Single cavity first tool
- Lower upfront tooling before volume proves out
- Simpler fill balance tuning
- Common when geometry is large or cosmetic risk is high
Glossary
Mold Cavity in Injection Molding: Definition, Single vs Multi, and Cosmetics
The mold cavity is the exterior forming half of the tool. Surface finish and layout drive cosmetics, cycle, and piece price.
Definition
What Is a Mold Cavity?
The mold cavity is the stationary half of the mold, usually the A side, that forms the exterior of the part.
It is the impression into which molten plastic is injected. Cavity surface finish sets the ceiling for exterior cosmetics after texture, resin, and pack.
While the core forms internal geometry such as ribs, bosses, and inside walls, the cavity defines the external shape customers see and touch.
Multi cavity tools duplicate the same cavity to raise output per cycle. Family tools combine unlike parts in one mold base when volumes justify shared tooling.
Tooling
Exterior surfaces start on cavity steel
Cavity side examples: polished land, parting lines, and multi cavity impressions.
- Mark Class A faces on CAD before SPI or texture scope is fixed.
- Multi cavity tools need fill balance and venting per impression.
- Pair cavity notes with core side rib and ejection review.
Design
What to show when cavity side cosmetics matter
Cavity side cooling removes most of the heat from the part. Uneven cooling can drive differential shrinkage, warpage, and longer cycles.
Class A surface identification on the CAD model
Texture spec or SPI finish target if already selected
Parting line location relative to visible edges
Draft direction tied to the cavity pull
Cooling layout notes when large cosmetic faces need uniform shrink
Layout
Single cavity versus multi cavity
Cavity count is a capital versus piece price trade, not a free multiplier. Multi cavity tools also need balanced runner geometry, consistent cooling cavity to cavity, clamp force sized to total projected area, and vent paths for every impression.
Multi cavity production
- Lower piece price when annual volume absorbs tooling
- Requires balanced runners and consistent cooling across impressions
- Each cavity needs ejection and venting justified in DFM
Cavity side red flags
Patterns to fix before cavity steel is cut
These issues often show up as flash, drag, burn, or cosmetic witness at T0.
Deep draw with insufficient draft leading to drag marks on cavity walls
Thin unsupported cavity steel that can deflect under injection pressure
Complex parting lines with flash risk at mismatch
Polished cavity land near the gate without a plan for gate blush or flow marks
No vent path at last fill areas risking burn or short shot on the cavity face
Go deeper
Where this term connects
Each link is a different angle on the same program, not a duplicate definition.
Core
Interior half and ejection context.
CoreA side and B side
Which mold half mounts where.
A side and B sideVent
Air must leave as plastic fills the cavity.
VentFamily molding
Unlike parts sharing one mold program.
Family moldingVisual reference
Photos from molding and tooling programs
Images are selected from the NetProto Forge media library for this topic.
Polished mold cavity surface on exterior housing tool
Polished mold cavity surface on exterior housing tool
Parting line on injection mold cavity half
Parting line on injection mold cavity half
Multi cavity mold impressions in cavity plate
Multi cavity mold impressions in cavity plate
FAQ
Frequently Asked Questions About Mold Cavities
What is the difference between a cavity and a core in injection molding?
The cavity forms exterior surfaces. The core forms interiors, ribs, and bosses. Many cosmetic faces and texture specs target cavity steel.
What is single cavity versus multi cavity molding?
Single cavity molds one part per cycle. Multi cavity molds duplicate impressions to raise output. Multi cavity adds runner balance, cooling consistency, and venting work per impression.
What is the cavity side in injection molding?
The cavity side is the mold half that forms the outside of the part. Shops often call it the A side, but confirm on the tool design print.
Is the cavity always the A side?
Shops often call the cavity the A side, but always confirm on the tool design print.
Can cavities be swapped as inserts?
Insert cavities are common when only one region of the tool changes between revisions.
Does cavity polish equal final part gloss?
Steel polish sets the ceiling. Texture, resin, and pack also shape what you see.
How does fill balance work in multi cavity molds?
Runners must deliver equal flow to each impression. Imbalance shows as size or weight variation part to part until steel or process is corrected.
How is cavity steel selected?
Choice depends on polish retention, wear at the gate, corrosion exposure, and repair strategy. Discuss pre hardened, hot work, or stainless mold tooling options with your mold maker rather than naming grades on the RFQ alone.
Next step
Mark visible surfaces on CAD
Upload CAD with Class A faces and parting line notes before steel cutting begins.