Presentation on theme: "Sodium Silicate-CO2 Molding"— Presentation transcript:
1 Chapter 12 Expendable-Mold Casting Processes (II) EIN 3390 Manufacturing Processes Spring, 2012
2 Sodium Silicate-CO2 Molding Molds and cores can receive strength from the addition of 3-6% sodium silicate (Water Glass)Remains soft and moldable until it is exposed to CO2Na2SiO3+CO2 ->Na2CO3+SiO2 (Colloidal)Hardened sands have poor collapsibilityDifficult for shakeout and core removalHeating from pour makes the mold stronger
3 No-Bake, Air-Set, or Chemically Bonded Sands Involves room-temperature chemical reactionsOrganic and inorganic resin binders can be mixed with the sand before the molding operationCuring reactions begin immediatelyNo-bake sand can be compacted by light vibrationsWood, plastic, fiberglass, or Styrofoam can be used as patternsSystem selections are based on the metal being poured, cure time desired, complexity and thickness of the casting, and the possibility of sand reclamation
4 No-Bake Sands Air-Set, or Chemically Bonded Sands High dimensional precision and good surface finishFor almost all engineering metalsGood hot strengthHigh resistance to mold-related casting defectsMolds decompose readily after the metal has been poured, providing good shakeoutCost of no-bake molding is about 20-30% more than green-sand moldingLimited to low-medium production quantities
5 Shell Molding Basic steps Individual grains of fine silica sand are precoated with a thin layer of thermosetting resin and heat-sensitive liquid catalyst.A metal pattern (usually some form of cast iron) is preheated to 230 – 3150cHeat from the pattern partially cures a layer of materialA strong, solid-bonded region adjacent to the pattern is formed in mm in thickness.2) Pattern and sand mixture are inverted and only the layer of partially cured material remains3) The pattern with the shell is placed in an oven and the curing process is completed
6 Shell Molding Basic steps (- continue) 4) Hardened shell is stripped from the pattern5) Shells are clamped or glued together with a thermoset adhesive6) Shell molds are placed in a pouring jacket and surrounded with sand, gravel, etc. for extra supportCasting Materials:Casting irons, alloys of aluminum, and copper
7 Shell Molding Advantages: Excellent dimensional accuracy with tolerance of 0.08 – 0.13 mmVery smooth surfacesExcellent Collapsibility and permeabilityLess cost of cleaning, and machiningLess amount of required mold materialHigh productivity, low labor costs.
8 Shell Molding Disadvantages: Cost of a metal pattern is often high Design must include the gate and the runnerExpensive binderLimited Part size
9 Dump-Box Shell Molding Figure Schematic of the dump-box version of shell molding. a) A heated pattern is placed over a dump box containing granules of resin-coated sand. b) The box is inverted, and the heat forms a partially cured shell around the pattern. c) The box is righted, the top is removed, and the pattern and partially cured sand is placed in an oven to further cure the shell. d) The shell is stripped from the pattern. e) Matched shells are then joined and supported in a flask ready for pouring.
10 Shell-Mold PatternFigure (Top) Two halves of a shell-mold pattern. (Bottom) The two shells before clamping, and the final shell-mold casting with attached pouring basin, runner, and riser. (Courtesy of Shalco Systems, Lansing, MI.)
12 Other Sand-Based Molding Methods V-process or vacuum moldingVacuum serves as the sand binderApplied within a specific vented pattern, drawing the sheet tight to its surfaceFlask is filled with vibrated dry, unbonded sandCompacts the sand and gives the sand its necessary strength and hardnessWhen the vacuum is released, the pattern is withdrawn
13 V-ProcessFigure Schematic of the V-process or vacuum molding. A) A vacuum is pulled on a pattern, drawing a heated shrink-wrap plastic sheet tightly against it. b) A vacuum flask is placed over the pattern and filled with dry unbonded sand, a pouring basin and sprue are formed; the remaining sand is leveled; a second heated plastic sheet is placed on top; and a mold vacuum is drawn to compact the sand and hold the shape. c) With the mold vacuum being maintained, the pattern vacuum is then broken and the pattern is withdrawn. The cope and drag segments are assembled, and the molten metal is poured.
