By H. H. JO N ES The m aking o f cores by blowing the sand
into the corebox under air pressure, is an art that has been developed in the last decade.
South A frican foundrym en have been quick to appreciate the advantages o f the m ethod, and already m ost o f the larger foundries are using core-blowing machines, some having as many as three, five, even six m achines at work. The science, however, is very new, and no text-books are available on the subject. Consequently, practical knowledge o f core blowing has been confined to those who have actually worked directly with core-blow ing m achines in the foundry. T he pneum atic core-blowing m achine is intended for producing small and medium size cores in quantities, and is prim arily a p ro duction machine. G iven a sufficient dem and for any particular core, equipm ent can be de
veloped to a very high degree and a trem endous rate of output achieved, with a corresponding reduction in cost per core.
In South Africa, several types of core are being blown in quantity, and the rate o f pro
duction in some of these cases does not com pare unfavourably with results obtained over
seas. H owever, as nearly all the foundries are of the jobbing type, cores are usually required in rather small quantities, and this entails fre
quent changing of coreboxes. Nevertheless, many core blowers are being successfully em
ployed in South Africa on, a t any rate, semi
jobbing work, and m ethods have been devised to adapt the m achines to this class of work.
Some of these m ethods are decidedly ingenious.
A num ber of m arked advantages result from producing cores by the blowing method.
Rapid Production
The most obvious gain is speed. This is in
fluenced very considerably by various factors, including shape, size, and num ber of cores re
quired, and also the degree o f efficiency o f the corebox and other equipm ent in use. In some cases the output will be forty o r fifty times as high as with hand work. In others, the advan
tage m ay not be nearly so great. Each core
m ust receive individual consideration, and it m ust always be decided whether a saving in the ultim ate cost of production can be m ade by blowing the core.
A lm ost as im portant as speed, however, is the im provem ent in quality o f the core. Blown cores will be m ore uniform in structure, fighter in weight and higher in strength than those p ro duced by other methods. They will be form ed m ore perfectly, have a higher degree of per
meability, and, due to their sm oother skins, will im part a better finish to the castings. A charac
teristic of a blown core is thqt the finer sand grains arrange themselves at the outside of the core with the larger grains in the centre. C on
sequently the skin will be o f m aximum sm ooth
ness and the core will have m aximum per
meability.
Less Venting and Reinforcing
Due to this openness, the cores do not require as many gas vents. However, on large chunky cores it is sometimes desirable to use vent rods.
These present no difficulty, for they may be built into the corebox in such a m anner that they can be withdrawn after the core has been blown. Alternatively, after blowing, a wire can be inserted through a print, and withdrawn, leaving a vent hole in the core.
The nature o f the blowing operation makes it possible, quickly and uniform ly, to fill core
box cavities having unusual o r intricate con
tours th at would be difficult, or impossible, to ram by hand. Since the filling is uniform , hard and soft spots are eliminated, and the core is of maxim um strength; thus, reinforcing wires and rods will no t be required to the extent necessary in a hand-ram m ed core. U sually it is possible to design coreboxes so they can be blown through a core print, o r so th a t a core print will extend through the wall o f the box to perm it any necessary nails or rods to be inserted before the corebox is drawn. Open
ings in the top, bottom o r sides o f the core
box will, of course, be autom atically sealed off during the blow, by the machine itself.
179 n2
If wires are fo u n d to be desirable in certain sm all o r sim ple cores, it is possible to place the fo rm e d w ire in the corebox cavity, and the whipping action o f the sand will pick up and em bed the w ire in th e body o f the core. On m ore difficult jobs the w ire can be bedded in a han d fu l o f sand before blowing; o r the rods can be supported in the core prints. In extrem e cases posts can be built in the lower half o f the box to support the rods, bu t there will be a corresponding opening in the finished core, w hich will have to be filled an d patched after drying. In the past, no t all South A frican
moisture. Incidentally this latter feature aids m aterially in m aking a core blow er payable in a jobbing foundry because oil-sand cores can be blown in quantities and kept on the shelf ready fo r use. This enables the core m aker to plan out his d a y ’s w ork and, to a large extent, keep ahead instead of w orking from hand to mouth, as it were, and having to keep m oulders waiting for cores.
T o blow cores successfully, one m ust have the m achine, com pressed air, suitable sand,- strong coreboxes, properly designed and vented;
the necessary drying plates o r carriers, a bench, a core oven, and som e shelves fo r storing the finished cores.
M achines
T h e m ain essentials o f a core-blowing machine are a sand reservoir w ith blow plate; some means o f filling the sand reservoir and sealing it; and a system of horizontal and vertical clamps to hold the corebox during the blow operations.
Fig. 1.-- Ty p i c a l De s i g n o f Co r e- Bl o w i n g Ma c h i n e.
foundries have m ade use o f the great advantages o f oil sand fo r cores.
