The most convenient hole shape is a circular hole. The core pin required, is a common part available in many different sizes and materials. Using core pins also offers some flexibility as replacing a core pin for a slightly bigger or smaller one often only requires minor tool modifications.
Obviously, holes can’t always be circular, and they don’t need to be. Differently shaped holes can be made using custom cores.
From a tool manufacturing point of view, it’s easiest to create a hole with a centerline parallel to the draw direction of the tool. The simple construction of a static core makes it sturdy, low-maintenance and relatively cheap.
Holes with an axis that isn’t parallel to the mold opening direction, are mostly made using retractable pins or split tools. Core pins should be draw polished and include draft to facilitate ejection. In some cases, retractable cores can be avoided: if the part design allows for an extreme taper in the wall, a hole perpendicular to the draw direction can be formed by the main static core.
A hole may be a through hole (or ‘thru-hole’) or a blind hole. A through hole goes all the way through a part's wall. In other words, there’s an opening on both sides. A blind hole, however, has a specific depth, it doesn’t break through to the other side of the workpiece.
Whether the hole goes all the way through or not makes a big difference from a manufacturing point of view. If it does, the core can be supported on both ends. The longer and/or thinner a core is, the more important this is. A core supported on both sides is less likely to bend or even break during injection molding.
When molten material flows around a core pin during injection molding, a weldline occurs on the opposite side of where it first reaches the core. If weldlines are not permissible due to strength or appearance requirements, holes may be partially cored to facilitate drilling as a post-molding operation.