Technical Guide8 min read

How to Calculate Clearance Between Ejector Sleeves and Center Pins

Key Takeaway: Clearance = Sleeve ID − Pin OD. Target 0.01–0.02 mm for general molding, and add thermal compensation for molds running above 200°C. For the complete reference table, see our Clearance Handbook.

Before You Calculate: What You Need

Before starting the clearance calculation, gather the following information from your mold design and resin supplier datasheet:

Sleeve bore diameter (nominal): From the mold design drawing
Center pin OD (nominal): From the mold design drawing or pin catalog
Sleeve material and pin material: For thermal expansion calculation
Mold operating temperature: From resin processing window
Resin type: For flash threshold reference
Required tolerance class: H6 or H7, based on part quality requirements

If any of these are unknown, use the defaults: H7 tolerance class, SKD61 sleeve, SKH51 pin, and general molding clearance (0.010–0.020 mm).

Step 1: Determine Nominal Dimensions

Tolerance classes referenced in this guide follow ISO 286 (IT grades). Thermal expansion coefficients are sourced from AZoM's M2 steel datasheet.

From the product catalog or your order specification, record:

Sleeve ID (inner diameter) — this is the bore that slides over the center pin
Pin OD (outer diameter) — the shaft diameter of the center pin
Tolerance class — H7 (standard) or H6 (high-precision) for the sleeve bore

Step 2: Calculate Cold Clearance

Cold Clearance (C_cold) = Sleeve ID_nominal − Pin OD_nominal

Application	Target C_cold
General molding	0.01 – 0.02 mm
High-precision	0.005 – 0.01 mm
High-temperature	0.015 – 0.025 mm

Step 3: Add Thermal Compensation

For molds operating above 200°C, thermal expansion closes the cold clearance. Compensate with:

C_operating = C_cold + (α_sleeve − α_pin) × D × ΔT

Worked Example

Given: Ø10mm bore, SKD61 sleeve (α=11.7×10⁻⁶), SKH51 pin (α=11.0×10⁻⁶), ΔT=250°C
Thermal differential: (11.7−11.0)×10⁻⁶ × 10 × 250 = 0.00175 mm ≈ 0.002 mm
Required cold clearance: 0.015 + 0.002 = 0.017 mm → Specify Sleeve ID = Pin OD + 0.017 mm

Step 4: Verify Against Flash Limit

Resin Type	Flash Threshold	Max Allowable Clearance
Nylon (PA6, PA66)	0.01 – 0.015 mm	0.012 mm
ABS, PS	0.02 – 0.03 mm	0.020 mm
Polycarbonate (PC)	0.025 – 0.035 mm	0.025 mm
PP, PE	0.02 – 0.025 mm	0.020 mm
Glass-filled (>20% GF)	0.008 – 0.015 mm	0.010 mm

Critical check: If your operating clearance exceeds the flash threshold for your resin, you must either tighten the bore tolerance (H6 instead of H7) or use pre-assembled matched sets.

Common Calculation Mistakes

Even experienced mold designers make these errors when calculating sleeve-pin clearance:

Mistake	Consequence	How to Avoid
Confusing radial and diametral clearance	Specifying 2× too much clearance → flash	Always clarify: this guide uses diametral clearance
Ignoring thermal expansion	Seizure at operating temperature	Always add thermal compensation for molds >200°C
Using room-temperature flash data	Clearance appears safe but flashes at process temp	Use resin supplier's flash thickness data at processing temperature
Same material for sleeve and pin	Galling and adhesive wear	Ensure ≥3 HRC hardness difference between components
Not accounting for wear	Flash develops after 100K shots	Specify clearance at the tight end of the range to allow for wear-in

Calculation Worksheet

Use this worksheet for each sleeve position in your mold. Fill in the values for your specific application:

