CH27FINISHING OF METAL MOILDS.doc

7/25/2019 CH27FINISHING OF METAL MOILDS.doc

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27.0 FINISHING OF METAL MOULDS

27.1 Introduction:Mould polishing is a very important step in producing a mould that works properly and apart that is functional and visually attractive. Proper finishing of the mould is important forthe release of the part. Polishing insures that the part is not damaged during ejection.For example, scratches from rough surfaces or dents from ejector pin. It also preventsdamage to the mould that could occur if the mould closed on a part stuck in the mould.The finish of the moulding largely depends on the finish of the cavity and core.

27.2 Ree!"e:elease is the a!ility of the mould to eject the component. To accomplish this, all cuttermarks must !e removed from the moulding surface of the cavities and cores. Further, thefinish should !e vertical, from !ottom of the cavity or core to the parting surface, to aid inthe ease of ejection. This is called draw stone. This also will speed up the mouldingcycle.

27.# A$$e!r!nce:There are different types of appearance re"uired after finishing. They are

!% Rou&' (ini"'in&:There are mainly three types of rough finish. The first is called draw stone. This is themost !asic of all finishes. It is strictly for release. The second is a sand !last or glass!ead. This produces a matte finish, creating non#slippery, non#reflective surfaces. Thethird type is a textured finish. This is done to create a specific appearance, such as woodgrain, leather or pe!!ly textures.

)% Mirror (ini"'in&:

$ mirror finish is called diamond finish. There are three !asic diamond finishes. The first,a rough diamond, is primarily used for release or as a preparation for a very fine texture.$ diamond, the most often specified, is used to enhance the appearance of opa"ue partsand the a!ility to see through clear and translucent parts. $ high shine or %& diamond isnecessary for optical or highly reflective parts.

c% Te*turin&:To prepare a surface for texturing, you must know what the texture will !e. This isnecessary to prevent over or under polishing. 'asically, the finer the texture, the finer thefinish needs to !e.

The product designer of a finished plastics product desires some sort of surface finish tothe component. $s a general rule it is considered that all moulding surfaces of a mouldmust !e provide a mirror polish. $ mirror polished moulding surface helps the moulds inmany ways as given !elow

(asy flow of molten plastics material ) melt* into the moulds (asy ejection of finished product from the mould +laing attractive surface for a commercial product +ood transparency for clear plastics materials -igh !rilliance and reflecta!ility for optical and automo!ile lamp reflectors ough surface of moulds would !e lia!le to retain acids and leading to localied

corrosion27.+ O,er,ie- o( t'e $roce"":The polishing process involves a num!er of steps that must !e performed in the properorder. This will ensure the !est finish in the least amount of time. emem!er, polishing is


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replacing one set of scratches with finer set of scratches until the desired finish isreached.Polishing can !e done !y following method

ap first with rough grade lapping paste )emery paper* to remove machiningmarks

$fter that medium and fine grade of lapping pastes )emery paper* are applied togive good finish

Final polishing is done !y chrome powder /iamond pastes of different grades are also used $mount of material removal is only a few hundredth 0are must !e taken to remove all machining marks initial stage itself to save time.

27. St!nd!rd "$eci(ic!tion o( (ini"':0urrently, there are three standards !eing used to define a finish. They are the 12/34PI which has six finishes, the 15(63 4PI which has &7 finishes and M4 values whichis the measurement of the depth of the scratches.

The mirror polish is achieved !y adopting polishing process, followed !y a properprocedure. The very first step is to remove the cutter marks left over !y the machiningoperation over the moulding surfaces. This is done !y using various type of hand toolslike files, needle files and raffle files. $fter o!tained surface finish corresponding to thefiner grade of files, a!rasive stones of various grades are used, which again followed !yvarious grades of emery papers. It is a la!orious and time consuming process whichdetermines the final re"uired final dimensions of mould cavity and core. Therefore thepolishing of moulds is to !e carried out under the close supervision of skilled mould

makers. The various se"uences of polishing process is illustrated in the following figures.To avoid manual strain and to speed up the polishing process there are polishing

machines availa!le with lot of accessories packed in a polishing kit which is alsoexplained with figures

27./ Mec'!nic! Eui$ent in Moud oi"'in&:Mechanical e"uipment )hand held* is important to the mould polisher for three reasons.

