Cold runner
- Lower manifold cost on many programs
- Produces trim scrap each cycle
- Color changes purge through frozen slug volume
Glossary
What Is a Runner? Injection Molding Explained
Runners distribute melt to each gate and set fill balance on multi cavity tools.
Glossary
Runners carry plastic from the sprue to each gate
Balance and scrap policy start in the runner layout.
Runner geometry sets how evenly each cavity fills on multi impression tools. Asymmetric layouts need careful sizing so the last cavity to fill still packs.
Cold runners become solidified slug that is trimmed and often reground. Hot runners keep plastic molten in the manifold and reduce feed path scrap.
Runner volume appears in resin purchase planning even when buyers only weigh the part.
Glossary
How runner shows up on real programs
Library photos illustrate molding and tooling context for this term.
- Use the term in RFQs with geometry and material context, not in isolation.
- Link to the design guide when the term affects steel or part design.
- Upload CAD when the term changes tooling scope or piece price.
Systems
Cold runner versus hot runner framing
Neither is free. The choice is scrap, capital, and color change behavior.
Hot runner
- Higher manifold capital and maintenance
- Less feed path scrap per shot
- Better fit when gate location must stay on cosmetic faces
Go deeper
Where this term connects
Each link is a different angle on the same program, not a duplicate definition.
Cold runner
Frozen slug trimmed each cycle.
Cold runnerHot runner
Heated manifold keeps plastic molten.
Hot runnerGate
Restricted entry where the runner meets the cavity.
GateDesign guide hub
Runner design and gate location rules.
Design guide hubVisual 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
Runner questions
Who decides runner layout?
Mold designers set runner and gate detail during tooling design on qualified programs.
Can runner balance be fixed after T1 samples?
Minor gate tweaks are possible. Large imbalance often needs steel change.
Does regrind change runner economics?
Regrind policy affects how much cold runner scrap hurts material cost.
Next step
Link runner talk to gate location
Upload CAD when fill imbalance or weld lines trace to feed path design.