Venting
The word "venting," as understood in foundry nomenclature, is a significant one, and means any or all of the various schemes which are being daily invented and practised to safely dispose of the gases produced in the moulds and cores, when brought in contact with the molten metal. It is unquestionably the most important phase of the moulder's art, and would likewise be the most interesting if the workmen fully understood all its niceties from the standpoint of the chemist. So far this advantage has been denied the average moulder, and there is every indication that he must for some time longer keep moulding castings the manipulation of which involves processes which are common only in the laboratory of the chemist. How he acquits himself of the task is an unsolved problem to every one at all conversant with the work.
All moulds and cores contain various proportions of organic and volatile matters, consisting of portions of roots, horse-dung, coal, straw, etc., all of which when decomposed by the hot metal generate inflammable gases; in addition to which must be added steam from the moist sand, which when decomposed gives rise to hydrogen, while its oxygen combines with whatever carbon may be present in the material to form carbonic oxides. These inflammable gases when mixed with atmospheric air produce a dangerously explosive compound, and it is in dealing with this objectionable substance that the moulder's judgment and skill are frequently taxed to the utmost in order to avoid the terrific explosions which would be sure to follow, in some instances, if it should be ignited prematurely.
The methods employed for venting are various--from the simple operation of making a small hole through the centre of an inch core, or perforating the sand in the top and bottom parts of a bench-flask, to the more complicated systems necessary for the successful production of high-grade castings. Nevertheless they all aim at the one object, viz., to convey the gas safely away as soon as it generates in the sand, and thus prevent it from forcing its way into the interior of the mould by breaking down such portions as are not of sufficient strength to resist the pressure. It is largely due to imperfect venting when the mould surface is destroyed in this manner, and what are technically called "scabs," "blisters," "blowheads," etc., may also be traced to this source, which in extreme cases may result in total disruption of the mould by explosion.
Very much of the venting practised on ordinary greensand work might, however, be dispensed with if those interested in the business were better informed with regard to the sand employed for moulding purposes. The worth of sands for foundry use are almost entirely dependent on their possessing certain chemical and physical properties; by the chemist's aid it is reasonable to anticipate a time in the near future when many of the evils we now attempt to obviate by increased venting will be more effectually remedied by a change in the materials employed.
While we admit that careful venting is a prime requisite in some cases, it is a fact beyond question that very much valuable time is wasted in venting some moulds which, if intelligently rammed with suitable material, would be equally good, or perhaps better, without a vent. Some moulders mix sea-coal with sand, believing that it imparts a quality thereto which makes venting unnecessary; whereas the sea-coal only serves to separate the clayey portions of sand, and introduces particles of refractory carbon, which prevent in some measure the partial fusing of the sand--to be noticed on castings that have been made in new sand. The coal burns and emits its smoke, forming a film of gas betwixt the sand and the metal; but this gas, like the rest, must be conveyed away as fast as it generates, otherwise it will seek an entrance to the mould, with the result above described.
To ascertain the effect of coal upon sand, and obtain a true estimate of the materials employed for making mould surfaces, prepare two open sand-plates on the floor, about 3 feet square and 1/2 inch thick, one bed to be made with ordinary coal facing-sand, the other in floor-sand free from coal; both to be equal in density and moisture, but neither one vented. The free-sand mould will permit the metal to spread uninterruptedly over its surface, because there is comparatively no gas-producing substances in the sand used for forming it. How different in the other case! The instant you begin to pour, gas is generated from the coal-sand surface, which cannot make its escape outwards because there are no vents provided: it must therefore force its way inwards; the result being that the whole surface of molten metal is converted into a mass of eruptive jets, which continue to bubble forth the imprisoned gases as long as the metal remains fluid. The solidified plate will show a honeycombed surface all over, and be worthless as a casting. If such a plate be prepared 2 inches thick instead of 1/2 inch, the metal will remain in a fluid condition for a longer space of time, and the quantity of gas generated will be augmented correspondingly; this naturally adds force to the gas, which in its effort to escape will carry along with it the sand crust, throwing it upwards through the metal with considerable force until the violent action is arrested by solidification of the mass.
The water contained in green-sand mould surfaces is at once converted to steam when the molten metal covers them; if this steam is not adequately drained off by venting, the result will be similar to that described for coal. Ordinarily this steam is pressed backwards into the porous mass of sand behind, but when this sand must necessarily be rammed so hard as to make it impossible for the steam to circulate through it, then recourse must be had to venting. Masses of green-sand almost entirely surrounded with metal require the most accurate venting, as there is no possibility of the steam and gas circulating; it must pass through a limited space, and special means are provided for guiding it out at that aperture, wherever it may be located.