&* Mechanical e"uipment greatly reduces the amount of time re"uired to reach thedesired finish.

7* Mechanical e"uipment provides flexi!ility )many solutions to one pro!lem* andversatility )a!ility to do more tasks* for the mould polisher.

8* 9sing hand held mechanical e"uipment ena!les the polisher to !e moreconsistent in finish and in the time re"uired from piece to piece and from jo! tojo!.

Od SI Ne- SI RMS De"cri$tion%& $ : & 8 micron +rade %8 /iamond !uff

# $ : 7 ; micron +rade %; /iamond !uff

%7 $ : 8 &< micron +rade %&< /iamond !uff

' : & 7= micron ;== +rit emery

' : 7 8= micron >== +rit emery

' : 8 >= micron 87= +rit emery

%8 0 : & 7< micron ;== /raw stone

0 : 7 8< micron >== /raw stone

%> 0 : 8 >< micron 87= draw stone

/ : & ? /ry 'last +lass 'ead % &&%< / : 7 ? /ry 'last % 7>= $luminum 2xide

%; / : 8 ? /ry 'last %7> $luminum 2xide


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!% He!,3 Grinder 4Die &rinder%:/ie grinders have a "uarter inch collets. They are used to drive large mounted points,car!ide !urrs, a!rasive discs, flap wheels, large !rushes, and felt !o!s. /ie grinders areused to install large radii, remove deep cutter marks, and to diamond polish largesurfaces. -eavy grinders, called die grinders, are availa!le in air drive and electric withflexi!le shafts.

)% enci Grinder" 4i&'t dut3%:Pencil grinders have interchangea!le &@A3 and 8@873 collets. They areused to drive small mounted points, car!ide !urs, a!rasive discs,diamond !rushes and felt !o!s. Pencil grinders are used to installsmall radii, removing deep cutter marks on small pieces or tightareas, and diamond finishing. ight duty grinders or pencil grindersare availa!le in air drive and electric with flexi!le shafts.

c% Ri&'t An&ed He!d:

ight angled head s is availa!le in large B3 and small &@A3 collets.

They are used with the same cutter and a!rasives as die and pencil grinders. They areuseful for getting into tight places like cavity side walls. They alsoafford the polisher more control when using a!rasive discs. ightangled heads are availa!le in air#drive and electric with flexi!leshafts.

d% + De&ree He!d:The >< degree head is a variation of the right angle head. It is usedwith the same cutters and a!rasives as the right angle head. Theyare good for getting into tight spots and the !ottom surfaces ofdeep cavities. >< degree heads are availa!le in air drive and

electric with flexi!le shafts.

e% ro(ier:The profiler is a reciprocating hand piece. It reciprocates 7=== to &==== times perminute. 4troke length is set on a cam inside the !ody. Profilers are the most fre"uentlyused piece of polishing e"uipment. They are used to push small files, polishing stones,


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wood and !rass laps, and felt !uffs. Profilers are availa!le in air and electric with flexi!leshafts.

(% Utr!"onic He!d iece:The true ultrasonic hand piece operates on a wave length. They are used with similartools and a!rasives as a profiler. 'ecause ultrasonic vi!rates in a > to 8= micron range,their use is limited to small areas and intricate details. They are very efficient when usedproperly. Tool length is critical for proper operation. It is availa!le in air and electric drive.It should !e noted that the air drive is not a true ultrasonic tool.

27.7 Moud oi"'in& eri"'!)e"

6hy are perisha!les importantCPerisha!les perform the actual cutting, smoothing, and shining of the mould. Mechanicale"uipment may make the perisha!les work faster, !ut it is the perisha!les that are doingthe polishing work. From the coarsest files and discs through stones and emery, andfinally to the fine diamond compounds, all perisha!les work together to refine themoulding surface to conform to the customerDs specifications.