Parameter	Symbol	Your Value	Source
Sleeve bore diameter (nominal)	D_bore	_____ mm	Mold design drawing
Pin OD (nominal)	D_pin	_____ mm	Mold design drawing
Sleeve material CTE	α_sleeve	_____ ×10⁻⁶/°C	Material datasheet
Pin material CTE	α_pin	_____ ×10⁻⁶/°C	Material datasheet
Operating temperature	T_op	_____ °C	Resin processing window
Ambient temperature	T_amb	25 °C	Standard assumption
Resin flash threshold	C_flash	_____ mm	Resin supplier datasheet
Target cold clearance	C_cold	_____ mm	Application table (this guide)
Thermal compensation	C_thermal	_____ mm	Calculated: (α_s−α_p)×D×ΔT
Total operating clearance	C_total	_____ mm	C_cold + C_thermal
Flash check	C_total < C_flash?	☐ Yes ☐ No	Must be Yes

Worked Examples by Application

Example 1: ABS Consumer Electronics Housing

Given: Ø8mm bore, SKD61 sleeve, SKH51 pin, ABS resin, mold temp 60°C

C_cold = 0.015 mm (general molding, Table 1 midpoint)
ΔT = 60 − 25 = 35°C
C_thermal = (11.7 − 11.0) × 10⁻⁶ × 8 × 35 = 0.0002 mm ≈ negligible
C_total = 0.015 mm
ABS flash threshold = 0.025 mm → 0.015 < 0.025 ✅
Specify: H7, target clearance 0.015 mm

Example 2: PA66-GF30 Automotive Connector

Given: Ø5mm bore, SKH51 sleeve, SKH51 pin, PA66-GF30, mold temp 85°C

C_cold = 0.010 mm (glass-filled, tight end of range due to low flash threshold)
ΔT = 85 − 25 = 60°C
C_thermal = (11.0 − 11.0) × 10⁻⁶ × 5 × 60 = 0 mm (same material)
C_total = 0.010 mm
PA66-GF30 flash threshold = 0.010 mm → 0.010 ≤ 0.010 ⚠️ borderline
Action: Reduce to 0.008 mm or switch to different materials to add thermal compensation safety margin

Example 3: PPS High-Temperature Mold

Given: Ø10mm bore, SKD61 sleeve, SKH51 pin, PPS resin, mold temp 140°C

C_cold = 0.020 mm (high-temperature, Table 1)
ΔT = 140 − 25 = 115°C
C_thermal = (11.7 − 11.0) × 10⁻⁶ × 10 × 115 = 0.0008 mm ≈ 0.001 mm
C_total = 0.021 mm
PPS flash threshold = 0.015–0.020 mm → 0.021 > 0.020 ⚠️ exceeds threshold
Action: Reduce C_cold to 0.015 mm → C_total = 0.016 mm < 0.020 mm ✅

Advanced Considerations

Clearance for Non-Circular Cross-Sections

While most ejector sleeves have circular cross-sections, some applications require non-circular profiles (oval, D-shaped, or rectangular). For non-circular sleeves, clearance calculation is more complex:

Measure clearance at multiple points: Check clearance at the widest point, narrowest point, and two intermediate points
Use the tightest point as the controlling dimension: The minimum clearance anywhere around the profile must exceed the seizure threshold
Thermal expansion is non-uniform: Different cross-section dimensions expand by different absolute amounts — a 20mm dimension expands twice as much as a 10mm dimension

Clearance for Stepped Sleeves

Stepped sleeves have two different bore diameters. Each diameter requires its own clearance calculation because:

Thermal expansion is proportional to diameter — the larger diameter section needs a slightly larger absolute clearance
The step transition creates a local stress concentration that affects wear patterns
Both clearances must stay within the flash threshold simultaneously

Calculate clearance independently for each diameter section, then verify that neither exceeds the resin's flash threshold at operating temperature.

Frequently Asked Questions

What is the maximum clearance before flash occurs?+

It depends on the resin. Low-viscosity resins like nylon can flash at 0.015mm. High-viscosity resins like polycarbonate tolerate up to 0.03mm. Glass-filled resins flash at even tighter clearances due to fiber bridging.

Should I use H6 or H7 tolerance for the sleeve bore?+

H7 is standard for general molding. Use H6 only for high-precision applications (medical, optical) where clearance must be held below 0.01mm.

How does temperature affect sleeve clearance?+

At 300°C operating temperature, a Ø10mm SKD61 sleeve expands approximately 0.003mm more than a SKH51 center pin. This differential must be added to the cold clearance specification.