weight is moved a t high speed, and yet the overshooting is not appreciable. Contact friction or detent effect is eliminated. This device can be applied to testing machines as well as production equipment.
Conclusion The rubber industry has provided a greatly increased field for instrument manufacturers in the last few years. A large part of the development of suitable instruments has been initiated by the industry itself. The days of trial and error in processing have ended, and the compounder can now take advantage of instrumentation by knowing that no matter what radical specifications he cares to write, he will have reasonable assurance that the equipment, under the guidance of instruments, will come through with the answer. With the call for more production, instruments will help to provide it
and a t the same time assure greater uniformity of product than can be obtained with manual control.
Literature Cited (1) Rosomworth, G . P. (to Firestone Tire & Rubber Co.), U. S.Patent 2,007,118 (July 2, 1935). (2) Brown, R. W., J. Scd. Inskuments, 9, 198 (1932). (2A) Davis, R. C . , and Graham, C . W. (to Firestone Tire and Rubber Co.), U. 8. Patent 2,234,911 (March 11, 1941). (3) Dillon, J . H., Physics, 1, 73 (1936); Dillon, J. H., and Allen, R. W. (to Firestone Tire & Rubber Co.), U. S.Patent 2,045,548 (June 23, 1936); Dillon, J. H., and Johnston, Norris, Zbid., 2,112,190 (March 22, 1938). (4) Karrer, E., IND.ENQ.CHEM.,21, 770 (1929). (5) Larrick, Lewis, A. S. 2'. M . Standards, 40, 1239-50 (1940). (6) Mooney, M . , IND.ENQ.CHEM., Anal. Ed., 6, 147 (1934). (7) Sommerville, A. A,, Vanderbilt News, Nov., 1940. (8) Williams, I., IND.ENQ.CHEM.,16, 362 (1925). (9) Zimmerman, E. C . , and Brown, R . W., Div, of Rubber Chem., A. C . S., Detroit, Mioh., 1927.
ANDREW HALE Fsrrel-Birmingham Company, Inc., Ansonia, Conn.
dislocation of employment. We have much greater reason to halt the improvement of aviation because of death and tragedy. The rubber industry can be justly proud of its record, and tribute should be paid to the farsighted managements of our factories, to scientists and product engineers, and to originators and builders of efficient equipment. This paper deals particularly with improvements made by the latter.
ITHIN our lives we have seen amazing progress in the W rubber industry. In 1900 the average automobile tire sold for over $50 and lasted for 2000 to 3000 miles. Today the average passenger tire costs about $13 and will easily operate without puncture or repair for 20,000 to 25,000 miles. Paralleling this record of reduced mileage cost we have also to see its effect on wealth. I n 1900 the average yearly wage of a rubber worker was $425, and only 36,000 men were employed in the entire rubber industry. Today the average wage is approximately $1600 and more than 175,000 are employed. Obviously the wage earner in 1900 couldn't afford the purchase of one tire a year. Today he can buy a full set of tires each year and have money left to acquire cars, radios, better homes, and other things which increase the comfort and pleasure of living. There are those who would unwittingly stop this progress because changes cause temporary
Bale Splitting Bales of rubber have been mutilated in various ways. They have been pulled apart by two men with cant hooks and knives. Barring frozen rubber, two men could probably pull a bale apart and reduce it to the necessary number of pieces in 6 or 7 minutes. This method soon gave way to power-driven saws. These mechanisms were already in wide
able of taking entire frozen n rubber will come forth in claim pellets from bins to preblended in unpreceII receive direct charges bins where each batch
The mill room of the future bales, will deliver warm and s pellet form. Remote con automatic scales and also dented quantities. Shee from mixers and deliver mix wi II await laboratory release 575
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use in the metal and wood-working industries. Two wellknown types were employed-a circular saw built for cutting logs and a reciprocating type used for steel bars. Production was somewhat improved but hazards were great and maintenance high. Some years later came the vertical hydraulic press with moving-up ram that carried the bale through a single fixed blade. These are still in use and a bale can be cut in four blocks in about 2 minutes, if proper feeding and take-away provisions are made. Then came the two intersecting and finally four intersecting knives fixed in a huge vertical press. Bales were brought in cutting position by hydraulic means, and after descent of the ram the cut pieces were pushed clear for the next bale. The latest type of bale cutter is a horizontal hydraulic press, specifically designed for and providing easier feed and disposal of cut pieces. Bales are reduced to ten pie-shaped pieces or, by electrically-heated knives in the cutting head, can be reduced to four or eight rectangular sections. Usually one man operate8 such a unit and can account for a t least 60 bales per hour. Still later developments indicate automatic feed and take-away.
