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 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 Review course objectives. Tell participants that all testing will be based on the course and chapter level objectives.

New Terms Billet Static metal pressure Thixotropy A small metal bar 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 Discuss sand, investment, and lost foam casting. Use gravity to fill the mold. The mold is destroyed to remove the casting. Mold making is as important part as is making the casting. Metal flow is slow. Walls are much thicker than in die casting. Cycle time longer because of inability of mold material to remove heat. Discuss permanent mold casting. A cousin to die casting. The mold is reused, not destroyed. The process uses gravity to fill the casting. Mold is steel with comparatively good thermal conductivity, hence the release agents used in this process are also insulators. This keeps the casting from freezing periodically and prevents filling. Machines for this process are smaller than machines used to die cast for similar sized castings. Discuss centrifugal casting. Frequently made by jewelers. The choice for low volume castings with a small amount of pressure. The molds are placed around the circumference of a centrifuge. As the centrifuge spins, metal is poured in at the center and centrifugal force distributes the metal to the molds.

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 Discuss sand, investment, and lost foam casting. Use gravity to fill the mold. The mold is destroyed to remove the casting. Mold making is as important part as is making the casting. Metal flow is slow. Walls are much thicker than in die casting. Cycle time longer because of inability of mold material to remove heat. Discuss permanent mold casting. A cousin to die casting. The mold is reused, not destroyed. The process uses gravity to fill the casting. Mold is steel with comparatively good thermal conductivity, hence the release agents used in this process are also insulators. This keeps the casting from freezing periodically and prevents filling. Machines for this process are smaller than machines used to die cast for similar sized castings. Discuss centrifugal casting. Frequently made by jewelers. The choice for low volume castings with a small amount of pressure. The molds are placed around the circumference of a centrifuge. As the centrifuge spins, metal is poured in at the center and centrifugal force distributes the metal to the molds.

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

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

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

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

Hot Chamber Machine Cycle 3-8

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

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

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

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

Cold Chamber Machine Cycle 3-14

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

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

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

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

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

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

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

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