ASTM B252-92(2014)
(Guide)Standard Guide for Preparation of Zinc Alloy Die Castings for Electroplating and Conversion Coatings
Standard Guide for Preparation of Zinc Alloy Die Castings for Electroplating and Conversion Coatings
SIGNIFICANCE AND USE
4.1 The performance and quality of electroplated or conversion-coated zinc alloy die casting depends upon the surface cleanliness and condition. Various metals are electroplated or conversion coatings are established on zinc alloys for decorative or engineering finish. The common electroplates applied are usually copper, nickel, and chromium for decorative and functional uses. The common conversion coatings applied are phosphates, chromates, and anodized coatings. Electroplated zinc die castings and conversion coatings on zinc die castings are used in many industries such as the marine, automotive, plumbing fixtures, and appliance industries.
SCOPE
1.1 This guide is intended as an aid in establishing and maintaining a procedure for preparing zinc alloy die castings for electroplating and conversion coatings. It is primarily intended for the preparation of Alloys UNS Z33521 (AG-40A) and UNS Z35530 (AC-41A) (Specification B86) for electroplating with copper, nickel, and chromium (Specification B456).
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: B252 − 92 (Reapproved 2014) Endorsed by American
Electroplaters’ Society
Endorsed by National
Association of Metal Finishers
Standard Guide for
Preparation of Zinc Alloy Die Castings for Electroplating
and Conversion Coatings
This standard is issued under the fixed designation B252; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 3. Summary of Practice
3.1 The normal sequence of preparation steps is as follows:
1.1 This guide is intended as an aid in establishing and
(1) smoothing of parting lines; (2) smoothing of rough or
maintaining a procedure for preparing zinc alloy die castings
defective surfaces, if necessary; (3) buffing, if necessary; (4)
for electroplating and conversion coatings. It is primarily
precleaning and rinsing; (5) alkaline electrocleaning and rins-
intended for the preparation ofAlloys UNS Z33521 (AG-40A)
ing; (6) acid dipping and rinsing; and (7) copper striking.
and UNS Z35530 (AC-41A) (Specification B86) for electro-
plating with copper, nickel, and chromium (Specification
4. Significance and Use
B456).
4.1 The performance and quality of electroplated or
1.2 The values stated in SI units are to be regarded as
conversion-coated zinc alloy die casting depends upon the
standard. No other units of measurement are included in this
surface cleanliness and condition. Various metals are electro-
standard.
plated or conversion coatings are established on zinc alloys for
1.3 This standard does not purport to address all of the decorative or engineering finish. The common electroplates
applied are usually copper, nickel, and chromium for decora-
safety concerns, if any, associated with its use. It is the
tive and functional uses. The common conversion coatings
responsibility of the user of this standard to establish appro-
applied are phosphates, chromates, and anodized coatings.
priate safety and health practices and determine the applica-
Electroplated zinc die castings and conversion coatings on zinc
bility of regulatory limitations prior to use.
die castings are used in many industries such as the marine,
automotive, plumbing fixtures, and appliance industries.
2. Referenced Documents
2.1 ASTM Standards:
5. Composition and Characteristics of Zinc Alloy Die
B6 Specification for Zinc
Castings
B86 Specification for Zinc and Zinc-Aluminum (ZA) Alloy
5.1 The alloys used in the manufacture of zinc alloy die
Foundry and Die Castings
castings are made with special high-grade zinc conforming to
B456 Specification for Electrodeposited Coatings of Copper
Specification B6, alloyed with about 4 % of aluminum, 0.04 %
Plus Nickel Plus Chromium and Nickel Plus Chromium
of magnesium, and either 0.25 (max) or 1.0 % copper (Alloys
2.2 Military Standard:
UNS Z33521 and UNS Z35530). Impurities such as lead,
MIL-S-13165C Shot Peening of Metal Parts
cadmium, tin, and iron are held at or below the specified low
levels in Specification B86.
