Sinker EDM
- Blind cavities and deep ribs without through access
- Shaped electrode copies three dimensional detail
- Common on cavity and core after heat treat
Glossary
Sinker EDM for Injection Molds: Electrodes, Ribs & Recast Layer
Sinker EDM uses a shaped electrode to spark erode blind details in hardened mold steel.
Definition
What Is Sinker EDM?
Sinker EDM (ram or die sinking EDM) erodes hardened steel with a shaped electrode that descends into the workpiece.
Each spark removes a tiny bit of steel. The electrode is the negative of the cavity detail being cut.
Mold makers use sinker EDM for blind pockets, deep ribs, sharp internal corners, logos, and textures where milling cannot reach after heat treat.
Wire EDM and hole drilling EDM are sibling processes. See theEDMhub for the full family overview.
Toolmaking
Shaped electrodes burn blind mold details
Ram EDM on hardened cavity and core steel.
- Deep ribs and pockets often need sinker EDM after heat treat.
- Skim passes manage recast before polish.
- Electrode design sets schedule and cost.
Electrodes
Electrode materials and design
Electrode count and skim strategy drive mold build hours.
Graphite electrodes are common for roughing and many detail burns
Copper or copper tungsten may be used when fine detail or wear resistance matters
Multiple electrodes often sequence from rough to skim to control recast layer
Applications
Typical sinker EDM on molds
These features often appear on core and cavity steel, not on the plastic part drawing alone.
Deep ribs and narrow pockets on the core half
Sharp internal corners tighter than cutter radius
Shutoffs and details on hardened cavity inserts
Texture or logo cavities that follow electrode shape
Finish
Recast layer after sinker EDM
Plan polish when plastic will touch the burned surface.
Rough burns leave thicker recast; skim passes reduce it
Cosmetic lands may need stoning before SPI gloss polish
See EDM finish and SPI spokes when RFQ defines texture or gloss
Versus wire
Sinker EDM vs wire EDM
Pick the process that matches open versus blind geometry.
Wire EDM
- Through cuts and insert outlines with a traveling wire
- Best when the path can enter and exit the steel block
- See the wire EDM glossary for slot and insert work
Go deeper
EDM family links
Return to the hub when you need all three processes in one view.
EDM overview
Sinker, wire, and hole drilling in one place.
EDM overviewWire EDM
Through cuts and insert profiles.
Wire EDMHole-drilling EDM
Small deep holes and wire starts.
Hole-drilling EDMEDM finish
Part surface after electrode texture.
EDM finishCore
Many ribs are burned on the core half.
CoreVisual reference
Photos from molding and tooling programs
Images are selected from the NetProto Forge media library for this topic.
Graphite sinker EDM electrode for mold cavity detail
Graphite sinker EDM electrode for mold cavity detail
Sinker EDM burning deep rib in mold core steel
Sinker EDM burning deep rib in mold core steel
EDM recast layer on sinker burned mold surface
EDM recast layer on sinker burned mold surface
FAQ
Frequently Asked Questions About Sinker EDM
What is the difference between sinker EDM and wire EDM?
Sinker EDM burns blind features with a shaped electrode. Wire EDM cuts through the part with a moving wire.
Why use sinker EDM instead of CNC milling?
Hardened steel, blind ribs, and sharp internal corners often need EDM when cutters cannot reach safely.
What is recast layer on sinker EDM?
A heat affected layer left by sparks. Skim burns and polish manage it on cosmetic or shutoff faces.
Graphite or copper electrode?
Graphite is common for productivity. Copper alloys help some fine detail and wear sensitive burns.
Does sinker EDM appear on my part quote?
Deep rib and pocket steel work belongs in tooling scope when CAD shows features that need burning.
Can sinker EDM achieve SPI A1 gloss?
Not in one step. Geometry is burned first, then stoned and polished to SPI targets.
Next step
Quote tooling with EDM features
Upload CAD with deep ribs, sharp corners, and hardened steel notes so sinker EDM scope is visible early.