What the process actually delivers
Silica sol is the binder, not the product. A wax pattern is dipped into a colloidal silica slurry, stuccoed with refractory sand, and repeated until the shell is thick enough. The wax is melted out, the shell is fired, metal is poured, and the shell is broken away. What matters to a buyer is the consequence of that binder choice: a finer shell face, so a smoother casting and a thinner wall than water glass can hold.
Below is the operating envelope our shortlisted foundries quote against. If your part falls outside it, tell us — we will say so rather than take the tooling order and discover the problem at first article.
| Parameter | Typical capability |
|---|---|
| Process | Silica sol (ethyl silicate) shell, lost wax, gravity or low-pressure pour |
| Finished weight | 0.01–100 kg. Comfortable band is 0.05–40 kg; above 60 kg shell handling risk rises sharply |
| Minimum wall | 1.5 mm on parts under 2 kg, 2.5 mm above 10 kg, 3.5 mm on large flat unsupported sections |
| Linear tolerance | ISO 8062-3 CT4–CT6 under 100 mm; add about 0.015 mm per extra 10 mm |
| Surface roughness | Ra 3.2–6.3 µm as-cast. Ra 1.6 µm achievable with a finer primary slurry, quoted as an adder |
| Materials | 304, 316, 316L, 17-4PH, 2205, CF8M, CF8, 42CrMo, plus low-carbon and low-alloy steel |
| Wax tooling | Aluminium die for volumes under 5,000 pcs, steel die above. 15–25 days to first shot |
| Sample lead time | 20–30 days after drawing approval and tooling sign-off |
| Production lead time | 35–50 days from sample approval, plus machining and finishing where specified |
| MOQ | 100–300 pcs per part number, size dependent |
| Certification | ISO 9001 as baseline; IATF 16949 for automotive; PED 2014/68/EU and AD 2000 on request |
Silica sol vs water glass vs sand casting
Most sourcing mistakes here are not supplier mistakes, they are process-selection mistakes. A buyer quotes a part as silica sol when sand casting would have done, and pays 2.5 times the per-kilo rate for tolerance nobody needed. The reverse also happens: a thin-wall part gets quoted in sand casting, fails first article on wall thickness, and the project loses a quarter.
| Dimension | Silica sol | Water glass | Sand casting |
|---|---|---|---|
| Binder | Colloidal silica | Sodium silicate | Clay / resin |
| As-cast surface | Ra 3.2–6.3 µm | Ra 6.3–12.5 µm | Ra 12.5–25 µm |
| Tolerance grade | CT4–CT6 | CT5–CT7 | CT8–CT10 |
| Minimum wall | 1.5 mm | 2.5 mm | 5–6 mm |
| Relative cost per kg | Highest | Middle | Lowest |
| Tooling cost | Medium (wax die) | Medium (wax die) | Low (pattern plate) |
| Right for | Thin-wall, high-finish, stainless and alloy parts | Similar geometry at lower finish requirement | Large, simple, heavy parts; loose tolerance |
| Wrong for | Parts above roughly 100 kg | Tight tolerance, thin walls | Anything under 4 mm wall |
The check that saves the most money: before you accept a silica sol quotation, ask what the part would cost in sand casting and what you would lose. If the answer is "the same tolerance", the silica sol premium is buying surface finish only — which may still be right, but you should be paying for it knowingly.
Materials we quote most often
316 / 316L stainless
Pump impellers, valve bodies, marine hardware, food-contact parts. The default where corrosion resistance matters more than strength.
17-4PH
Precipitation-hardening grade for shafts, linkages and parts needing H900 condition strength after casting plus heat treatment.
2205 duplex
Chloride environments, desalination and chemical process parts. Needs tighter pour control than austenitic grades.
CF8M / CF8
Cast equivalents of 316 and 304, common for valve and pump bodies specified to ASTM A351.
42CrMo and low-alloy steel
Gear blanks, yokes, brackets where hardness and fatigue life drive the specification.
Carbon steel
Structural and general engineering parts where cost dominates and corrosion is handled by coating.
Where silica sol castings fail — and what stops it
These five defects account for most first-article rejections on Chinese silica sol parts. Each one has a process control that should be visible in the foundry's records, not just promised in an email.
1. Shell cracking during dewax
Caused by a shell built too thin, or steam pressure spiking during autoclave dewax. Ask for the shell build schedule — number of coats and stucco grade per layer — and whether dewax is autoclave or furnace. A foundry that cannot produce the schedule has no control over it.