5.2 Die castings made of Alloys UNS 233521 and UNS
This guide is under the jurisdiction of ASTM Committee B08 on Metallic and
235530 are usually dense and fine grained but do not always
Inorganic Coatings and is the direct responsibility of Subcommittee B08.02 on Pre
have smooth surfaces. Defects sometimes encountered in the
Treatment.
surfacelayersincludecracks,crevices(coldshut),skinblisters,
Current edition approved Nov. 1, 2014. Published November 2014. Originally
approved in 1951. Last previous edition approved in 2009 as B252 – 92 (2009).
and hemispherical pores. Burrs are usually left at parting lines
DOI: 10.1520/B0252-92R14.
where fins and gates are removed by die trimming.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
5.3 Cast surfaces are frequently contaminated with parting
Standards volume information, refer to the standard’s Document Summary page on
compounds applied at frequent intervals to die surfaces to
the ASTM website.
facilitate the ejection of the castings and with water-soluble
AvailablefromStandardizationDocumentsOrderDesk,Bldg.4SectionD,700
Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS. oils added to quenching tanks for corrosion inhibition.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B252 − 92 (2014)
5.4 Zinc alloy die castings are chemically active and are conveyor through successive belts or wheels to polish different
dissolvedoretchedduringprolongedcontactwithconcentrated areas,amanualoperationmayberequiredlatertocompletethe
solutions of many mineral or organic acids or strongly alkaline smoothing of parting lines if they are too curved. The finish
solutions with a pH greater than 10. Immersion periods in such ranges from 0.2 to 0.6 µm, depending on the abrasive and the
solutions should be of short duration to avoid roughening. pressure.
7.3 Smoothing by spinning in abrasives is accomplished by
6. Smoothing of Parting Lines
attaching die castings to spindles or drums rotated with a
6.1 Parting lines are smoothed by (1) mechanical polishing
peripheral speed of about 600 m/min in a slurry of abrasive
with abrasive-coated wheels or belts, (2) tumbling with abra-
material such as ground corn cobs or nut shells mixed with a
sive media, or (3) vibration with abrasives.
small amount of grease or other lubricant. Times usually range
from 5 to 10 min and the finish from 0.1 to 0.2 µm, depending
6.2 Abrasiveswithasizerangeof220to300meshgluedon
on the abrasive.
cloth wheels or continuous cloth belts that run over flexible
back-up wheels are usually used for mechanical polishing of
7.4 Vibrating tubs loaded with plastic chips (such as poly-
parting lines. Wheel diameters range from 5 to 40 cm,
urethane) impregnated with an abrasive (such as aluminum
depending on the complexity of the shape. Wheels are rotated
oxide) smooth the surfaces of die castings in 2 to 4 h when
with a minimum peripheral speed of 2500 m/min.Aperipheral
frequencies are in the range of 1700 to 2100 cpm and
speedof2100m/minshouldnotbeexceededwithbelts.Lower
amplitudes are adjusted to 3.2 to 6.4 mm. Vibratory machines
speeds of the order of 1100 to 1400 m/min are fairly common
produce a finish of 0.15 to 0.25 µm, with a cutting rate of 5
for small die castings polished on small diameter wheels.
µm/h.Asmoother finish of 0.075 to 0.125 µm can be obtained
Abrasive belts should not be used dry but should be lubricated
with plastic media containing finer abrasive, which removes
withasmallamountofgrease.Diecastingsusuallyarehandled
metal at a slower rate. Media and zinc parts are usually loaded
individually to polish parting lines smooth. This may require
with a ratio of 5:1 or 6:1. Surface gouges may occur with a
30 s or less for small castings, and sometimes 5 or 6 min for
smaller ratio.
larger ones.
