Production of Zinc Diecastings for Electroplating

Relative costs of fabricating, finishing and plating zinc diecastings

Some Design RulesMimimum crown of 0.15cm per cmIf flat surfaces required, use satin instead of bright finish to hide wavinessAll edges should be rounded off to radius of at least 0.4mm, preferably 0.8mmReduce depth of concave recesses as much as possible, avoid depths greater than 50% of widthIf sharply angled grooves are needed, paint the bottom of the grooves, it is cheaper than plating the bottom of the groovesSlots and holes shown have widths at least 2X their depthSpaces between slots should be spaced so that spacing between their centers is 4X their widthBlind hole depths should be less than ½ their width and blind holes <5.6mm diameter should be avoidedThreaded holes should be countersunk to minimize buildup on their outsideThe height of fins and ribs should be reduced as much as possible with radius>1.6mm at baseParallel fins should be spaced so distance between centers is >4X fin widthRecessed letters preferred to raised letter. Raised letter heights should be <50% of their widthIf studs threaded before plating, max thickness is 5µmDrain holes should be provided in cup-like contours to avoid hand rinsing

Design factors influencing platability of zinc diecastings

Minimum radii for angles defined by indentations

Checklist for High Quality Castings

Properly designed and constructed diesSmooth working, run-in casting machinesCorrect alloy compositionGood melting and delivery practice, proper die lubricationCorrect injection and trimming procedures

Die Design Guidelines

Plan for location of ejector pins to prevent marks in visible areas, or place on areas that can be easily polishedFill thick sections before thin to allow progressive coolingAlloy should reach vents and overflows last to allow complete die cavity fillingPlace vents at parting line to allow easy removal of flashSurfaces required to slide on cavity during ejection should be taperedCastings with defects >50µm are not salvageable

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

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.

Ragone F

luid

ity,

Inch

es

Aluminum, Weight Percent

Fluidity of Zinc Die Casting Alloys

Solidification Ranges

Alloy Solidification Range ºC ( ºF )

Alloy 3 6 (11)

ZA-8 29 (52)

ZA-12 55 (100)

ZA-27 112 (202)

Zamak alloys have smaller freezing ranges than ZA alloys

Therefore, shrinkage porosity rarely occurs in Zamak alloys

Casting Limits

Casting LimitsZamak 3 Zamak 5

Pb (max) 50 ppm 50 ppm

Cd (max) 40 ppm 40 ppm

Sn (max) 30 ppm 30 ppm

Integranular corrosion can be caused by high levels of Pb, Cd, Sn

ZA contaminant levels are similar

Effect of humidity test on zinc-aluminum alloy containing cadmium

As cast plate

Discolored &Crackedhumidity-testedpanels

As-polished structure of humidity-tested zincaluminum alloy containing cadmium showing a

crack and intergranular corrosion

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.

Surface dross laden with large and smallFeAl3 intermetallic particles

Drilling Time (min.)

Tool Wear – ZA-27 Die Casting

Mean

Wear

Lan

d W

idth

(in

.)

Mean

Wear

Lan

d W

idth

(m

m )

Many large FeAl3 particles(0.07% Fe)

Many small FeAl3 particles(0.22% Fe)

Cosmetic Defects

• Cold Shuts• Blisters• Die Soldering• Surface

Shrinkage

• Flaking or waving

• Solidification cracking

• Hot tearing• Internal porosity

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

Cold Shuts

• Important Control Variables:– Cavity fill time– Gate velocity– Die & metal temperatures– Flow pattern in cavity.

• Cold shuts cannot be removed by intensification.

( a) (b)

Cold Shut Regions

(a)Surface view of a cold-Shut region of a casting

(b) Higher Magnification view of center field in“(a)”

Cold shut in a zinc diecasting

conventionally electroplated after

polishing and buffing

Cold lap in a zinc diecasting electroplated

conventionally after mechanical buffing

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.

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.

Examples of thin laps beingLifted by plating layer stresses

Views of shallow laps that usuallycan be removed by buffing

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.

Exfoliation of a zinc diecasting

conventionally plated after mechanical

buffing

Skin blister in a zinc diecasting

conventionally electroplated after

polishing and buffing

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.

Small surface pores in a zinc diecasting conventionally

electroplated after polishing and buffing

Large surface pores in a zinc diecasting conventionally

electroplated after polishing and buffing

Examples of blistering of castingduring paint baking

As-Cast and Plated After a paint bakingheat treatment

Views of Castings with ExtensiveSurface Lapping

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.

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.

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 cracking2. Laking or Waving 4. Hot tears

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.

