1、Designation: B252 92 (Reapproved 2009)Endorsed by AmericanElectroplaters SocietyEndorsed by NationalAssociation of Metal FinishersStandard Guide forPreparation of Zinc Alloy Die Castings for Electroplatingand Conversion Coatings1This standard is issued under the fixed designation B252; the number im
2、mediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 Th
3、is guide is intended as an aid in establishing andmaintaining a procedure for preparing zinc alloy die castingsfor electroplating and conversion coatings. It is primarilyintended for the preparation of Alloys UNS Z33521 (AG-40A)and UNS Z35530 (AC-41A) (Specification B86) for electro-plating with cop
4、per, nickel, and chromium (SpecificationB456).1.2 The values stated in SI units are to be regarded asstandard. No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility
5、of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2B6 Specification for ZincB86 Specification for Zinc and Zinc-Aluminum (ZA) AlloyFoundry and Die Castin
6、gsB456 Specification for Electrodeposited Coatings of Cop-per Plus Nickel Plus Chromium and Nickel Plus Chro-mium2.2 Military Standard:MIL-S-13165C Shot Peening of Metal Parts33. Summary of Practice3.1 The normal sequence of preparation steps is as follows:(1) smoothing of parting lines; (2) smoothi
7、ng of rough ordefective surfaces, if necessary; (3) buffing, if necessary; (4)precleaning and rinsing; (5) alkaline electrocleaning and rins-ing; (6) acid dipping and rinsing; and (7) copper striking.4. Significance and Use4.1 The performance and quality of electroplated orconversion-coated zinc all
8、oy die casting depends upon thesurface cleanliness and condition. Various metals are electro-plated or conversion coatings are established on zinc alloys fordecorative or engineering finish. The common electroplatesapplied are usually copper, nickel, and chromium for decora-tive and functional uses.
9、 The common conversion coatingsapplied are phosphates, chromates, and anodized coatings.Electroplated zinc die castings and conversion coatings on zincdie castings are used in many industries such as the marine,automotive, plumbing fixtures, and appliance industries.5. Composition and Characteristic
10、s of Zinc Alloy DieCastings5.1 The alloys used in the manufacture of zinc alloy diecastings are made with special high-grade zinc conforming toSpecification B6, alloyed with about 4 % of aluminum, 0.04 %of magnesium, and either 0.25 (max) or 1.0 % copper (AlloysUNS Z33521 and UNS Z35530). Impurities
11、 such as lead,cadmium, tin, and iron are held at or below the specified lowlevels in Specification B86.5.2 Die castings made of Alloys UNS 233521 and UNS235530 are usually dense and fine grained but do not alwayshave smooth surfaces. Defects sometimes encountered in thesurface layers include cracks,
12、 crevices (cold shut), skin blisters,and hemispherical pores. Burrs are usually left at parting lineswhere fins and gates are removed by die trimming.5.3 Cast surfaces are frequently contaminated with partingcompounds applied at frequent intervals to die surfaces tofacilitate the ejection of the cas
13、tings and with water-solubleoils added to quenching tanks for corrosion inhibition.1This guide is under the jurisdiction of ASTM Committee B08 on Metallic andInorganic Coatings and is the direct responsibility of Subcommittee B08.02 on PreTreatment.Current edition approved Sept. 1, 2009. Published D
14、ecember 2009. Originallyapproved in 1951. Last previous edition approved in 2004 as B252 92 (2004).DOI: 10.1520/B0252-92R09.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, r
15、efer to the standards Document Summary page onthe ASTM website.3Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, Unite
16、d States.5.4 Zinc alloy die castings are chemically active and aredissolved or etched during prolonged contact with concentratedsolutions of many mineral or organic acids or strongly alkalinesolutions with a pH greater than 10. Immersion periods in suchsolutions should be of short duration to avoid
17、roughening.6. Smoothing of Parting Lines6.1 Parting lines are smoothed by (1) mechanical polishingwith abrasive-coated wheels or belts, (2) tumbling with abra-sive media, or (3) vibration with abrasives.6.2 Abrasives with a size range of 220 to 300 mesh glued oncloth wheels or continuous cloth belts
18、 that run over flexibleback-up wheels are usually used for mechanical polishing ofparting lines. Wheel diameters range from 5 to 40 cm,depending on the complexity of the shape. Wheels are rotatedwith a minimum peripheral speed of 2500 m/min. A peripheralspeed of 2100 m/min should not be exceeded wit
19、h belts. Lowerspeeds of the order of 1100 to 1400 m/min are fairly commonfor small die castings polished on small diameter wheels.Abrasive belts should not be used dry but should be lubricatedwith a small amount of grease. Die castings usually are handledindividually to polish parting lines smooth.