7.5 Controlled shot peening will plastically deform and
6.3 Tumbling in horizontal barrels, loaded with abrasive
densify the casting surface and near-surface layers. Shot
stones such as limestone, preformed and fused aluminum
peening can seal surface pores, which can create problems in
oxide, ceramic shapes or abrasive-loaded plastic chips, and a
electroplatingandconversioncoating.Theprocessisdescribed
lubricant such as soap or detergent solution, removes parting-
in MIL-S-13165C. The process is also effective in removing
line burrs from die castings in 4 to 12 h. The barrels may be
fins, burrs, and flash from the surface. The casting
rotated at 4 r/min. Higher speeds reduce the time cycles and
configuration, including the smallest size radii and wall
costs, but also increase the danger of impingement of parts
thickness, as well as the required finish and contamination
against zinc surfaces.Ahexagonal barrel with a capacity of 0.5
limits, will dictate the proper selection of peening media, shot
m can be loaded with 450 kg of abrasive stones or chips and
size, intensity, and coverage, as is detailed in MIL-S-13165C.
90 kg of zinc die castings.
8. Buffing
6.4 Vibration in a bed of resin-bonded abrasive chips
removes parting-line burrs, typically in 1 to 4 h. Frequencies
8.1 Die castings are buffed to produce a mirror-like finish,
range from 700 to 2100 cpm and amplitudes from 0.8 to 6.4
suitable for plating with conventional solutions, when good
mm. A vibrating tub with a capacity of 0.5 m can be loaded
leveling plating solutions are not available. Buffing can be
with about 900 kg of abrasive media and 180 kg of zinc die
omitted, however, for die castings which have good surfaces or
castings.Adilute solution of detergent or soap is continuously
which can be uniformly polished to a finish of 0.25 µm, if
metered through the bed of media and parts to keep their
solutions with good leveling power are used for plating copper
surfaces clean and maximize surface smoothing. Parting lines
and nickel.
may be mechanically polished before vibratory processing
8.2 Die castings are buffed on cloth wheels rotated at a
when a large amount of flash must be removed.
peripheral speed not exceeding 2150 m/min. Slower speeds, of
the order of 1100 to 1600 m/min, are used for small die
7. Smoothing of Rough or Defective Surfaces
castings.Buffingcompoundsshouldbemadewithabinderthat
7.1 Rough or defective surfaces are smoothed by (1) me-
is readily emulsified or saponified during alkaline cleaning.
chanical polishing on rotating wheels or continuous, abrasive-
The abrasive may be tripoli (amorphous silica) or lime, mixed
coated belts, (2) spin finishing, (3) vibratory finishing, or (4)
with about 25 % of tallow or other lubricants. Compounds
controlled shot peening. Fissures, skin blisters, and other
suspended in a liquid are preferred for automatic buffing
defects with a depth of 25 to 50 µm can usually be erased with
machines that advance die castings through a succession of
these metal-removal methods. Deeper defects are infrequent.
buffs of varying diameter and width, which individually
7.2 Mechanical polishing for smoothing rough or defective smooth different surface areas. Buffs are usually made of cloth
surfaces is similar to mechanical polishing for smoothing withathreadcountof34to37/cm.Afinishof0.025to0.05µm
partinglineareas(see6.2).Partinglinesandroughordefective can be produced by buffing. The smoothing rate is influenced
surfaces are frequently polished by the same operator. If bythetemperatureofthemetalsurface(fasteratapproximately
polishing is mechanized to advance die castings attached to a 150°C than at lower temperatures).
B252 − 92 (2014)
8.3 After buffing, surfaces with impacted buffing compound 9.2.2.2 These emulsions normally are used hot, about 80°C,
can be improved by passing them over a dry wheel to remove as a soak, sometimes with agitation, for about 2 to 5 min. A
buffing compound. This will reduce the demand placed on the warm water spray rinse should follow the emulsion soak
precleaning solution. cleaning. Buffing compound not removed in the emulsion soak
is sufficiently softened so that it is easily removed in an
9. Precleaning and Rinsing
alkaline spray wash operation that normally follows.