O il-Sand
A core blow er has often been the m eans of introducing oil-sand cores into a foundry, yet at the sam e tim e greatly reducing the overall cost per core. Oil-sand cores have great strength when dried, and exceptional perm eability; they virtually elim inate the need for nails, wires, rods, irons o r vents, except fo r certain large cores; and the cores can be kept in stock for m onths w ithout their drying out or absorbing
Fi g. 2 . — Ar r a n g e m e n t o f Ve n t e d Bo a r d Ac t i n g a s Fl o o r a n d c a p a b l e o f Ve n t i n g Ma n y Di f f e r e n t Ty p e s o f Co r e.
M achines of several different designs are m anu
factured. T he m achine shown in Fig. 1 has a sand reservoir which travels back and forth from the filling position to the blowing position, on a carriage fitted with rollers running on rails in the upper structure o f the fram e. In the filling position, the th ro a t o f th e sand reservoir is open to receive sand by gravity from the hopper. O perating a small hand-lever causes the reservoir to move under pow er to the blow position directly over the corebox, w hen blow
1 8 0 FLOOR LINE ^ 4. SAND RESERVOIR IN
LOADING POSITION
holes, drilled in the blow plate, will correspond with blow holes in the corebox. When the foot pedal is depressed, the m achine autom atic
ally goes through the following cycle of opera
tio n s :—
The m achine table and corebox are raised by the vertical clam p diaphragm , thus forcing the sand reservoir to rise also, and form a seal against a rubber sealing ring in the head of the machine. An autom atic safety valve then makes contact, and adm its air behind the m ain blow diaphragm , which lifts off its seat, perm itting the compressed air to enter the sand reservoir. This forces the sand into the corebox. T he pressure is then autom atically exhausted, leaving the core
box filled. T he whole operation occupies about 6 secs., no m atter w hether the core be small or large; or w hether one core is blown at a time or several dozen.
With this type of m achine a free-flowing sand must be used. If the sand is not flowable, craters will form over each blowhole, and air, only, will enter the corebox. Some core blowers incorporate an electrically-driven impeller in the sand reservoir, which permits the use of stronger sands by reducing the tendency to clog. One design employs a valve over the blowhole, which closes after each blow, thus retaining the air- pressure in the reservoir, the object being to reduce air consum ption. The disadvantage of these latter types is that the machine has to be stopped, the pressure exhausted, and a cover removed after every few blows, in order to re
plenish the sand reservoir; and the blow area in the blow plate is strictly limited, so that it is frequently uneconom ical to blow small cores (like screen cores) on machines of this design, since only one or two can be blown at a time.
Sands
The question of the sand to be used depends on the size and shape of the core and the type of metal being poured. M any kinds of sand are being em ployed on the Reef, from ordinary dum p sand mixed with about 2 \ per cent, of raw linseed oil, to quite strong mixtures con
taining percentages of red sand, yellow sand, bentonite, molasses, core gum, or other proprie
tary binders. The core sand should be thoroughly mixed, preferably in a machine, which will do the job far better and with a re
duced percentage o f core oil. The sand should be placed in the mill first, the dry binder, if any, added, and after a short time, the core oil.
A fter mixing for several minutes, water may be added to bring the m ixture to the desired moisture content, and the mixer run for several minutes longer. Over-m ixing should be avoided.
C oreboxes
N o m atter how excellent a core-blowing m achine m ay be, it cannot blow perfect cores
unless the corebox is satisfactory. Coreboxes m ust be properly designed, with suitable blow holes, correctly located, for the entry o f the stream o f sand and air, and adequate vents for the air to escape.
F o r high production the best results are obtained from first-class m etal coreboxes having well-fitted joints throughout, and sufficient land at the parting join t and around the blowholes to ensure a good, tight seal. T he outside sur
faces of the box should be squared, and all opposite clam ping surfaces m achined parallel so that the box will clam p squarely in the machine. The boxes should be o f cast iron, alum inium , or other metal, or of hard wood, depending on cost, weight, rate of wear, and am ount of m achining entailed. If necessary, alum inium or wooden boxes can be fitted with steel bushes or inserts at the places m ost sub
ject to wear. H ard wood coreboxes have given
Fi g. 3 .— Ty p i c a l Sc r e e n Ve n t e d Co r e b o x.
very good results in m any South A frican foun
dries, especially where the num ber of cores re
quired at any one time is fairly small. Some wooden boxes have been in frequent use for as long as three or four years; they are now som ew hat battered and scarred. The cores they now m ake have flash on them that has to be cut off with a trowel. D uring all this time the attitude has been that the cores were being made much m ore cheaply than could be done by hand, and therefore the position was satis
factory; but a certain am ount o f touching-up and finishing-off was always required, due to defects in the coreboxes, and undoubtedly m uch time would have been saved if first-class metal boxes had been m ade initially. If a t all pos
sible, the equipm ent should be such that the trowel is never needed, and can be dispensed with. M any of the cores at present used in this country have a flat surface and can be draw n on to a flat plate. Others, such as hand-grenade cores, require driers or carriers, w hich m ust 181
be cast o r pressed to shape, an d are a n essen blow er can often effect substantial econom ies.
