{"id":312,"date":"2026-04-20T01:53:45","date_gmt":"2026-04-20T01:53:45","guid":{"rendered":"https:\/\/kurtfoundry.com\/?p=312"},"modified":"2026-07-05T09:04:49","modified_gmt":"2026-07-05T09:04:49","slug":"make-pattern-casting","status":"publish","type":"post","link":"https:\/\/kurtfoundry.com\/fr\/blog\/make-pattern-casting\/","title":{"rendered":"Comment r\u00e9aliser un mod\u00e8le pour le moulage"},"content":{"rendered":"\n

A pattern that\u2019s off by half a percent in shrinkage produces castings that miss the machining envelope \u2014 and the error scales with part size. I\u2019ve pulled 50-inch ductile iron castings that measured over a tenth of an inch oversize because the shop used textbook shrinkage instead of testing actual contraction. Every decision during pattern construction \u2014 type, material, allowances \u2014 cascades directly into casting quality and cost.<\/p>\n\n\n\n

How Patterns Create the Mold Cavity<\/h2>\n\n\n\n

The pattern is a physical replica of your finished part, oversized to compensate for metal shrinkage and machining stock. A moldmaker presses it into sand to create the negative cavity, removes it, and that cavity receives molten metal. It\u2019s the first tooling decision in the sand casting process, and every downstream step \u2014 gating, risering, shakeout \u2014 inherits whatever the pattern gets right or wrong.<\/p>\n\n\n\n

\"Wood<\/figure>\n\n\n\n

Simple parts with no undercuts can use a one-piece (loose) pattern \u2014 press it into one half of the mold and pull it straight out. Most real-world parts need a split pattern that divides along a parting line, with each half sitting in its own flask (cope and drag). Splitting along the right centerline keeps constant cross-sections, simplifies sand packing, and gives you clean extraction every time.<\/p>\n\n\n\n

If your part has internal cavities, the pattern also needs core prints \u2014 extensions that create seats where sand cores<\/a> will be placed during mold assembly.<\/p>\n\n\n\n

Pattern Types and When Each One Pays Off<\/h2>\n\n\n\n

A match plate pattern isn\u2019t \u201cbetter\u201d than a loose pattern. Each type is justified at specific volume thresholds, and specifying the wrong one wastes money in both directions.<\/p>\n\n\n\n

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

Loose Patterns<\/h3>\n\n\n\n

A two-piece wood or plastic pattern, hand-placed in the flask. Best for prototypes and short runs under 50 pieces. Cycle times are slow, but tooling cost is minimal \u2014 often under $1,000 for simple geometries.<\/p>\n\n\n\n

Match Plate Patterns<\/h3>\n\n\n\n

Both cope and drag halves mounted on a single plate that aligns them automatically. Justified at roughly 100-500 molds per year when you need dimensional consistency between halves. Tooling runs $2,000-$8,000 depending on complexity.<\/p>\n\n\n\n

Cope-and-Drag Patterns<\/h3>\n\n\n\n

Each half mounted on its own plate. Standard for larger castings and no-bake or floor-molding operations where a single match plate won\u2019t fit the mold<\/a>. Higher tooling cost, but throughput on large parts is unmatched.<\/p>\n\n\n\n

Gated Patterns<\/h3>\n\n\n\n

Multiple part cavities on a single plate with integrated runners and gates. Reserved for high-volume runs of small parts where the upfront investment pays back through multi-cavity pouring.<\/p>\n\n\n\n

Pattern Materials<\/h2>\n\n\n\n

A wood pattern running production volumes wears out of tolerance before you finish the order \u2014 I\u2019ve seen loose patterns lose a full thousandth per side after 100 molds. Material choice locks in how many good castings you\u2019ll get before the pattern needs rebuilding.<\/p>\n\n\n\n