9.2.2.3 Emulsion cleaning is an effective method for remov-
9.1 Itisstronglyrecommendedthatthepreliminaryremoval
ing buffing compound. Its principal disadvantage is the danger
ofmostofthebuffingcompoundandothersoilinaprecleaning
of carryover of hydrocarbon solvent into plating baths because
operation be done as soon as possible after buffing and
of incomplete rinsing. For this reason, it is very important that
polishing. Most buffing compounds become substantially more
proper alkaline cleaning and rinsing follow to ensure solvent
difficult to remove after aging several days.
removal from blind holes, defects in rack coatings, and
9.2 There are several methods by which soils can be
recesses.
removed from zinc die castings prior to final alkaline electro-
9.2.2.4 All federal, state, and local regulations for the use
cleaning.Generallyspeaking,thesefallintothreemainclasses:
and disposal of solvents should be followed.
solvent degreasing, emulsion cleaning, and cleaning with
9.2.3 Aqueous Base Detergents—In recent years, hot mix-
aqueous base detergents.
tures of emulsifiers and surfactants (wetting agents), some-
9.2.1 Solvent Degreasing—Before considering the use of
times combined with mild alkaline phosphates or borates, are
solvent degreasing, federal and state safety and environmental
usedforsoakcleaningtosoftenandremovebuffingcompound.
laws and regulations should be consulted. Many of the com-
Combining soak cleaning with ultrasonics is particularly effec-
monly used solvents are now being banned from use. Exposure
tive on impacted buffing compound. Such detergent soaks
to their vapors (VOC) is being strictly regulated for health,
should be followed by spray cleaning with an alkaline cleaner.
safety,andenvironmentalreasons.Currentsafeexposurelevels
If a spray cleaning step is not needed, then the soak cleaning
for various solvents should be obtained before use. Cold
step should be followed by a spray rinse with warm water
solvents, such as mineral spirits, methylene chloride,
before electrocleaning. Sometimes conventional alkaline soak
trichloroethylene, perchloroethylene and trichloroethane, are
cleaners are used for precleaning die castings with little or no
used with brushing to loosen packed buffing compound, but
buffing compound on them. These alkaline cleaners must be
this method usually is not practical for mass production
mild and inhibited since strong alkali will attack the castings.
conditions. Simple dipping in cold solvent is often ineffective.
9.3 Power Spray Alkaline Washing—Alkaline spray clean-
Vapor degreasing with trichloroethylene or perchloroethylene
ers are widely used, during the initial cleaning operation or
is widely practiced. Often the buffed die castings are sprayed
following initial presoaks in emulsions, solvents, or detergents.
with, or immersed in, hot solvent for mechanical removal of
This is accomplished with conveyerized units equipped with
heavy soil deposits. This is followed by condensation of hot,
washing, draining, rinsing, and draining sections. The solution
clean solvent vapors on the work; this removes the last traces
heated to a temperature range of 50 to 80°C is sprayed with a
of grease and compound. The method is very effective,
pressure of 170 to 205 kPa through nozzles on 20 to 30 cm
provided adequate measures are taken to remove the very fine
centers in t
...
This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: B252 − 92 (Reapproved 2009) B252 − 92 (Reapproved 2014) Endorsed by American
Electroplaters’ Society
Endorsed by National
Association of Metal Finishers
Standard Guide for
Preparation of Zinc Alloy Die Castings for Electroplating
and Conversion Coatings
This standard is issued under the fixed designation B252; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope
1.1 This guide is intended as an aid in establishing and maintaining a procedure for preparing zinc alloy die castings for
electroplating and conversion coatings. It is primarily intended for the preparation of Alloys UNS Z33521 (AG-40A) and UNS
B86) for electroplating with copper, nickel, and chromium (Specification B456).
Z35530 (AC-41A) (Specification
1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory
limitations prior to use.
