introduction-to-die-casting-session-8 by panniuniu


									Chapter 8 Overview
 The quality of a   die casting is more than
  skin deep
 A quality casting is free of defects
 In order to determine the quality of a casting,
  you must be able to identify the defects
 There are three common types of defects

Chapter 8 Objectives
 Correctly identify the   common surface
 Correctly identify the common internal
 Correctly identify the common types of
  dimensional defects

New Terms
 Inclusions
  • Materials that have been included in the alloy
    that should not be there, such as aluminum
    oxide, silicon carbide, fluxes and sludge
 Polymorphic
  • The ability of, in certain environments, the
    properties of the alumina crystals to change
New Terms cont.
 Porosity
  • A void in the casting, caused by trapped gas or
 Viscous
  • The state of being semi-fluid; not flowing freely

Surface Defects
                    Flow Defects
 •   Cold flow                      •   Poor-fill
 •   Cold shut                      •   Laps
 •   Flow marks                     •   Flow lines
 •   Cold                           •   Swirls
 •   Chill                          •   Knit lines
 •   Severe chill                   •   Mis-run
 •   Non-fill
                    Other Defects
 • Blisters                         • Soldering
 • Cracks
Flow Defects
 Result from how metal
  flows to and within the die
 Adjusting process
  variables can sometimes
  impact their occurrence
    • The alloy begins to freeze
      before the casting is
      completely filled out
    • Several alloy flows converge
      but do not weld or fuse completely together
Flow Defects cont.
       8 Factors Affecting Flow Defects

 Fill time                 Flow distance
 Wall thickness            Gate velocity
 Die temperature           Alloy type
 Alloy temperature         Venting

8 Factors: Fill Time
 The maximum allowable time to    fill the die
  cavity that results in an acceptable casting
  • If exceeded, the casting will have some defect
 Fill time calculation based on several    factors
  •   Die temperature
  •   Alloy temperature
  •   Casting geometry
  •   Alloy being cast
8 Factors: Wall Thickness
 Part of   the casting’s geometry
  • Heavy wall sections equate to a lot of heat and
    high cooling requirements
  • Thin walls equate to very little heat and minimal
    cooling requirements

Flow Defects: Die Temperature
 Time-averaged temperature of        the die during
  sustained production
 Cannot be measured any time at any place in
  the die
 Ideally, it will:
  • Be as high as possible
  • Still permit making the casting
  • Vary as little as possible over the entire cycle
Flow Defects: Alloy Temperature
 Temperature of   the alloy as it begins to fill
  the die cavity, as it passes through the gate
 Hard to measure in real time as the casting is
  being made
 Estimated to determine fill time calculations
 Avoiding delays in alloy transfer can
  minimize temperature losses
8 Factors: Flow Distance
 The distance that the   metal must flow once it
  passes through the gate
 Alloy should flow to its terminal location
  without freezing
 If the flow distance is too long and if the alloy
  speed is too slow, it’s difficult for the metal to
  fill the cavity without beginning to freeze

8 Factors: Gate Velocity
 The speed the alloy travels as   it passes
  through the gate
 If not controlled, can be detrimental to the
  tooling causing washout and erosion
 If too low, the alloy may not atomize and not
  have enough energy to reach the ends of the
  casting or to properly weld together

8 Factors: Alloy Type
 Can make   difference in the surface finish
 Zinc, Zamak 7 was designed to have the best
  fluidity and surface finish
 Silicon content in aluminum aids fluidity
 Alloys closer to the eutectic will be more fluid
 Eutectic alloys are regarded as harder to cast

8 Factors:
Venting and Vacuum
 Trapped air causes blisters and gas porosity
  and backpressure in the cavity
 Back pressure can change the flow enough
  to cause surface defects
 Most noticeable in blind features
 May be necessary to add vacuum to remove
Other Defects: Blisters
 Bubble-like bumps
  on the casting
 Gases trapped in the
  casting near the casting
  surface cause them
 When casting is ejected
  and the casting surface
  is not strong enough to withstand the gas pressure,
  the surface yields and the blister forms
Other Defects: Cracks
 Two major   causes for cracks are:
  • Heat
     – Insufficient
     – Excessive
  • Externally applied stresses

Other Defects: Soldering
   The fusion of aluminum in the
    alloy with iron from the steel
    surface of the die cavity
   When soldering occurs, the
    casting sticks to the cavity;
    casting must be torn away
   Aggravated by higher than usual
    die temperatures, high gate velocities
    and high metal pressures
   Enhanced if the iron content in alloy is low
   Can be caused by insufficient draft angles     8-18
Impact of Internal Defects
 Mechanical properties include:
  • Tensile strength, elongation, hardness, impact
    strength and others
 Measured on samples; results are   published
  to help designers pick best suited material
 Internal defects reduce mechanical