Wood<\/h3>\n\n\n\n

The default for prototypes and low-volume work. Mahogany and sugar pine machine cleanly and resist warping better than softwoods. Dense particle-board (craft-wood) holds dimensions under sandpaper better than balsa, forcing intentional sizing rather than accidental material removal. Typical lifespan: 50-200 molds before wear requires repair.<\/p>\n\n\n\n

Plastic and Urethane<\/h3>\n\n\n\n

Urethane tooling board machines to tight tolerances and holds up for 500-2,000 molds without absorbing moisture like wood. Cost runs two to three times a comparable wood pattern.<\/p>\n\n\n\n

Metal (Aluminum and Cast Iron)<\/h3>\n\n\n\n

Production patterns for long-run programs. Aluminum patterns last 5,000-10,000+ molds; cast iron is heavier but nearly indestructible for high-volume green sand lines.<\/p>\n\n\n\n

One critical detail when switching from wood to metal: metal patterns require double shrinkage allowance. The metal pattern itself was cast and already shrank once. Apply single shrinkage to a cast metal pattern, and every gray iron or steel casting it produces comes out undersize. I\u2019ve seen shops miss this on their first metal pattern job \u2014 it\u2019s an expensive rework that sets the whole program back weeks.<\/p>\n\n\n\n

3D-Printed Patterns<\/h3>\n\n\n\n

Printed patterns fill a niche: complex geometries at low volumes where traditional lead time or cost can\u2019t be justified. They work for prototyping, but build volume constraints and material limitations<\/a> make them impractical for production runs.<\/p>\n\n\n\n

Pattern Allowances<\/h2>\n\n\n\n

Five separate adjustments get built into every pattern, and applying them out of order compounds the error on every dimension. The sequence below is what I walk through on every new pattern job.<\/p>\n\n\n\n

The Allowance Sequence<\/h3>\n\n\n\n

Apply in this order: (1) start with final required dimensions, (2) add shrinkage, (3) add machining stock, (4) apply draft, (5) modify for distortion, (6) reduce for shake. Reversing steps two and three inflates machining allowance by the shrinkage factor \u2014 small on a 6-inch part, meaningful on a 36-inch one.<\/p>\n\n\n\n

Shrinkage Allowance<\/h3>\n\n\n\n

Every metal contracts as it cools from pouring temperature to room temperature. The values below are starting points by alloy family:<\/p>\n\n\n\n

Alloy<\/th>Shrinkage Rate<\/th><\/tr><\/thead>
Gray cast iron (ASTM A48)<\/td>0.7-1.05%<\/td><\/tr>
Ductile iron (ASTM A536)<\/td>0-1.0%<\/td><\/tr>
Carbon cast steel (ASTM A216)<\/td>2.0-2.1%<\/td><\/tr>
Stainless cast steel (ASTM A351)<\/td>2.0-2.3%<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n

(Non-ferrous families \u2014 aluminum ~1.3%, brass\/bronze ~1.5% \u2014 sit between iron and steel, but the four ferrous rows above cover what a steel and iron foundry actually pours.)<\/p>\n\n\n\n

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

Actual shrinkage varies with section thickness, geometry constraints, and mold rigidity. Take a large ductile iron casting where the pattern shop applies a textbook 1.0% only to measure 0.7% actual contraction: on a 12-inch feature the error is 0.036 inches oversize \u2014 machinable. On a 50-inch feature, that same 0.3% difference produces a 0.15-inch error exceeding the machining envelope. Always pour a trial run on new patterns and measure actual shrinkage before production.<\/p>\n\n\n\n

Ductile iron is especially unpredictable \u2014 the range spans from nearly zero to a full percent depending on graphite nodule count and section modulus.<\/p>\n\n\n\n

Machining Allowance<\/h3>\n\n\n\n

Extra material on surfaces that will be machined to final dimensions:<\/p>\n\n\n\n

    \n
  • Small castings (under ~300 mm): 1.5-3 mm per surface<\/li>\n
  • Medium castings (300-800 mm): 3-6 mm per surface<\/li>\n
  • Large castings (over 800 mm): 6-12 mm per surface<\/li>\n<\/ul>\n\n\n\n