2. Ceramic inclusion from slurry contamination
Old slurry, wrong pH, or stucco dust settling into the prime coat. Shows up as black speckling after machining. The control is slurry viscosity and pH logged daily, plus a settling tank. Ask to see the log, not the certificate.
3. Dimensional drift on long parts
Anything over 300 mm in one direction wanders because wax and shell expand differently. The fix is a wax injection temperature window and a checked first-article CMM report on the actual tool, not a generic tolerance statement.
4. Gas porosity in heavy sections
Sections over 25 mm in a silica sol shell cool slowly and trap gas. Requires chilled molds, controlled pour temperature, or a design change to a cored section. If your part has a heavy boss on a thin wall, raise it before tooling.
5. Hot tearing at sharp internal corners
Restraint during solidification. Rounded fillets of at least 1.5 mm radius and a modified gating design solve it. This is a drawing conversation that costs nothing before tooling and a rework quote after.
How we verify a foundry before you pay for tooling
This is the part most sourcing agents skip. A certificate PDF proves a company passed an audit on one day; it does not prove the line running your part is under that system.
- Business licence and export record — legal entity name, registered capital, and whether the company has actually exported castings under its own name.
- Certificate validity — ISO 9001 or IATF 16949 checked against the issuing body's public register, with scope and expiry, not just the PDF.
- Equipment on the floor — spectrometer, X-ray or film capability, CMM, and heat-treat furnace with chart recorder. Photographed on a live call.
- Someone else's parts — photographs of work in progress for other buyers, with identifying marks removed. A shop with only stock photos has not run your part type.
- Live walkthrough — a scheduled video call through the wax room, shell line, pour floor and finishing. We run it and report what we see.
If a foundry fails any of these, we say so and drop it from the shortlist. That is the service, and it is why the shortlist is three foundries rather than thirty.
What to send with your RFQ
Quotations come back faster and closer when the first message carries five things. Without them, every foundry quotes a different assumption and you cannot compare the numbers.
- Drawing or 3D file — STEP or IGES preferred, with the tolerance block and any critical dimensions marked.
- Material grade and standard — 316L to ASTM A351 CF8M, not "stainless".
- Annual volume and order pattern — annual quantity plus typical release size. A 12,000/year part released 1,000 at a time prices differently from one released 4,000 at a time.
- Finishing scope — as-cast, machined, polished, passivated or coated, and which surfaces.
- Inspection and documentation — whether you need a dimensional report, material certificate to EN 10204 3.1, and third-party inspection.
Send a drawing, get a costed sheet in 24 hours
Metal, molding, machining, tooling, freight and inspection each on its own line — so you can see where the money goes instead of arguing with one blended per-kilo number.
Or email cfwc@163.com · WhatsApp +86 159 4126 7000
Frequently asked questions
What is silica sol investment casting?
Silica sol casting uses a colloidal silica binder instead of sodium silicate to build the ceramic shell around a wax pattern. The finer binder gives a smoother surface, tighter dimensional tolerance and thinner minimum wall than water glass, which is why it dominates stainless and high-alloy precision parts.
What is the minimum order quantity?
Typically 100 to 300 pieces per part number depending on size and material. Small parts under 0.5 kg often start at 300 pcs, parts above 20 kg at 50 to 100 pcs. Tooling is billed separately, usually USD 300 to 2,500 for an aluminium wax die.
How tight a tolerance can it hold?
ISO 8062-3 grade CT4 to CT6 for dimensions under 100 mm. Above that, add roughly 0.015 mm per 10 mm of additional length. Anything tighter needs machining, which we plan into the quotation rather than promising from the as-cast surface.
How long does a quotation take?
A line-item quotation comes back within 24 hours of receiving a drawing or 3D file with material grade and annual volume. Tooling cost, sample lead time and production lead time are quoted as separate lines.
Can you verify the foundry before I place an order?
Yes. We check business licence and export record, certificate validity against the issuing body, spectrometer and X-ray capability, and we can run a live video walkthrough of the shop floor before you commit to tooling.
Related reading
- Silica sol investment casting: the complete process guide — how the shell is built, step by step.
- Gray iron casting pricelist — benchmark bands if your part is iron rather than steel.
- How to verify a Chinese foundry certificate — the checks behind the shortlist above.
- China foundry audit checklist — what to look at on the shop floor.