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Defects and electroplating

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Defects and electroplating

  1. 1. Production of Zinc Diecastings for Electroplating
  2. 2. Relative costs of fabricating, finishing and plating zinc diecastings
  3. 3. Some Design Rules Mimimum crown of 0.15cm per cm If flat surfaces required, use satin instead of bright finish to hide waviness All edges should be rounded off to radius of at least 0.4mm, preferably 0.8mm Reduce depth of concave recesses as much as possible, avoid depths greater than 50% of width If sharply angled grooves are needed, paint the bottom of the grooves, it is cheaper than plating the bottom of the grooves Slots and holes shown have widths at least 2X their depth Spaces between slots should be spaced so that spacing between their centers is 4X their width Blind hole depths should be less than ½ their width and blind holes <5.6mm diameter should be avoided Threaded holes should be countersunk to minimize buildup on their outside The height of fins and ribs should be reduced as much as possible with radius>1.6mm at base Parallel fins should be spaced so distance between centers is >4X fin width Recessed letters preferred to raised letter. Raised letter heights should be <50% of their width If studs threaded before plating, max thickness is 5µm Drain holes should be provided in cup-like contours to avoid hand rinsing
  4. 4. Design factors influencing platability of zinc diecastings
  5. 5. Minimum radii for angles defined by indentations
  6. 6. Checklist for High Quality Castings Properly designed and constructed dies Smooth working, run-in casting machines Correct alloy composition Good melting and delivery practice, proper die lubrication Correct injection and trimming procedures
  7. 7. Die Design Guidelines Plan for location of ejector pins to prevent marks in visible areas, or place on areas that can be easily polished Fill thick sections before thin to allow progressive cooling Alloy should reach vents and overflows last to allow complete die cavity filling Place vents at parting line to allow easy removal of flash Surfaces required to slide on cavity during ejection should be tapered Castings with defects >50µm are not salvageable
  8. 8. Cross sections of rough surface diecastings plated with bright copper in cyanide and acid baths, then with leveling duplex nickel
  9. 9. Casting Fluidity • Zamak Alloys are more fluid than ZA Alloys. • Aluminum increases fluidity for Zamak Alloys – keep Al to high side of range. • Magnesium decreases fluidity, but not as much as aluminum changes.
  10. 10. Fluidity of Zinc Die Casting Alloys Ragone Fluidity, Inches Aluminum, Weight Percent
  11. 11. Solidification Ranges Zamak alloys have smaller freezing ranges than ZA alloys Alloy Solidification Range ºC ( ºF ) Alloy 3 6 (11) ZA-8 29 (52) ZA-12 55 (100) ZA-27 112 (202) Therefore, shrinkage porosity rarely occurs in Zamak alloys
  12. 12. Casting Limits Integranular corrosion can be caused by high levels of Pb, Cd, Sn Casting Limits Zamak 3 Zamak 5 Pb (max) 50 ppm 50 ppm Cd (max) 40 ppm 40 ppm Sn (max) 30 ppm 30 ppm ZA contaminant levels are similar
  13. 13. Effect of humidity test on zinc-aluminum alloy containing cadmium Discolored & As cast plate Cracked humidity-tested panels
  14. 14. As-polished structure of humidity-tested zinc aluminum alloy containing cadmium showing a crack and intergranular corrosion
  15. 15. Intermetallics • Intermetallics are mostly Fe-Al: – Leave “comet tails” after buffing. – Can be removed by stirring, letting the bath stand and skimming. – Machining (tool wear) problems can also result.
  16. 16. Surface dross laden with large and smallFeAl3 intermetallic particles
  17. 17. Tool Wear – ZA-27 Die Casting Many large FeAl3 particles Mean Wear Land Width (mm ) (0.07% Fe) Mean Wear Land Width (in.) Many small FeAl3 particles (0.22% Fe) Drilling Time (min.)