All green-sand surfaces--which for obvious reasons must be made very hard--require special treatment; such, for instance, as the bottom of bed-plates, lathe and planer beds, and all similar moulds. For work of this class the ordinary wire venting must be supplemented by the use of a cinder-bed, which acts as a general receiver of all the gases generated on the outside walls and bottom surfaces of the mould, and for the inside too in some instances. This consists of digging down from 12 to 16 inches below the bottom surface of the pattern and placing a layer of coarse cinders down on the bottom from 6 to 8 inches deep, the interstices to be filled with finer ones. Over this a thin layer of hay or straw serves to prevent the sand from entering. Pipes must be set at convenient places, to which the cinder-bed is connected, and through which the collected gases will escape to the surface. Over this a layer of old sand is firmly rammed to within one inch of the intended surface, when the whole is vented with a 3/8-inch wire down through the sand to the cinders; after which the facing-sand is spread over in sufficient quantity to admit of treading or ramming down enough to leave the surface somewhat above the straight-edges by which the bed is formed. Before striking off the superfluous sand, it is requisite in some particular cases to supplement the previous venting with the large wire to the cinders by another course of very fine vents, giving them a little slant in order to make sure of striking the large vents. The large vents may be 2 inches apart, but the smaller ones should be much closer. By using an extremely fine wire in the latter venting, there will be no open vents by the time the bed has been strickled off and made smooth. Fig. 1 illustrates the processes herein explained.
Cinder-beds offer many inducements for their more general adoption, as by this means all venting required on the sides and elsewhere may be effectually done by either pushing a wire down to the cinders or ramming up rods from thence. This enables the moulder to make his mould free of vent-holes at the joint--"a consummation most devoutly to be wished," as every intelligent moulder knows. While it is freely admitted that gas will rise easier than it will descend, there is no question about the efficiency of down venting when the passageways are kept clear.
All large areas, especially such as must receive cores, etc., over which the metal will rest, can be easily and most effectually vented by means of the cinder-bed when any of the other methods usually employed might render the operation more than doubtful. See Fig. 2.
Deep green-sand work, such as tanks, cisterns, etc., round or square, offer very few difficulties when the cinder-bed is employed as a basis for venting. If such castings be plain, and are moulded bottom up, the open bed below will readily receive the vents from the wire direct; but should there be branches or other attachments, which make it necessary to lift out the core, an intermediate layer of cinders inside the core will intercept the vent, and a convenient hole in the lifting-plate serves to convey the gas downward to the original bed. See Figs. 3 and 4.
There are, however, a large number of moulds that can be very readily vented by the wire alone. Thin flat work is particularly adapted for direct wire-venting. A shallow channel cut in the joint some distance from the pattern serves as a starting-point for the wire, which when bent a little may be thrust in under the pattern (see Fig. 5); or, should the pattern be more complicated, as a beam or lintel, the sand below the casting may be perforated by means of a bent wire thrust in from the outside, after the ramming has reached some distance from the bottom, and these again pierced by vertical vents from the joint. It only remains to vent down the lower vents through the core and a somewhat imperfect communication is made. The success of this method depends largely on the sand under and around the pattern being evenly tempered, and sufficiently porous to permit the gases to circulate freely. See Fig. 6.
The value of working green-sands with the least possible amount of water is forcibly demonstrated by the following illustrations: When making cast-iron flasks with an upper and lower web on the sides, it invariably happens that more or less repairing needs to be done at the edges after the pattern has been drawn out of the sand. Should it happen that a careless or ignorant moulder attempts this, he will try to facilitate the operation by a plentiful application of water, the steam generated from which, when the metal rises to that part, no ordinary venting is able to carry away. Now there are very few moulders of any experience whatever who have not seen more than one flask utterly spoiled on this account, and yet they insist upon a free use of water, for the same reason, on other important moulds, evidently persuading themselves that because it is hidden under a flask no such harm can ensue. It most assuredly does; and only the added pressure in the covered moulds prevents a complete disaster always, but even that fails in eradicating the scabs and dirt.
