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Diecasting Basics

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Title: Diecasting Basics


1
Diecasting Basics
  • Presented by Eastern Alloys
  • Derek Cocks
  • Ryan Winter

2
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!

3
Some Early Die-Cast Machines First Plunger
machine Patented by 1905 by Doehler
4
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
10
The Cold Chamber Process
Used for higher melting point alloys e.g. ZA 12
27 (Zinc), Aluminum, Brass Magnesium Alloys
11
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

12
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

16
  • 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.

17
  • 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.

18
Process Control
19
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.

20
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.

21
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.

22
Cavity Fill Time
  • Long fill times give cold shuts, unless a high
    die temperature is used. The following rough
    values may be used

23
Minimum Die Temp. vs. Max. fill time
For Zamak alloys with plated surfaces
24
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.

25
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)

26
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.

27
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.

28
Die Filling Process Control Values
PQ2 Machine Capability
Shot Valve Calibration
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Die Casting Process
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Process Selection
32
1 1 1 1 1 1 1 1 1 1 1 1 1
Casting Process Selection
PRESSURE DIE CASTING     PERMANENT MOLD
CASTING     SAND CASTING
33
Diecasting
  • Net Shape Process
  • High Volumes (low piece price)
  • Complex Shape (part consolidating)
  • High Strength (compared to gravity casting)

34
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
35
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
36
Permanent Mould (Gravity) Casting
  • Cast Iron, or high density, fine grained graphite
    moulds
  • Small to medium sized volumes
  • Good for prototyping

37
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

41
Primary Process Variables for Diecasting
42
Engineering Properties of Die cast Alloys
CONTENTS of This Section - Health
Issues Physical Properties Mechanical Properties
43
Zinc is non-toxic essential to life
A daily dietary Zn intake of 15 mg.is recommended
44
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.
50
Mechanical Properties
51
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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