When we talk about mold base structure, the first decision is almost always two-plate versus three-plate versus hot runner. The two-plate Type A/B configuration remains the workhorse for the majority of jobs—simple parting line, straightforward ejection, and lower cost. Three-plate Type C/D adds a second parting line to allow pin-point gating at the part center, which matters when gate location is constrained by appearance or warp. For hot runner systems, the mold base must accommodate a manifold plate, and you need to plan for thermal expansion of the manifold—typically 0.2–0.4 mm over a 400 mm span—so the pocket dimensions and locating dowels must leave room for growth. In all cases, the A/B plate thickness should be calculated from cavity pressure, not guessed; a common rule is to keep deflection under 0.05 mm at the cavity face.
Guide pillar layout and ejection design are where many mold base failures originate. For a standard rectangular mold base, four guide pillars are placed at the corners, but on long, narrow parts you should add a fifth pillar or offset the pillar positions to prevent plate skew during opening. Ejection plate thickness follows the ejector force: for a 250 mm × 250 mm mold base, an ejector plate of 20–25 mm is typical, but if you run high ejector stroke or use a stripper plate, go thicker. Also check that the return pins are at least as long as the ejector plate travel plus 5 mm clearance. On hot runner molds, route water lines in the cavity plate away from the manifold pocket—at least 15 mm wall thickness—to avoid hot spots and cracking.
Selection comes down to part geometry, gate type, and production volume. A 24-configuration matrix covers most cases: two-plate A/B for simple parts, three-plate C/D for center gating, and hot runner variants with manifold plates for high-cavitation runs. Before releasing the design, verify plate thickness against injection pressure, confirm guide pillar clearance (H7/g6 is standard), and check that the ejector plate has enough room for the ejector bar. These details save you from rework at T1. For more mold base sourcing and structure references, visit MoldSo at www.moldso.com.