Pins for Ejector Block

Pins for ejector block (also called ejector plate guide pins or return pins) serve a structural function in the ejection system — they guide the ejector plate assembly during its forward and return stroke, ensuring all ejector pins move in perfect alignment. Unlike ejector pins that contact the molded part, block pins contact the mold plate face (A-plate or B-plate) during mold close to push the ejector plate back to its home position. They also absorb the closing force impact, protecting the ejector pins from compressive damage. Typically 4 block pins are used per mold (one at each corner of the ejector plate).

Ejector block pins are available in two primary materials: (1) S45C (AISI 1045) carbon steel — HRC 28–32, the standard choice for general-purpose molds. The lower hardness is intentional — the block pin should be softer than the mold plate it contacts, acting as a sacrificial wear component. (2) SUJ2 (AISI 52100) bearing steel — HRC 58–62, used for high-cycle molds (1 million+ cycles) where the standard S45C pins would wear too quickly. Both materials are available with or without nitrided surface treatment. Block pins are not available in M2 or H13 because these materials are too hard and would damage the mold plate contact surface.

Ejector block pins are installed by press-fit into the ejector plate (interference fit of 0.01–0.02mm). The pin head sits in a counterbore in the ejector retainer plate, and the shaft extends through the ejector plate and protrudes beyond the ejector pin tips. Installation steps: (1) Ream the ejector plate bore to H7 tolerance, (2) press-fit the pin using an arbor press (do not hammer — this damages the pin head), (3) verify the pin protrusion length — it must be 2–3mm longer than the longest ejector pin to ensure the ejector plate is fully returned before mold close. The protruding tip is typically flat or slightly crowned (R50–R100mm radius) to distribute the closing impact force.