How to Set Up Secondary Ejection with Two-Step Center Pins and Stepped Sleeves

Understanding Secondary Ejection

Standard (single-stage) ejection pushes all ejector components forward at the same time. For most parts, this works fine. But certain part geometries — deep draws, undercuts that require collapsing cores, or parts with features at different heights — need the part to be released in two separate stages.

Two-step center pins are the mechanical heart of secondary ejection systems. The first step (smaller diameter section) pushes the part off the core in Stage 1. Then, the second step (larger diameter section) engages and ejects the part completely in Stage 2. This prevents the tearing, warping, or sticking that would occur if the part were ejected in a single stage.

Getting the timing and force distribution right is critical. This guide walks through the design process step by step.

Step 1: Design the Ejection Sequence

Two-stage ejection is a standard technique in injection mould design. The timing mechanics follow mechanical advantage principles applied to ejector plate sequencing.

Secondary ejection splits the demolding process into two distinct stages:

Step 2: Size the Center Pin Steps

The two-step center pin has two diameter sections that correspond to the two ejection stages:

• First step (larger diameter): Guides through the sleeve bore during primary stroke. Must maintain clearance with sleeve ID at all temperatures.
• Second step (smaller diameter): Extends beyond the sleeve tip during secondary stroke. Step height = secondary ejection stroke length.

Step 3: Set Timing Clearances

Common Pitfalls and Solutions

Timing Mechanism Options

The two ejection stages must be precisely timed so that Stage 2 doesn't begin until Stage 1 has completed. There are three common timing mechanisms:

Common Design Mistakes

Secondary ejection systems are more complex than single-stage designs. These are the most frequent errors:

• Insufficient Stage 1 travel: If the part hasn't fully cleared the undercut or core feature before Stage 2 begins, the part tears. Allow at least 2mm of clearance beyond the deepest feature before triggering Stage 2.
• Mismatched pin and sleeve materials: Two-step pins experience higher bending stress than straight pins due to the diameter transition. Use SKD61 for toughness, not SKH51 — the same shoulder fatigue issue applies here.
• Ignoring return stroke sequence: The ejector plates must return in the reverse order (Stage 2 retracts first, then Stage 1). If both retract simultaneously, the stepped pin can jam in the sleeve bore.
• Undersized second step diameter: The second step must provide enough contact area to distribute ejection force without marking the part. Minimum wall contact length: 2× part wall thickness.

Pairing Two-Step Pins with Sleeves

Two-step center pins work inside stepped ejector sleeves. The sleeve's step diameter must match the pin's step diameter with proper clearance at both diameters. This means two clearance specifications per assembly:

• Clearance at small diameter (Stage 1): Standard 0.010–0.020 mm
• Clearance at large diameter (Stage 2): Standard 0.010–0.020 mm

Both clearances must be verified independently. A mismatch between the two clearances causes the sleeve to bind or wobble during transition between stages. For critical applications, use factory-matched sets.

Secondary Ejection vs. Alternative Approaches

Two-step center pins aren't the only way to handle difficult ejection scenarios. Here's how they compare to alternative approaches:

Two-step center pins are the best choice when the part has cylindrical features at multiple diameters that need sequential release. For other ejection challenges, the alternatives listed above may be simpler and more cost-effective.

Specification Template for Two-Step Pins

When ordering two-step center pins, provide all of the following dimensions:

• Small diameter (D1): The diameter that forms the first (deepest) section of the part feature
• Large diameter (D2): The diameter that forms the second (shallower) section
• Step position (L1): Distance from the pin tip to the diameter transition
• Overall length (L): Total pin length from head to tip
• Head diameter and thickness: Must match the ejector plate pocket
• Fillet radius at step: Specify R ≥ 0.5mm to prevent shoulder fatigue
• Material: SKD61 recommended for toughness at the step shoulder
• Mating sleeve bore ID at D1 and D2: For clearance verification

Incomplete specifications are the #1 cause of non-conforming two-step pins. The step position (L1) is especially critical — a 0.5mm error in step position causes the diameter transition to misalign with the part feature, creating a visible step mark on the molded part.

Cost Analysis for Two-Step Systems

Secondary ejection systems are more expensive than single-stage systems, both in initial tooling and ongoing maintenance. Here's a realistic cost comparison:

The 2–3× higher cost of secondary ejection is justified when the part geometry makes single-stage ejection impossible or when single-stage ejection causes unacceptable part quality (tearing, sticking, warpage). Do not use secondary ejection "just in case" — it adds complexity and maintenance burden that isn't warranted for simple part geometries.