Injection mold ejection systems account for 15-25% of mold manufacturing cost and are the most common source of production downtime after cooling issues. A poorly designed ejector system produces cosmetic defects (ejector pin marks, surface blush), dimensional distortion, or outright part sticking. This guide covers the engineering parameters behind ejection system design: pin sizing, layout density, force calculation, and alternative ejection methods.
Ejection Force Calculation
Ejection force depends on three factors: shrinkage grip (the part contracts onto the core during cooling), surface friction between the polymer and the mold steel, and the draft angle of the cavity. The basic equation is: F_eject = P_shrink × A_contact × (mu × cos(alpha) – sin(alpha)) / (1 – mu × tan(alpha)), where P_shrink is the shrinkage pressure (3-7 MPa for unfilled PP/PE, 5-15 MPa for semicrystalline polymers like POM or PA), A_contact is the contact area, mu is the friction coefficient (0.3-0.5 for steel/polymer dry contact), and alpha is the draft angle in degrees.
| Polymer | Shrinkage Rate (%) | Typical Shrinkage Pressure (MPa) | Min Draft Angle (deg) |
|---|---|---|---|
| PP (unfilled) | 1.5-2.5 | 3-5 | 0.5-1.0 |
| ABS | 0.4-0.7 | 4-6 | 0.5-1.0 |
| PA66 (unfilled) | 1.5-2.0 | 5-10 | 1.0-2.0 |
| POM | 1.8-2.5 | 8-15 | 1.0-2.0 |
| PC | 0.5-0.7 | 5-8 | 0.5-1.5 |
| PBT+30%GF | 0.3-0.8 | 6-10 | 1.0-2.0 |
For a core 100 mm tall with a 50 mm × 50 mm cross-section and 1 deg draft, using PP, the ejection force is approximately 3-5 kN. The total machine ejector force should be 2-3× the calculated ejection force to account for uneven shrinkage and mold wear over the production run.
Ejector Pin Sizing and Layout
Standard ejector pins range from 1 mm to 25 mm diameter. The rule of thumb: use the largest diameter that fits the available space. Pin wall thickness (the material between the pin OD and the core surface) should be at least 3-5 mm for pins under 6 mm diameter, and 5-8 mm for larger pins. Pin spacing should not exceed 40-60 mm in flat areas. Each pin should be positioned within 10-15 mm of a rib, boss, or side wall to prevent local deflection of the part.
The total cross-sectional area of all ejector pins should be 1-3% of the projected part area. For a part with 0.1 m2 projected area, use 10-30 cm2 of total pin cross-section. This prevents the pins from punching through the part or creating visible surface indentations.
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Alternative Ejection Methods
Ejector sleeves are preferred when ejecting around a fixed core pin (e.g., cylindrical bosses). The sleeve ID must be 0.1-0.3 mm larger than the core OD. Stripper plates provide the most uniform ejection for thin-walled parts or parts with a large flat surface area. They distribute the ejection force across the entire part edge, eliminating pin marks entirely. The stripper plate thickness should be at least 20-30 mm for mold widths under 400 mm, and 30-50 mm for larger molds.
Lifters and slides with built-in ejection handle undercuts. For a lifter, the angle should be 5-15 deg, and the lifter body width at least 10 mm. Hydraulic ejection cylinders are used when the machine ejector force is insufficient or when a delayed ejection sequence is needed. Typical hydraulic cylinder bore: 32-63 mm at 10-15 MPa working pressure.
FAQ
How many ejector pins does a typical mold need?
A rough guideline: one ejector pin per 25-40 cm2 of projected area for flat parts, one per 10-15 cm2 for parts with deep ribs. A 300 x 200 mm flat panel (600 cm2) needs 15-24 pins. Place pins near ribs, bosses, and side walls first.
What causes ejector pins to break during production?
Pin breakage is almost always caused by insufficient draft angle, misalignment between the pin and the ejection plate, or pins that are too small for the required ejection force. Pins under 3 mm diameter are particularly prone to buckling. Increase pin diameter or add more pins to distribute the force.
What is the standard ejector pin clearance?
Standard clearance between the ejector pin and the pin hole is H7/f7 fit (0.01-0.04 mm for a 6 mm pin, 0.02-0.06 mm for a 16 mm pin). Tighten to H7/g6 for high-cavity-pressure applications to prevent flash around the pin.
When should you use a stripper plate instead of ejector pins?
Use a stripper plate when: (1) the part is thin-walled (<1.5 mm) and pins would puncture it, (2) the part has a large flat surface where pin marks are cosmetically unacceptable, or (3) the core has a deep draw that creates high shrinkage grip. Stripper plates also prevent part warpage during ejection.
See our Injection Mold Design Guide and Mold Problem Troubleshooting Guide. Industry standards: SPI Mold Classification and DME ejector pin standards.
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Standards & References
The following industry standards are referenced in this article: