Casting Processes Cheatsheet

Cheatsheet Content

### Investment Casting (Lost Wax) - **Process:** 1. **Wax Pattern:** Create a wax replica of the part. 2. **Shell Building:** Dip wax in ceramic slurry, then sand, repeat. 3. **Dewaxing:** Heat to melt out wax, leaving ceramic mold. 4. **Pouring:** Pour molten metal into hot mold. 5. **Finishing:** Break ceramic, remove gates. - **Advantages:** High accuracy, excellent surface finish, complex shapes, versatile materials. - **Disadvantages:** Expensive, time-consuming, limited size/weight. - **Why Use It?** When high precision, intricate details, and excellent surface finish are critical, especially for parts made from difficult-to-machine alloys (e.g., aerospace, medical, jewelry). ### Patterns & Allowances - **Pattern:** A replica of the casting (slightly larger) used to create the mold cavity. - **Functions:** Defines cavity, forms gating system, supports cores, ensures mold filling. - **Types:** Single piece (simple, low volume), Split (easier removal), Match Plate (accurate alignment, moderate-high volume), Cope & Drag (large castings), Skeleton (very large, cost-effective), Sweep (symmetrical shapes). - **Materials:** Wood, metal, plastic, plaster. - **Allowances (Compensations on pattern):** - **Shrinkage:** Pattern made larger to compensate for metal contraction during cooling. Most significant allowance. - **Draft:** Tapered surfaces (0.5°-2°) on pattern for easy removal from mold without damage. - **Machining:** Extra material (2-10mm) on surfaces to be machined later for precision/finish. - **Distortion:** Pattern intentionally distorted opposite to expected warpage for complex shapes. - **Shake (Rapping):** Small allowance for mold cavity enlargement during pattern removal (often negligible). ### Sand Mold & Casting - **Sand Mold Components (Green Sand):** - **Flask:** Holds sand (Cope - top, Drag - bottom, Cheek - middle for 3-part). - **Molding Sand:** Silica sand, clay (binder), water. - **Facing Sand:** Fresh, fine, high refractoriness (contacts metal). - **Backing Sand:** Coarser, recycled (fills flask). - **Gating System:** - **Sprue:** Vertical channel for pouring metal. - **Pouring Basin:** Funnel at sprue top. - **Runner:** Horizontal channels from sprue to gates. - **Gate:** Entry point from runner to mold cavity. - **Riser:** Reservoir of molten metal to feed casting during shrinkage. - **Core:** Pre-formed sand shape for internal features/hollows. - **Vents:** Passages for gases to escape, preventing porosity. - **Sand Casting Steps:** 1. **Pattern Placement:** Pattern in drag, sand rammed. 2. **Parting Line:** Drag inverted, cope placed, parting line defined. 3. **Gating System:** Sprue/riser pins and runner/gate impressions formed. 4. **Mold Compaction:** Sand rammed in cope. 5. **Pattern Removal:** Cope lifted, pattern/pins removed. 6. **Core Setting:** Cores placed in core prints (if needed). 7. **Mold Closing:** Cope aligned on drag. 8. **Clamping/Weighting:** Secure flask against metal pressure. 9. **Pouring:** Molten metal poured into basin, fills mold. 10. **Cooling & Solidification:** Metal cools, riser feeds shrinkage. 11. **Shakeout:** Casting/sand removed from flask. 12. **Cleaning & Finishing:** Remove gates/risers/flash, clean surface, inspect. ### Casting Defects - **Surface Defects:** - **Scabs/Washes:** Rough patches or shallow depressions. **Cause:** Poor sand strength, high pouring temp, excessive moisture, high metal velocity. **Remedy:** Improve sand properties, control temp/velocity. - **Blowholes/Pinholes:** Spherical/elongated cavities. **Cause:** Trapped gases (high moisture, poor venting). **Remedy:** Dry molds, improve vents, increase sand permeability. - **Drops:** Irregular projections from sand fall. **Cause:** Insufficient cope sand strength. **Remedy:** Proper ramming, stronger sand. - **Dimensional Defects:** - **Distortion/Warpage:** Deviation from shape. **Cause:** Uneven cooling, improper design, high internal stress. **Remedy:** Redesign for uniform thickness, controlled cooling, distortion allowance. - **Shrinkage Cavities:** Voids from metal