Closed and ground die springs have both end faces ground so they sit square to the spring axis, giving the pocket a flat, perpendicular surface to carry load. Closed un-ground die springs keep closed end coils but are not ground, and they remain an available option inside the same ISO 10243, JIS B5012 and American Standard series. The right choice is a statement about die set alignment and load path, not a matter of habit.

This guide compares the two end types the way a tool designer or maintenance buyer actually meets them: at the drawing stage, when the spring pocket is being defined and the die set is already fixed. It sets out what each end face does under load, how to match the end type to guide-post alignment and load class, and where a closed un-ground spring still fits without creating a weak point in the tool.

The Problem: An End-Face Detail That Decides How the Load Travels

A die spring does one job inside a press tool: it stores and returns energy along a defined axis. The spring pocket, the guide pin and the spring have to agree on that axis. When they do not, the spring is no longer being compressed purely axially, and the tool starts to compensate in places that were never designed for it.

The end face is where that agreement is either made or left to the surrounding hardware. A ground end face presents a flat, perpendicular bearing surface to the pocket. A closed but un-ground end face presents closed coils, but the face itself has not been prepared to that condition. Both are legitimate products; they simply place the responsibility for squareness in different places.

The symptoms of getting this wrong are rarely dramatic at first. They show up as uneven wear on guide components, as springs that do not return to a clean seated position, and as load behaviour that no longer matches the load-deflection curve the tool was designed around. By the time those symptoms are visible, the conversation has usually moved from springs to rework.

Industry Background: Why Rectangular Die Springs Carry Two End Options

Rectangular wire die springs are the default elastic element in press tools because of energy density. Per TopWay product documentation, the rectangular cross-section stores up to 30% to 50% more energy than round wire springs of the same footprint, which is why a die spring pocket can stay compact while still delivering the force a stamping or ejection sequence needs.

That density sits inside a standardised envelope. ISO 10243:2019 standardises colour-coding and dimensions for rectangular-section compression springs used in press tools, and JIS B5012 specifies allowable tolerances on the inner and outer diameters of springs, with a different focus from ISO 10243's mounting-hole orientation. Because the load class, colour and envelope are pinned by these systems, buyers can compare springs across suppliers — but the standards-driven envelope is also why the end type is often carried along as a catalogue option rather than re-examined for each tool.

Demand keeps pushing that envelope harder. Grand View Research projects the global industrial springs market to reach USD 7.1 billion by 2033, and the automotive stamping market alone was valued at USD 87.98 billion in 2024 according to Market Research Future — a large end-use base for die springs used in automotive stamping dies, plastic injection molds and die casting tooling. More cycles, tighter clearances and higher load classes all make the seating condition of the spring end face matter more, not less.

TopWay springs are manufactured in 55CrSi chrome silicon steel, shot-peened and multi-stage tempered, and are rated for a fatigue life of 3 to 5 million high-frequency compression cycles within a 12%–40% compression stroke (TOPWAY SPRINGS published data). That service window only holds if the spring is loaded the way the curve assumes — axially, with a preload, in a pocket that is square to the axis.

What Each End Type Actually Does Under Load

Closed and ground: a prepared bearing face

In a closed and ground spring, the end coils are closed and the end faces are then ground. The functional result, as described in TopWay product data, is precision ground parallelism: the ground end faces ensure squareness and perpendicular load distribution. That combination is what removes lateral deflection from the load path and protects the mold guide components from side loading they were not sized for.

Practically, this means the spring arrives able to define its own seating plane. The pocket still has to be machined correctly, but the spring no longer depends on the pocket edges or on installation clearance to keep the load travelling along the axis. In guided die sets, where the guide post and bushing set an alignment the tool cannot deviate from, that is the condition the design assumes.

Closed un-ground: a closed coil without a prepared face

In a closed un-ground spring, the end coils are closed but the end faces are not ground. It remains an available option within the same TopWay series — the ISO 10243, JIS B5012 and American Standard catalog ranges are all specified with End Type: Closed and Ground / Closed Un-ground. Nothing about the material, the 55CrSi grade, the powder coating or the load class changes.

What changes is where squareness comes from. Because the end face is not ground, squareness and perpendicular load distribution are not established by the spring end itself; they have to be established by how the spring seats in the pocket. That is acceptable where the seat geometry is controlled and the tool does not depend on the spring's end face to hold alignment — and it is a genuine risk where it does.

Load class is chosen first, end type second

End type never overrides load class. In the ISO 10243 range the classes run CIF super minimal load (light green), CIL light duty load (green), CIM medium duty load (blue), CIH heavy duty load (red) and CIB extra heavy duty load (yellow), across outer diameters of 10–63 mm, inner diameters of 5–38 mm and free lengths of 25–305 mm. The JIS B5012 range runs CSWF minimal load (yellow), CSWL light load (blue), CSWM medium load (red), CSWH heavy load (green), CSWB extra heavy load (brown) and CSWG super heavy load (black), across outer diameters of 6–70 mm, inner diameters of 3–38.5 mm and free lengths of 10–350 mm. The American Standard range covers medium duty (blue), medium heavy duty (red), heavy duty (gold) and extra heavy duty (green) springs, with outer diameters of 10–51 mm, inner diameters of 4.76–25.4 mm and free lengths of 25–305 mm.

Select the load class and the deflection first, then decide the end type within that class. Reversing the order is how tools end up with a correctly ground spring that is in the wrong load band.

Step-by-Step: Matching End Type to Die Set Alignment and Load

Use Cases: Where the Decision Changes

Automotive stamping dies

High-cycle automotive stamping runs combine tight guide clearances with continuous preloaded operation. This is the classic closed and ground application: the spring must return the stripper or pressure pad without introducing sideways force into guide posts and bushings, and the tool is expected to run for millions of strokes.

Injection molds and die casting tooling

Ejection systems rely on even force distribution across the ejector plate. Where the plate and pocket are machined square and the assembly is guided, closed and ground ends keep the load path perpendicular. In die casting tooling, the same requirement meets higher ambient temperatures, which is where multi-stage tempering and resistance to thermal relaxation become part of the end-type decision rather than a separate topic.

Automation, machine tools and general machinery

TopWay Series springs are also used in industrial automation, elevator systems, machine tools, engineering machinery, environmental filtration equipment, high-voltage electrical switches, valves and heavy-duty energy systems. In these assemblies the spring pocket is often part of a fabricated housing rather than a precision die plate, and the seating condition — not the catalogue default — should decide the end type.

Maintenance and MRO replacement

When a spring is replaced during a press stoppage, the safest approach is to match the original specification, including end type, unless the failure analysis points to a seating problem. Upgrading an un-ground position to a ground spring does not fix a pocket that is out of square; it only removes one possible contributor.

Comparison Table: Closed and Ground vs Closed Un-ground

Frequently Asked Questions

Conclusion: Let the Pocket Decide the End Face

The comparison between closed and ground and closed un-ground die springs does not end in a universal winner. It ends in a rule: the end face should match the way the tool establishes alignment. Where the die set is guided and the pocket is machined as a perpendicular seat, a closed and ground spring carries the load the way the design assumes — square to the axis, without lateral deflection, and without transferring side load into guide components. Where the seat geometry itself controls the spring's position, closed un-ground remains a valid, available option within the same standards and load classes.

Both decisions become safer when the rest of the specification is disciplined: correct load class by model code rather than colour habit, a preload of 5%–10% of free length, deflection planned inside the 12%–40% stroke window, 55CrSi material with heat-number traceability, and documentation that states the end type on the order and the label.