!% Mi )!"t!rd"arge files, called mill !astards, are rarely used in polishing !ut they deserve a mention!ecause on occasion they are used during the !enching process. Mill !astards came infour !asic shapes. Flat, round, triangle and oval. Typically A to &7 inches long, they have

coarse teeth and can remove larger amount of steel. 'ecause of this, caution should !eused with these tools. They are used for flattening, initial smoothening or large radii andcleaning out corners. They are always pushed !y hands.

)% Re((er"4mall files, called refflers, are used much more fre"uently than larger files. efflers areused in the !enching phase of polishing. They come in many shapes and configurations.


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Typically rifflers are six inch long with a small cutting surface on each end. They are usedfor initial smoothing in hard to reach and detail areas. ifflers can !e used !y hand or

with a profiler.

c% Di!ond (ie"The name diamond file refers to the cutting medium not the shape. They are used in !oth!enching and smoothening phases. /iamond files are 83 to ;3 long with a cuttingsurface if &3 to >3. They come in four shapes flat, round, half round and triangle. /iamondfiles are made !y electroplating diamond particles to a file !lank. They are excellent foruse during an (/M process. /iamond files can also !e used to install allradii. They can !e used !y hand removing cutter marks from thin ri!s, especially whenthe ri!s have !een !urned during (/M process. /iamond files can !e used !y hand !utare most effective when driven !y a profiler.

d% Grindin& -'ee"

These are wheels mounted at the tip of a shank. /ifferent shapes are availa!le dependson the usage. These wheels are clamped on air grinders and can !e used for removingmaterial during !enching. /eep cutter marks can !e removed !y this process.

e% F!$ -'ee"Flap wheels are strips of emery with one end epoxied to an ar!or. Flap wheels are &3 t=>3 in diameter. They are used with the die grinders. 'ecause they are resilient, flapwheels works well on flat and contoured surfaces. Flap wheels are not aggressiveenough to remove cutter marks. They are used to take out disc and cartridge roll marks.Flap wheels work !est when only light pressure is applied.


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(% Stone"4tones are manufactured 1sticks3. $luminum oxide and silicon car!ide are the a!rasiveparticles used. They are used in the final finishing phase of polishing. 4tones are s"uare,rectangular or round. 4ies range from B3 x &@A3 to & s"uare inch, !ut all are ;3 long.+rits range from &7= very coarse to &7== very fine. 0oarse stones )&7= : >==* are forremoving cutter marks and !enching marks. Finer stones are used to refine the finishand to prepare for diamond finishing. 4tones are used with a profiler and !y hand for adraw stone finish. 4tones should !e used with a lu!ricant such as mineral spirit toprevent 1loading up.3

&% Eer3(mery is a cloth with a!rasive material glued to one side and is used in the final finishingphase. (mery comes Iength &3 and 73 widths on rolls of varying lengths. +rits range from;= to ;==. (mery is pushed !y hand or !y wrapping it over a stick. 6hen wrapped over astick, a ru!!er pad should !e glued to the contact surface. The emery should !e usedwith lu!ricant to prevent loading and to produce a finer finish. (mery is used to smooth

large surfaces and refine a stone finish prior to diamond finishing.

'% Di!ond co$ound/iamond compound is a mixture of diamond particles and a grease !ase. /iamond

particles are graded for sie !y filtering through a mesh.They are used in the final finishing phase. It is availa!le ingrades from E= : very coarse )A= : &== microns*, to &@&= :the finest )= : B microns*. The coarse grades are used for

cutting and the finer ones are used for shining. This can !eused with !rass, wood, nylon and felt laps pushed !y aprofiler. /iamond compound is also used with !rushes and felt !uffs driven !y a die or


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pencil grinder. /iamond polishing removes stone and emery scratches and is the finalstep in producing a 1mirror3 finish.

i% 5ru"'e"

'rushes are made from animal hair, nylon and !rass. 'rushes are availa!le from coarseto soft. They come in three shapes# cup, wheel and end. They are availa!le in manysies. 'rushes are the most common way of driving diamond compound. 0up !rushesare used to diamond corners and flat surfaces. 6heel !rushes are used in insidediameters, side walls, parting lines, radii and round pins. (nd !rushes are used on flat orlightly curved surfaces. 'rushes are driven with die or pencil grinders, right angle and >

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CH27FINISHING OF METAL MOILDS.doc