Ellipsoidal Head Shell Tooling Concept Development — Macrodyne Presses & Automation

The Brief

What Macrodyne Needed Evaluated

Macrodyne Presses & Automation is supplying a hydraulic press to a customer seeking to manufacture ellipsoidal pressure vessel head shells — a common formed component in pressure vessels, storage tanks, and industrial cylinders. Two shell types are required, both sharing the same outer envelope geometry (Ø16.000″ diameter, 3.900″ height) but differing in wall thickness and flange configuration.

The customer's key constraint is that no press cushion or dedicated blank holder is available on the press. In conventional deep drawing, a blank holder applies controlled downward force on the blank's flange zone to prevent wrinkling as material flows into the die cavity. Without one, an alternative must be engineered into the tooling itself. Macrodyne engaged Luckee to evaluate two tooling approaches and assess their suitability for each shell type.

Client	Macrodyne Presses & Automation Inc. — Ontario, Canada
Product	Ellipsoidal pressure vessel head shells — two types
Press	Macrodyne hydraulic press — supplied by Macrodyne
Key Constraint	No press cushion or blank holder available
Concept A	Self-contained nitrogen gas spring / cylinder system within the tooling
Concept B	Simplified draw-form — no active blank control (geometry permitting)
Scope	Tooling concept feasibility evaluation — both types

Shell Geometry

Two Shell Types — Shared Envelope, Different Walls

Both shell types conform to a near-standard 2:1 semi-ellipsoidal profile — the most common head geometry in pressure vessel design, defined by a height-to-diameter ratio of 1:4 (H/D = 0.25). The heads specified here have H/D = 0.244 — slightly shallower than the true 2:1 standard, which is a favorable condition for forming. The generously proportioned knuckle radius (R2.675″ = 0.167D, well above the ASME minimum of 0.06D) further reduces the risk of thinning or cracking in the knuckle zone during forming.

Despite sharing the same outer geometry, the two types differ meaningfully in wall thickness and flange width — differences that directly affect forming behaviour and the relative suitability of each tooling concept.

Type 1 — Profile View

Cross-section profile: Ø16.000″ × 3.900″ H, crown R14.080″, knuckle R2.675″. Wall 0.177″, flange 1.031″. Near-standard 2:1 ellipsoidal — slightly shallow at H/D = 0.244.

Type 2 — Section E-E (Scale 1:3)

Section view: same Ø16.000″ outer envelope, 3.900″ height, R14.080″ crown, R2.675″ knuckle. Thinner wall at 0.132″, narrower flange at 0.396″.

Type 1

Heavier wall — wider flange

Outer Diameter16.000″

Height3.900″

Crown RadiusR14.080″

Knuckle RadiusR2.675″

Wall Thickness0.177″

Flange Width1.031″

Type 2

Thinner wall — narrower flange

Outer Diameter16.000″

Height3.900″

Crown RadiusR14.080″

Knuckle RadiusR2.675″

Wall Thickness0.132″

Flange Width0.396″

⚠ Thinner wall reduces natural wrinkling resistance — blank control more critical. Nitrogen Concept A preferred.

Tooling note: Because both types share the same outer geometry (Ø16.000″, R14.080″ crown, R2.675″ knuckle), a common die cavity can potentially serve both shell types. The punch geometry and blank holder force would need to be adjusted for the wall thickness difference — but this opens the possibility of a single set of primary die components with interchangeable punch inserts, reducing tooling cost.

The Core Challenge

No Blank Holder — Why It Matters and How to Solve It

In conventional ellipsoidal head forming, a blank holder ring presses down on the peripheral flange of the blank as the punch descends. This controlled restraint is what prevents the flange from buckling (wrinkling) as material is drawn radially inward into the die cavity. Without it, the compressive hoop stress in the flange zone has no counterforce — and wrinkling is the likely result.

The no-cushion, no-blank-holder constraint does not make forming impossible — but it does mean the blank-holding function must be engineered into the tool itself, or the geometry must be evaluated to determine whether active holding is even necessary given the relatively shallow draw ratio. Both paths are explored below.

Concept A

Self-Contained Nitrogen Gas Spring System

Nitrogen gas springs (or cylinders) are integrated directly into the lower die body. As the punch descends, the springs engage the blank flange and apply a controlled downward force — replicating the function of a press cushion without requiring one in the press itself.

Force is progressive — increases with compression, aligned with draw-in requirements
Multiple spring units distributed around the flange perimeter for even hold-down force
Estimated total force for Ø16″ blank: 8,000–15,000 lbf depending on material gauge and friction
Commercially available nitrogen spring units fit within standard die envelope
Suitable for both Type 1 and Type 2

Suitability — Type 1 (0.177″ wall)

Suitability — Type 2 (0.132″ wall)

Concept B

Simplified Draw-Form (No Active Blank Control)

Given the relatively shallow ellipsoidal geometry (H/D = 0.244) and generous knuckle radius (R2.675″ = 0.167D), it may be possible to form the shell without any active blank holding — relying on the blank's natural resistance to buckling (a function of its thickness-to-diameter ratio) and careful die geometry design.

