How to Choose the Right Core Pin for Your Injection Mold
Key Takeaway: Core pin selection follows six steps: (1) match structural type to cavity geometry, (2) select material for operating conditions, (3) verify tolerance/standard, (4) add surface treatment if needed, (5) evaluate gas venting, (6) plan maintenance economics. When uncertain, start with SKH51 straight core pins — they cover 80% of standard applications.
Step 1: Determine the Structural Type
The internal feature geometry of your molded part dictates which core pin type to use. Start with the simplest option that meets your requirements:
| Feature Type | Core Pin Type | When to Use |
|---|---|---|
| Simple round hole | Straight Core Pins | Default for 80% of applications |
| Counterbored hole (2 diameters) | One-Step Round | Screw recesses, alignment bores |
| Multi-diameter hole (3 sections) | Two-Step Round | Connector housings, complex fittings |
| Any above, in 16+ cavity mold | Taperless Variants | Saves ~15 min/bore in plate machining |
| Boss pillar for screw fastening | Round for Boss | Electronics housings, structural parts |
| High-wear (>500K shots) | Inlay Core Pins | Tip-only replacement saves 60-80%/cycle |
Rule of thumb: If unsure, start with a straight core pin. Upgrade only when the part design specifically requires it.
Step 2: Select the Material Grade
Material selection is driven by operating temperature, required service life, and corrosion environment. For detailed material comparison, see our SKH51 vs SKD61 vs STAVAX ESR guide.
| Material | Hardness | Max Temp | Best For | Avoid When |
|---|---|---|---|---|
| SKH51 (M2) | HRC 60-62 | ~600°C | General-purpose, high-cycle | Corrosive resins (PVC, POM) |
| SKD61 (H13) | HRC 48-52 | ~600°C | High-temp, die casting | Ultra-high hardness needed |
| STAVAX ESR | HRC 48-52 | ~250°C | Medical, optical, corrosive | High-temp applications |
| Hotwork Steel | HRC 44-48 | ~550°C | DIN-standard, budget | Need maximum hardness |
Default: SKH51 covers the broadest range. Only deviate when operating conditions specifically require it. Material grades are defined by JIS G 4403 (high-speed steels) and JIS G 4404 (hot-work steels).
Step 3: Specify Tolerance & Head Thickness
Two critical dimensions determine interchangeability and fit:
- Shaft diameter tolerance: Standard JIS tolerance suits most applications. Extra-precision (+0/−0.005 mm) is required for minimal-clearance bores where the pin seats with near-zero play.
- Head thickness (T): Standardized at T = 4 mm for JIS pins per JIS B 6131. DIN-standard pins use different head thickness conventions — verify before ordering.
Critical check: Confirm JIS or DIN standard before ordering. Mixing standards causes fit problems that are expensive to fix after mold assembly.
Step 4: Choose Surface Treatment
Surface treatments extend service life and improve molded part quality. Match treatment to application:
| Treatment | Effect | When to Specify |
|---|---|---|
| None (as-ground) | Standard surface finish | General-purpose, cost-optimized |
| Tip Lapping | Mirror-finish tip (Ra < 0.1 μm) | Visible features, cosmetic parts |
| TiCN Coating | 2–5× life extension (HV 3000+) | High-cycle (>500K), abrasive resins |
| Nitriding | Surface hardening + mild corrosion resistance | High-wear environments |
Step 5: Evaluate Gas Venting Requirements
Gas trapping occurs when the melt front seals air with no escape path. Use gas release core pins when:
- Depth-to-diameter ratio > 3:1 → gas release recommended
- Blind holes (no through-hole exit for air) → gas release strongly recommended
- Boss pillars → gas release often necessary
- Standard through-holes with adequate parting-line venting → usually not needed
Three gas release geometries are available:
| Type | Venting Capacity | Best For |
|---|---|---|
| Flat-Sided | Moderate | Standard-depth, light venting |
| SVC (Semi-circular Vent Channel) | High | Deep cavities, severe gas trapping |
| GVFC (with Sintered Filter) | High + flash prevention | High-pressure fill, flash-sensitive areas |
Step 6: Consider Maintenance Economics
For high-volume production, total maintenance cost often exceeds the initial purchase cost of the pins:
| Production Volume | Recommended Approach | Rationale |
|---|---|---|
| <100K shots | Standard solid pins | Lowest initial cost, replacement unlikely |
| 100K – 500K shots | Standard pins + spare set | Plan for one replacement cycle |
| >500K shots | Inlay core pins | Tip-only swap saves 60-80% per cycle |
| >500K + high-cavity | Taperless + inlay | Maximum maintainability |
Selection Checklist
- ☐ Internal feature type identified → structural type selected
- ☐ JIS or DIN standard confirmed for the mold
- ☐ Material grade selected (temperature + corrosion environment)
- ☐ Tolerance class confirmed (standard vs extra-precision)
- ☐ Surface treatment specified if cosmetic or high-cycle
- ☐ Gas release evaluated (depth ratio, blind holes, bosses)
- ☐ Maintenance approach determined (solid vs inlay)
- ☐ Quantity and lead time confirmed
Frequently Asked Questions
What is the most important factor when choosing a core pin?+
Should I always use the highest hardness material?+
No. SKH51 (HRC 60-62) provides the highest hardness but is brittle on very thin pins (<1 mm) and has no corrosion resistance. Choose SKD61 for toughness or STAVAX ESR for corrosion resistance when conditions demand it.
When is gas release necessary?+
When hole depth exceeds 3× pin diameter, when the hole is blind, or when forming boss pillars. Without venting, trapped gas superheats and causes burn marks, short shots, or surface discoloration. See gas release core pins.
How do I decide between solid and inlay core pins?+
Use solid pins for molds under 500K shots. For molds exceeding 500K shots where tip wear is the primary failure mode, inlay core pins reduce per-replacement cost by 60-80%.
Continue Reading
All Core PinsBrowse the full product range by categoryStraight Core PinsDefault for 80% of standard moldsStepped RoundMulti-diameter internal featuresTaperlessFast plate machining, universal swapBoss & InlaySpecialty geometries & modular tipsMaterial GuideSKH51 vs SKD61 vs STAVAX ESRCore Pin vs Ejector PinFunction & specification differences