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Chapter 3 Overview

Chapter 3 Overview. Several variations to die casting The variations depend on several factors: Temperature of the metal pump Consistency of the metal when it is injected Metal velocity Gating configuration Condition of die cavity at moment of injection . Chapter 3 Objectives.

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Chapter 3 Overview

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  1. Chapter 3 Overview • Several variations to die casting • The variations depend on several factors: • Temperature of the metal pump • Consistency of the metal when it is injected • Metal velocity • Gating configuration • Condition of die cavity at moment of injection

  2. Chapter 3 Objectives • Identify the two major methods of injecting metal into the die • List advantages of hot chamber die casting • Explain why cold chamber die casting is used • Explain how vacuum die casting can reduce defects • List two emerging die casting technologies

  3. New Terms • Billet • A small metal bar • Static metal pressure • The metal pressure in the die cavity at the instant that the cavity is full • Thixotropy • The property of a fluid mixture to become more fluid as the mixture is agitated

  4. Two Major Processes • Hot chamber and cold chamber die casting • Get name from temperature of metal pump relative to temperature of the metal • In hot chamber, the metal pump is submerged in the metal and is same temperature as the metal • In cold chamber, the metal pump is outside the furnace and is cold relative to the metal ladled into it

  5. Hot Chamber Components • A Frame • Suspends shot components above and in the furnace; mounted to stationary platen • Shot cylinder • Actuates vertically; metal is injected with downward stroke of the shot cylinder

  6. Hot Chamber Components • Coupling • Connects shot cylinder to plunger rod and tip • Plunger rod and tip • Pumps the metal; piston tip has 2 or 3 grooves in it for piston rings

  7. Hot Chamber Components • Rings • Prevents metal from bypassing tip; helps maintain metal pressure after die cavity is filled • Gooseneck • Combination sleeve and metal path out of the metal pump

  8. Hot Chamber Components • Nozzle • Tube connecting gooseneck to die cast die; extends from gooseneck, through stationary platen, to die cast die; it’s heated to keep the metal liquid in the nozzle • Sprue bushing • What the nozzle seats against; cooled to assure the metal in it freezes

  9. Hot Chamber Machine Cycle 3-8

  10. Hot Chamber Process Uses • For low melting point alloys and alloys with a small aluminum constituent • These alloys include those made from: • Lead • Tin • The Zamak family of zinc alloys, ZA8 zinc alloy and a small amount of AZ91D magnesium alloy

  11. Cold Chamber Components • C-frame • Structural framework that supports the shot components; mounted to stationary platen • Shot cylinder • Mounted to the C-Frame; injects metal with its horizontal stroke

  12. Cold Chamber Components • Coupling • Connects the shot cylinder to the plunger rod and tip • Cold chamber • Shot sleeve or tube that the plunger slides in to pump the metal

  13. Cold Chamber Components • Plunger rod and tip • Pumps the metal; made from highly conductive material and is water-cooled • Conventional cold chamber plunger tips do not have rings; newest technology indicates that the tip may benefit from a design with rings

  14. Cold Chamber Machine Cycle 3-14

  15. Cold Chamber Process Uses • Used for high melting point alloys and alloys with a significant aluminum constituent: • Aluminum • Copper • Magnesium • Iron • Titanium • Composite materials

  16. Hot Chamber Advantages • Metal temperature control is better maintained • Metal transfer not required • Cooling of piston tip and sleeve not required • Fewer oxidation losses

  17. Process Limitations/Variations • Conventional process limited by internal porosity • Due to trapped gases or solidification shrinkage • New technologies developed to provide denser castings • Use vacuum, squeeze casting, or semi-solid and thixotropic melting/casting methods

  18. Vacuum Process • Die cavity evacuated using a vacuum pump • Several commercially available systems • Limitations to how complete a vacuum can be achieved; 26-27 inches of mercury seems adequate for most applications

  19. Squeeze Casting • Gate velocity is much lower • Gate thickness is much higher • Metal pressures at the end of cavity filling are much higher

  20. Semi-Solid Casting • Alloy cast is part liquid and part solid • Premise: defects form during solidification; solids therefore should be free of defects • Billet of material is preheated to casting temp. in a specially constructed induction heater • Billet placed in cold chamber, then injected • Uses low gate velocities and high metal pressures to make very dense castings

  21. Thixotropic Casting • Takes advantage of the thixotropy of alloy • When injected and forced through the gate, literally sheared and agitated, it flows like a plastic material • Injection system is a combination of the screw used in plastic injection and the plunger used in conventional die casting

  22. Summary • Two major processes • Components are similar • Process used is based on alloy’s melting point • Hot chamber process has several advantages • Process variations are based on new technologies • Try to overcome conventional die casting’s limitation of internal porosity

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