2. Referenced Documents
2.1 ASTM Standards:
B6 Specification for Zinc
B86 Specification for Zinc and Zinc-Aluminum (ZA) Alloy Foundry and Die Castings
B456 Specification for Electrodeposited Coatings of Copper Plus Nickel Plus Chromium and Nickel Plus Chromium
2.2 Military Standard:
MIL-S-13165C Shot Peening of Metal Parts
3. Summary of Practice
3.1 The normal sequence of preparation steps is as follows: (1) smoothing of parting lines; (2) smoothing of rough or defective
surfaces, if necessary; (3) buffing, if necessary; (4) precleaning and rinsing; (5) alkaline electrocleaning and rinsing; (6) acid
dipping and rinsing; and (7) copper striking.
4. Significance and Use
4.1 The performance and quality of electroplated or conversion-coated zinc alloy die casting depends upon the surface
cleanliness and condition. Various metals are electroplated or conversion coatings are established on zinc alloys for decorative or
engineering finish. The common electroplates applied are usually copper, nickel, and chromium for decorative and functional uses.
The common conversion coatings applied are phosphates, chromates, and anodized coatings. Electroplated zinc die castings and
conversion coatings on zinc die castings are used in many industries such as the marine, automotive, plumbing fixtures, and
appliance industries.
This guide is under the jurisdiction of ASTM Committee B08 on Metallic and Inorganic Coatings and is the direct responsibility of Subcommittee B08.02 on Pre
Treatment.
Current edition approved Sept. 1, 2009Nov. 1, 2014. Published December 2009November 2014. Originally approved in 1951. Last previous edition approved in 20042009
as B252 – 92 (2004).(2009). DOI: 10.1520/B0252-92R09.10.1520/B0252-92R14.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
Available from Standardization Documents Order Desk, Bldg. 4 Section D, 700 Robbins Ave., Philadelphia, PA 19111-5094, Attn: NPODS.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
B252 − 92 (2014)
5. Composition and Characteristics of Zinc Alloy Die Castings
5.1 The alloys used in the manufacture of zinc alloy die castings are made with special high-grade zinc conforming to
Specification B6, alloyed with about 4 % of aluminum, 0.04 % of magnesium, and either 0.25 (max) or 1.0 % copper (Alloys UNS
Z33521 and UNS Z35530). Impurities such as lead, cadmium, tin, and iron are held at or below the specified low levels in
Specification B86.
5.2 Die castings made of Alloys UNS 233521 and UNS 235530 are usually dense and fine grained but do not always have
smooth surfaces. Defects sometimes encountered in the surface layers include cracks, crevices (cold shut), skin blisters, and
hemispherical pores. Burrs are usually left at parting lines where fins and gates are removed by die trimming.
5.3 Cast surfaces are frequently contaminated with parting compounds applied at frequent intervals to die surfaces to facilitate
the ejection of the castings and with water-soluble oils added to quenching tanks for corrosion inhibition.
5.4 Zinc alloy die castings are chemically active and are dissolved or etched during prolonged contact with concentrated
solutions of many mineral or organic acids or strongly alkaline solutions with a pH greater than 10. Immersion periods in such
solutions should be of short duration to avoid roughening.
6. Smoothing of Parting Lines
6.1 Parting lines are smoothed by (1) mechanical polishing with abrasive-coated wheels or belts, (2) tumbling with abrasive
media, or (3) vibration with abrasives.
6.2 Abrasives with a size range of 220 to 300 mesh glued on cloth wheels or continuous cloth belts that run over flexible back-up
wheels are usually used for mechanical polishing of parting lines. Wheel diameters range from 5 to 40 cm, depending on the
complexity of the shape. Wheels are rotated with a minimum peripheral speed of 2500 m/min. A peripheral speed of 2100 m/min
should not be exceeded with belts. Lower speeds of the order of 1100 to 1400 m/min are fairly common for small die castings
polished on small diameter wheels. Abrasive belts should not be used dry but should be lubricated with a small amount of grease.
