PRELIM INARY CONSIDERATIONS Machining and Contraction Allowance

Before describing m oulding m ethods in detail there are one or two prelim inary factors w orthy of consideration. In the first place, adequate, but not excessive, m achining allowances should be m ade on patterns. As a general rule, ts to i in. on the seat, and | to tV in. per side on pintle, boss and other m achined diam eters will be sufficient, and wherever possible the seat should be cast down, as any entrapped dirt, slag or sand, however small, on this surface will lead to rejection. T he m ain body of the valve lid should be cast as nea.r to size as possible. The need o f greater machining allowances than those stated above indicates something wrong else­

where, such as gassy or sluggish metal, too low a pouring tem perature, poor sand o r

imper-Fi g. 3.— Bo i l e r St o p Va l v e Li d s.

m eable mould, incorrect m ethod o f running, etc., and an attem pt should be m ade to find out the cause of the trouble rather than to add extra m achining and hope for the best. A con­

traction allowance of tc to } in. per ft. should be allowed on patterns, according to the type of alloy employed.

Pouring Temperature

Pouring tem perature for all the nickel alloys used for valve lids is high, being between 1,450 and 1,550 deg. C., according to the com position employed. U nlike gunmetal, however, there is a wide latitude in this direction, and the relation between pouring tem perature and casting section

e2

is no t critical. Every endeavour should be m ade to pour as hot as possible. T he tem perature range required is outside the scope o f the m ost suitable foundry type of im m ersion pyrom eter, b u t a very good guide can be obtained by test­

ing the m etal w ith a | in. m ild steel bar. If the m olten m etal just melts the end o f this bar, bringing it to a point, it can be assum ed th a t the tem perature is aro u n d 1,500 to 1,550 deg. C.

If the bar does not quite m elt, bu t evolves sparks, then the tem perature is in the lower range of 1,450 to 1,500 deg. C. N o m etal of the types m entioned should be poured into cast­

ings lower than 1,450 deg. C.

Moulding Sands

In view o f the high pouring tem peratures entailed, care m ust be taken to ensure fairly refractory and perm eable moulds. T h e best results are obtained by the use of dry sand moulds, but green sand can be utilised for small castings if necessary. T he m ajority o f natural

best dry sand for the job is, however, one of the m ore open m ixtures as used for the p ro ­ duction of iron castings m ade from naturally bonded or synthetic sand having an A .F .A . p e r­

m eability n um ber of n o t less than 80 with a dried com pression strength o f n o t less than 50 lbs. per sq. in. G o o d results can also be obtained by m aking the m ould in oil sand using fo u r parts o f sea sand plus one p a rt o f red sand as a base.

F o r the p roduction of the sm aller valve lids in green sand the usual finely textured brass- foundry sands are no t satisfactory unless they are opened up w ith sharp o r sea sand and incorporate an addition o f aro u n d 5 per cent, plum bago o r a good grade o f blacking in the facing to im prove refractoriness. G o o d results are again obtained by the use o f an open iron- foundry type o f green sand containing coal dust, bu t the latter ingredient should n o t be added to the m ore com m on brassfoundry green sands as their perm eability as a rule will not take care o f the additional gas evolved at the high casting tem peratures entailed. Lightly dusting the m ould surface w ith plum bago and p rinting back the pattern is a useful precaution w here this can be conveniently arranged.

F o r the production o f the m ajority o f the

6 'W

Fi g. 4 .— Gr a d u a l Op e n i n g Va l v e Li d s.

m oulding sands in brassfoundry use, such as Mansfield, Bromsgrove, Belfast, E rith, etc., by themselves are neither refractory nor perm eable enough to give the best results, but it by no means follows th at they cannot be employed.

Satisfactory results can be obtained by ensur­

ing dry sand m oulds m ade from the above are well wire vented, thoroughly dried, and have their surfaces protected with a good plum bago wash. T he latter should be m ade from the highest grade plum bago bonded with the neces­

sary am o u n t of china clay and w ater to which a sm all am ount o f core gum, dextrin, molasses or wood extract has been added. lu s t enough o f the la tte r should be used to prevent the dried wash com ing aw ay on the fingers w hen rubbed.

