An undercut prevents a casting from leaving the die in its main pull direction. In high-pressure aluminum die casting, it may require a moving side core, also called a slide, core pull or side action. This raises die cost, yet may replace CNC machining. The right choice depends on part function, tool complexity, cycle time and repeat cost.
This article covers slides in a conventional aluminum die. It is not the same as a multi-slide hot-chamber process used for many small zinc castings; tool layout, alloy and machine choice differ.
Which Die Cast Features Require a Side Core?
A fixed core can form a hole when it withdraws as the die opens. A movable core is needed when the feature locks the part across that path. Before quoting, identify each pull direction rather than count features alone.
Common die casting undercuts include:
- Closed side holes that run parallel to the parting line
- Lateral grooves or recessed connector openings
- Inward-facing lips, hooks and retention features
- Side bosses or internal cavities that trap the casting
Features facing the same way may share one die casting slider if their position, travel and shutoff layout permit it. Opposing features normally need separate core pulls. Four holes do not always mean four slides, while two holes in different directions may need two mechanisms.
Moving the parting line through an external feature may remove its undercut. Flash, trim access, appearance and tolerance still need review. See how parting lines and ejector pin marks affect functional surfaces.
How Do Side Cores Increase Tooling and Part Cost?
A side core needs a guided slide body, wear surfaces, locking support, actuation and a shutoff that resists metal pressure. Cooling, runners and ejector pins must fit around it. The layout may enlarge the die or limit cavity count.
The main die casting tooling cost drivers are:
- Pull directions: Each independent direction may need another slide assembly.
- Core size and travel: Large cores or long strokes need more support and force.
- Actuation: Angle pins suit many short strokes. Hydraulics give controlled timing for some top or long-travel slides but add controls and service.
- Shutoff and tolerance: Thin shutoffs and critical locations need precise fitting.
- Die layout: Slides compete with cooling, gating and ejection and may affect die size or cavity count.
- Cycle and service: Core movement can add time; guides, locks and shutoffs need planned care.
A slide does not always create the higher total cost. A cast side hole may remove drilling, fixtures and inspection from every part, offsetting higher tooling over repeat orders. For low volume or a tight bore, cast a pilot and finish it by CNC. Define the datum and machining allowance for die cast parts before quoting.
Avoid a generic “price per slide.” Cost depends on die size, stroke, cored area, life, tolerance and service access. The quote should show each pull direction and what it forms.
For a sound comparison, ask the supplier to quote both the side-core route and the cast-pilot-plus-machining route against the same annual volume.
Should the Undercut Be Redesigned, Machined or Cast With a Slide?
The lowest tool quote may not give the lowest program cost. Compare annual volume, tolerance, appearance, function and die life.
| Option | Usually suitable when | Main trade-off |
|---|---|---|
| Rotate the feature | It can follow the main pull | May affect assembly |
| Move to the parting line | Die halves can split it | Review flash and mismatch |
| Open it to an edge | A closed feature is not vital | May change strength or looks |
| Cast a pilot; machine | Volume is low or tolerance tight | Adds repeat cost |
| Use a side core | Volume supports added tooling | Adds die complexity |
| Split the part | Assembly remains acceptable | Adds a joining step |
Approve a slide when the repeat work and risks it removes are worth more than its tool complexity. Include fixtures, inspection and scrap risk—not only CNC minutes. A near-net feature may still need machining for a precision fit, thread or seal.
Review this while the CAD can change. The die casting DFM checklist brings draft, wall thickness, parting line, ejection and machining into the same review.
Review Undercuts Before Tooling
Send Yongzhu your 2D drawing and 3D model before die design. Mark side holes, grooves, recesses, key tolerances, cosmetic faces and machined surfaces. Include the alloy, order volume and assembly function.
We can map pull directions, check whether features can share a side core, and compare redesign, as-cast and CNC options using your actual geometry.
Frequently Asked Questions
Can a side core be added after the die has already been built?
Sometimes, if space, tool steel, cooling, runners and ejection allow it. Retrofitting can require major rework, so the existing die data must be reviewed first.
Can an internal thread be formed with a die casting side action?
A straight slide cannot release a full internal thread. A rotating core may work but adds complexity. A cored pilot followed by tapping is often more robust.
Will a side-core shutoff leave a visible line on the casting?
It can leave a fine witness line where tool surfaces meet. Agree its position before tooling if the area is cosmetic, sealing, sliding or hard to finish.
Is a hand-loaded loose core practical for repeat aluminum die casting?
Usually not for stable high-volume output. Manual loading adds time, cycle variation and placement risk. It may suit rare geometry or limited runs, but a slide or redesign is more repeatable.