The writer remembers a foundry that made a specialty of pistons, the two rings and spring for which were made as separate castings. As these were turned all over, and ought to present an absolutely clean face throughout their entire surface, it was considered by all to be a critical job, and many castings were rejected because of the pin-holes and dirt which, no matter how careful the moulding, would be revealed when the skin was broken. One man in the shop, by some considered a crank, kept reminding them that they were using too much water and coal-facing, and that as long as they did this they would never make a permanent success of the job. How he was answered by the indignant failures around him need not be related here. The foreman, fearing that this crank's boast of being able to make them clean might reach the ears of his superiors, thought to silence him forever by giving him one of the largest springs to make, fully expecting that he would fail in making good his boast, and intending to use that as a means for ridding himself of an intolerable nuisance. In this, however, he was deceived. The crank dug his hole deep and wide, and filled it to within a few inches of the pattern with dry old sand from the scrap-pile, after which he prepared his facing-sand, which consisted of finely sifted old sand just moist enough to bind together. With the exception of that portion immediate to the runner, the whole was faced with the dry mixture, and as the gate which he used was a very fine drop-gate, very little of the coal-facing sufficed. With a sharp, fine vent-wire he pricked through the cope to the pattern, and with a larger one round and under it. After finishing clean with absolutely no water, he returned the cope and made his runner-basin, which held almost all the iron required for the casting. He flooded this basin instantly with the hottest iron procurable, in a manner which made it impossible for any dirt to enter the small gate he had made. In went the iron at its leisure, and out through every little hole rushed the hot air and gas, until the mould filled, when the iron spurted upward in a hundred tiny sprays. Result: The first large spring ever made at that place without a flaw. It is needless to say that the crank remained. Fig. 7 illustrates the crank's mode of procedure.
There can be no question that copes need venting to permit the escape of steam and gas; otherwise, if not led upward, they may force an entrance into the mould below, carrying a crust of sand along. The holes should be small. Large holes act too much like open risers, and rob the mould of that steady pressure so desirable to maintain for the support of other surfaces besides the cope. When copes that have been vented buckle, the true cause will be found in the sand.
It is criminal to suppose that a core or piece of mould, because it is far removed from the upper surface, may be left unvented, and trust to the pressure above preventing future trouble from that source. If the certain commotion created at that precise part by such neglect be not immediately apparent, it is probable that more or less of this gas which has entered the mould, instead of passing outside by a suitably provided vent, is held imprisoned in some part of the casting, and is likely at some time or other to reveal itself unpleasantly.
One reason why large surfaces in open-sand moulds can, as a rule, be made without any other venting than a moderately soft bed affords, arises from the fact that most castings, including foundry-plates made after this manner, are not required to be very correct, a slight swell or scab not materially affecting their usefulness. Nearly all beds for open-sand castings can be made moderately soft, as before stated, and without any admixture of coal. The latter condition limits the gas present to whatever gas-producing elements are contained in the old sand, which is very little; the former condition is favorable to a free absorption of the little that is made. It must be remembered also that such beds are not called upon to resist the same amount of pressure that is common in covered work. A plate 2 inches thick in open sand exerts a pressure downward equal to 1/2 pound per square inch; the same plate covered, with head-pressure of 2 feet, would be 6 1/2 pounds per square inch.
Relieving moulds of expanded atmospheric air and accumulated gases is sometimes as difficult an operation as any that are connected with venting the sands and loam used for making them. Leaving risers open in order to free the moulds of these accumulations is not to be thought of where the materials are in any sense deficient; and some adequate means must otherwise be provided for the expulsion of these offending gases. One manner of accomplishing this is to make large basin riser-heads at the highest point of the mould, fill the basin with soft hay well pressed down, and place thereon a riddle, with weights to keep it there; or make good-sized plug-risers, and place over each a piece of fine wire-cloth, securing it in such a manner as that nothing shall pass out except through the netting. By either of these means the mould is effectually relieved without any of the roar and friction which usually attend open pouring when the riser area is limited.
When large volumes of gas must necessarily be relieved by a very limited passageway, either from cores or moulds, extra precautions should be taken, and one great help is to make sure that the atmosphere in the immediate neighborhood of the vent be as hot as possible. A considerable body of molten iron poured down under the mouth of the vent is better than lighted shavings, as it insures a steady heat which precludes the possibility of cold air interfering with the easy egress of the outcoming gas. When the gas from one core must necessarily pass through another core to reach its place of exit there should be no hesitation about making such connections as will convert the two cores into one practically. This may be easily accomplished by making pipe-connections, and, whether the final exit be through the side, top, or bottom, if the mould be an important one, the pipe method of securing vents should be strictly adhered to. See Fig. 8. Not unfrequently large core-barrels in horizontal moulds will explode with disastrous effect during the process of casting. This is an instance where the dangerous accumulations spoken of at the outset are made possible within the hollow barrel. There are several ways of preventing these explosions: a few shavings scattered along the bottom and ignited when pouring commences serve to burn off the gases as they exude; but if by any means the light should cease suddenly before the casting is well poured, the danger is not removed. Where practicable, it is advisable to fill the barrel with straw or shavings, and thus exclude the atmosphere, or place a netting of wire-cloth at each end that will exactly fill the space: this acts like a Davy-lamp, prevents the flame from entering the barrel, and allows the gas to burn harmlessly away at each end.
Large round or flat bottomed tanks and compound cylinders cast with their open ends down, making it necessary to convey the gas from the bottom of the mould, furnish an interesting phase of venting. While there are many methods for accomplishing this, it is certain that filling the core with sand, coke, straw, etc., is by all means the safest, and should always be adopted with castings of magnitude that are costly to produce.