Blacking, boulder or com m on red clay should on no account be em ployed as alternative con­

stituents, as they lower the refractoriness of such a protective wash. F urtherm ore, the addition o f the organic binder should no t be om itted, as this also m aterially im proves refractoriness. The

Fi g. 5.— Me t h o d o f Mo u l d i n g Ba rf o r Ma k in g V e r y Sm a l l Li d s (1 -i n. Bo r e Va l v e).

castings under review, cores will n o t usually be required. A lthough draw ings m ay outline a design w here a sim ple core could be used, it will be found cheaper and better in the long run to m achine from the solid as the in tro d u c­

tion of cores invariably prevents progressive solidification being obtained w ith consequent porosity on the hydraulic test which is on an average around 600 lbs. per sq. in.

Valve Lids o f Simple Design

Before com m encing production, the foundry- m an should have in his hands a draw ing o f the finished p a rt showing all m achining limits in order to check th a t allow ed on the castings.

F u rth e rm o re any outside patterns o r core-boxes should be carefully exam ined to ensure that they agree with subsequent rem arks relative to 52

the best m ethod of moulding. In many instances, for example, it has been found that patterns and core-boxes are designed in order to facilitate m achining operations rather than the production of sound castings. In such cases the foundrym an should no t commence m ould­

ing until satisfactory modifications have been effected. This particularly applies to the pro­

vision of core-boxes, the use of which has often to be om itted if the best results are to be obtained.

Fig. 1 illustrates a com m on and fairly simple design of lid for the larger type of boiler stop valves. Such castings will have overall diam eters between 10 to 12 in., with a 4-in.

diam eter centre boss bored ou t to I f in. dia., and a body section tapering from I f in. down to f in. adjacent to the seat portion. The stresses these lids have to w ithstand are often very high, and the im portance of obtaining cast­

ings of the best quality cannot be too strongly emphasised. N o attem pt should be m ade to core o u t the hole through the centre boss, as

Fi g. 6 .— He a v y Li d s ( Pi n t l e Ty p e).

this may interfere with the proper feeding of the casting at the junction of the body with the boss, a point where m axim um solidity and strength is m ost needed.

Very satisfactory results have been obtained in practice by m oulding the seat face down in dry sand and casting the boss solid, as shown in Fig. 1. A large tapered riser is attached for feeding purposes, and the casting is poured through this riser from a runner bush by a num ber of | in. jets, arranged circum ferentially so th at the m etal streams do not strike the mould wall. The height o f the riser should be greater than the depth of the boss underneath.

Direct pouring into the feeding head is the only way of ensuring the hottest metal at this point, and the use of a pouring bush with jet runners minimises the ingress of slag and also has the additional advantage of acting as a “ hot-top ” which materially assists in keeping the metal in the riser fluid until the casting has solidified.

There is, of course, no need to allow the runner

bush to retain metal, and pouring can be stopped before the feeding head is full so that the runner bush empties itself into the casting.

Figs. 2 and 3 illustrate smaller types o f lid castings for boiler stop and manoeuvring valves.

The m oulding m ethods em ployed in principle are similar to those already described, namely, to cast solid and pour direct through a feeding head, with the exception that recourse should be m ade to the use o f chills in order to assist progressive solidification. F o r example, in the design shown in Fig. 2, there is some doubt as to w hether the head will feed the entire depth of the long centre boss, and the use of an external base chill is indicated. An internal chill, m ade from a steel bar, acting as a core for the threaded section B (Fig. 2) would be

still m ore effective, and if necessary this can remain in the casting and be removed in subsequent m achining operations. In the case of dom ed type lids as outlined at A, in Fig. 3, the section of the casting at the centre is much too heavy in relation to the diam eter of the boss to obtain satisfactory results by top feed­

ing alone, and progressive solidification should be again arranged with the assistance of chills or carborundum sand.1 It is interesting to note that the use o f the latter is steadily increasing, and quite a few foundries are finding it a more convenient substitute fo r chills in the produc­

tion o f pressure castings. If design can be modified to give a m ore even section, as shown at B (Fig. 3), the use o f chills, etc., obviously becomes unnecessary.

