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, preferably0.8mmReduce depth of concave recesses as much as possible, avoid depths greaterthan 50% of widthIf sharply angled grooves are needed, paint the bottom of the grooves, it ischeaper 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 theircenters is 4X their widthBlind hole depths should be less than ½ their width and blind holes <5.6mmdiameter 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 withradius>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 CastingsProperly designed and constructed diesSmooth working, run-in casting machinesCorrect alloy compositionGood melting and delivery practice, proper dielubricationCorrect injection and trimming procedures
Die Design GuidelinesPlan for location of ejector pins to prevent marksin visible areas, or place on areas that can beeasily polishedFill thick sections before thin to allow progressivecoolingAlloy should reach vents and overflows last toallow complete die cavity fillingPlace vents at parting line to allow easy removalof flashSurfaces required to slide on cavity duringejection should be taperedCastings with defects >50µm are not salvageable
Cross sections of roughsurface diecastingsplated with bright copperin cyanide and acidbaths, then with levelingduplex 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.
Fluidity of Zinc Die Casting AlloysRagone Fluidity, Inches Aluminum, Weight Percent
Solidification RangesZamak 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
Casting LimitsIntegranular corrosion can be caused by high levels of Pb, Cd, Sn Casting Limits Zamak 3 Zamak 5Pb (max) 50 ppm 50 ppmCd (max) 40 ppm 40 ppmSn (max) 30 ppm 30 ppm ZA contaminant levels are similar
Effect of humidity test on zinc-aluminum alloy containing cadmium Discolored &As cast plate Cracked humidity-tested panels
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
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.)
Cosmetic Defects• Cold Shuts • Flaking or waving• Blisters • Solidification• Die Soldering cracking• Surface Shrinkage • 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.
Cold Shut Regions ( a) (b)(a)Surface view of a cold- (b) Higher MagnificationShut region of a casting view of center field in “(a)”
Cold shut in a zinc Cold lap in a zinc diecasting diecasting electroplated conventionally conventionally after electroplated after mechanical buffingpolishing and 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 usually can 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 Skin blister in a zinc diecasting diecastingconventionally plated conventionally after mechanical electroplated after buffing 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 Large surface pores in zinc diecasting a zinc diecasting conventionally conventionally electroplated after electroplated after polishing and buffing polishing and buffing
Examples of blistering of casting during paint baking
Views of Castings with Extensive Surface Lapping As-Cast and Plated After a paint baking heat treatment
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 cracking 2. 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.
Views of Surface Shrinks on a ZA Casting Surface Shrinks Close-up View of Surface within a Shrinkage area
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
Views of Lake Areas in CastingA BExample of a lake Microstructure inon a plated casting lake area of casting in Fig. “A”
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
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 CrackingA BSolidification cracking Solidification cracks atof a bulky & complex inside surface ofcasting casting in Fig. “A”
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 A B As-polished Higher magnification View of crack etched view of crack at lower arrow location in Fig. “A”
Hot tear cracks BAEdge 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
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 trimmingGate pore withsmall opening; noplating of innersurfaceGate pore withplating of innersurfaces & corrosionlower down
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)
The depth of surface defects in a sample of defective zinc diecastings
Inspecting Zn DiecastingsNeed to identify defects requiring excessive polishing orbuffingInspection should be nondestructive and rapidDye penetrant is best of non-visual methods, butimproved lighting techniques allow visual inspection tobe preferred methodBest for first inspection to occur after trimming. Need tosort into-Diecastings with no plating problems-Salvageable castings using economicalpolishing, buffing or vibratory milling-Castings that would still show defects after finishingand plating that should be scrapped
Evaluation of 9 nondestructive methods forinspecting zinc diecastings for surface defects
As-cast surface illuminated to alevel of more than 2700 Lux (250 foot candles) with a mixture of direct and diffused light
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
Visual InspectionAlmost all fissures and pits on a typicaldiecasting are< 50µm, at limit of human eye,but good lighting can allow visual inspectionPolarized light reduces glare but preventsviewing of fissures and pitsLaser lighting produced granular surfaceappearance, limiting its sensitivitySmooth castings, including those inspectedafter polishing and buffing, give high reflectivitysurfaces and therefore different lightingrequirements than as-cast surfaces.
Preparation for Electroplating
Design for FinishingPosition parting line, gates, vents, overflowsand ejectors on insignificant surfacesLocate gates to produce sound castings withgood surface quality, in locations avoidingmarks left after breaking or shearingAvoid sharp edges, corners or protrusions thatcan cause excessive wear on polishing wheelsor beltsFor barrel plating, avoid plain flat surfaces thathay cause castings to stick togetherDesign for fixturing to allow use of automatic orsemi-automatic equipment
Die PreparationPolishing of die to reduce roughness to maximum of0.2µm will increase die cost moderarately but cansubstantially reduce expensive polishing and buffingOxide films on the die surface are beneficial foreliminating soldering and reducing heat lossA thin crack-free Cr plating layer can be inexpensivelystripped and replaced. Cr plate must be compressivelystressed to prevent cracking and spalling. Solutions ofchromic, sulfuric and fluosilic acid used at 40-43°C todeposit compressively stressed Cr with minimumthickness of 10µmElectroless Ni on clean die surfaces can also producedurable surface
Polishing Belts and WheelsRemoval of metal with abrasive, especially rough edgesafter trimmingSlurry finishing involves rapid movement of castings, ieby spinning, in abrasiveUse of coarser (240 grit) abrasive followed by fineallow for polishing of both jagged, wide burrs and finerparting lines etcVibratory finishing faster than barrel tumbling(abrasive-loaded plastic chips)Finishes of 3-5µm possible with vibratory finishing, canbe 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°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
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
Metal removal rate for salvaging defective diecastings in vibratory machines
Metal removal rate during vibratory finishing with chemical accelerators
Alkaline electropolishingbath for zinc diecastingsAcid electropolishingbath for zincdiecastings
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 robbersof each corner of theworkpiece are inelectrical contactwith the workpiece4 curved plasticshields areplaced one ateach corner ofthe 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 platingSection of rack rack equippedequipped with auxiliary with auxiliarynickel anodes nickel anodes to to increase coating improve thicknessthickness of Ni and Cr uniformity onaround automobileheadlamp doors handles
Distribution of nickel on an automobile doorhandle 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)