  18. 18. Cosmetic Defects • Cold Shuts • Flaking or waving • Blisters • Solidification • Die Soldering cracking • Surface Shrinkage • Hot tearing • Internal porosity
  19. 19. Cold Shuts • Defined as surface lappings of solidified metal on die castings. • Caused by premature solidification of flowing metal. • Results in line defects at stream intersections
  20. 20. Cold Shuts • Important Control Variables: – Cavity fill time – Gate velocity – Die & metal temperatures – Flow pattern in cavity. • Cold shuts cannot be removed by intensification.
  21. 21. Cold Shut Regions ( a) (b) (a)Surface view of a cold- (b) Higher Magnification Shut region of a casting view of center field in “(a)”
  22. 22. Cold shut in a zinc Cold lap in a zinc diecasting diecasting electroplated conventionally conventionally after electroplated after mechanical buffing polishing and buffing
  23. 23. Eliminating Cold Shuts • Cavity fill time should be 20 ms or less for casting 2 mm (0.080 in) or thinner for chrome plating. • Painted castings can tolerate fill times up to 40 ms. • Die temperature should be at least 200ºC (390ºF) on the surface. • Runner and gates should be designed to produce uniform cavity fill.
  24. 24. Eliminating Cold Shuts • Heat transfer can be retarded by auxiliary heaters, textured dies & die coatings. • Cold shuts shallower than 0.05 mm (0.002 in) can be removed by buffing. • Excessive buffing or sanding can expose subsurface porosity. • Cold shuts act like “notches” can cause brittle fracture.
  25. 25. Examples of thin laps being Lifted by plating layer stresses
  26. 26. Views of shallow laps that usually can be removed by buffing
  27. 27. Blisters • Caused by expansion of gases or corrosion products trapped in pores near plated surface. • Gas in pores is nitrogen or hydrogen (from mold lubricant). • Usually form during premature ejection from die or baking or heat treatment of casting. • Blisters can also occur if a lap is not completely removed – plating stresses lift off the poorly-bonded joint.
  28. 28. Exfoliation of a zinc Skin blister in a zinc diecasting diecasting conventionally plated conventionally after mechanical electroplated after buffing polishing and buffing
  29. 29. Surface Porosity: Blisters • Minimize blistering due to subsurface porosity by limiting ejection temperature. • Minimize blistering due to gas porosity by minimizing trapped gases in casting. Improve feed system, eliminate sharp corners. • Gas should be forced into less critical regions of the casting. • Increase gate velocity to decrease size of pores. • Cooler dies will make pores form more in center of casting.
  30. 30. Small surface pores in a Large surface pores in zinc diecasting a zinc diecasting conventionally conventionally electroplated after electroplated after polishing and buffing polishing and buffing
  31. 31. Examples of blistering of casting during paint baking
  32. 32. Views of Castings with Extensive Surface Lapping As-Cast and Plated After a paint baking heat treatment
  33. 33. Die Soldering • Defined as fusion of cast metal to die steel during casting – sometimes referred to as buildup. • Can be caused by direct impingement of molten metal stream on a flat surface, die erosion, high die temperature or insufficient draft angles. • Soldering due to die erosion usually occurs near the gate – eroded or pitted areas occur.
  34. 34. Die Soldering • Insufficient draft angles or high die temperatures can also roughen the die surface, encouraging soldering. • Best solution is to use a good die lubricant, combined with good metal flow and uniform die temperatures.
  35. 35. Defects Cause by Hot Spots • High die temperatures used to improve surface quality. – Each increase in die temperature of 11ºC (20ºF) above 200ºC (390ºF) has same effect as increasing fill time by 2 ms. • Defects include: 1. Surface Shrinks 3. Solidification cracking 2. Laking or Waving 4. Hot tears
  36. 36. Surface Shrinkage • Usually coincides with hot surface spots on die. • Caused by delayed solidification in this area compared to surrounding areas, hence increased contraction. • Shrinkage areas are shiny on Zamak alloys, frosty on ZA alloys.
  37. 37. Views of Surface Shrinks on a ZA Casting Surface Shrinks Close-up View of Surface within a Shrinkage area
  38. 38. Laking or Waving • Defined as large, irregular patches on die casting surface – can be sunken or raised. • Vary in size & shape, but always in same general area of casting – can have height difference of 0.025 mm (0.001 in.) • Higher lakes are more rapidly cooled than surrounding areas.
