When we talk about mold structure, the first decision is parting line and cavity layout, because everything downstream — gating, ejection, cooling — hangs off that choice. For a typical two-plate injection mold, a balanced runner layout keeps fill pressure within roughly 5–10% across cavities, and that directly affects shrinkage consistency. On the steel side, P20 (about 30 HRC) suits general-purpose cores and cavities, while 718H or S136 (roughly 50 HRC after heat treatment) is the norm for high-polish or corrosive-resin jobs. Wall thickness in the cavity insert should not drop below about 1.5 times the cavity depth without adding support pillars, or you will see deflection under injection pressure and flash at the parting line. These are not textbook numbers — they come from the shop floor, and ignoring them shows up as scrap.
Process planning is where structure meets manufacturability. A cavity block with deep ribs is usually roughed, stress-relieved, semi-finished, heat-treated, then finish-machined — skipping the stress relief is a common cause of dimensional drift after heat treatment. For fits, guide pins typically run H7/g6, and ejector holes H7 with a clearance of 0.02–0.05 mm depending on diameter; too tight and you gall the bore, too loose and you get misalignment and wear. Cooling lines should sit about 1.5–2.5 times their diameter from the cavity surface — closer cools faster but risks temperature spikes and weld-line issues. Venting deserves equal attention: 0.02–0.03 mm deep vents at the parting line and 0.01–0.02 mm at ejector pins prevent burn marks and short shots without flashing.
Assembly and tryout are the final reality check. Blue-check the parting surfaces, verify shut-off heights, and confirm ejector plate travel before the first shot. During tryout, start with short shots to map the fill pattern, then adjust holding pressure and cooling time in small steps — a 10% change in holding pressure can shift part weight by 1–2%, which tells you a lot about gate freeze. If flash appears, check clamp tonnage and parting-surface contact before touching the process window. Debugging is mostly about isolating one variable at a time. For more mold sourcing and supplier information, visit MoldSo at www.moldso.com.