Surface Finish Standards for Core Pin Tips: Ra Values & Processing Methods
Key Takeaway: Specify Ra 0.4–0.8 μm (ground) as the default for production molds. Upgrade to Ra 0.1–0.2 μm (lapped) only for cosmetic internal features. Never leave pins as-machined (Ra 1.6+) in production molds — the rough surface increases ejection force and causes drag marks.
Surface Finish Levels by Processing Method
| Processing Method | Achievable Ra (μm) | Cost Level | Best For |
|---|---|---|---|
| As-machined (turned/milled) | 1.6–3.2 | $ | Prototype molds, non-visible features only |
| Ground (cylindrical grinding) | 0.4–0.8 | $$ | Standard production — 80% of applications |
| Lapped (precision lapping) | 0.1–0.2 | $$$ | Cosmetic internal features, optical parts |
| Mirror polished | 0.02–0.05 | $$$$ | Optical lenses, light pipes, medical devices |
| TiCN coated (over ground) | 0.8–1.0 (matches substrate) | $$ + coating | Wear resistance for filled resins |
How Ra Affects Part Quality
The core pin tip surface finish transfers directly to the internal surface of the molded part. A rough pin produces a rough hole; a polished pin produces a smooth hole.
- Ra > 1.6 μm — Visible machining marks on the internal surface. Acceptable only for structural holes that are not cosmetic and not load-bearing contacts.
- Ra 0.4–0.8 μm — Smooth surface with no visible tool marks. Standard for connector pin seats, screw bosses, and general functional holes.
- Ra 0.1–0.2 μm — Near-optical surface. Required for medical syringe barrels, optical fiber ferrules, and any internal feature visible to the end user.
- Ra < 0.05 μm — Mirror finish. Reserved for optical components (light pipes, lens housings) where surface imperfections scatter light.
According to ISO 20457 (plastics molding), surface finish requirements should be specified on the mold drawing at each forming surface with the corresponding Ra symbol.
Ejection Force vs Surface Finish
Surface finish directly influences the force required to eject the molded part from the core pin:
| Tip Surface Ra | Relative Ejection Force | Risk Level |
|---|---|---|
| Ra 3.2 (as-machined) | 1.8× baseline | High — drag marks, pin marks likely |
| Ra 0.8 (ground) | 1.0× baseline (reference) | Low — standard production |
| Ra 0.2 (lapped) | 0.7× baseline | Very low — smooth ejection |
| Ra 0.8 + TiCN coating | 0.6× baseline | Lowest — coating reduces friction |
According to ScienceDirect's ejection force reference, the coefficient of friction between solidified plastic and steel decreases approximately linearly with surface roughness in the Ra 0.1–3.2 μm range.
Selecting the Right Finish
- Step 1 — Determine if the internal feature is cosmetic (visible to end user) or functional (hidden in assembly)
- Step 2 — Check the part wall thickness at the pin location. Thin walls (< 0.8 mm) require smoother finish to reduce ejection force
- Step 3 — Check the resin type. Sticky resins (TPE, soft PVC) require smoother finish than commodity resins (PP, PE)
- Step 4 — Order tip-processed core pins with the appropriate finish level, or order standard pins and specify tip processing as a secondary operation
Frequently Asked Questions
What Ra value should I specify for core pin tips?+
Ra 0.4–0.8 μm (ground) for standard production. Ra 0.1–0.2 μm (lapped) for cosmetic or optical features. As-machined (Ra 1.6+) only for prototypes or non-visible features.
How does surface finish affect part ejection?+
Rough surfaces (Ra > 1.6) increase ejection force by 30–80% compared to ground surfaces. This causes pin marks, drag marks, and potential part distortion, especially on thin sections.
Does TiCN coating improve surface finish?+
No — TiCN replicates the substrate finish. Finish the tip to the required Ra before coating. TiCN's benefit is wear resistance (2–5× life extension), not surface quality improvement.
Related Product Categories
Need Specific Surface Finish?
Specify your target Ra value — we offer ground, lapped, and mirror-polished tip processing with measured Ra verification.
✓ Ra measured & reported✓ MOQ 1 piece✓ TiCN coating available