Surface Shrinks Close-up View of Surface within a Shrinkage area

Views of Surface Shrinks on a ZA Casting

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.

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

Example of a lakeon a plated casting

Microstructure in lake area of

casting in Fig. “A”

Views of Lake Areas in Casting

A B

Surface waviness on a zinc diecasting after electroplating with leveling copper and

nickel

Small nodules on a zinc diecasting

electroplated with leveling copper and

nickel

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

Solidification crackingof a bulky & complexcasting

Solidification cracks at inside surface of casting in Fig. “A”

Solidification Cracking

A B

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

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

Hot tear crack along the base of a ridge on a casting

As-polishedView of crack

Higher magnificationetched view of crack

at lower arrowlocation in Fig. “A”

A B

Hot tear cracks

Edge view of hot-tear crackalong the length of a gate

after trimming

View of similar castingas shown in Fig. “A” but with

gate attached

A

B

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

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.

Gate pores exposedBy trimming

Gate pore withsmall opening; noplating of inner surface

Gate pore with plating of innersurfaces & corrosionlower down

Original small gate poreEnlarged by action of

Accelerated corrosion test.Upper polished view (X100); lower

Etched view (X200)

Large gate pores in waterHose gun casting exposedBy machining to create a

“leaker.” (X10)

B

Gate Pores (cont’d.)

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

Inspecting Zn Diecastings

Need to identify defects requiring excessive polishing or buffingInspection should be nondestructive and rapidDye penetrant is best of non-visual methods, but improved lighting techniques allow visual inspection to be preferred methodBest 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

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

Recommended lighting arrangements for as-cast surfaces

Recommended lighting arrangements for polished surfaces

Recommended lighting arrangements for plated surfaces

As-cast surface illuminated to a level of more than 2700 Lux (250

foot candles) with a mixture of direct and diffused light

Buffed surface illuminated to a level of more than 2700 Lux (250 foot candles) with a mixture of direct

and diffused light

Well-diffused light source Patterned light source

Visual Inspection

Almost all fissures and pits on a typical diecasting are< 50µm, at limit of human eye, but good lighting can allow visual inspectionPolarized light reduces glare but prevents viewing of fissures and pitsLaser lighting produced granular surface appearance, limiting its sensitivitySmooth castings, including those inspected after polishing and buffing, give high reflectivity surfaces and therefore different lighting requirements than as-cast surfaces.

Preparation for Electroplating

Design for Finishing

Position parting line, gates, vents, overflows and ejectors on insignificant surfacesLocate gates to produce sound castings with good surface quality, in locations avoiding marks left after breaking or shearingAvoid sharp edges, corners or protrusions that can cause excessive wear on polishing wheels or beltsFor barrel plating, avoid plain flat surfaces that hay cause castings to stick togetherDesign for fixturing to allow use of automatic or semi-automatic equipment

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 buffingOxide films on the die surface are beneficial for eliminating soldering and reducing heat lossA 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 10µmElectroless Ni on clean die surfaces can also produce durable surface

Polishing Belts and Wheels

Removal of metal with abrasive, especially rough edges after trimmingSlurry finishing involves rapid movement of castings, ie by spinning, in abrasiveUse of coarser (240 grit) abrasive followed by fine allow for polishing of both jagged, wide burrs and finer parting lines etcVibratory 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

Conditions for mechanically polishing and buffing zinc diecastings

Buffing – moves metal from microprojections to microdepressions

Surface temperature must be >120°C, preferably >150°CSurface roughness after buffing is 2-3µmGood vibratory finishing and levelling plating can make buffing unneccessaryRemoval 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 poresSubsurface pores can be completely filled with leveling copper

Surface roughness variations resulting from some polishing and buffing operations

Surface roughnesses after plating refer to leveling electroplate in all cases1 microinch= 0.0254µm

Metal removal rate for salvaging defective diecastings in vibratory machines

Metal removal rate during vibratory finishing with chemical accelerators

Alkaline electropolishing bath for zinc diecastings

Acid electropolishing bath for zinc diecastings

Alkaline cleaning solutions for zinc diecastings

Typical cycles for plating of zinc diecastings

Plating costs vs plating time to deposit a specified minimum thickness

The 4 basic kinds of plating rack construction

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

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

Variations in electroplate thickness over various shapes

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

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

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

electrodeposits

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

diecasting

Section of rack equipped with auxiliary nickel anodes to increase coating thickness of Ni and Cr around headlamp doors

Section of plating rack equipped with auxiliary nickel anodes to improve thickness uniformity on automobile handles

Distribution of nickel on an automobile door handle resulting in 400% waste of metal on high

current density areas

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)

Recommendations for plating zinc diecastings