Diecasting Basics

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Diecasting Basics

• Presented by Eastern Alloys
• Derek Cocks
• Ryan Winter

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A Brief history of Die Casting

• First machines were developed for line-o-type
  printing using Lead alloys
• Die casting machines for engineered parts was
  developed in USA by Doehler. Patented in 1905
• First machines were hand operated, often using
  compressed air directly on the metal, lead or
  zinc
• Modern machines use hydraulics to develop high
  pressures (several thousand psi ) very fast
  fill times
• Cold chamber machines were developed for high
  melting point alloys Al, Mg, Brass, Stainless
  steel Uranium!

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Some Early Die-Cast Machines First Plunger
machine Patented by 1905 by Doehler
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The Diecasting Machine
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Hot Chamber Diecasting (Large to Medium sized
Machines)
Production rates 120 to 500 shots per hour
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Hot Chamber Diecasting (Miniature Components)
Shot rates in excess of 1500 shots/hr
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The Cold Chamber Process
Used for higher melting point alloys e.g. ZA 12
27 (Zinc), Aluminum, Brass Magnesium Alloys
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ZA 27 Winch Gear Housing

• ZA27 replaced Diecast Aluminium
• Same dies used with no changes
• The max. load was increased from 2500 to 3500
  pounds
• With bigger motor load was further increased to
  4500 pounds

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Metallurgy of Diecasting Alloys

• All alloys have low melting points
• Short freezing range i.e.. close to eutectic
• May be Binary or Ternary based
• Properties are decided by fast solidification
  rate

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Diecasting Sequence
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• Start of Cycle
• As soon as die locking force is confirmed, the
  shot may start. Plunger is above the inlet port
  and metal has filled the shot sleeve.
• Plunger Descends at slow stroke speed but no
  metal is pumped through the gooseneck until the
  port is covered

• Start of Slow Stroke
• After the inlet port is covered, the metal is
  pumped upwards through the gooseneck and into the
  nozzle
• Providing the speed is not too high the air is
  pushed ahead of the metal and through the vents
• Slow stroke should continue until metal reaches
  the nozzle or slightly into the runner system.
  It must be adjusted carefully using shot
  monitoring where available

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• Fast Shot Start
• Metal is now at the nozzle, or just into the
  runner. The Fast shot switch opens the valve to
  the accumulator and the plunger accelerates to a
  high speed.
• The speed of the plunger is related to the cavity
  fill time and the gate velocity therefore, it
  must be controlled
• 4. Die Full Packing and Freeze
• At the end of the die filling, the plunger
  decelerates rapidly giving high metal inertia
  pressures that may cause flashing.
• At this point, the metal pressure should be
  maintained as the casting freezes so to densify
  the part.

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• Plunger Creep
• If the rings or sleeve is worn, blow-by occurs,
  giving plunger creep.
• If the plunger bottoms out as shown in the
  diagram, the packing pressure will not be
  maintained, and porous castings will result.
  Blow-by will increase the wear rate of both
  plunger and sleeve, that are costly to replace.
  Therefore look for signs and replace rings often
  and regularly.
• Low Metal Levels
• If the the metal level is low, the gooseneck
  contains more air so the slow-stroke length will
  be too short to vent it before fast shots cuts
  in.
• If cold ingots are added to the machine pot to
  make up the level the pot temperature will drop
  and defective castings will result.
• The diagram shows the position at the end of
  slow-stroke.

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Process Control
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Die Filling Process Control Values

• As a shot is made, several things happen in
  sequence. The most important features are listed
  in order which they occur, with a description of
  the sort of effects each has on casting quality.

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Slow Stroke Speed

• It gives time for the gasses in the nozzle (or
  cold chamber) and runner system to be vented
  through the die. It conserves the energy in the
  accumulator for the die filling process.
• Too Low Increases the cycle time and may allow
  metal to freeze in the cold chamber or nozzle.
• Too High Gives insufficient time for venting.
• Normal speed ½ to 2 seconds total.

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Fast Shot Change Over Position

• Normally activated by a limit switch, the
  changeover does not occur instantly. First, the
  solenoid operated pilot valve opens, followed by
  the main valve, pressure builds up and the
  plunger and oil column accelerate. The time
  taken varies with the machine, and shot
  monitoring is the only way to check exactly when
  full fast shot speed is attained.
• Too late Some cavity filling occurs with slow
  velocity and metal may freeze at the gate.
• Too early Insufficient time for venting. The
  energy in the accumulator may be used before the
  casting is made.
• Normal position Change over at the nozzle ( or
  cold chamber full position) so die changes do not
  affect it.
• Ensure that fast shot velocity is attained before
  metal reaches the gate.