  39. 39. Laking or Waving (Cont’d.) • Buffing reveals transition lines between different solidified zones. • Usually caused by over-heated dies, inadequate filling, poor die lubrication. • Better fill times can also reduce laking
  40. 40. Views of Lake Areas in Casting A B Example of a lake Microstructure in on a plated casting lake area of casting in Fig. “A”
  41. 41. Surface waviness on a Small nodules on a zinc zinc diecasting after diecasting electroplating with electroplated with leveling copper and leveling copper and nickel nickel
  42. 42. Solidification Cracking • Occurs if feeding of area is restricted. • Usually occurs when thick sections are fed by thin ones – shrinkage occurs in the last area to freeze (hottest area). • Rare in Zamak alloys because of low freezing range & normal presence of entrapped gas. Gas maintains pressure and feeding
  43. 43. Solidification Cracking A B Solidification cracking Solidification cracks at of a bulky & complex inside surface of casting casting in Fig. “A”
  44. 44. Hot Tearing • Begins along inside corners of casting if thermal contraction is hindered • Occurs when an outside corner of the die is over-heated • Solidification of the corner is retarded, freezing & contraction of metal on either side applies stress, resulting in cracks to semi-solid metal
  45. 45. Hot Tearing (Cont’d.) • Can occur with bosses and along length of a gate, where it is confused with trimming damage • To eliminate, control die temperature, die cooling methods, make part inside radii as large as possible • A minimum radius of 2 mm (0.08 in.) is desired
  46. 46. Hot tear crack along the base of a ridge on a casting A B As-polished Higher magnification View of crack etched view of crack at lower arrow location in Fig. “A”
  47. 47. Hot tear cracks B A Edge view of hot-tear crack View of similar casting along the length of a gate as shown in Fig. “A” but with after trimming gate attached
  48. 48. Internal Porosity • Distinct from subsurface porosity that causes blisters • Internal porosity revealed by trimming, machining. Must be removed before plating • Can also cause leaks in fluid handling components. • Important factors for porosity size and distribution are metal flow system, venting & die temperature
  49. 49. Internal Porosity (Cont’d.) • Fill patterns must be uniform. • Gate velocity should exceed 35 m/s (115 ft/sec) for atomized flow • Vents remove entrapped gas. • Die & metal temperature, together with cooling system, also affect porosity. • Rapid solidification traps gas throughout the casting.
  50. 50. Gate pores exposed By trimming Gate pore with small opening; no plating of inner surface Gate pore with plating of inner surfaces & corrosion lower down
  51. 51. Gate Pores (cont’d.) B Large gate pores in water Original small gate pore Hose gun casting exposed Enlarged by action of By machining to create a Accelerated corrosion test. “leaker.” (X10) Upper polished view (X100); lower Etched view (X200)
  52. 52. The depth of surface defects in a sample of defective zinc diecastings
  53. 53. Inspecting Zn Diecastings Need to identify defects requiring excessive polishing or buffing Inspection should be nondestructive and rapid Dye penetrant is best of non-visual methods, but improved lighting techniques allow visual inspection to be preferred method Best for first inspection to occur after trimming. Need to sort into -Diecastings with no plating problems -Salvageable castings using economical polishing, buffing or vibratory milling -Castings that would still show defects after finishing and plating that should be scrapped
  54. 54. Evaluation of 9 nondestructive methods for inspecting zinc diecastings for surface defects
  55. 55. Recommended lighting arrangements for as-cast surfaces Recommended lighting arrangements for polished surfaces
  56. 56. Recommended lighting arrangements for plated surfaces
  57. 57. As-cast surface illuminated to a level of more than 2700 Lux (250 foot candles) with a mixture of direct and diffused light
  58. 58. Well-diffused light source Patterned light source Buffed surface illuminated to a level of more than 2700 Lux (250 foot candles) with a mixture of direct and diffused light
  59. 59. Visual Inspection Almost all fissures and pits on a typical diecasting are< 50µm, at limit of human eye, but good lighting can allow visual inspection Polarized light reduces glare but prevents viewing of fissures and pits Laser lighting produced granular surface appearance, limiting its sensitivity Smooth castings, including those inspected after polishing and buffing, give high reflectivity surfaces and therefore different lighting requirements than as-cast surfaces.
