How to Select Pin-Point Gate Tip Diameter and Length for Your Mold

Step 1: Determine Base Gate Diameter from Wall Thickness

The fundamental rule for pin-point gate sizing is based on the part wall thickness at the gate location. The gate diameter should allow the resin to fill the cavity before the gate freezes off, while remaining small enough to minimize the vestige.

The empirical formula used across the industry:

• Gate diameter (D_g) = Wall thickness (t) × 0.6 to 0.8
• Minimum practical gate diameter: 0.5 mm
• Maximum standard pin-point gate diameter: 2.0 mm

*For wall thicknesses above 2.5 mm, pin-point gate diameter is capped at 2.0 mm. Beyond this size, consider switching to a direct sprue or tunnel gate for better packing and reduced vestige.

Step 2: Adjust for Resin Type

Different resins have different flow characteristics that require gate diameter adjustments. The key parameter is the resin's melt flow index (MFI) — higher MFI resins flow more easily through smaller gates.

For glass-filled resins, always use the upper range of gate diameter. The abrasive filler causes progressive gate enlargement over the mold's life, so starting with a slightly larger gate extends the window before the vestige becomes unacceptable. According to the ASTM D1238 standard for melt flow rate measurement, MFI values must be tested at the specific temperature and load conditions specified for each resin family.

Step 3: Calculate Gate Tip Length

Gate tip length is the portion of the bushing that extends beyond the bushing body into the cavity plate. It must be calculated precisely — too short and the gate does not reach the cavity surface; too long and the tip protrudes into the part.

The calculation:

• Tip length (L_t) = Cavity plate thickness (T_p) − Bushing body seated depth (D_s) + Offset (0.0 to +0.2 mm)

The offset value depends on the desired gate vestige style:

• +0.0 mm (flush) — Tip is exactly flush with the cavity surface. Produces the flattest vestige.
• +0.1 to +0.2 mm (protruding) — Tip extends slightly into the cavity. Creates a small dimple in the part surface that helps center the gate break point.
• −0.1 mm (recessed) — Tip sits slightly below the cavity surface. Produces a raised vestige nub on the part. Avoid this unless specifically required.

Step 4: Verify Against Process Limits

After calculating the initial gate diameter, verify it against process constraints:

• Injection pressure check — Gate pressure drop (ΔP) ≈ 8ηQL / (πr⁴), where η is melt viscosity, Q is volumetric flow rate, L is gate land length, and r is gate radius. If ΔP exceeds 30% of your machine's maximum injection pressure, increase gate diameter.
• Shear rate check — Gate shear rate (γ) ≈ 4Q / (πr³). For most resins, keep γ below 50,000 s⁻¹ to avoid degradation. According to ScienceDirect's shear rate engineering reference, excessive shear at the gate causes molecular degradation, discoloration, and reduced mechanical properties.
• Fill time check — The gate must remain open long enough for the cavity to fill and pack. Gate freeze-off time ≈ (D_g/2)² / (4α), where α is the resin's thermal diffusivity (~0.1 mm²/s for most plastics).

Common Mistakes to Avoid

• Using the same gate diameter for different wall sections — If a part has varying wall thickness, gate at the thickest section and check that the thinnest sections can fill before gate freeze-off.
• Ignoring gate land length — The gate orifice is not a thin-wall restriction. The gate land (the parallel section at tip diameter) should be 0.5–1.5 mm long. Too short causes inconsistent degating; too long increases pressure drop.
• Forgetting thermal expansion — The mold operates at 40–120°C, causing 0.01–0.03 mm expansion of the gate diameter. This is usually negligible but matters for sub-0.8 mm gates on precision parts.