Die castings usually are handled individually to polish parting lines smooth. This may require 30 s or less for small castings, and
sometimes 5 or 6 min for larger ones.
6.3 Tumbling in horizontal barrels, loaded with abrasive stones such as limestone, preformed and fused aluminum oxide,
ceramic shapes or abrasive-loaded plastic chips, and a lubricant such as soap or detergent solution, removes parting-line burrs from
die castings in 4 to 12 h. The barrels may be rotated at 4 r/min. Higher speeds reduce the time cycles and costs, but also increase
the danger of impingement of parts against zinc surfaces. A hexagonal barrel with a capacity of 0.5 m can be loaded with 450
kg of abrasive stones or chips and 90 kg of zinc die castings.
6.4 Vibration in a bed of resin-bonded abrasive chips removes parting-line burrs, typically in 1 to 4 h. Frequencies range from
700 to 2100 cpm and amplitudes from 0.8 to 6.4 mm. A vibrating tub with a capacity of 0.5 m can be loaded with about 900 kg
of abrasive media and 180 kg of zinc die castings. A dilute solution of detergent or soap is continuously metered through the bed
of media and parts to keep their surfaces clean and maximize surface smoothing. Parting lines may be mechanically polished before
vibratory processing when a large amount of flash must be removed.
7. Smoothing of Rough or Defective Surfaces
7.1 Rough or defective surfaces are smoothed by (1) mechanical polishing on rotating wheels or continuous, abrasive-coated
belts, (2) spin finishing, (3) vibratory finishing, or (4) controlled shot peening. Fissures, skin blisters, and other defects with a depth
of 25 to 50 μm can usually be erased with these metal-removal methods. Deeper defects are infrequent.
7.2 Mechanical polishing for smoothing rough or defective surfaces is similar to mechanical polishing for smoothing parting
line areas (see 6.2). Parting lines and rough or defective surfaces are frequently polished by the same operator. If polishing is
mechanized to advance die castings attached to a conveyor through successive belts or wheels to polish different areas, a manual
operation may be required later to complete the smoothing of parting lines if they are too curved. The finish ranges from 0.2 to
0.6 μm, depending on the abrasive and the pressure.
7.3 Smoothing by spinning in abrasives is accomplished by attaching die castings to spindles or drums rotated with a peripheral
speed of about 600 m/min in a slurry of abrasive material such as ground corn cobs or nut shells mixed with a small amount of
grease or other lubricant. Times usually range from 5 to 10 min and the finish from 0.1 to 0.2 μm, depending on the abrasive.
7.4 Vibrating tubs loaded with plastic chips (such as polyurethane) impregnated with an abrasive (such as aluminum oxide)
smooth the surfaces of die castings in 2 to 4 h when frequencies are in the range of 1700 to 2100 cpm and amplitudes are adjusted
to 3.2 to 6.4 mm. Vibratory machines produce a finish of 0.15 to 0.25 μm, with a cutting rate of 5 μm/h. A smoother finish of 0.075
to 0.125 μm can be obtained with plastic media containing finer abrasive, which removes metal at a slower rate. Media and zinc
parts are usually loaded with a ratio of 5:1 or 6:1. Surface gouges may occur with a smaller ratio.
7.5 Controlled shot peening will plastically deform and densify the casting surface and near-surface layers. Shot peening can
seal surface pores, which can create problems in electroplating and conversion coating. The process is described in MIL-S-13165C.
The process is also effective in removing fins, burrs, and flash from the surface. The casting configuration, including the smallest
B252 − 92 (2014)
size radii and wall thickness, as well as the required finish and contamination limits, will dictate the proper selection of peening
media, shot size, intensity, and coverage, as is detailed in MIL-S-13165C.
8. Buffing
8.1 Die castings are buffed to produce a mirror-like finish, suitable for plating with conventional solutions, when good leveling
plating solutions are not available. Buffing can be omitted, however, for die castings which have good surfaces or which can be
uniformly polished to a finish of 0.25 μm, if solutions with good leveling power are used for plating copper and nickel.
