Open-frame linear DC solenoids use a bobbin-wound coil that is either overmolded or taped, with the winding visible on at least two sides of a C- or D-shaped laminated steel frame, and they are typically benchmarked at 50,000 to 100,000 cycles [S1][S7].
Tubular (molded/encapsulated) solenoids fully enclose that same coil inside a sealed metal casing, which pushes life expectancy above 25 million cycles in equivalent duty [S1]. The two constructions diverge on cost, thermal path, environmental sealing, force density, and modification flexibility, and each is the correct choice only inside a specific application envelope.
Construction Anatomy: What Each Variant Actually Contains
Open-frame solenoids consist of three functional parts: an open iron frame (C-Frame style encloses the coil on one side, Box-Frame style on two sides), a bobbin-wound coil that is either overmolded or taped, and a movable plunger at the center of the coil [S5]. The frame is typically a "C" or "D"-shaped stack of laminated steel that completes the magnetic circuit around the exposed coil [S6].
Molded (tubular/encapsulated) constructions take that same coil-and-plunger set and pot it inside a cylindrical metal tube, sealing the winding, the bobbin, and the lead exit against the environment [S1]. The reference solenoid coil build uses closely wound copper or aluminum wire on a hollow core; the choice of conductor and insulation class is identical between constructions, but only the molded version adds the outer mechanical shell that the open frame intentionally omits [S3].
Life Expectancy and Duty Cycle: The Numbers Behind the Trade-Off
Open-frame solenoids are typically benchmarked at 50,000 to 100,000 cycles, while tubular varieties are more durable with a life expectancy above 25 million cycles [S1]. Higher-end open-frame platforms, such as Johnson Electric's linear series, are published with a life rating of up to 5 million cycles, with custom designs credited for further gains [S5].
Open frames are well suited to intermittent duty and to high-volume OEM builds where the application budget is for a 6 to 18 month service life; tubular/encapsulated constructions are the right pick when the part will see continuous or high-cycle duty, or when it must be qualified to a long warranty window. Continuous or intermittent duty is supported by both constructions, but only the open frame can be repaired or re-taped in the field; once the molded shell is potted, the coil is effectively a service-replaceable unit [S1][S5].
Thermal Path, Sealing, and Insulation Class
Open frame solenoids have a visible coil and exposed magnetic structure, making them more economical and suitable for applications where cost-effectiveness and heat dissipation are priorities [S4]. The exposed winding sheds heat directly to ambient air, so an open frame is preferred when the duty cycle pushes the coil close to its thermal limit; molding the coil in a sealed tube traps that same heat inside the casing, which is acceptable when the duty is light or when the design intentionally oversizes the coil [S4].
Insulation class is independent of construction, but is the second thermal lever. Standard solenoid insulation classes run A, B, F, and H, with H class rated to 180°C for harsh-environment and continuous-operation designs [S4]. In a molded package the H-class coil is the only thermal safety net; in an open frame the H-class coil can be downgraded one or two classes because the open geometry already removes a large fraction of the waste heat through convection. For a primer on how a solenoid coil converts current into linear force, the solenoid coil reference page lays out the operating principle. For process engineers also weighing a different actuator topology, the comparison on air solenoid valve architectures is a useful parallel read.
Force, Stroke, and Response Time
Latch and open frame solenoids have very similar statistics in terms of holding force and stroke length; however, unlike open frames, magnetic latch solenoids do not require current in the end position [S1]. The Box-Frame open style delivers slightly higher force output and higher resistance to environmental conditions than the C-Frame style, but both still trail a comparably sized tubular solenoid in raw force density because the closed steel shell of the tubular variant forms a tighter magnetic circuit [S5].
Response time is a function of electrical, magnetic, and mechanical time constants, not of the outer shell. Specialized fast-response solenoid designs can actuate in roughly 0.3 ms, and the open frame's efficient heat dissipation lets it hold that speed over rapid cycling because the coil does not thermally saturate [S4]. Tubular solenoids can match the electrical response but will hit a thermal ceiling sooner under continuous rapid-cycling duty, unless the design derates the force or enlarges the coil.