    Add machining allowance only to surfaces requiring finishing \u2014 not every face.<\/p>\n\n\n\n

    Draft Allowance<\/h3>\n\n\n\n

    Taper on vertical surfaces so the pattern withdraws without tearing the mold. External surfaces need 1-3 degrees; internal surfaces need 2-5 degrees because the sand grips internal features tighter.<\/p>\n\n\n\n

    On a 100 mm vertical wall with 2-degree draft, the base will be approximately 3.5 mm wider than the top \u2014 a change that affects as-cast wall thickness and stress calculations.<\/p>\n\n\n\n

    Distortion and Shake Allowance<\/h3>\n\n\n\n

    Distortion allowance is pre-distortion \u2014 building the opposite error into the pattern to cancel real warping. An L-shaped bracket prone to inward bending gets made with outward-bent arms. No universal table exists; this requires experience with the specific alloy and geometry.<\/p>\n\n\n\n

    Shake allowance is the one allowance that makes the pattern smaller, not larger. When the moldmaker raps the pattern to loosen it before extraction, the cavity enlarges slightly \u2014 typically 0.5-1 mm per 100 mm. On small, precise castings, ignoring shake puts you consistently oversize.<\/p>\n\n\n\n

    Pattern Mistakes That Cost You Castings<\/h2>\n\n\n\n

    The most common mistake I see in new patterns isn\u2019t wrong dimensions \u2014 it\u2019s design decisions that create defects no amount of gating optimization can fix.<\/p>\n\n\n\n

    Sharp Corners and Hot Spots<\/h3>\n\n\n\n

    Sharp rectangular interior corners create hot spots where metal pools and solidifies last. Shrinkage porosity forms there \u2014 voids hiding exactly where stress concentrates. Adding radius to all interior corners ensures uniform metal flow and consistent cooling. Most experienced pattern shops won\u2019t build sharp interior corners regardless of the print, because they know the castings come back as scrap.<\/p>\n\n\n\n

    Missing or Insufficient Draft<\/h3>\n\n\n\n

    Flat surfaces without adequate draft cause the pattern to drag across the sand during extraction, tearing sand into the mold cavity. Those sand inclusions show up in the finished casting as rough patches that destroy tooling during machining. Internal surfaces are worse: skimping from 3 degrees down to 1 degree can turn an easy mold release into a destroyed cavity.<\/p>\n\n\n\n

    Using a Core as the Single Datum Point<\/h3>\n\n\n\n

    Designers sometimes reference all critical dimensions from a single core position. Cores shift during mold assembly and pouring. When the datum core moves even slightly, every dimension referenced from it moves too \u2014 producing a dimensionally scattered casting no single machining setup can save. Pin your cores to minimize movement, and add machine stock on core-referenced surfaces for correction room during finish machining.<\/p>\n\n\n\n

    Before You Build: A Pattern Checklist<\/h2>\n\n\n\n

    Pattern quality determines casting quality \u2014 no gating system compensates for a poorly designed pattern. Before committing to tooling:<\/p>\n\n\n\n

      \n
    • Match pattern type to production volume. A $6,000 match plate for 20 pieces is as wasteful as running 1,000 pieces off loose patterns.<\/li>\n
    • Run a DFM review with your foundry before pattern construction. Changes at the CAD stage cost a fraction of what rework costs after the pattern is built.<\/li>\n
    • Apply allowances in sequence \u2014 shrinkage, machining, draft, distortion, shake \u2014 not in isolation.<\/li>\n
    • Radius every interior corner. Flag sharp corners before the pattern shop builds exactly what you drew.<\/li>\n
    • Budget for a trial pour. Measure the first casting against the print and adjust before production.<\/li>\n<\/ul>\n\n\n\n

      The pattern is the most upstream tooling decision in your casting program \u2014 every shortcut here multiplies through every mold it stamps out. Get it right once, and you\u2019ve solved the problem for the life of the program.<\/p>\n\n\n\n