Cavity
Exterior half versus interior half.
CavityGlossary
Mold Core in Injection Molding: Definition, DFM, and Red Flags
The mold core is the interior forming half of the tool. It carries ribs, bosses, ejection, and many shutoffs that affect part quality.
Definition
What Is a Mold Core?
The mold core is the internal forming half of an injection mold, usually on the moving B side.
It shapes interior geometry: ribs, bosses, undercuts, and inside walls.
While the cavity forms the exterior surface, the core defines internal features and often carries most of the ejection system.
Core steel must survive ejector pin contact, vent channels, and sliding shutoffs when side actions exist.
When buyers ask for zero draft on an interior wall, core pull (drag on release) and scuff marks become the negotiation.
Tooling
Core side responsibilities in DFM review
Most ejection and vent issues show up 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.
- Deep ribs and tall bosses on the core side often lack direct cooling. That can extend cycle time and cause localized sink or warp.
- 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
Core side features with no practical cooling path. Long cycle time and uneven shrinkage can follow.
Go deeper
Where this term connects
Each link is a different angle on the same program, not a duplicate definition.
A side and B side
Which mold half mounts where.
A side and B sideVent
Air must leave as plastic enters core 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
Frequently Asked Questions About Mold Cores
What is the difference between a mold core and a cavity?
The cavity forms exterior surfaces customers see. The core forms interiors, ribs, bosses, and inside walls. Ejection pins and many vents usually live on the core half.
Can the core be replaced without replacing the cavity?
Yes. Insert cores and interchangeable pins are standard maintenance paths.
Why does plastic stick to the core side?
Stick or drag often traces to insufficient draft, polish drag on deep ribs, vacuum in blind features, or resin shrink gripping the core. Venting and ejection layout review usually come first.
What is the difference between core pull marks and ejector pin marks?
Core pull marks are scuff or drag on interior walls during release. Ejector pin marks are localized push points where pins contact the part. Both belong in cosmetic acceptance when interiors are visible.
What draft angle is needed on the core side?
Interior walls and ribs need draft aligned with the core pull direction. Exact degrees depend on depth, resin, and texture. Document draft on the drawing and confirm in DFM before steel cut.
Why do sink marks show on the cavity side?
Thick boss roots on the core side pull material and can show as sink opposite the boss on the exterior.
Does core steel need to match cavity steel?
Not always. Wear and polish requirements differ by region and can use inserts.
How does core side venting work?
Vents let air escape as melt fills ribs and pockets on the core side. Blind ribs without a vent path are a common short fill or burn risk.
Next step
Review core side DFM early
Upload CAD with rib and boss stacks visible before tooling deposit.