Core Pins for Injection Molds
Core pins form the internal geometry of a molded part - holes, bosses and undercuts - and they take injection pressure on every shot.
Selection comes down to hardness class and how the pin is retained. Both decide whether the pin runs for years or comes out of the press every few months.
Start with the function
Hardness class
Standard, high-hardness and copper alloy core pins solve three different problems: general wear, abrasive resin, and hot spots.
Ejection around the core
An ejector sleeve slides over the core pin and pushes the part off without drag marks - the answer for thin-wall bosses and standoffs.
Retention
How the pin is held in the retainer plate decides whether it seats square shot after shot, and whether it can come back out for service.
What to look for
Standard hardness. General-purpose molding where wear is moderate. Easier to machine and modify in house, which matters when a pin has to be shortened or stepped on the bench.
High-hardness. Specified for abrasive resins - glass-filled nylon, PBT, PC/ABS blends - and for high-cavity pressures. The harder surface resists erosion at the pin tip and scoring along the body.
Copper alloy. The heat-transfer specialists. Copper alloy pins pull heat out of hot spots at the core tip, which cuts cycle time and prevents localized shrinkage or sink. They are the go-to for thin-wall parts and hot-runner molds where cooling is the bottleneck.
Retention: counterbore versus retainer. A plain counterbore seats the pin head and stacks tolerances - if counterbore depth or head thickness is off, the head is forced against the bottom and the pin pulls out of square. A dedicated core pin retainer (catalog designation CPR) captures the head instead of pinching it, so the pin seats consistently and the toolmaker can pull it without levering against the plate.
Retainer material and sizing. Stock core pin retainers are commonly 12L14 steel with black oxide finish - free-machining, so it holds the tolerance the pin head needs, and corrosion-resistant in storage. Size by pin head diameter, retainer plate thickness and pin length.
Ejector sleeves. Nitrided parallel sleeves in inch and metric, with through-hard versions for extreme cycles and oversize sleeves for hole repair. Matching the sleeve ID to the core OD and the sleeve OD to the plate bore is the critical fit.
Tell us the resin, the cycle count and the core geometry - we will recommend the grade. Request a quote or call 814-438-7657.