirm that the installed insert and its surrounding boss geometry meet the creepage and clearance requirements for the applicable voltage class. This is primarily a geometry and placement issue, but it can constrain insert diameter and boss design in space-constrained areas.
Service cycle life. Define the number of installation and removal cycles expected at each interface over the vehicle’s service life. Battery pack service intervals are not yet well-established across the industry, but design for at least ten cycles at any interface that could be accessed during battery service, and more at diagnostics and inspection points.
What This Means for Your Design
Insert selection for EV battery housings is more consequential than it is in most automotive plastic applications, because the loads, the thermal environment, the sealing requirements, and the serviceability expectations are all more demanding. The right answer depends on the specific joint: its load, its thermal exposure, its sealing role, and how many times it will be cycled in service. It should be validated against those conditions rather than carried over from conventional automotive practice.
The insert types that perform best across the broadest range of EV battery applications are heat-set inserts with multi-directional retention geometry for mid-load interfaces and molded-in inserts at primary structural attachment points. Self-tapping and expansion inserts have appropriate niches but should not be the default in a thermal-cycling, high-load environment.
Where Does CFI Fit In?
For over 30 years, Components for Industry has been a worldwide provider of industrial components and a trusted partner for companies in industrial plastics and composites, including Automotive at the OEM level. If you want more information about CFI and our industrial components including threaded inserts, brackets, and clips, contact us at 847-918-0333 or sales@componentsforindustry.com.
