Chapter 3 Overview

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 its 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 alloys melting point
• Hot chamber process has several advantages
• Process variations are based on new technologies
• Try to overcome conventional die castings
  limitation of internal porosity