Definition
Former holes (hole-forming features, core pins) are the parts of the mold whose job is to create the holes, bores and openings called for by the part design. Where the Cavity forms the outer shape, a former hole is the positive steel — a pin or core — that the plastic flows around, leaving a hole in the finished Molded Part.
How they are built
- Integral (same formation): machined directly into the core or cavity block when the hole runs in the mold's opening direction, so it clears as the mold opens.
- Sliders / side actions: when a hole runs across the opening direction (a side hole or undercut), the hole-forming pin is mounted on a slider (side-action core) that retracts sideways before ejection, then returns for the next shot.
Why they matter
- Function & assembly: molded-in holes for screws, snaps, shafts and ports avoid secondary drilling and support clean Design for Assembly.
- Flow & defects: melt splitting around a pin rejoins on the far side, forming a weld line; pins also need support, or they deflect under Melt pressure and shift the hole. Worn or mismatched pins let plastic seep and create Flash around the hole.
- Cooling & wear: thin pins run hot and wear fastest, so they are a common maintenance and dimensional-control point.
Design notes
Holes are placed and sized with draft and steel strength in mind; through-holes are often formed by a pin meeting the opposite face ("shut-off"), while blind holes use a single supported pin. Their location relative to the Parting Line decides whether a simple integral pin or a slider is needed.
Related terms
- See also: Cavity, Molded Part, Parting Line, Design for Assembly, Flash
What are former holes in a mold?
Mold features — pins or cores — that form the holes and openings in a molded part; the plastic flows around the steel pin, leaving a hole when the part is ejected.
How are holes made in injection-molded parts?
By hole-forming pins or cores in the mold: integral pins for holes in the opening direction, and sliders (side-action cores) that retract sideways for holes or undercuts running across the opening direction.
Why do molded holes sometimes have weld lines or flash?
The melt splits around the pin and rejoins on the far side, leaving a weld line; if the pin is worn, unsupported or mismatched to its mating face, plastic seeps past it and forms flash around the hole.