制造工程及技术(热加工)11[1].2

1、CHAPTER 11PART IIMetal Casting Processes Investment Casting Vacuum Casting Permanent-Mold Casting Slush Casting Pressure Casting Die Casting Centrifugal Casting Also called lost-wax process First used 4000 3000 BC The pattern is made of wax or of a plastic by molding or rapid prototyping techniques

2、Term investment derives from the fact that the pattern is invested with the refractory material Need careful handling because they are not strong enough to withstand the forces involved in mold making Wax can be recovered and reused One-piece mold Dried in the air Heated to 90 175 C Held inverted fo

3、r 12 hrs to melt out wax The mold is then heated to 650 1150 C for about 4 hrs depending on the metal to be cast to drive off the water of crystallization After the metal has been poured the mold is broken up and the cast is removed A number of patterns can be joined to make one mold called a tree w

4、hich increases production rateOne-piece mold - cont Small parts The tree can be inserted on to a flask and filled with slurry investment The investment is then placed into a chamber and evacuated to remove air bubbles Next it is placed in a vacuum drawing machine to produce fine detail Not a cheap p

5、rocess Produces fine details Good surface finish Few or no finishing operations Can produce intricate parts from parts weighing 1g 35Kg Ex : Investment die casting examples 1. WAX INJECTION : Wax replicas of the desired castings are produced by injection molding. These replicas are called patterns.2

6、. ASSEMBLY : The patterns are attached to a central wax stick, called a sprue, to form a casting cluster or assembly.3. SHELL BUILDING : The shell is built by immersing the assembly in a liquid ceramic slurry and then into a bed of extremely fine sand. Up to eight layers may be applied in this manne

7、r.4. DEWAX : Once the ceramic is dry, the wax is melted out, creating a negative impression of the assembly within the shell.5. CONVENTIONAL CASTINGIn the conventional process, the shell is filled with molten metal by gravity pouring. As the metal cools, the parts and gates, sprue and pouring cup be

8、come one solid casting. 6. KNOCKOUTWhen the metal has cooled and solidified, the ceramic shell is broken off by vibration or water blasting.7. CUT OFFThe parts are cut away from the central sprue using a high speed friction saw.8. FINISHED CASTINGSAfter minor finishing operations, the metal castings

9、-identical to the original wax patterns-are ready for shipment to the customer.Fig : Investment casting of an integrally cast rotor for a gas turbine. (a) Wax pattern assembly. (b) Ceramic shell around wax pattern. (c) Wax melted out and the mold is filled under a vacuum, with molten super alloy. (d

10、) The cast rotor produced to net or near-net shape. Variation of the investment-casting process Uses same type of wax or plastic pattern as investment casting Patten is then dipped into fluidized bed of Fine- grained fused silica Zircon flour Pattern is then dipped into coarser grained silica to bui

11、ld up additional coatings and proper thickness to withstand the thermal shock of pouring The rest of the procedure follows the investment casting processMixture of fine sand and urethane is molded over metal dies a cured with amine vaporThe mold is partially immersed into molten metal held in an ind

12、uction furnaceThe metal is melted in air or in a vacuumThe molten metal is usually 55 C above the liquidus temperature begins to solidify within a fraction of a second5. Alternative to investment, shell-mold, and green-sand casting6. Relatively low costCalled hard-mold castingTwo halves of a mold ar

13、e made from materials such as iron, steel, bronze, or other alloysThe mold cavity and gating system are machined in to the moldSand aggregate are placed in to the mold prior to casting for producing cavitiesTypical core materials are Oil-bonded or resin-boned sand Plaster Graphite Gray iron Low-carb

14、on steel Hot-worked die steelMold cavity surfaces are coated with refractory slurry to increase the life of the mold every few castingsMechanical ejectors are used to remove complex partsCan produce high production ratesGood surface finishMolten metal is poured into the metal moldA desired thickness

15、 of the solidified skin is obtainedThe remaining metal is poured outThe mold halves are then opened and the casting is removedUsed a graphite or metal moldMolten metal is forced into the mold by gas pressureThe pressure is maintained until the metal solidifies in the moldUsed for high-quality castin

16、gs(a) The bottom-pressure casting process utilizes graphite molds for the productin of steel railroad wheels. (b) Gravity pouring method of casting a railroad wheel. Note that the pouring basin also serves as a riser. Further example of permanent-mold casting Molten metal is forced into the die cavi

17、ty at pressures ranging from .7MPa 700MPa Parts made from here range from: Hand tools Toys Appliance components There are two basic types of die casting machines Hot-chamber - involves the use of a piston to push molten metal in to the die cavity Cold-chamber molten metal is poured in to the injecti

18、on chamber & the shot chamber is not heated 1. The die is closed and the piston rises, opening the port and allowing molten metal to fill the cylinder. 2. The plunger moves down and seals the port pushing the molten metal through the gooseneck and nozzle into the die cavity, where it is held und

19、er pressure until it solidifies. 3. The die opens and the cores, if any, retract. The casting remains in only one die, the ejector side. The plunger returns, allowing residual molten metal to flow back through the nozzle and gooseneck. 4. Ejector pins push the casting out of the ejector die. As the

20、plunger uncovers the filling hole, molten metal flows through the inlet to refill the gooseneck, as in step (1). 1. The die is closed and the molten metal is ladled into the cold-chamber shot sleeve.2. The plunger pushes the molten metal into the die cavity where it is held under pressure until soli

21、dification.3. The die opens and the plunger advances, to ensure that the casting remains in the ejector die. Cores, if any, retract.4. Ejector pins push the casting out of the ejector die and the plunger returns to its original position. Dies are rated according to their clamping force that is neede

22、d Factors involved in selection of die cast machines are Die size Piston stroke Shot pressure Cost Die-casting dies Single cavity Multiple-cavity Combination-cavity Unit dies Ratio of Die weight to part weight is 1000 to 1 Surface cracking is a problem with dies due to the hot metal that is poured i

23、n to them Has ability to produce strong high- quality parts with complex shapes Good dimensional accuracy and surface detailsSingle cavity dieMultiple cavity diec) Combination died) Unit die800 ton hot chamber die casting machine, DAM 8005. This is the largest hot chamber machine in the world and co

24、sts about $1.25 million.Utilizes the inertial forces caused by rotation to distribute the molten metal in to the mold cavitiesFirst used in the 1800;sThree types of centrifugal casting True centrifugal casting Semi centrifugal casting CentrifugingSchematic illustration of the centrifugal casting process. Pipes, cylinder liners, and similarly shaped parts can be cast with this process. (a) Sche