20、This may require30 s or less for small castings, and sometimes 5 or 6 min forlarger ones.6.3 Tumbling in horizontal barrels, loaded with abrasivestones such as limestone, preformed and fused aluminumoxide, ceramic shapes or abrasive-loaded plastic chips, and alubricant such as soap or detergent solu
21、tion, removes parting-line burrs from die castings in 4 to 12 h. The barrels may berotated at 4 r/min. Higher speeds reduce the time cycles andcosts, but also increase the danger of impingement of partsagainst zinc surfaces. A hexagonal barrel with a capacity of 0.5m3can be loaded with 450 kg of abr
22、asive stones or chips and90 kg of zinc die castings.6.4 Vibration in a bed of resin-bonded abrasive chipsremoves parting-line burrs, typically in 1 to 4 h. Frequenciesrange from 700 to 2100 cpm and amplitudes from 0.8 to 6.4mm. A vibrating tub with a capacity of 0.5 m3can be loadedwith about 900 kg
23、of abrasive media and 180 kg of zinc diecastings. A dilute solution of detergent or soap is continuouslymetered through the bed of media and parts to keep theirsurfaces clean and maximize surface smoothing. Parting linesmay be mechanically polished before vibratory processingwhen a large amount of f
24、lash must be removed.7. Smoothing of Rough or Defective Surfaces7.1 Rough or defective surfaces are smoothed by (1) me-chanical polishing on rotating wheels or continuous, abrasive-coated belts, (2) spin finishing, (3) vibratory finishing, or (4)controlled shot peening. Fissures, skin blisters, and
25、otherdefects with a depth of 25 to 50 m can usually be erased withthese metal-removal methods. Deeper defects are infrequent.7.2 Mechanical polishing for smoothing rough or defectivesurfaces is similar to mechanical polishing for smoothingparting line areas (see 6.2). Parting lines and rough or defe
26、ctivesurfaces are frequently polished by the same operator. Ifpolishing is mechanized to advance die castings attached to aconveyor through successive belts or wheels to polish differentareas, a manual operation may be required later to complete thesmoothing of parting lines if they are too curved.
27、The finishranges from 0.2 to 0.6 m, depending on the abrasive and thepressure.7.3 Smoothing by spinning in abrasives is accomplished byattaching die castings to spindles or drums rotated with aperipheral speed of about 600 m/min in a slurry of abrasivematerial such as ground corn cobs or nut shells
28、mixed with asmall amount of grease or other lubricant. Times usually rangefrom 5 to 10 min and the finish from 0.1 to 0.2 m, dependingon the abrasive.7.4 Vibrating tubs loaded with plastic chips (such as poly-urethane) impregnated with an abrasive (such as aluminumoxide) smooth the surfaces of die c
29、astings in 2 to 4 h whenfrequencies are in the range of 1700 to 2100 cpm andamplitudes are adjusted to 3.2 to 6.4 mm. Vibratory machinesproduce a finish of 0.15 to 0.25 m, with a cutting rate of 5m/h. A smoother finish of 0.075 to 0.125 m can be obtainedwith plastic media containing finer abrasive,
30、which removesmetal at a slower rate. Media and zinc parts are usually loadedwith a ratio of 5:1 or 6:1. Surface gouges may occur with asmaller ratio.7.5 Controlled shot peening will plastically deform anddensify the casting surface and near-surface layers. Shotpeening can seal surface pores, which c
31、an create problems inelectroplating and conversion coating. The process is describedin MIL-S-13165C. The process is also effective in removingfins, burrs, and flash from the surface. The casting configura-tion, including the smallest size radii and wall thickness, aswell as the required finish and c
32、ontamination limits, willdictate the proper selection of peening media, shot size,intensity, and coverage, as is detailed in MIL-S-13165C.8. Buffing8.1 Die castings are buffed to produce a mirror-like finish,suitable for plating with conventional solutions, when goodleveling plating solutions are no
33、t available. Buffing can beomitted, however, for die castings which have good surfaces orwhich can be uniformly polished to a finish of 0.25 m, ifsolutions with good leveling power are used for plating copperand nickel.8.2 Die castings are buffed on cloth wheels rotated at aperipheral speed not exce
34、eding 2150 m/min. Slower speeds, ofthe order of 1100 to 1600 m/min, are used for small diecastings. Buffing compounds should be made with a binder thatis readily emulsified or saponified during alkaline cleaning.The abrasive may be tripoli (amorphous silica) or lime, mixedwith about 25 % of tallow o