Fig. 4 outlines the m ethod of m oulding small valve lids of somewhat different design as re­

quired for certain types of control valves. The m ain point to note in both the examples given is the fairly wide pattern modifications entailed

over the finished m achined product. It will be observed th a t in the design show n at A the p a t­

tern has been altered to facilitate the use o f chill segments, whilst th e corfe has been om itted. In the case o f exam ple B, there is little alternative bu t to cast the valve w ith the seat up, so that the heavier section can be directly fed by the head. In this instance there is an obvious

Fi g. 8 .— Su g g e s t e d Me t h o df o r “ Ca s t i n g-i n ” Pi n t l e.

seat area (x), little trouble need be anticipated regarding the possibility o f dirt, etc., being trapped a t this point. I t m ight be noted that the design o f the feeding head has been modified in Fig. 4, over the examples previously given, to em brace a sudden thickening as it leaves the casting instead of a gradual taper. T here is no d o u b t th a t this m aterially im proves feeding action and also provides a guide for cutting off operations in the fettling or m achine shop.

In the m anufacture of very sm all lids, say, for 1-in. bore valves, there is little advantage to be gained in m aking individual castings. T he m ost convenient m ethod is to m achine from sand cast bars m oulded vertically, as show n in Fig. 5. Chill casting is no t recom m ended, due to the tendency tow ards centre porosity, and even w ith sand cast bars care m ust be ta k en to ensure th a t the length o f the bar is n o t too long, otherw ise trouble o f a sim ilar kind will arise.

N orm ally a b ar 6 to 7 in. long, with a head of sim ilar length, is ab o u t the m axim um which can be em ployed in conjunction w ith any guarantee o f soundness.

Valve Lids o f M ore Difficult Design All the lid designs so fa r dealt w ith have been free fro m pintles, an d founding difficulties increase w hen this fitm ent is incorporated as an integral p a rt of the casting. Fig. 6 illustrates a typical pintle type valve lid, an d provides an interesting com parison o f good and bad m ould­

ing m ethods. If the casting is poured as shown at B (Fig. 6), i.e., w ith a heavy runner on the seat portion, porosity will invariably be found in the centre o f the lid a t A, even w hen am ple feeding heads are em ployed. T his is due to the fact that, when pouring is com pleted, the m etal at this p o int is ho tter th a n th a t in the head, which consequently solidifies before fully feeding the central portion o f the castings. The tendency to porosity is fu rth e r aggravated by the use o f a feeding head having an open top and by the extra m etal som etim es allow ed on a line xx, w here th e pintle joins the valve body.

By adopting m ethod A (Fig. 6) solidity and pressure tightness are ensured.

T h e latter m ethod w orks extrem ely well when the pintle is n o t too long and w hen it is attached to a fairly heavy body section. U nfortunately, certain designs o f lids exist in w hich m ost of the body is o f lighter section th a n th e pintle, as outlined in Fig. 7, and it becom es necessary to cast the pintle u p in o rd er to get the centre of the casting properly fed. T h e m atter is fu rth er com plicated by the fact th a t the valve seat is nearly always on the pintle side, an d this m ust be cast up as well. F ortunately, by exercising a little foresight in slightly m odifying the de­

sign at this point, as show n in A (Fig. 7), or increasing th e m achining allow ance to ab o u t

| in., asi in B (Fig. 7), any trouble due to dirty seats can be elim inated. T h e business o f get­

ting the pintle and central body section sound is n o t so easy a m atter. In the design shown at A in Fig. 7, incorporating a relatively light pintle and body section attached to a heavy hexagonal shaped centre, it is necessary to in ­ crease the diam eter o f the pintle to equal th a t o f the centre section and subsequently m achine the pintle dow n to size. In castings having no

heavy central portion, as illustrated at B (Fig. 7), the pintle need only be thickened enough to ensure progressive solidification. In any case of doubt, however, one should err on the safe side by over-thickening, as on no account should the pintle, when cast up, be allowed to solidify before the body section below.

