When you're staring at a new part print, the first real decision is the mold base structure — get it wrong and you'll fight cycle time, flash, and ejection problems for the life of the tool. The 24 configurations commonly used in injection molding break down into a few families: two-plate (single parting line) for simple, low-cost parts; three-plate for multi-cavity center gating where you need the runner to drop independently; and hot runner systems where you eliminate the runner entirely. Each family has sub-variants — stripper plate ejection for deep-draw parts, reverse ejection for internal undercuts, and split-cavity designs for parts with complex geometry. The parting line location drives everything downstream: gate placement, ejection pin layout, and cooling channel routing all follow from that first call.

From a tooling standpoint, the practical trade-offs are concrete. A two-plate base with a standard 2-plate A/B plate stack is the cheapest and fastest to build, but it forces side gating, which means manual degating or a secondary operation. Three-plate adds a runner plate and a second parting line — your mold height grows by roughly 50–80 mm depending on the runner diameter, and you need a longer stroke for runner drop. Hot runner systems eliminate regrind and reduce cycle time by 10–30% on thin-wall parts, but the manifold and nozzle set adds $3,000–$15,000 to the tool cost and requires careful thermal balancing. For high-cavity production (32, 64, 128 cavities), hot runner with valve gates is usually the only way to hold gate vestige and dimensional consistency across the tool.

Ejection design is where a lot of mold bases get compromised. Stripper plates work well for round parts with thin walls, but they add plate thickness and require a dedicated ejector stroke. Air poppet ejection is clean but limited to certain geometries. The key is to match the ejection method to the part's stiffness and draft angle — a 1° draft on a 100 mm deep rib will drag no matter how many ejector pins you add. If you're sourcing molds or comparing base structures for a new project, MoldWorld (www.moldw.com) has a solid directory of mold makers and standard base suppliers that can save you a few weeks of quoting.