Modification, Customization, and Unit Cost Ceiling
Open-frame solenoids are suited to high-volume production, and modification may add substantially to pricing in small (typically <5k unit) volumes [S2]. Common modifications include adding a return spring (external with a staked disc, e-ring, shouldered plunger, or machined groove; or internal, at the cost of plunger cross-section and force), plunger re-termination (threaded hole, e-ring groove, stop washer to defeat residual magnetism), pushrod fitting, coil rewinds for low-voltage operation, diode clamps for back-EMF, and frame or mounting-hole changes that usually need new tooling below roughly 10k units per year [S2].
The standard configuration for most open-frame solenoids is pull configuration with a clevis connection to the plunger (a slot with cross-hole running through it at 90°), with smaller parts using just a cross-hole in the plunger, and the unit is mounted with threaded holes in one or more flat sides [S2]. Molded/encapsulated constructions can be customized, but each tooling change costs an order of magnitude more because the steel tube and potting fixture are involved, so they are normally only economical at higher annual volumes or when the specification is frozen. For a related OEM cost-versus-volume trade-off in a different product class, the spec guide on Structural Epoxy vs Mechanical Fastening: 2026 Load-Bearing Joint Spec walks through a similar volume-versus-tooling decision.
Decision Matrix: Which Construction to Specify
On cost, the open frame is the lower-cost option for high-volume production; molded/encapsulated parts carry a cost premium of typically 30% to several times the open-frame price for the same force class [S1][S7]. On cycle life, open frame is rated 50,000 to 100,000 cycles baseline (up to 5 million on premium platforms with custom design [S5]), while molded/tubular is rated above 25 million cycles [S1]. On environmental sealing, open frame exposes the coil on at least two sides and is intended for clean, indoor duty; molded/encapsulated seals the winding against dust and moisture [S1][S4]. On thermal management, open frame dissipates heat through the exposed coil; molded/encapsulated traps heat inside the casing and therefore usually requires an H-class (180°C) insulation to compensate [S4]. On modification flexibility, open frame accepts spring, plunger, pushrod, coil, and connector changes in lots down to a few thousand; molded/encapsulated changes need new tooling and are economical only at higher volumes [S2].
Specify open frame for vending, ATM, smart lock, gaming, cash drawer, and medical-cabinet applications where the duty is intermittent, the environment is clean, and unit cost dominates the BOM [S1][S5][S9]. Specify molded/tubular for fuel-injection, outdoor access control, industrial valve actuation on dirty or wet sites, and any application that demands >25 million cycles or that must be potted against contamination [S1][S4]. For the broader question of when an open-frame solenoid even fits a system, the related solenoid valve reference lays out the fluid-handling boundary case. For a different material-class trade-off that follows a similar "encapsulated vs exposed" logic, the Coconut Shell vs Coal-Based Activated Carbon spec guide is a useful analogue.
Limitations and Failure Modes
The known failure modes of open-frame solenoids are residual magnetism at full plunger closure, contamination of the exposed winding, and mechanical wear of the clevis/cross-hole plunger termination under high-cycle duty [S2]. Residual magnetism is commonly seen on open-frame parts because the plunger is normally allowed to close fully against the pole piece, and it is usually handled by adding a stop washer, a non-magnetic shim, or a small air gap at the closed position [S2].
Molded/encapsulated solenoids do not have residual magnetism as a service problem, but they can fail by coil burnout that is non-repairable, by lead-wire fatigue at the potting entry, and by moisture ingress through any breach in the seal. The open frame is the right pick when the design can tolerate those field-serviceable failure modes; the molded version is the right pick when the design cannot tolerate them at all. For related decision logic on a different component family, the Soft-Seated vs Metal-Seated Butterfly Valve Temperature Limits guide applies a similar "sealed vs exposed" trade-off to valve trim.
Trackable near-term signals for specifiers: open-frame OEM cost curves (copper and grain-oriented electrical steel price moves), new H-class and 180°C-class molded platforms released for continuous-duty industrial valves, and any move by major vending and ATM OEMs to derate the open-frame 50k to 100k cycle baseline in published datasheets.