Foseco Ferrous Foundryman''''s Handbook Part 12 doc

Unfiltered, choke at ingates 6.7 SEDEX filter at base of sprue 5.6 SEDEX filter in pouring bush 6.1 Both methods of using the filter are effective in removing inclusions.. Combined filte

Filtration and the running and gating of iron castings 265

Using filters in vertically parted moulds

Example 1: Grey iron brake disc casting

Brake disc casting are frequently made on DISAMATIC moulding machineswith vertically parted moulds The disc castings are subsequently machinedover a large part of their surface, any inclusions revealed by machining lead

to both high tool wear and scrap castings Filtration is clearly desirable butthere are a number of problems of application:

Mould production rates are up to 350/hour

Access to the open mould is not easy, filters must be placed by the coresettermachine to avoid slowing the mould rate

Space to locate filter prints is limited

Two systems of placing filters are possible (Fig 17.18a,b,c)

(c)

Figure 17.18 Use of a SEDEX filter in vertically parted moulds (a) unfiltered; (b) filter placed at base of sprue; (c) filter placed in pouring bush.

266 Foseco Ferrous Foundryman’s Handbook

Figure 17.18a shows the unfiltered running system, as is usual withDISAMATIC systems, the system is pressurised, being choked at the ingates.Figure 17.18b shows the SEDEX filter placed in a ‘crush print’ at the base ofthe sprue In this position, the filter can be placed using the coresetter Thechoke is prior to the filter Figure 17.18c shows the filter placed in the pouringbush, here it is possible to place it by hand after the mould has been closed.The sprue base acts as choke

The pour time is shorter with the SEDEX filter than without

Unfiltered, choke at ingates 6.7

SEDEX filter at base of sprue 5.6

SEDEX filter in pouring bush 6.1

Both methods of using the filter are effective in removing inclusions

Combined filter, feeder and pouring cup,

the KALPUR direct pouring system

The concept of direct pouring into the top of a mould cavity has long beenrecognised as desirable, with the potential benefits of:

Improved yield

Simplified sprue, gating and feeding design

Reduced fettling costs

Unfortunately, it was frequently found to introduce defects due to theturbulent flow of the metal in all but the simplest of castings In addition,the impingement of high velocity metal streams caused erosion of moulds

or cores These objections can be overcome by pouring the metal through aceramic foam filter situated at the base of an insulating pouring/feedingsleeve, the KALPUR unit Clean metal, free from turbulence and oxide, fillsthe mould cavity and readily feeds the casting through the filter Directionalsolidification and casting soundness is promoted and gates and sprues madeunnecessary The impingement problem is reduced because the metal velocity

is reduced as it passes through the filter (Fig 17.19)

Application to horizontally parted moulds

For manual moulding and simple moulding machines, the open pouringcup shape of Fig 17.20 can be used; an example of its use is shown in Fig.17.21 In horizontally parted automatic moulding lines, the KALPUR typeshown in Fig 17.22 can be used as in Fig 17.23

Filtration and the running and gating of iron castings 267

Figure 17.19 A schematic view of the cleaning and flow-smoothing effect of pouring directly through a KALPUR unit.

Figure 17.24 shows a ductile iron vice base, casting weight 26 kg made on

a 20/24 Hunter moulding machine

268 Foseco Ferrous Foundryman’s Handbook

Filtration and the running and gating of iron castings 269

270 Foseco Ferrous Foundryman’s Handbook

Application of direct pouring to vertically parted moulds

Increasingly complicated grey and ductile iron automotive castings are beingproduced in vertical moulds for hydraulic, brake, suspension and transmissionsystems Components such as conrods, hubs, brake drums, flywheels, brakediscs, brake brackets and steering knuckles are now common productioncastings in vertical moulds Since many of these are safety castings, theymust usually be filtered For ductile iron, feeders must also be incorporated

so that sound castings can be obtained

Figure 17.25a shows a conventional pattern layout for a ductile iron carhub casting made with three castings on the pattern plate The running

(a)

(b)

Figure 17.25 Use of the KALPUR unit in vertically parted automatic moulding lines: (a) conventional layout; (b) using KALPUR.

Filtration and the running and gating of iron castings 271

system allows mould erosion due to high metal velocity Figure 17.25b shows

a direct pouring system using a KALPUR direct pour unit at the top, andbelow it, a KALMIN S feeder sleeve ensuring correct feeding of the lowertwo castings Casting yield increased from 61% to 78% and productivity by33% while casting quality was improved

Figure 17.26 shows a ductile iron differential housing, casting weight 30

kg made on a DISAMATIC 2070 moulding machine

Figure 17.26 Ductile iron differential housing made on a DISAMATIC 2070 moulding machine with a KALPUR unit.