In this connection, prospective purchasers of core blow ers, after inspecting existing installa
tions in other foundries, have been heard to blowholes opposite these points. T he blowholes in the m achine blow plate should be counter blowholes, strategically located, plugged when
no t in use, can alm ost elim inate the need for approaches an explosive condition, causing the sand grains to reb o u n d from the p o in t o f con contained by the corebox cavity originally, m ust be released. If it is no t released, the box
m unicate with holes drilled through to a t
m osphere, at an angle. These holes can be joined together by a channel and the system m ust be able to bleed air quickly from the grooves or slots. This scheme can be adapted in m any ways to suit different coreboxes; but direct outlets to the atm osphere should be dis
couraged. At least one abrupt change of direc
tion should be introduced to retard the velocity of the venting stream , this m aterially reducing the cutting action which tends to enlarge the vents.
Venting Data
The size of vent em ployed is governed by the average grain size of the core sand in use;
the vents should always resist the passage of the sand grains. If they do not, sand will stream out and leave a hollow place in the core.
V ent plugs for coreboxes are available com mercially. They are m anufactured in diameters of in. to i in., and some are o f a type that can be m achined to conform with the contour
production core blowing, it is sometimes pos
sible to provide suitable vents in the actual blow plate of the m achine itself. This saves having to vent each corebox where several are in use at a time.
Limiting Conditions
Core-blowing machines are available in various sizes based on (a) the size of corebox they will accom m odate; and (b) the weight of core they can blow. The size of box accom modated is governed by the size of the blow plate, machine table, and clamps. A part from these mechanical lim itations it is noteworthy that if a corebox has a greater area than the opposing clam p diaphragm (say in the case of blowing an impeller core), the pressure tending to force the box open will be greater than the pressure tending to keep it closed. The result will be that the box will gape open, and sand will blow violently in all directions.
However, if latches be fitted to the box itself, to hold it together, this lim itation is overcome.
In the case of boxes that are split horizontally, the side clamps are not required and can be removed from the machine if desired; much
Fi g. 4 . — Ty p i c a l Sc r e e n Ve n t e d Co r e b o x.
of the corebox cavity. These vents are most useful and there are few venting troubles that cannot be overcome with their aid. They are applied quite simply, by pressing them into holes drilled in the wall of the corebox, a shoulder being left in the hole to save the vent plug from being forced out by the air pressure.
Pieces of fine mesh cloth can also prove m ost useful for venting. They are m ade to form p art o f the floor or walls o f the corebox and should be backed and faced with thin plate plentifully perforated. W hen blowing a num ber o f cores at a time in a long box, open at both top and bottom , a useful m ethod is to m ake up a board, faced with this fine mesh screen cloth, and cut away to provide a free escape fo r the released air. This vented board acts as a floor for every core in the box, and will serve to vent m any different coreboxes very efficiently, as is shown in Figs. 3 to 6. In
mass-Fi g. 5.— Va r io u s Ty p e s o f Co r e b o x Ve n t s i n Co m m o n Us e.
wider coreboxes can then be accom modated.
The weight of core that can be blown at one operation is usually a little over one-third of the weight of sand in the reservoir. Thus, if the reservoir contains, say, 120 lb. of sand, the heaviest core that could be satisfactorily blown would weigh between 4 0 and 5 0 lb. in the green state.
In a jobbing coreshop, a core blower capable of blowing cores of about the size mentioned will be able to produce a great variety and q uan
tity or cores. A smaller machine would be less useful for jobbing because of its m ore limited capacity.
Auxiliary Plant
V arious developments of the standard core blower as described above are available. One type incorporates a built-in pattern-draw fea-183
ture, either pneum atically or hydraulically con
trolled, w hich is ideal for certain work.
A nother developm ent is a fully autom atic operating valve which aids continuous high
speed production by tim ing every phase o f the blow cycle to the m ost efficient speed for the one particular job. It is n o t suitable fo r jobbing work.
O ther “ extras ” are autom atic core sand feeders and elevators, corebox draw ing machines; and conveyor loops or turntables, etc., which assist in handling the coreboxes. A simple, but very useful device for use when draw ing coreboxes is a vertical plate m ounted a t the corner of the bench, and fitted w ith a vibrator controlled by a knee valve. This device greatly facilitates the draw operation, and saves the usual destructive rapping o f the
cessful, however, because the necessity for vent
ing had no t been realised. Eventually, a job was undertaken for which an old corebox split at the back was used, and an excellent core was produced. Even then it was n o t realised that this result was due to the use o f a split corebox. A fter considerable experim ent they tried using fairly coarse sand and oil only. was particularly interesting because his firm were experiencing their teething troubles with a core blow er, and he could see th a t it took
"ome tim e to obtain really satisfactory opera
tion. H e asked w hat binder was generally re
cognised as giving the best results. W ere the com position binders, generally speaking, more satisfactory than oil, molasses, dextrin, etc.?
Mr. H. H. Jo n e s s u g g e s t e d th a t s o m e should be pooled. Different foundries used different mixtures. In one establishm ent a dozen m ixtures were tried, but the cores were not successful because o f insufficient pressure.
They started with a pressure of 110, but after usually being blown with com plete regularity.
In m any foundries blown cores were being separately carried out. V ariations in perform ance were usually traceable to variation in the giving satisfaction seemed clearly indicated by the fact that a num ber of foundries which