14 Advantages and Disadvantages of the V-Process Absence of moisture-related defectsBinder cost is eliminatedSand is completely reusableFiner sands can be usedBetter surface finishNo fumes generated during the pouring operationExceptional shakeout characteristics
15 Advantages and Disadvantages of the V-Process Relatively slow processUsed primarily for production of prototypesLow to medium volume partsMore than 10 but less than 50,000
16 12.3 Cores and Core MakingComplex internal cavities can be produced with coresCores can be used to improve casting designCores may have relatively low strengthIf long cores are used, machining may need to be done afterwardsGreen sand cores are not an option for more complex shapes
17 Dry-Sand Cores Produced separate from the remainder of the mold Inserted into core prints that hold the cores in positionDump-core boxSand is packed into the mold cavityScrap level with top surface (like paring line)Invert box and leave molded sand on a plateSand is baked or hardenedSingle-piece cores in a split-core boxTwo-halves of a core box are clamped together
18 Dry-Sand CoresFigure V-8 engine block (bottom center) and the five dry-sand cores that are used in the construction of its mold. (Courtesy of General Motors Corporation, Detroit, MI.)
19 Additional Core Methods Core-oil process (1% vegetable oil)Sand is blended with oil to develop strengthWet sand is blown or rammed into a simple core boxIn convection ovens at 200 – 2600c for curingHot-box methodSand is blended with a thermosetting binderHeat to 230 0c for curingCold-box processBinder coated sand is packed and then sealedGas or vaporized catalyst polymerizes the resin
20 Additional Core Methods Figure (Left) Four methods of making a hole in a cast pulley. Three involve the use of a core.Figure (Right) Upper Right; A dump-type core box; (bottom) core halves for baking; and (upper left) a completed core made by gluing two opposing halves together.
21 Additional Core Considerations Air-set or no-bake sands may be usedEliminate gassing operationsReactive organic resin and a curing catalystShell-moldingCore making alternativeProduces hollow cores with excellent strengthSelecting the proper core method is based on the following considerationsProduction quantity, production rate, required precision, required surface finish, metal being poured
22 Casting Core Characteristics Sufficient strength before hardeningSufficient hardness and strength after hardeningSmooth surfaceMinimum generation of gasesAdequate permeabilityAdequate refractorinessGood collapsibility
23 Techniques to Enhance Core Properties Addition of internal wires or rodsVent holes formed by small wire into coreCores can be connected to the outer surfaces of the mold cavityCore printsChaplets- small metal supports that are placed between the cores and the mold cavity surfaces and become integral to the final casting
24 ChapletsFigure (Left) Typical chaplets. (Right) Method of supporting a core by use of chaplets (relative size of the chaplets is exaggerated).
25 Mold ModificationsCheeks are second parting lines that allow parts to be cast in a mold with withdrawable patternsInset cores can be used to improve productivityFigure (Right) Molding an inset section using a dry-sand core.Figure (Left) Method of making a reentrant angle or inset section by using a three-piece flask.
26 12.4 Other Expendable-Mold Processes with Multiple-Use Patterns Plaster mold castingMold material is made out of plaster with additives to improve green strength, dry strength, permeability, and castabilitySlurry is poured over a metal patternHydration of plaster produces a hard moldBake plaster mold to remove excess waterImproved surface finish and dimensional accuracyLimited to the lower-melting-temperature nonferrous alloys
27 12.4 Other Expendable-Mold Processes with Multiple-Use Patterns Antioch processVariation of plaster mold casting50% plaster, 50% sand mixed with waterImprovement of permeability and reduce solidification time
29 Ceramic Mold Casting Mold is made from ceramic material Ceramics can withstand higher temperaturesGreater cost and not reusable for moldShaw processReusable pattern inside a slightly tapered flaskMixture sets to a rubbery state that allows the part and flask to be removedMold surface is then ignited with a torch
30 Ceramic Mold CastingFigure Group of intricate cutters produced by ceramic mold casting. (Courtesy of Avnet Shaw Division of Avnet, Inc., Phoenix, AZ)
31 Other Casting Methods Expendable graphite molds Rubber-mold casting Some metals are difficult to castTitaniumReacts with many common mold materialsPowdered graphite can be combined with additives and compacted around a patternMold is broken to remove the productRubber-mold castingArtificial elastomers can be compounded in liquid form and poured over the pattern to produce a semirigid moldLimited to small castings and low-melting-point materials
32 12.5 Expendable-Mold Processes Using Single-Use Patterns Investment castingOne of the oldest casting methodsProducts such as rocket components, and jet engine turbine bladesComplex shapesMost materials can be castedFigure Typical parts produced by investment casting. (Courtesy of Haynes International, Kokomo, IN.)
