As cut EDM on visible land
- Matte gray appearance from recast layer and pitting
- Flow marks may look worse on high gloss expectations
- Requires stoning, polish, or texture spec to match SPI targets
Glossary
EDM Injection Molding: Sinker, Wire & Hole-Drilling EDM
EDM erodes conductive mold steel with controlled sparks. Sinker EDM, wire EDM, and hole drilling EDM each solve different geometry problems in tool making.
Definition
What Is EDM in Injection Mold Tooling?
EDM (electrical discharge machining) erodes conductive materials with controlled sparks.
Electrical discharge machining removes material by vaporizing tiny bits of steel with repeated sparks. Mold makers use it where cutters cannot reach or where steel is already hard.
It covers three processes used in injection mold tooling: sinker EDM for blind cavities and deep ribs, wire EDM for through cut profiles, and hole drilling EDM for small deep holes.
This page is the EDM family overview and how each process fits mold making. Sinker EDM details below apply to ram EDM on cavity and core steel.
Wire EDM is a sibling process that uses a moving wire for through cuts and insert outlines. See the wire EDM page for details. For small deep holes such as wire start holes and vent paths, see hole-drilling EDM . For blind cavities and deep ribs with shaped electrodes, see sinker EDM .
Toolmaking
Sinker EDM in mold making
Sinker EDM cuts blind ribs and pockets; wire EDM cuts through profiles. Recast layer affects finish on both.
- Recast layer may need polish when cosmetics demand gloss.
- Wire and sinker EDM solve different geometry problems.
- Graphite electrodes define sinker EDM before ram burning.
Processes
Three EDM processes for mold geometry
Each process solves a different geometry problem. See the dedicated pages for sinker EDM, wire EDM, and hole-drilling EDM.
Sinker EDM (ram or die sinking): shaped electrode erodes hardened steel for blind cavities, deep ribs, sharp corners, logos, and textures. Most common in cavity and core making. See the sinker EDM glossary entry.
Wire EDM: traveling wire cuts through profiles for inserts, parting plates, ejector pin holes, and sliders. See the wire EDM glossary entry.
Hole drilling EDM (small hole or fast hole): tubular electrode drills deep holes for wire start holes, vent paths, cooling channels, and broken tap removal. See the hole-drilling EDM glossary entry.
Toolmaking
Sinker EDM in mold making
These apply primarily to sinker EDM on cavity and core steel.
Sinker EDM cuts what milling cannot on blind cavities, deep ribs, and sharp internal corners.
Spark erosion details show up on ribs, pockets, and shutoffs when cutters cannot reach hardened steel.
Graphite or copper electrodes define sinker EDM geometry before ram burning.
EDM time and electrode work belong in tooling quotes when CAD shows deep ribs or zero draft shutoffs.
Finish
EDM surface versus polished cavity land
All EDM processes leave a recast layer. The finish strategy below applies to sinker and wire EDM surfaces that will contact plastic.
EDM plus planned polish or texture
- Geometry preserved with controlled final finish steps
- Deep ribs achievable without weakening steel with only thin cutters
- Schedule includes EDM time plus finish labor explicitly
Recast
Recast layer on EDM surfaces
Critical shutoffs and high stress features may need recast removal.
Recast layer (white layer) forms on all EDM surfaces. Thickness depends on amperage and pulse settings.
Roughing passes leave thicker recast; skim cuts reduce it.
Stoning or polishing may be required before high gloss SPI targets or tight shutoffs.
Selection
EDM vs CNC milling
Process choice follows steel hardness, geometry, and finish plan. Full mold steel routing table lives on the EDM vs CNC milling compare page.
EDM often wins when
- Steel is already hardened after heat treatment
- Features need internal corners tighter than standard cutter radius
- Depth to width ratio exceeds typical end mill reach
- Geometry includes narrow deep ribs or blind pockets
CNC often wins when
- Steel is still soft before heat treatment
- Features are open with generous corner radii
- High surface finish is required without heavy post EDM polish
Go deeper
EDM and finish links
Open the process spoke that matches your geometry question.
Sinker EDM
Blind ribs, pockets, and electrodes.
Sinker EDMWire EDM
Through cuts and insert profiles.
Wire EDMHole-drilling EDM
Start holes, vents, and deep holes.
Hole-drilling EDMEDM finish
RFQ notes for EDM texture on parts.
EDM finishSPI A1 finish
Gloss targets after steel geometry is cut.
SPI A1 finishCore
Deep ribs often cut on the core half.
CoreCavity
Cavity side shutoffs and cosmetic lands.
CavityEDM vs CNC milling
Mold steel routing compare guide.
EDM vs CNC millingVisual 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
Wire EDM cutting hardened mold insert profile
Wire EDM cutting hardened mold insert profile
EDM recast layer on mold steel surface close up
EDM recast layer on mold steel surface close up
FAQ
Frequently Asked Questions About EDM
What is the difference between sinker EDM and wire EDM?
Sinker EDM uses a shaped electrode to burn blind features in hardened steel. Wire EDM uses a moving wire to through cut profiles and insert outlines.
Does EDM work on non conductive materials like plastic?
No. EDM requires conductive workpieces. Plastic part geometry is formed by the mold steel cut with EDM or milling.
What is recast layer and do I need to remove it?
Recast is a heat affected layer left by sparks. Remove or polish it when cosmetics, seal shutoffs, or fatigue sensitive steel require clean parent metal.
How much does EDM add to mold lead time and cost?
Depends on rib depth, electrode count, and finish plan. Deep sinker EDM and skim polish add shop hours beyond simple milling.
When should I choose EDM over CNC milling?
Choose EDM for hardened steel, blind ribs, and sharp internal details. Choose milling for open soft steel features with reachable cutters.
Can EDM achieve SPI A1 finish directly?
Not as a single as cut step. Class A gloss needs planned stoning and polish after EDM geometry is complete.
What is hole drilling EDM used for in molds?
Wire start holes, vent paths, cooling line access, and broken tap relief without starting from an open edge.
Does EDM affect steel hardness or wear resistance?
Recast can differ from parent steel hardness. Mold makers manage recast on wear and shutoff faces when the program requires it.
Next step
Confirm EDM scope on your tool
Upload CAD with geometry notes such as deep ribs, sharp corners, and hardened steel to confirm whether EDM is the right path for your program.