surface finish green sand resin shell
Who this helps: Design Engineers / Buyers who must specify and verify Ra on iron castings (gray/grey & ductile).
What you’ll get: measured-style ranges for as-cast Ra by process/size, what moves those numbers (sand/coating/temperature/blast), and copy-paste drawing notes so suppliers quote apples-to-apples.
Prepared by YB Metal Solution. Want a surface plan for your part? Upload your drawing via /rfq—YB Metal will return the right process, Ra targets by zone, and inspection steps.
Author: YB Metal Solution Engineering Team (hereafter YB Metal)
Table of contents
- Why surface finish matters
- Real numbers: Ra by process & size
- What moves Ra (and how to control it)
- Shot-blast profile: what to expect
- How to measure Ra correctly
- What to put on the drawing (copy–paste)
- Mini case: same housing, 3 processes
- What YB Metal delivers
- FAQs
Why surface finish matters
- Sealing & leakage: high Ra hurts gasket sealing; even on non-pressure parts, roughness drives coating consumption.
- Machining stock: cleaner as-cast faces allow less stock and shorter cycles.
- Paint/powder: consistent blast profile improves adhesion and cosmetics.
Real numbers: Ra by process & size
Ranges below are typical for iron castings, measured as-cast (pre-blast) on accessible faces with a stylus profilometer (see §5). Use µm / µin.
Part size bands
- Small: ≤ 300 mm (12 in) major dimension
- Medium: 300–800 mm (12–31 in)
- Large: ≥ 800 mm (31 in)
| Process | Small (µm / µin) | Medium (µm / µin) | Large (µm / µin) | Notes |
|---|---|---|---|---|
| Shell molding | 3.2–6.3 / 125–250 | 4–7.5 / 160–295 | 5–9 / 200–355 | Finest as-cast. Sensitive to core spans & coating dryness. |
| Resin sand | 6.3–12.5 / 250–490 | 8–16 / 315–630 | 10–20 / 395–790 | Good balance; core freedom; LOI/coating control matters. |
| Green sand | 12.5–25 / 490–980 | 14–28 / 550–1100 | 16–32 / 630–1260 | Most economical; surface follows GFN/moisture/compactability. |
Rule of thumb：larger parts trend rougher; thin-wall zones (faster cooling) can look cleaner if burn-on is controlled.
Quick converter: 1 µm ≈ 39.37 µin.
What moves Ra (and how to control it)
| Factor | Tends to… | Control window / tip |
|---|---|---|
| Sand fineness (AFS GFN) | ↑GFN → smoother; too fine → gas defects | Green: ~40–70; Resin: 50–100; Shell: 70–120. Balance with venting/LOI. |
| Coating type & DFT | Zircon/Alumina smooth high-heat faces; too thick → orange peel/cracking | Target DFT 0.15–0.30 mm (6–12 mil), full dry. |
| Metal temperature / superheat | Too hot → penetration/burn-on (rough); too cold → misruns | Hold a narrow window; coordinate with gating fill time. |
| Mold hardness / compactability (green) | Low hardness → scabs/tears (rough); too hard → erosion | Keep moisture & compactability within set KPIs. |
| Binder & LOI (resin/core) | High LOI → blows/veins (rough) | Control LOI; ensure full cure and adequate dry. |
| Core venting | Poor vent → blisters/veins (rough) | Add vents, avoid long blind pockets. |
| Blast media/coverage | Can increase micro-roughness; evens texture visually | Match profile to coating spec; don’t “over-polish.” |
Deeper reads on your site：
Shot-blast profile: what to expect
Typical wheel-blast with S280–S330 steel shot at adequate coverage gives a blast profile roughly:
| Base process | Ra after standard shot-blast (µm / µin) |
|---|---|
| Shell | 4–8 µm / 160–315 µin |
| Resin | 6–12 µm / 240–475 µin |
| Green | 8–16 µm / 315–630 µin |
Important: Ra on blasted surfaces reflects blast profile, not the true as-cast skin. Many paint/powder systems want 5–15 µm profile—set by media mix, velocity and time. If you need low Ra for sealing, finish-machine that face and specify Ra 1.6–3.2 µm (63–125 µin) after machining.
How to measure Ra correctly
Instrument: contact stylus profilometer (≈ 2 µm tip).
Cut-off/trace: λc = 0.8 mm, sampling length ≥ 4 mm (ISO 4287/4288 practice).
Direction: traverse perpendicular to lay/flow marks if possible.
Locations: specify 3–5 zones per part family; avoid parting lines and blast shadows.
Report: Ra (and Rz if requested) with raw traces attached.
What to put on the drawing (copy–paste)
Surface / zones
Z1 As-cast (pre-blast) target:
Shell: Ra ≤ 6.3 µm (250 µin)
Resin: Ra ≤ 12.5 µm (490 µin)
Green: Ra ≤ 25 µm (980 µin)
Z2 Sealing faces (after machining):
Ra 1.6–3.2 µm (63–125 µin)
Z3 Painted/powdered exterior:
Blast profile Ra 6–12 µm (240–475 µin), no embedded media
Measurement
Profilometer, tip ≈ 2 µm; λc = 0.8 mm; trace ≥ 4 mm; 3 traces/zone.
Report Ra (and Rz if required) with raw traces.
Process notes
Coating DFT 0.15–0.30 mm (6–12 mil) on high-heat faces; full dry.
Supplier to control AFS GFN, LOI, moisture and mold hardness to window.
Wheel-blast S280–S330; media cleanliness & coverage logged.
Mini case: same housing, 3 processes
- Geometry: ductile-iron housing, envelope 420×280×80 mm (medium).
- Green sand: as-cast Ra 18–24 µm; blast 10–14 µm; machining stock 2.5–3.0 mm on sealing faces.
- Resin sand: as-cast Ra 9–12 µm; blast 7–10 µm; stock 1.5–2.0 mm.
- Shell molding: as-cast Ra 4–6 µm; blast 4–7 µm; stock 0.8–1.5 mm; tooling cost ↑; unit cost ↓ at volume.
Outcome: shell cut cycle time −18% in machining and improved powder-coat cosmetics; breakeven achieved at ~5–7k pcs/year.
What YB Metal delivers
YB Metal Solution sets up your surface plan end-to-end:
- Select process & sand system, define Ra targets by zone.
- Specify coating type/DFT and blast parameters to hit paint specs.
- Provide Ra measurement plan, stylus settings, and first-article traces.
- Back with process KPIs (AFS GFN, LOI, moisture, hardness) and corrective actions if out of window.
Want a surface plan for your part? Upload your drawing—YB Metal will respond with targets, process notes and a quote.
FAQs
CTA — specify with proof, not guesses
Set real Ra targets and hit them the first time. Upload your drawing to /rfq—YB Metal will propose process, coating and blast settings with a measurement plan.