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Cavity Fill Time

• Long fill times give cold shuts, unless a high
  die temperature is used. The following rough
  values may be used

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Minimum Die Temp. vs. Max. fill time
For Zamak alloys with plated surfaces
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Gate Velocity

• During die filling all the pressure is converted
  to velocity at the gate. The atomized metal
  fills the die with sufficient energy to
  redistribute the gas, prevent concentration of
  porosity, and go into every crevice of the die.
• Too low (lt100 FPS) Porosity, lack of fill,
  blisters and gate porosity.
• Too high (gt160/200 FPS) Gate erosion, excessive
  vortexing, which may give cold shuts produced at
  the end of cavity fill.

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Final Metal Pressure

• This will help to densify the casting by feeding
  molten metal to fill voids produced as
  liquid-to-liquid shrinkage occurs. It is no
  substitute for filling die castings under low
  pressure (hence low velocity) since pressure
  cannot be transmitted to thick sections once the
  gate has been frozen.
• Too high flash, lamination and movement of
  sliding cores.
• Too low porosity, particularly in thick
  sections and close to the gate.
• Normal range 2000-5000 psi (up to 10000 psi if
  an intensifier is used)

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Plunger Creep/ Intensifier Delay Time

• In a hot chamber machine plunger creep is mainly
  due to metal flowing past the plunger after the
  shot is made and may result in the plunger
  bottoming. In the cold chamber process the
  final metal pressure is increased by an
  intensifier. In order to reduce flashing the
  intensifier may not be triggered until the
  casting is partly solidified.

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Dry Shot Speed and Fast Shot Valve Calibration

• By measuring these two values the PQ2 technique
  may be used to predict the effects of making
  changes to the machine settings without the need
  to monitor more shots.
• Remove the plunger (hot chamber) or dont fill
  the shot well (cold chamber) and measure the
  plunger speed at max. speed setting and at
  successively lower valve settings using the same
  hydraulic pressure.

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Die Filling Process Control Values
PQ2 Machine Capability
Shot Valve Calibration
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Die Casting Process
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Process Selection
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1 1 1 1 1 1 1 1 1 1 1 1 1
Casting Process Selection
PRESSURE DIE CASTING     PERMANENT MOLD
CASTING     SAND CASTING
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Diecasting

• Net Shape Process
• High Volumes (low piece price)
• Complex Shape (part consolidating)
• High Strength (compared to gravity casting)

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Sand Casting

• Greatest configurational flexibility
• Lowest mould cost
• Low volume ? min 100s castings

Most Common alloys ZA12 ZA27
ZA12 castings for Petroleum Industry -NON SPARKING
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Permanent Mould (Gravity) Casting

• Near net-shape process
• Medium volumes ? min 1,000s castings
• Alloy choice based on the end-use requirements

ZA8 Emergency Vehicle Ladder Bracket Assembly
Spir-o-lizer in ZA12
ZA27 Motor Mounts
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Permanent Mould (Gravity) Casting

• Cast Iron, or high density, fine grained graphite
  moulds
• Small to medium sized volumes
• Good for prototyping

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Process Comparisons
5 best 1 worst
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Primary Process Variables for Diecasting
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Primary Process Variables for Diecasting

• MATERIAL
• Purchase on spec
• Save certificates run charts
• Check pots regularly
• METAL MELTING HANDLING
• Metal Level
• Metal Temperature
• Drossing
• Segregation
• DIE TEMPERATURE
• Surface contact pyrometer available
• Nozzle temperature control
• Die face temperature measurement
• Die temperature control

• DIE LUBRICANT
• Purchase from quality supplier ( tech service)
• Control mix
• Control of pressures times
• Spray pattern set/control
• DIE FILLING
• Principles of PQ2 (machine die)
• Runner and feed design
• Design for fast automatic running
• Control of machine settings
• Shot monitoring

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Primary Process Variables for Diecasting
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Engineering Properties of Die cast Alloys
CONTENTS of This Section - Health
Issues Physical Properties Mechanical Properties
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Zinc is non-toxic essential to life
A daily dietary Zn intake of 15 mg.is recommended
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Physical Properties
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Examples of mass being useful
Zinc alloy gives this razor a Qualityfeel
Lead weights had to be added to this plastic
phone
Zinc alloys' relatively high density gives the
parts sufficient mass to prevent the mixer
moving on the counter service while in use. The
high damping capacity of the Zinc No 3 alloy used
reduces noise and vibration.
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Mechanical Properties
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DESIGN HINT Designers must bear in mind that
hardness is not directly related to strength or
wear properties and cannot be used to assess the
quality of a die casting.
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Strength to Weight Ratios
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Creep Data - Zamak 3
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