By this means suitable provision can be made for carrying off what little gas is formed by the brick core, etc., and all danger from admixture with atmospheric air successfully avoided.
For ordinary pan-castings, however, much quicker methods must be devised, even if some risks are taken. It is therefore no uncommon thing to see such castings made without any particular attention to the vent other than to cut a single gutter from the middle, underneath, and connect with a pipe which leads it to the floor-level. An explosion once in a while prompts the moulder to observe greater care, but it is for a short time only. A bad feature at some foundries is to place large quantities of shavings and wood in the interior, and set them on fire before casting commences: this creates an instant expansion of the core, and very often loosens the loam from the bricks. A little iron run down a sloping gutter to the middle will heat the atmosphere within to create a draught which, if there be two opposite pipes, will convey the gas harmlessly away. By leading a good-sized pipe up to the surface, and covering it with wire-cloth, all communication with the inside is shut off, and the gas may be lighted as any ordinary vent. The same result is obtained when the gutter leading from the middle is filled with cinders or straw. A dumb-vent is a channel constructed from the pit through the wall of the foundry, or to some part within that is remote from the possibility of sparks igniting the gas, and thus causing an explosion.
A remarkable incident occurred at a foundry in England, where the writer was engaged moulding a large purifier in loam. The foreman, a self-willed fellow, with little knowledge or experience in that class of work, strenuously opposed any special measures being taken for carrying off the vent, and the mould, rammed within iron curbs which rested on the plate lugs outside the slings, was duly prepared for casting, leaving the vent-hole to take care of itself down at the bottom between the pit-wall and the curbs. As I had a decided objection to pouring a piece so inadequately vented, the foreman took the ladle and poured it with great pomp, exclaiming, as he passed on his way to the cupola, "I told you so!" The words had hardly escaped his lips when a most terrific explosion occurred. The mould and fastenings, being contained within the curbs, were lifted entirely from the floor and fell back again in a leaning position, scattering the runner in all directions. But, strange to relate, the casting was comparatively uninjured, the fine gates having congealed in the interval. The foreman was very much surprised. When shallow pans are cast in casings or moulds supported above the floor level no special venting is required, as the free circulation of air prevents any accumulation of gases. A few shavings will serve to light at once the vents of deeper close-moulds cast after this manner, and the gas will burn away freely in the air.
Brick walls of loam-work are best vented by choosing such material for the loam as will be sufficiently porous when dry to permit a free circulation of the gases outwards, and this is why as much care should be practised in making the building-loam porous as there is for the facing-loam; otherwise, the gas must enter the mould and be forcibly ejected at the riser and runners. This is why there is such a rush of air at the moment vertically cast moulds are filled where no attention is given to this particular. All connections, such as branches, flanges, brackets, etc., should be vented direct with wires or straws, and these be carefully connected with the upright vents, which should in all cases be set at intervals around the mould when it is rammed in the pit. Gases generated in loam covering-plates may either pass through holes in the plate or be led to the edge by layers of straw set down at the bottom of the prickers before it is covered with loam. Core covering-plates for cylinders, condensers, cisterns, etc., are vented by means of holes cast therein to lead the gases inside the core. Flat brick surfaces need only to be openly built and the spaces filled with fine cinders, connecting them with whatever means for outlet may be provided.
Dry-sand moulds, if they are made in suitable materials and well dried, require little or no venting, except in confined parts that are remote from the ordinary means of exit for escaping gas. Projections of sand that are almost surrounded with metal, and such portions of the mould as are least likely to be dry, need some special venting--the former for the escape of gases, the latter for steam. If the ordinary coal-facing is used to make dry-sand moulds, then the venting needs to be in every respect as particular as for green-sand. But if the facing be simply a refractory sharp sand with just sufficient clay and flour to make it cohesive, venting, as before stated, may be almost dispensed with.
Venting-cores might be very much simplified if the sands used for making them were chosen with the view of meeting the necessities of every case; but too frequently they are chosen at haphazard, and every core is made from the same pile of sand, no matter what it may be required for. Cores that are difficult to vent on account of their diminutiveness may sometimes be used successfully without vents if they are made from washed sand stiffened with a little glue-water. The reason for this is that the gas-producing substances are eradicated from the sand by washing, and the small quantity of glue required to make it cohesive is too slight to seriously affect it. While cores, generally speaking, may be considered as a kind of dry-sand mould, there must be every attention given to core-venting, as in the majority of cases they are surrounded with the molten metal, which drives the gases to the centre from all directions, and if instant egress is not given to this constant flow, the core is shattered and an eruption occurs within the mould.