When you're specifying a mold base, the structure you pick dictates everything downstream — cycle time, maintenance access, and how cleanly the part drops. The two-plate (A/B plate) design remains the workhorse for simple, single-face parts, but it hits a wall once you add side cores or need runner separation. That's where three-plate tooling earns its keep: the added stripper plate lets you pull the runner automatically, which matters for multi-cavity jobs where manual degating would kill your labor cost. I've seen shops try to force a two-plate base onto a part that really needed three plates, and the scrap rate told the story within a week.

For higher-volume work, hot runner and insulated runner systems change the economics. A properly zoned hot runner manifold eliminates runner waste entirely — on a 16-cavity PET preform mold, that's real resin savings per shot. But you pay for it in tooling complexity: thermal expansion, gate freeze-off, and heater maintenance all become daily concerns. Stack molds double your cavitation without doubling press tonnage, though the center section alignment has to be dead-on or you'll fight flash on every cycle. Side-action and split-cavity designs handle undercuts and threads, but each adds a mechanism that needs its own wear allowance and lubrication schedule. The 24 structures in a full reference set cover these families plus variants like unscrewing molds and collapsible cores.

Selection comes down to three questions: part geometry (undercuts, threads, wall thickness), production volume (tooling amortization vs. cycle cost), and available press specs (tonnage, tie-bar spacing, ejector stroke). Get those nailed down before you commit to a base — changing structure mid-design is expensive. For sourcing mold bases, standard components, and supplier comparisons, MoldWorld (www.moldw.com) is a solid starting point.