  60. 60. Preparation for Electroplating
  61. 61. Design for Finishing Position parting line, gates, vents, overflows and ejectors on insignificant surfaces Locate gates to produce sound castings with good surface quality, in locations avoiding marks left after breaking or shearing Avoid sharp edges, corners or protrusions that can cause excessive wear on polishing wheels or belts For barrel plating, avoid plain flat surfaces that hay cause castings to stick together Design for fixturing to allow use of automatic or semi-automatic equipment
  62. 62. Die Preparation Polishing of die to reduce roughness to maximum of 0.2µm will increase die cost moderarately but can substantially reduce expensive polishing and buffing Oxide films on the die surface are beneficial for eliminating soldering and reducing heat loss A thin crack-free Cr plating layer can be inexpensively stripped and replaced. Cr plate must be compressively stressed to prevent cracking and spalling. Solutions of chromic, sulfuric and fluosilic acid used at 40-43°C to deposit compressively stressed Cr with minimum thickness of 10µm Electroless Ni on clean die surfaces can also produce durable surface
  63. 63. Polishing Belts and Wheels Removal of metal with abrasive, especially rough edges after trimming Slurry finishing involves rapid movement of castings, ie by spinning, in abrasive Use of coarser (240 grit) abrasive followed by fine allow for polishing of both jagged, wide burrs and finer parting lines etc Vibratory finishing faster than barrel tumbling (abrasive-loaded plastic chips) Finishes of 3-5µm possible with vibratory finishing, can be reduced to 1-2µm by level plating
  64. 64. Conditions for mechanically polishing and buffing zinc diecastings
  65. 65. Buffing – moves metal from microprojections to microdepressions Surface temperature must be >120°C, preferably >150°C Surface roughness after buffing is 2-3µm Good vibratory finishing and levelling plating can make buffing unneccessary Removal of buffing compound from recesses can be difficult Electropolishing can be used to remove burrs and fissure-like defects up to 50-75µm, but can expose subsurface pores Subsurface pores can be completely filled with leveling copper
  66. 66. Surface roughness variations resulting from some polishing and buffing operations Surface roughnesses after plating refer to leveling electroplate in all cases 1 microinch= 0.0254µm
  67. 67. Metal removal rate for salvaging defective diecastings in vibratory machines
  68. 68. Metal removal rate during vibratory finishing with chemical accelerators
  69. 69. Alkaline electropolishing bath for zinc diecastings Acid electropolishing bath for zinc diecastings
  70. 70. Alkaline cleaning solutions for zinc diecastings
  71. 71. Typical cycles for plating of zinc diecastings
  72. 72. Plating costs vs plating time to deposit a specified minimum thickness
  73. 73. The 4 basic kinds of plating rack construction
  74. 74. Single spline rack showing door handles jigged to prevent excessive buildup of plate at the tips
  75. 75. Thickness variations for electroplated nickel in a a groove with a width-to-depth ratio of 0.85
  76. 76. Variations in electroplate thickness over various shapes
  77. 77. Effect of anode size and position on the thickness variations on electroplate
  78. 78. The cathode robbers of each corner of the workpiece are in electrical contact with the workpiece 4 curved plastic shields are placed one at each corner of the workpiece
  79. 79. Plating rack with integrated, hinged current shields for improving coating thickness of electrodeposits
  80. 80. Integrated plating rack showing auxiliary anode for obtaining uniform coating thickness on a diecasting
  81. 81. Section of plating Section of rack rack equipped equipped with auxiliary with auxiliary nickel anodes nickel anodes to to increase coating improve thickness thickness of Ni and Cr uniformity on around automobile headlamp doors handles
  82. 82. Distribution of nickel on an automobile door handle resulting in 400% waste of metal on high current density areas
  83. 83. Copper-nickel-chromium coatings on zinc diecastings (ASTM B456) All applied on undercoat of copper or yellow brass with thickness of at least 5µm (0.2mil)
  84. 84. Recommendations for plating zinc diecastings