This document discusses design guidelines and best practices for producing zinc diecastings for electroplating. It provides rules for minimum radii, recessed features, hole sizes and spacing to improve platability. It also covers factors that influence quality like alloy composition, melting practices, and die design. Defects that can occur like cold shuts, blisters, die soldering, shrinkage and laking are described along with techniques to prevent them.

1. Production of Zinc Diecastings for
Electroplating

2. Relative costs of fabricating, finishing and plating
zinc diecastings

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. Design factors influencing platability of zinc
diecastings

5. Minimum radii for angles defined by indentations

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. 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. Cross sections of rough
surface diecastings
plated with bright copper
in cyanide and acid
baths, then with leveling
duplex nickel

10. 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.

11. Fluidity of Zinc Die Casting Alloys
Ragone Fluidity, Inches
Aluminum, Weight Percent

12. 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

13. 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

14. Effect of humidity test on zinc-aluminum
alloy containing cadmium
Discolored &
As cast plate Cracked
humidity-tested
panels

15. As-polished structure of humidity-tested zinc
aluminum alloy containing cadmium showing a
crack and intergranular corrosion

16. 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.

17. Surface dross laden with large and smallFeAl3
intermetallic particles

18. 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.)

19. Cosmetic Defects
• Cold Shuts • Flaking or waving
• Blisters • Solidification
• Die Soldering cracking
• Surface Shrinkage • Hot tearing
• Internal porosity

20. 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

21. Cold Shuts
• Important Control Variables:
– Cavity fill time
– Gate velocity
– Die & metal temperatures
– Flow pattern in cavity.
• Cold shuts cannot be removed by
intensification.

22. 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)”

23. Cold shut in a zinc Cold lap in a zinc
diecasting diecasting electroplated
conventionally conventionally after
electroplated after mechanical buffing
polishing and buffing

24. 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.

25. 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.

26. Examples of thin laps being
Lifted by plating layer stresses

27. Views of shallow laps that usually
can be removed by buffing

28. 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.

29. Exfoliation of a zinc Skin blister in a zinc
diecasting diecasting
conventionally plated conventionally
after mechanical electroplated after
buffing polishing and buffing

30. 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.

31. 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

32. Examples of blistering of casting
during paint baking

33. Views of Castings with Extensive
Surface Lapping
As-Cast and Plated After a paint baking
heat treatment

34. 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.

35. 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.

36. 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

37. 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.

38. Views of Surface Shrinks on a ZA Casting
Surface Shrinks Close-up View of
Surface within a
Shrinkage area

39. 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.

40. 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

41. 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”

42. 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

43. 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

44. Solidification Cracking
A B
Solidification cracking Solidification cracks at
of a bulky & complex inside surface of
casting casting in Fig. “A”

45. 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

46. 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

47. 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”

48. 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

49. 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

50. 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.

51. 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

52. 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)

53. The depth of surface defects in a sample of
defective zinc diecastings

54. 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

55. Evaluation of 9 nondestructive methods for
inspecting zinc diecastings for surface defects

56. Recommended
lighting
arrangements for
as-cast surfaces
Recommended
lighting
arrangements for
polished surfaces

57. Recommended
lighting
arrangements for
plated surfaces

58. As-cast surface illuminated to a
level of more than 2700 Lux (250
foot candles) with a mixture of
direct and diffused light

59. 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

60. 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.

61. Preparation for Electroplating

62. 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

63. 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

64. 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

65. Conditions for mechanically polishing and buffing
zinc diecastings

66. 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

67. 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

68. Metal removal rate for salvaging defective
diecastings in vibratory machines

69. Metal removal rate during vibratory finishing with
chemical accelerators

70. Alkaline electropolishing
bath for zinc diecastings
Acid electropolishing
bath for zinc
diecastings

71. Alkaline cleaning solutions for zinc diecastings

72. Typical cycles for plating of zinc diecastings

73. Plating costs vs plating time to deposit a specified
minimum thickness

74. The 4 basic kinds of plating rack construction

75. Single spline rack showing door handles jigged to
prevent excessive buildup of plate at the tips

76. Thickness variations for electroplated nickel in a a
groove with a width-to-depth ratio of 0.85

77. Variations in electroplate thickness over various
shapes

78. Effect of anode size and position on the thickness
variations on electroplate

79. 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

80. Plating rack with integrated, hinged current
shields for improving coating thickness of
electrodeposits

81. Integrated plating rack showing auxiliary anode
for obtaining uniform coating thickness on a
diecasting

82. 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

83. Distribution of nickel on an automobile door
handle resulting in 400% waste of metal on high
current density areas

84. 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)

85. Recommendations for plating zinc diecastings