Cavity
Exterior half versus interior half responsibilities.
CavityGlossary
What Is a Mold Core? Injection Molding Explained
The core half carries interior detail, ejection, and many shutoffs that affect part quality.
Glossary
The core forms ribs, bosses, and the inside of housings
Ejection, venting, and shutoffs concentrate on the core half.
Core steel must survive repeated ejector pin contact, vent channels, and sliding shutoffs if side actions exist.
Thin cores between ribs or deep bosses are snap risks during fill if venting or steel support is weak.
When buyers ask for zero draft on an interior wall, core pull and scuff marks become the negotiation.
Tooling
Core side responsibilities in DFM review
Most ejection problems are visible on the core before the first sample ships.
- Pin locations should land on hidden faces when cosmetics are strict.
- Deep cores need draft or split lines that buyers accept on the drawing.
- Insert cores can localize wear without rebuilding the entire cavity block.
Core side red flags
Patterns NetProto DFM calls out on interior geometry
Fix these in CAD before approving steel cut.
Blind ribs with no vent path to atmosphere
Boss stacks thicker than the primary wall without coring
Sharp internal corners that drive stress and polish drag
Undercuts on the core pull without side action budget
Go deeper
Where this term connects
Each link is a different angle on the same program, not a duplicate definition.
Vent
Air must leave as plastic enters core side details.
VentEjector pin
How parts release from the core after cooling.
Ejector pinVisual reference
Photos from molding and tooling programs
Images are selected from the NetProto Forge media library for this topic.
Injection molded production parts on a tray
Injection molded production parts on a tray
Engineer reviewing a molded part drawing
Engineer reviewing a molded part drawing
Plastic housing components from a steel tool
Plastic housing components from a steel tool
FAQ
Core questions
Can the core be replaced without replacing the cavity?
Yes. Insert cores and interchangeable pins are standard maintenance paths.
Why do sink marks show on the cavity side?
Thick boss roots on the core side pull material and show as sink opposite the boss.
Does core steel need to match cavity steel?
Not always. Wear and polish requirements differ by region and can use inserts.
Next step
Review core side DFM early
Upload CAD with rib and boss stacks visible before tooling deposit.