{"id":336,"date":"2026-04-26T01:53:45","date_gmt":"2026-04-26T01:53:45","guid":{"rendered":"https:\/\/kurtfoundry.com\/?p=336"},"modified":"2026-07-05T09:04:33","modified_gmt":"2026-07-05T09:04:33","slug":"blank-sand-cast","status":"publish","type":"post","link":"https:\/\/kurtfoundry.com\/fr\/blog\/blank-sand-cast\/","title":{"rendered":"Comment fabriquer une \u00e9bauche moul\u00e9e au sable"},"content":{"rendered":"\n

Sand casting produces roughly 70% of all metal castings worldwide. The blank you receive from a foundry arrives rough \u2014 gate stubs, parting line flash, and sand texture still attached. Your tolerances, surface finish, and machining stock are all locked in during the production sequence from pattern to shakeout, and each stage gives you specific DFM leverage if you know where to push.<\/p>\n\n\n\n

Pattern and Core Preparation<\/h2>\n\n\n\n
\"Wooden<\/figure>\n\n\n\n

Every sand casting starts with a pattern \u2014 a physical replica of the finished part, oversized to compensate for metal shrinkage. Shrinkage is not one number you add uniformly. Metal contracts in three distinct phases: liquid contraction as the melt cools to its freezing point, solidification shrinkage as it transforms from liquid to solid (roughly 3% by volume for carbon steel), and solid-state contraction as the casting cools to room temperature. Pattern allowance only compensates for that last phase. The first two require proper gating and riser design \u2014 no amount of pattern scaling fixes inadequate feeding.<\/p>\n\n\n\n

For carbon steel, expect about 2% linear shrinkage built into the pattern; gray iron runs lower at roughly 1%, which is one reason the foundry needs your alloy locked before pattern making starts. Draft angles \u2014 typically 1-3 degrees on vertical surfaces \u2014 let the pattern pull cleanly from the sand without tearing the mold cavity. Skip the draft, and your foundry either adds it for you (changing your dimensions) or fights broken mold edges on every pull.<\/p>\n\n\n\n

If your part has internal passages, the foundry builds sand cores separately and sets them into the mold before closing. Before you finalize that drawing, check whether your internal geometry actually needs a core or can be machined from solid \u2014 cores add cost and tolerance uncertainty.<\/p>\n\n\n\n

Molding and Gating<\/h2>\n\n\n\n

The mold is built in two halves: the cope (top) and drag (bottom), packed with sand around the pattern. Where these halves meet creates the parting line \u2014 a feature visible on every blank you receive. Parting line location is not cosmetic. It determines where flash forms, where dimensional variation is highest, and where machining fixtures will reference.<\/p>\n\n\n\n

The gating system \u2014 sprue (vertical entry channel), runners, and gates \u2014 controls how metal fills the cavity. Good gating design prevents 70% of casting defects. Too fast, and turbulent flow traps air and erodes sand into the casting. Too slow, and metal freezes before filling thin sections. Risers \u2014 reservoirs of extra metal at thick sections \u2014 feed shrinkage as the casting solidifies. Every gate and riser leaves a stub on the blank that must be cut after shakeout.<\/p>\n\n\n\n

\"Cross-section<\/figure>\n\n\n\n

I have seen more blanks rejected from bad gating than from wrong alloy selection. If your foundry asks to move a gate location, listen \u2014 they are protecting your part quality.<\/p>\n\n\n\n

Pouring and Cooling<\/h2>\n\n\n\n
\"Molten<\/figure>\n\n\n\n

Gray iron pours below 1300 degrees C; carbon and stainless steel pour hotter, around 1550-1600 degrees C. The exact temperature matters because metal poured too hot absorbs hydrogen from moisture in the sand, creating gas porosity \u2014 small round voids just below the surface that show up only after machining.<\/p>\n\n\n\n

Once the mold is full, cooling rate determines both microstructure and dimensional stability. Controlled cooling inside the sand mold is the professional standard. Quenching freshly cast ferrous parts in water to speed handling causes thermal shock cracks and locked-in residual stress \u2014 the casting must cool in the mold to below its critical temperature before shakeout.<\/p>\n\n\n\n

Cycle times for small-to-medium parts run 5-30 minutes from pour to shakeout. Large or thick-section castings can take hours.<\/p>\n\n\n\n

Shakeout and Cleanup<\/h2>\n\n\n\n
\"As-cast<\/figure>\n\n\n\n

Shakeout is exactly what it sounds like \u2014 the sand mold is broken apart to free the casting. What comes out is not a finished part. The blank carries gate stubs, riser pads, parting line flash (thin metal fins where the cope and drag did not seal perfectly), and an as-cast surface with sand texture embedded in it.<\/p>\n\n\n\n

Gate stubs and risers get cut off with saws or grinders. Flash gets chipped or ground. The remaining surface is shot-blasted. Surface finish on as-cast faces typically runs 250-500 Ra microinches \u2014 far rougher than any machined surface.<\/p>\n\n\n\n

Most defects trace back to three root causes: moisture in the sand (gas porosity), inadequate riser feeding (shrinkage cavities with jagged internal surfaces), or temperature problems during pour (cold shuts where two metal fronts meet without fusing, or hot tears from uneven cooling). When you receive a blank with defects, work backwards from the riser \u2014 the answer is usually there.<\/p>\n\n\n\n

Common Spec Mistakes That Add Cost<\/h2>\n\n\n\n

The most common mistake I see from design engineers is specifying tolerances tighter than the process can deliver. Standard sand casting achieves CT8-12 tolerance class per ISO 8062, with CT9 typical. Anything tighter than CT8 on an as-cast feature means you are paying for machining whether you intended to or not. Specify as-cast tolerances where the function allows, and call out machined surfaces explicitly \u2014 2 mm is a common machining allowance on medium-diameter features.<\/p>\n\n\n\n

Second: ignoring section thickness transitions. Aim for gradual wall transitions with less than a 1:4 thickness ratio. A thick boss next to a thin wall cools at different rates, pulling shrinkage porosity into the junction.<\/p>\n\n\n\n

\"Diagram<\/figure>\n\n\n\n

Third: placing critical features across the parting line. Dimensional variation is always worst at the parting line due to cope-drag alignment tolerance. Move critical datums to one mold half whenever possible.<\/p>\n\n\n\n

Your DFM Checklist Before Ordering a Blank<\/h2>\n\n\n\n

Specify as-cast tolerances at CT9 or wider on non-machined surfaces, and add explicit machining stock on features that need precision. Check that all vertical surfaces have at least 1 degree of draft. Keep section thickness ratios below 1:4, and use generous fillets at every wall junction. Confirm that your critical dimensions sit on one side of the parting line, not across it.<\/p>\n\n\n\n

One step most engineers skip: talk to the foundry before releasing the drawing. A 15-minute conversation about parting line location and gating access saves weeks of sample iteration and thousands in pattern revisions.<\/p>\n\n\n\n