Simpler tooling — lower cost, easier to build and maintain
Requires precise blank diameter and die geometry optimisation
Wrinkling risk is real but manageable for thicker Type 1 (0.177″)
Not recommended as primary approach for Type 2 (0.132″) — thinner wall reduces natural wrinkling resistance significantly

Suitability — Type 1 (0.177″ wall)

Suitability — Type 2 (0.132″ wall)

Engineering Analysis

Detailed Findings — Geometry, Concepts, and Springback

Geometry Assessment — Near-Standard 2:1 Ellipsoidal, Favorable for Forming

Confirmed

The specified shell geometry (Ø16.000″, H = 3.900″, R14.080″ crown, R2.675″ knuckle) conforms closely to the standard 2:1 semi-ellipsoidal profile used throughout the pressure vessel industry. At H/D = 0.244 — marginally shallower than the theoretical 2:1 ratio of 0.250 — the draw depth is slightly reduced compared to standard, which is a favorable condition. The knuckle radius at R2.675″ (0.167D) is well above the ASME minimum of 0.06D, significantly reducing the concentration of bending and thinning strain at the knuckle zone. Both characteristics are positive indicators for forming feasibility.

Concept A: Nitrogen Gas Spring System — Feasible for Both Types

Feasible

Integrating nitrogen gas springs into the lower die is the highest-confidence path for both shell types. Commercial nitrogen spring units (e.g., Nitrogen-technology springs in the 50–150 kN range per unit) can be arranged in a circular pattern within the die body, engaging the blank periphery as the punch descends. The progressive force characteristic of nitrogen springs is well-matched to drawing requirements — initial engagement at low force reducing wrinkling tendency, rising through the stroke as material draws in. For a Ø16″ blank in carbon steel, an estimated 6–8 spring units distributed around the periphery would provide adequate and even hold-down force. The key design parameter requiring validation is the initial spring preload relative to the onset of blank movement — too high and the blank tears; too low and wrinkling initiates before enough draw-in has occurred. This is calibrated during tooling trials.

Concept B: Draw-Form — Viable for Type 1, Not Recommended as Primary for Type 2

Conditionally Viable

For Type 1 (0.177″ wall), the t/D ratio of 0.011 sits within the range where free draw-forming without active blank holding has been demonstrated in practice for similar geometries — particularly for single-piece production runs where tooling cost sensitivity is high. The approach relies on careful blank diameter optimisation (excess blank material increases wrinkling risk), tight control of die radii, and lubrication. For Type 2 (0.132″ wall), the t/D ratio falls to 0.00825 — at this thickness relative to diameter, the blank's natural buckling resistance is insufficient to reliably prevent wrinkling at the flange without some form of active restraint. Concept B is assessed as not suitable as the primary approach for Type 2, though it may be trialled as a secondary option if Type 1 trials are successful.

Common Die Cavity Strategy — Single Tool Set for Both Types Feasible

Confirmed

Since both types share identical outer geometry, a single female die cavity (defining the external shell surface) can serve both types. The wall thickness difference (0.177″ vs. 0.132″) is accommodated by the punch diameter — a smaller-diameter punch for Type 1 (thicker wall) and a larger-diameter punch for Type 2 (thinner wall), with both punches sharing the same crown profile. This reduces tooling investment significantly. The nitrogen spring arrangement within the die body would need to accommodate different hold-down force requirements for the two thicknesses — achievable by adjusting spring preload settings between production runs rather than requiring separate spring assemblies.

Springback Assessment — Predictable and Manageable

Low Risk

Springback in shallow ellipsoidal forming is generally predictable and well-understood. The large crown radius (R14.080″) produces low bending strain on the crown surface — springback here is minimal and compensated by a slight reduction in die crown radius (typically 3–5% for carbon steel). The knuckle zone (R2.675″) is the more active springback area due to the tighter bend radius; die knuckle radius should be reduced by approximately 5–8% to compensate, with exact values determined from first-article measurements. No unusual springback conditions are anticipated for either shell type given the standard ellipsoidal geometry.

Blank Material Specification — To Be Confirmed by Customer

Pending Confirmation

The forming analysis above assumes a low-to-medium carbon steel blank (e.g., ASTM A1011 or equivalent) in the specified thicknesses. The final blank material specification has not yet been confirmed by the customer. If stainless steel is required, the higher work-hardening rate and lower friction coefficient will increase nitrogen spring force requirements by approximately 30–40%, and springback compensation values will need to be revised upward. If the material is confirmed as stainless, the Concept B draw-form approach should be deprioritised further — stainless is significantly more prone to wrinkling without active blank control than carbon steel at equivalent thicknesses.

Recommended path forward: Proceed with Concept A (nitrogen gas spring system) as the primary tooling design for both shell types — it offers the highest confidence of consistent, wrinkle-free forming across both Type 1 and Type 2. Design the tooling to allow Concept B (simplified draw-form, no nitrogen springs) to be trialled on Type 1 during first-article press trials, to determine if the simpler approach is viable for that type. Luckee to manage Chinese tooling partner sourcing and verification for the die set manufacture.

Tooling Concept Development for
Ellipsoidal Pressure Vessel Head Shells