KALPUR direct pouring filter/feeder systems reduce mould and core erosionand ensure that there is hotter metal in the feeders than in the castings,giving ideal directional solidification from the casting to the feeder

Avoids turbulence of metal entering the mould

Prevents slag and other inclusions from entering the mould

Avoids high velocity impingement of the metal stream onto cores ormould surfaces

Encourages thermal gradients within the casting which help to producesound castings

Enables the casting to be separated from the running/gating system easily

Controlling the flow of metal

Ideally the gating system should control the flow of metal into the mould

If lip pouring ladles are used, this can be readily achieved since the pourer

is able to match the requirements of the gating system by altering the tilt ofthe ladle It is more difficult with bottom pour ladles The flow rate from abottom pour nozzle/stopper rod system is determined by the nozzle sizeand the metal height in the ladle according to the formula:

where t = pouring time (s)

D = mean ladle diameter (cm)

M1 = initial weight of metal (kg)

∆M = weight poured (kg)

d = nozzle diameter (cm)

ρ = density of liquid steel (7.7 g/ml)

Filtration and the running and gating of steel castings 273

(From C.S Blackburn and B Blair; Gating system design for bottom pourladles, 35th SCRATA Conference, May 1992)

The flow rate varies considerably depending on the level of metal in theladle This is illustrated in Fig 18.1 which shows the variation in pouringtime of seven 750 kg castings from a ladle initially containing 5750 kg ofsteel A gating system designed to accept the initial pouring rate of 750 kg

in 18.23 seconds will not be suitable to accept the last 750 kg which isdischarged from the ladle in 44.95 seconds, 2.5 times longer!

Figure 18.1 The variation in pouring time of seven 750 kg castings poured from a bottom-pour ladle initially containing 5750 kg of steel.

(From Blackburn C.S and Blair B 35th SCRATA Conference, May 1992, courtesy CDC.)

Weight in Pouring ladle (kg) time (sec.) 5750

The nozzle/stopper rod system of a bottom pour ladle is an excellent

‘on/off’ valve but ideally should not be used as a flow control valve Attempts

to use it to control flow result in breakup of the metal stream with consequentrisk of reoxidation of the steel and possible erosion of the stopper rod itself.There is, however, no alternative if the gating system is to control the flowrate A compromise must be reached for each cast

274 Foseco Ferrous Foundryman’s Handbook

Conventional running systems without filters

The elements of a running/gating system for a horizontally parted mouldare shown in Fig 17.1

Pouring bush The use of conical shaped bushes which direct flow straight

down the sprue is discouraged as not only will air and dross be entrainedand carried down into the system, but also the high velocity of the metalstream will result in excessive turbulence in the gating system Incorrectalignment between nozzle and pouring cup will also cause metal splashing(Fig 18.2)

A pouring bush designed as in Fig 18.3 provides a larger target area, ashape which minimises splashing and sufficient volume to accommodatethe maximum flow of metal obtained by opening the stopper rod fully thusreducing the need to throttle The exit from the pouring bush should beradiused and match up with the sprue entrance

Figure 18.2 Simulation of metal splashing due to incorrect alignment of nozzle and pouring cup.

(From Blackburn C.S and Blair B 35th SCRATA Conference, May 1992,

Courtesy CDC.)

Filtration and the running and gating of steel castings 275

avoid the possibility of slag being drawn into the mould cavity rather thanbeing retained in the pouring cup The metal stream exiting the bush narrows

in diameter as it falls and its velocity increases To avoid air aspiration, thesprue should taper with the smaller area at the bottom

Sprue base Because stream velocity is at its maximum at the bottom of the

sprue it is important that a sprue base be used to cushion the stream andallow the flow to change from vertical to horizontal with a minimum ofturbulence Recommended sizes of the sprue base are, a diameter two–three times the sprue exit diameter and depth at least equal to the depth ofthe runner bar (Fig 18.4)

rectangular (with some taper to allow for moulding), or circular (if refractoryhollow-ware is used) It is presumed that a tall runner allows slag and dross

Figure 18.3 Simulation showing the elimination of metal splashing due to optimisation

of pouring cup design.

(From Blackburn C.S and Blair B 35th SCRATA Conference, May 1992,

Courtesy CDC.)