8.2 Die castings are buffed on cloth wheels rotated at a peripheral speed not exceeding 2150 m/min. Slower speeds, of the order
of 1100 to 1600 m/min, are used for small die castings. Buffing compounds should be made with a binder that is readily emulsified
or saponified during alkaline cleaning. The abrasive may be tripoli (amorphous silica) or lime, mixed with about 25 % of tallow
or other lubricants. Compounds suspended in a liquid are preferred for automatic buffing machines that advance die castings
through a succession of buffs of varying diameter and width, which individually smooth different surface areas. Buffs are usually
made of cloth with a thread count of 34 to 37/cm. A finish of 0.025 to 0.05 μm can be produced by buffing. The smoothing rate
is influenced by the temperature of the metal surface (faster at approximately 150°C than at lower temperatures).
8.3 After buffing, surfaces with impacted buffing compound can be improved by passing them over a dry wheel to remove
buffing compound. This will reduce the demand placed on the precleaning solution.
9. Precleaning and Rinsing
9.1 It is strongly recommended that the preliminary removal of most of the buffing compound and other soil in a precleaning
operation be done as soon as possible after buffing and polishing. Most buffing compounds become substantially more difficult to
remove after aging several days.
9.2 There are several methods by which soils can be removed from zinc die castings prior to final alkaline electrocleaning.
Generally speaking, these fall into three main classes: solvent degreasing, emulsion cleaning, and cleaning with aqueous base
detergents.
9.2.1 Solvent Degreasing—Before considering the use of solvent degreasing, federal and state safety and environmental laws
and regulations should be consulted. Many of the commonly used solvents are now being banned from use. Exposure to their
vapors (VOC) is being strictly regulated for health, safety, and environmental reasons. Current safe exposure levels for various
solvents should be obtained before use. Cold solvents, such as mineral spirits, methylene chloride, trichloroethylene,
perchloroethylene and trichloroethane, are used with brushing to loosen packed buffing compound, but this method usually is not
practical for mass production conditions. Simple dipping in cold solvent is often ineffective. Vapor degreasing with
trichloroethylene or perchloroethylene is widely practiced. Often the buffed die castings are sprayed with, or immersed in, hot
solvent for mechanical removal of heavy soil deposits. This is followed by condensation of hot, clean solvent vapors on the work;
this removes the last traces of grease and compound. The method is very effective, provided adequate measures are taken to remove
the very fine abrasive and metallic particles from the work. Trichloroethylene and perchloroethylene are nonflammable as used in
vapor degreasing and still must be used in systems designed to protect personnel from inhalation of vapors. Suppliers of solvents
should be consulted as to the safety of a given installation.
9.2.1.1 All federal, state, and local regulations for the disposal of solvents should be followed.
9.2.2 Emulsion Cleaning:
9.2.2.1 Impacted buffing compound may be loosened, and to some extent removed, by immersion in various hydrocarbon-water
emulsions. These emulsions are available in several forms, including unstable emulsions (diphase cleaners), invert type emulsions,
mixtures of emulsions and alkaline cleaners, and stable emulsions. Such emulsion cleaners usually have a suitable hydrocarbon
base such as kerosene or a higher flashpoint solvent to which is added emulsifiers, soaps, and inhibitors to prevent etching of the
die castings. The pH of the emulsion cleaner should be kept between 7 and 10 to avoid damage to the castings.
9.2.2.2 These emulsions normally are used hot, about 80°C, as a soak, sometimes with agitation, for about 2 to 5 min. A warm
water spray rinse should follow the emulsion soak cleaning. Buffing compound not removed in the emulsion soak is sufficiently
softened so that it is easily removed in an alkaline spray wash operation that normally follows.
9.2.2.3 Emulsion cleaning is an effective method for removing buffing compound. Its principal disadvantage is the danger of
carryover of hydrocarbon solvent into
...
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