Impact of Internal Defects
 Pressure tightness
  • An important property for some applications
  • The process has to be controlled while making
    solid, low porosity castings
  • Internal defects can cause loss of pressure
 Machineability
  • Affected by porosity and inclusion defects, the
    two types of internal defects                   8-20
Internal Defects:
 Most inclusions are
  non-metallic aluminum
  oxide (corundum)
 Oxides get into the bath
 Most is removed, but
  some remains and ends
  up in castings
 Size and shape of the individual corundum particles
  varies widely
Internal Defects:
Inclusions-Oxide Films and Dross
 Inclusions of oxide films
  and dross are major
  cause for leakers and
  excessive tool wear
 This is generally gamma
  aluminum oxide
 Oxide films prevent divergent
  alloy steams knitting together
  properly as the cavity fills
Internal Defects:
 Silicon carbide
  refractories get into
  castings if furnace-
  cleaning practices
  not maintained
 As damaging
  as corundum
 Encountered infrequently compared to corundum
 Distinguished by its very black, glass-like coloring
Internal Defects:
 Not usually recognized during a   cursory
  visual inspection
 Casting must be submerged in city water
 If flux inclusions are present, they will grow
  crystals on the casting surface
 Appears as light mottling on all surfaces

Internal Defects:
 Composed of complex
  inter-metallic compounds
  of Al-Si-Fe-Mn-Cr
 Is quite hard and will
  damage cutter tooling
 Under high magnification
  sludge is easily recognized by the extremely fine
  primary crystals and their pentagonal shape

Internal Defects:
 A void in thecasting
 Has two root causes:
  • Trapped gas
  • Shrinkage

Internal Defects:
Porosity-Trapped Gas
To solve a gas porosity problem, look at all
  sources of gas generation
 Trapped air
  • Always present because of the turbulent method
    used to fill the die cavity
 Air in cold chamber
  • minimized by filling the cold chamber with alloy

Internal Defects:
Porosity-Trapped Gas
 Turbulence: when alloy is subjected to
  turbulence in the presence of air
  • Minimize when picking up and transporting alloy
    to the cold chamber through ladling practices
  • Slow portion of the shot cycle must be controlled
     – Optimize timing of plunger
     – Accelerate plunger tip when past pour hole
     – When sleeve is filled, follow with a smooth
       acceleration to the fast shot speed
Internal Defects:
Porosity-Trapped Gas
 Improper venting: another cause       for trapped
  • Vents must be open to allow air trapped above
    the alloy in cold chamber to escape
  • If vent is working, a puff of air coming out can be

Internal Defects:
Porosity-Trapped Gas
 Excessive lubricants: can result     in gas from
  two sources
  • Release of combustion products when some of
    the die lube burns when the alloy hits
  • Most releases are diluted with water
  • Water in lube will turn to steam and produce a
    great volume of gas
  • Gas forms when alloy runs over puddled plunger
    tip lube                                       8-30
Internal Defects:
Porosity-Trapped Gas
 Other sources of trapped gas
  • If die cavity cracked, it might allow fluid from the
    cooling line to leak into die cavity
  • Water or oil in the cavity, when hit by the alloy,
    will form gas

Internal Defects:
 Shrinkage: porosity that occurs if    the alloy
  solidifies without pressure on it
  • All alloys shrink a certain percentage
 High pressure die casting
  • Uses intensifiers/other methods to increase alloy
    pressure once cavity has been filled with alloy
  • Alloy pressure must be transmitted from the
    biscuit through the runner to the gate
Internal Defects:
 Shrink defects:
  occur at the last place
  in the casting to freeze
  • characterized by a rough
    and jagged appearance
  • tends to be continuous
    by nature

Dimensional Defects
 Dimensional variations covered:
  • Linear variation, across parting line variation,
    shift and mismatch, warpage
 Most dimensional defects related to:
  • Die temperatures
  • Condition of the die
  • Force of injection

Dimensional Defects:
Die Temperature
 Thermal expansion/contraction: objects
  lengthen when heated, get smaller when
  • Castings get smaller when cooled
  • Dimensional problem can occur when one half of
    die is much hotter than other half
  • Can be a problem for the die and the casting

Dimensional Defects:
Die Condition
 Flash Buildup at   parting line
  • Prevents the die from closing properly
  • May cause an oversize dimension
  • Prevents wedgelock from holding slide in place
 Flash buildup at   front of slide
  • Prevents slide from going to “ready to cast”

Dimensional Defects:
Die Condition
 Soldering
  • Small core pins can be very susceptible; solder
    buildup can cause an oversize out-of-tolerance
  • May occur in walls; could cause an undersize or
    thin wall

Dimensional Defects:
Force of Injection
 Force of   injection
  • Overcomes locking capability, causing tie bars
    stretch and allowing die to flash
  • Flashing adds to size, cause slides to backout
  • Normal injection force, impact, and intensification
  • Can:
     – Reduce the mass and speed
     – Minimize impact
     – Apply intensification before gates freeze
Dimensional Defects
 Statistical dimensional control
  • Product gets larger or smaller over time
  • Process variables that contribute to the
    dimensional variation need to be identified
  • A control technique, such as the average and
    range chart, needs to be applied

 3 categories of defects: surface, internal, and
  dimensional, and many defects in each
 The operator may or may not have control
  over them
 2 subcategories of surface defects:
  flow and other
 There are many types of flow defects

Summary cont.
 2 subcategories of internal defects:
  inclusions and porosity
 Dimensional defects are related to die
  temperatures, die condition, and the injection


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