contraction. **Cause:** Inadequate/improper risers. **Remedy:** Optimize riser design/placement. - **Sand Casting Specific Defects:** - **Misrun:** Incomplete filling. **Cause:** Low pouring temp, poor fluidity, thin sections, inadequate gating. **Remedy:** Increase temp, improve fluidity, optimize gating. - **Cold Shut:** Incomplete fusion of metal streams. **Cause:** Low pouring temp, slow pouring. **Remedy:** Increase temp/speed, optimize gating. - **Hot Tear:** Cracks during cooling. **Cause:** Restricted contraction, stress concentration, poor mold collapsibility. **Remedy:** Improve mold collapsibility, redesign corners. - **Inclusions:** Trapped non-metallic particles. **Cause:** Dirty metal, mold/core erosion. **Remedy:** Proper melting, filters, improved mold material. ### Molding Sand Properties - **Permeability:** Allows gases to escape. (High is good; prevents blowholes). Factors: grain size, clay/moisture content, ramming. - **Refractoriness:** Withstands high temps without fusing. (High is good; prevents metal penetration). Factors: sand type, purity. - **Cohesiveness (Green Strength):** Holds shape when moist. (Prevents mold erosion/collapse). Factors: clay/moisture content, binder. - **Adhesiveness:** Sticks to flask. (Keeps sand in place). - **Plasticity (Flowability):** Flows and compacts around pattern. (Ensures accurate cavity, good finish). Factors: grain shape, moisture/clay. - **Collapsibility:** Breaks down easily after solidification. (Prevents hot tears, easy casting removal). Factors: binder, additives. - **Types:** Green (moist, common), Dry (baked, higher strength), Loam (high clay, large castings), Core (special binders, baked), System (reclaimed). ### Core Support Methods - **Core:** Pre-formed sand to create internal features/hollows. - **Core Prints:** Extensions on pattern create recesses in mold; core fits into these for primary support/positioning. - **Chaplets:** Small metal supports placed in mold to hold core in place, especially for heavy/long cores. Fuse with molten metal. Must be clean for proper fusion. ### Centrifugal Casting - **Principle:** Uses centrifugal force to distribute and compact molten metal. - **Types:** - **True Centrifugal:** Mold rotates fast horizontally. Creates hollow cylindrical parts (pipes, tubes) without a core. High density, impurities at inner surface. - **Semi-Centrifugal:** Mold rotates vertically. Produces solid castings with central hole (wheels, gears). Density highest at outer periphery. - **Centrifuging (Pressure Casting):** Entire flask rotates slowly with multiple molds. Force aids filling intricate cavities. - **Advantages:** Dense castings, fine grain, good mechanical properties, excellent surface finish (true centrifugal). - **Disadvantages:** Limited to symmetrical shapes (true), high equipment cost. ### Die Casting - **Process:** Molten metal forced under high pressure into reusable metal dies. - **Types:** - **Hot Chamber:** Melting pot integrated. Piston forces metal directly into die. **Metals:** Low melting point (Zn, Pb, Sn). **Pros:** High rate. **Cons:** Limited metals. - **Cold Chamber:** Metal melted separately, ladled into unheated chamber, then forced into die. **Metals:** High melting point (Al, Mg, Cu). **Pros:** High temp metals. **Cons:** Slower rate, more oxidation. - **Advantages:** High production rate, excellent surface finish, good dimensional accuracy, thin walls. - **Disadvantages:** High tooling cost, limited part size, primarily for non-ferrous, risk of porosity. ### Metal Casting Flowchart 1. **Pattern Making:** Design and create pattern with allowances. 2. **Mold Making:** Prepare molding material, create mold cavity and gating system. 3. **Core Making:** Produce and place cores (if needed). 4. **Melting:** Melt, refine, and bring metal to pouring temperature. 5. **Pouring:** Transfer molten metal to mold. 6. **Cooling & Solidification:** Metal cools, risers feed shrinkage. 7. **Shakeout/Mold Removal:** Separate casting from mold. 8. **Cleaning & Finishing:** Remove excess material, clean, heat treat (if needed), machine. 9. **Inspection:** Check for defects (visual, NDT, dimensional).