35、r other lubricants. Compoundssuspended in a liquid are preferred for automatic buffingmachines that advance die castings through a succession ofbuffs of varying diameter and width, which individuallysmooth different surface areas. Buffs are usually made of clothwith a thread count of 34 to 37/cm.Afi
36、nish of 0.025 to 0.05 mcan be produced by buffing. The smoothing rate is influencedby the temperature of the metal surface (faster at approximately150C than at lower temperatures).8.3 After buffing, surfaces with impacted buffing compoundcan be improved by passing them over a dry wheel to removebuff
37、ing compound. This will reduce the demand placed on theprecleaning solution.B252 92 (2009)29. Precleaning and Rinsing9.1 It is strongly recommended that the preliminary removalof most of the buffing compound and other soil in a precleaningoperation be done as soon as possible after buffing andpolish
38、ing. Most buffing compounds become substantially moredifficult to remove after aging several days.9.2 There are several methods by which soils can beremoved from zinc die castings prior to final alkaline electro-cleaning. Generally speaking, these fall into three main classes:solvent degreasing, emu
39、lsion cleaning, and cleaning withaqueous base detergents.9.2.1 Solvent DegreasingBefore considering the use ofsolvent degreasing, federal and state safety and environmentallaws and regulations should be consulted. Many of the com-monly used solvents are now being banned from use. Exposureto their va
40、pors (VOC) is being strictly regulated for health,safety, and environmental reasons. Current safe exposure levelsfor various solvents should be obtained before use. Coldsolvents, such as mineral spirits, methylene chloride, trichlo-roethylene, perchloroethylene and trichloroethane, are usedwith brus
41、hing to loosen packed buffing compound, but thismethod usually is not practical for mass production conditions.Simple dipping in cold solvent is often ineffective. Vapordegreasing4with trichloroethylene or perchloroethylene iswidely practiced. Often the buffed die castings are sprayedwith, or immers
42、ed in, hot solvent for mechanical removal ofheavy soil deposits. This is followed by condensation of hot,clean solvent vapors on the work; this removes the last tracesof grease and compound. The method is very effective,provided adequate measures are taken to remove the very fineabrasive and metalli
43、c particles from the work. Trichloroethyl-ene and perchloroethylene are nonflammable as used in vapordegreasing and still must be used in systems designed toprotect personnel from inhalation of vapors. Suppliers ofsolvents should be consulted as to the safety of a giveninstallation.9.2.1.1 All feder
44、al, state, and local regulations for thedisposal of solvents should be followed.9.2.2 Emulsion Cleaning:9.2.2.1 Impacted buffing compound may be loosened, and tosome extent removed, by immersion in various hydrocarbon-water emulsions. These emulsions are available in severalforms, including unstable
45、 emulsions (diphase cleaners), inverttype emulsions, mixtures of emulsions and alkaline cleaners,and stable emulsions. Such emulsion cleaners usually have asuitable hydrocarbon base such as kerosene or a higherflashpoint solvent to which is added emulsifiers, soaps, andinhibitors to prevent etching
46、of the die castings. The pH of theemulsion cleaner should be kept between 7 and 10 to avoiddamage to the castings.9.2.2.2 These emulsions normally are used hot, about 80C,as a soak, sometimes with agitation, for about 2 to 5 min. Awarm water spray rinse should follow the emulsion soakcleaning. Buffi
47、ng compound not removed in the emulsion soakis sufficiently softened so that it is easily removed in analkaline spray wash operation that normally follows.9.2.2.3 Emulsion cleaning is an effective method for remov-ing buffing compound. Its principal disadvantage is the dangerof carryover of hydrocar
48、bon solvent into plating baths becauseof incomplete rinsing. For this reason, it is very important thatproper alkaline cleaning and rinsing follow to ensure solventremoval from blind holes, defects in rack coatings, andrecesses.9.2.2.4 All federal, state, and local regulations for the useand disposa
49、l of solvents should be followed.9.2.3 Aqueous Base DetergentsIn recent years, hot mix-tures of emulsifiers and surfactants (wetting agents), some-times combined with mild alkaline phosphates or borates, areused for soak cleaning to soften and remove buffing compound.Combining soak cleaning with ultrasonics is particularly effec-tive on impacted buffing compound. Such detergent soaksshould be followed by spray cleaning with an alkaline cleaner.If a spray cleaning step is not needed, then the soak cleaningstep should be followed by a spray rinse with warm waterbefore ele