There are a few designs of lids in existence incorporating a very long pintle w here d oubt will exist as to w hether any m ethod of m ould­

ing will produce a satisfactory casting. In such cases it is w orth w hile taking the m atter up with the designer as to w hether the pintle cannot be m ade from w rought bar, and subsequently fitted by m echanical means, o r cast into the fid in the foundry. In the past there seems to have been very little effort m ade to “ cast-in ” pintles, but it should be quite a practical operation providing the body section is heavy enough to ensure the passage of enough m etal to effect welding o f the pintle with the body. The pintle (A, Fig. 8) can be m ade from M onel bar, and the end going into the casting should be m achined with a series of deep knife-edged parallel grooves to facilitate its union with the m olten m etal in a similar m anner to that em­

ployed for a certain type o f chaplet, as shown in Fig. 8. T he high pouring tem peratures em­

ployed fo r the type of castings under review should prom ote satisfactory fusion, bu t if any difficulty is experienced recourse could be m ade to coating the grooved end of the pintle section with pure tin. In m any designs o f lids the attachm ent o f pintles by a casting-in operation will also m aterially simplify m oulding methods and assist the production of sound castings by their acting as an internal chill.

Test-Bars

T he m ethods outlined in this Paper for m ould­

ing valve fids do no t readily lend themselves to the production of “ cast-on ” test-bars, and the supply o f separately cast bars should be adopted. W hilst some controversy m ay arise on this latter point in connection w ith speci­

fication requirem ents, certain facts stand out very clearly in defence o f this argum ent. In the first place the foundrym en’s prim ary con­

sideration is to produce a sound casting, and this is seriously jeopardised by enforcing the use o f “ cast-on ” bars, particularly in view of the type o f alloy employed and the size and design o f casting being made. Secondly, all valve fids are invariably subjected to a hydraulic test of a t least double their norm al working pressure, which in conjunction with an indica­

tion o f m etal quality as provided by a separately cast test-bar, should be amply suffi­

cient to differentiate between suitable and u n ­ suitable material.

Fig. 9 illustrates various m oulding methods which can be adopted for the production of test

bars required in conjunction with valve fids.

T he I.B.F. bar, as shown a t A, recom m ended for gunm etal castings, is n o t altogether satis­

factory fo r nickel alloys, as there is a tendency for th e bars to break a t the shoulder and give erratic results on test. This is caused through the presence o f slight porosity a t the end of the bar nearest the riser brought about by inadequate feeding due to the colder m etal at this point, and also probably due to strains arising through restricted contraction.

The modified m ethod o f m oulding the I.B.F.

u

-Fi g. 9 .— Mo u l d in g Me t h o d s f o r Te s t-Ba r s.

bar outlined at B undoubtedly improves feeding by delivering hot m etal to both ends of the test- bar, bu t contraction strains and cracks can still arise unless the sand between the risers is eased soon after pouring. If this operation is not carefully done there is still the danger o f the test-bar being dam aged through rough handling, and probably the simplest way o f producing consistently sound bars is to ad o p t either the wedge or clover leaf2 type o f pattern, as shown at C and E. Furtherm ore, these two m ethods produce test-bars moulded ip » sim ilar m anner

to the actual lid castings, and the test figures attained a re therefore m ore nearly representa­

tive of th e m etal quality in the actual casting.

Som e foundrym en m ay grum ble ab o u t the increased m achining entailed in rem oving the head from the test bar p o rtio n in the latter types, b u t this becom es a relatively sim ple m a tte r if a high speed slitting wheel o r one of the latest types of fast m etal cutting circular saws is available. A dm ittedly, w ithout some such assistance the rem oval of the test-bar portion is a tedious operation, and m atters can be facilitated by adopting the m oulding m ethod illustrated a t D in Fig. 9. This m ethod is widely used in the U nited States, and providing the test-bars- a re kept short good results are obtained. T he m ould is gated a t the base of th e head portion as shown and two bars poured from a com m on runner.

O ne last thought in conclusion. Rem em ber th a t steam valve lids fo r m arine service are re­

quired to stand u p to arduous conditions these days, an d only th e best is good enough. I f a casting is no t wholly satisfactory, then scrap it w ithout com punction, and on no account attem pt to patch it up by welding, n o r disguise its solidity by peening o r any other means.

R E F E R E N C E S . 1 M etal In d u s tr y , M ay 21, 1937, p ag e 573.

R E F E R E N C E S . 1 M etal In d u s tr y , M ay 21, 1937, p ag e 573.