276 Foseco Ferrous Foundryman’s Handbook

to collect in the upper part of the runner The distance between sprue andthe first gate should be maximised for effective inclusion removal Therunner should extend beyond the last gate so that the first cold, slag-richmetal is trapped at the end of the runner

Ingates The ingate should introduce clean metal into the mould cavity

with the minimum of turbulence Low turbulence is best achieved by ensuringthat the gating system is unpressurised by sizing the gates to have a totalcross-section at least as large as that of the runner bar The gating systemmust always be full which is achieved by taking ingates of the top of therunner bar and gating into the lowest part of the mould cavity The ingateshould also be sized so that its modulus is smaller than that of the section

of the casting into which it enters This prevents the formation of a smallshrinkage cavity at the ingate/casting interface If this cannot be achieved,the metal should be introduced into a feeder head

any metal shrinkage occurring during solidification of the casting

Figure 18.4 Flat bottomed sump located at sprue base minimises turbulent metal flow.

(From Blackburn C.S and Blair B 35th SCRATA Conference, May 1992,

Courtesy CDC.)

Filtration and the running and gating of steel castings 277

Riser An opening leading from the mould cavity which relieves air pressure

in the mould cavity as it fills with metal It also acts as a flow-off, allowingcold or dirty metal to be removed from the mould cavity If an open toppedfeeder head is used, a riser is not necessary

Gating ratio The gating system should be unpressurised with sprue : runner:

ingate areas 1 : 2 : 2 If a bottom pour ladle is used, then the sprue sizeshould be related to the nozzle size, with the nozzle bore controlling theflow although, as mentioned above, this is generally not possible because ofthe variable flow from a bottom pour ladle as the level in the ladle falls.Gating of steel castings is not a precise science and each foundry developsits own preferred methods

The use of ceramic foam filters

The primary function of the gating system is to introduce metal quickly intothe mould without turbulence or non-metallic inclusions Conventional gatingdesign is always a compromise When effective ceramic foam filters suitablefor steel were developed, they led to immediate improvements in the quality

of steel castings since, not only do filters allow inclusions in the steel to beeliminated more certainly than in the past, but they also reduce turbulence

in the metal stream allowing the two main functions of a gating system to

be readily achieved in a way that allows significant yield increases

Inclusions in steel castings

The cleanliness of steel is, in the main, defined in terms of non-metallicinclusions Improved properties of castings result from reducing the number

of inclusions Inclusions are introduced into steels during every stage ofprocessing, from melting, trimming additions, deoxidation, tapping andteeming, as well as being generated within the mould itself Inclusions aregenerally termed:

Exogenous – which arise from external sources and are typically particles of

sand, refractory, moulding materials, melting and ladle slags and agglomerates

of any of these

Indigenous – which originate from chemical reaction between elements within

the steel itself during melting and deoxidation, and reoxidation, which occursduring tapping and teeming Such inclusions are essentially silicates, oxides,nitrides and sulphides, or more often complexes of these

278 Foseco Ferrous Foundryman’s Handbook

It has been estimated that 1 tonne of steel contains between 1012 and 1015indigenous oxide inclusions Each inclusion has associated with it a localisedstress field, with inclusions of the order of 20 microns capable of nucleatingfatigue cracks, and effecting reduction in fracture toughness and ductility

A study of macro-inclusions in steel castings from 14 foundries in theUSA found:

83% were reoxidation products which could be up to 5–10 mm in size14% were from mould materials (sand and coating)

2% were slag

1% were refractory

1% were deoxidation products, always small in size, typically less than

15 µm

(Svoboda J.M et al Trans AFS 95 187–202 (1987))

Reoxidation occurs during pouring from the ladle and within the runningsystem as a result of turbulence, illustrating the importance of designingrunning and gating systems with care

Effect of inclusions

Exogenous inclusions: Usually found in areas of rapid solidification, on upper

cast surfaces or trapped under cores or adjacent to vertical walls They can

be surface or subsurface, revealed on machining and may also be associatedwith ‘gassy’ cavities The size of these inclusions can vary from fine micro-particles to gross macro inclusions up to about 75 mm diameter and 10 mmthick Exogenous inclusions give:

Impaired surface finish

Poor machinability

Reduced mechanical properties

Indigenous inclusions: are very varied and are predominantly of the following

types:

Silicates – Usually glasslike inclusions which are relatively hard Can have

an adverse effect on fatigue and impact properties Silicates have a deleteriouseffect on machinability by drastically reducing tool life

Alumina – Primary source is from the deoxidation practice where aluminium

is used as the principal deoxidiser In general appearance the alumina crystalshave a branched dendritic form Alumina affects toughness and ductility,reduces the ‘polishability’ of surfaces and is abrasive to cutting tools It alsoreduces fatigue strength Aluminium is probably the most common deoxidant