33 Investment Casting Sequential steps for investment casting 1) Produce a master pattern2) Produce a master die3) Produce wax patterns4) Assemble the wax patterns onto a common wax sprue5) Coat the tree with a thin layer of investment material6) Form additional investment around the coated cluster
34 Investment CastingSequential steps for investment casting (- continue)7) Allow the investment to harden8) Remove the wax pattern from the mold by melting or dissolving9) Heat the mold10) Pour the molten metal11) Remove the solidified casting from the mold
35 Advantages and Disadvantages of Investment Casting Complex shapes can be castThin sections, down to 0.4 mm can be madeExcellent dimensional precisionVery smooth surfaceMachining can be eliminated or reducedEasy for process steps automationDisadvantageComplex processCostly for dieQuantity of investment casting 100 – 10,000/year
36 Investment CastingFigure Investment-casting steps for the flask-cast method. (Courtesy of Investment Casting Institute, Dallas, TX.)
37 Investment CastingFigure Investment-casting steps for the flask-cast method. (Courtesy of Investment Casting Institute, Dallas, TX.)
38 Investment CastingFigure Investment-casting steps for the shell-casting procedure. (Courtesy of Investment Casting Institute, Dallas, TX.)
39 Investment CastingFigure Investment-casting steps for the shell-casting procedure. (Courtesy of Investment Casting Institute, Dallas, TX.)
41 Counter-Gravity Investment Casting Pouring process is upside downVacuum is used within the chamberDraws metal up through the central sprue and into the moldFree of slag and drossLow level of inclusionsLittle turbulenceImproved machinabilityMechanical properties approach those of wrought materialSimpler gating systemsLower pouring temperaturesImproved grain structure and better surface finish
42 Evaporative Pattern (Full-Mold and Lost-Foam) Casting Reusable patterns can complicate withdrawalMay mandate design modificationsEvaporative pattern processesPattern is made of expanded polystyrene (EPS)or polymethylmethacrylate (EPMMA)Pattern remains in the mold until the molten metal melts away the patternIf small quantities are required, patterns may be cut by handMaterial is lightweight
43 Evaporative PatternsMetal mold or die is used to mass- produce the evaporative patternsPattern: 2.5% polymer, 97.5% airFor multiple and complex shapes, patterns can be divided into segments or slicesAssembled by hot-melt gluingFull-mold processGreen sand is compacted around the pattern and gating system
44 Evaporative Patterns Lost-foam: (see Figure 12-32) Make polystyrene pattern assemblyMake a thin refractory coating for Polystyrene patternPlace dried pattern into a flask surrounded by fine unbounded sandCompact sand by vibrationPour molten metal onto patternDump sand and remove casting from flaskBackup sand can be reused
45 Lost Foam ProcessFigure Schematic of the lost-foam casting process. In this process, the polystyrene pattern is dipped in a ceramic slurry, and the coated pattern is then surrounded with loose, unbonded sand.
46 Advantages of the Full-Mold and Lost-Foam Process Sand can be reusedCastings of almost any sizeBoth ferrous and nonferrous metalsNo draft is requiredComplex patternsSmooth surface finishCores are not requiredAbsence of parting linesHigher metal yield
47 Lost-Foam CastingFigure The stages of lost-foam casting, proceeding counterclockwise from the lower left: polystyrene beads→ expanded polystyrene pellets → three foam pattern segments → an assembled and dipped polystyrene pattern → a finished metal casting that is a metal duplicate of the polystyrene pattern. (Courtesy of Saturn Corporation, Spring Hill, TN.)
49 12.6 Shakeout, Cleaning, and Finishing Final step of casting involves separating the molds and mold materialShakeout operationsSeparate the molds and sand from the flasksPunchout machinesForce entire contents of a flask from a contanerVibratory machinesRotary separatorsRemove sand from casting (iron, steel, brass)Blast cleaningRemove sand, oxide scale, parting line burrs.
50 12.7 SummaryDifferent expendable-mold casting processes are developed to create shaped containers, and then utilize liquid fluidity and subsequent solidification to produce desired shapes of casting products.Each process has unique advantages and disadvantagesBest method is chosen based on the product shape, material and desired properties
51 Homework for Chapter 12:Review questions: 6, 11, 34, 42, 48, 49 (on page 311 – 312)Problems: 1-b, 1-d (on page 122)