Chapter 8 Overview

1
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

2
Chapter 8 Objectives

• Correctly identify the common surface defects
• Correctly identify the common internal defects
• Correctly identify the common types of
  dimensional defects

3
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
  drastically

4
New Terms cont.

• Porosity
• A void in the casting, caused by trapped gas or
  shrinkage
• Viscous
• The state of being semi-fluid not flowing
  freely

5
Surface Defects
Flow Defects

• Cold flow
• Cold shut
• Flow marks
• Cold
• Chill
• Severe chill
• Non-fill

• Poor-fill
• Laps
• Flow lines
• Swirls
• Knit lines
• Mis-run

Other Defects

• Soldering

• Blisters
• Cracks

6
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

7
Flow Defects cont.
8 Factors Affecting Flow Defects

• Fill time
• Wall thickness
• Die temperature
• Alloy temperature

• Flow distance
• Gate velocity
• Alloy type
• Venting

8
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

9
8 Factors Wall Thickness

• Part of the castings 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

10
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

11
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

12
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, its difficult for the metal
  to fill the cavity without beginning to freeze

13
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

14
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

15
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 gasses

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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

17
Other Defects Cracks

• Two major causes for cracks are
• Heat
• Insufficient
• Excessive
• Externally applied stresses

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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

19
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 properties

20
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
  tightness/leaks
• Machineability
• Affected by porosity and inclusion defects, the
  two types of internal defects

21
Internal DefectsInclusions

• 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

22
Internal DefectsInclusions-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

23
Internal DefectsInclusions-SiC

• 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

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Internal DefectsInclusions-Flux

• Not usually recognized during a cursory visual
  inspection
• Casting must be submerged in city water overnight
• If flux inclusions are present, they will grow
  crystals on the casting surface
• Appears as light mottling on all surfaces

25
Internal DefectsInclusions-Sludge

• 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

26
Internal DefectsPorosity

• A void in the casting
• Has two root causes
• Trapped gas
• Shrinkage

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Internal DefectsPorosity-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

28
Internal DefectsPorosity-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

29
Internal DefectsPorosity-Trapped Gas

• Improper venting another cause for trapped air
• 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 seen

30
Internal DefectsPorosity-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

31
Internal DefectsPorosity-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

32
Internal DefectsPorosity-Shrinkage

• 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

33
Internal DefectsPorosity-Shrinkage

• 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

34
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

35
Dimensional DefectsDie Temperature

• Thermal expansion/contraction objects lengthen
  when heated, get smaller when cooled
• 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

36
Dimensional DefectsDie 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
  position

37
Dimensional DefectsDie Condition

• Soldering
• Small core pins can be very susceptible solder
  buildup can cause an oversize out-of-tolerance
  condition
• May occur in walls could cause an undersize or
  thin wall

38
Dimensional DefectsForce 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

39
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

40
Summary

• 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

41
Summary cont.

• 2 subcategories of internal defects inclusions
  and porosity
• Dimensional defects are related to die
  temperatures, die condition, and the injection
  force