Rubber Breakdown Now it is generally accepted that premastication of rubber is advisable both from the standpoints of final product plasticity and economy in mixing. In a 16 X 40 inch mill probably 40 or 50 pounds can be broken down in 15 minutes, an output of 160-200 pounds per hour.
Discharge of Dusted Pellets from Pellet Machine
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The mills grew in size until it became possible for one man to operate two 84-inch mills, each with corrugated back roll and equipped with an apron. Production per man reached 1600 pounds per hour. I n recent years Banbury mixers operating a t double speed are doing an acceptable job in 3 or 4 minutes. This is the so-called hot breakdown, and it has many advocates. The size 11 Banbury, capable of a 300pound batch, can produce as high as 6000 pounds hourly. Size 27’s in one large factory are producing 12,000 pounds per hour. A machine built specially for rubber breakdown is the Gordon plasticator, furnished in three commercial sizes. The 12-, 15-, and 20-inch sizes are capable of approximately 2500,5000, and 9000 pounds per hour, respectively, a t existing normal speeds. Future capacities may be materially increased with higher screw speeds. The plasticator can be fed and discharged continuously and offers possibilities of automatic operation.
Compounding and Mixing Probably the most amusing evidences of the ancient mill room are pictures of old compound rooms. They remind us of country grocery stores with open barrels of sugar, pickles, crackers, etc. The hand scoop for pigment, a can for oil, a set of beam scales, and fifty or one hundred battered batch pans comprised the necessary equipment. High-priced materials and a better and more uniform produ c t soon demanded more accurate weighing. A sense of good housekeeping compelled storage of raw materials in bins or steam-heated tanks, piped t o mixers, and this change-over was accelerated when pigment suppliers offered free-flowing materials. Many compound rooms today are built t o suit Banbury mixing. The Banbury operator rides on a moving tramcar with scale and one large batch pan. He is able to pass by each bin with the necessary ingredient and weigh directly into the single-batch pan. In many mill rooms the weighing of pigment is part of the Banbury operator’s duty. The open mill grew from the 16 X 40 to the 26 X 84 and now the 28 X 84 inch size. Hence, a 30-minute mix stock was produced in larger batches in direct ratio to growth of mill size. The Banbury mixer was first put on the market in 1916 and in the decade following 1920 came into widespread use. At first it was built in only the No. 3 size; then the KO,9 and No. 27 were developed. Later came a popular demand for the size 11, capable of a batch size which could be handled by a single 84-inch sheeting mill. This machine was actually a modified design of the No. IOW, built specially for warming cold-aged stock, and thus requiring greater rigidity throughout. It is a favorite of plants carrying hard and continuous schedules. Generally, a size 11 with its sheeting mill is capable of producing twice the tonnage per man-hour, and with half the horsepower per 100 pounds, than can be obtained from 8 4 inch mills. Some plants have doubled the speed of their Banbury mixers with a consequent decrease in cycle time. A new internal mixer, which challenges the prestige of the Banbury, is the single-rotor type. This has already proved itself in several
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mill rooms and is awaiting final approval by one of -. our largest tire plants. Potentiometers are now provided with recorders such as will show temperature of stock during the mixing period. This is an aid to uniformity control. In some mill rooms, particularly those operating a t high speed, the Banbury discharges a t a certain temperature rather than after a definite elapsed period. Automatic cycle controls are being employed which eliminate the operator.
Stock Cooling The old rack is still in evidence in a great many places. One concern can now supply a housed circular rack driven by individual motor and equipped with spray unit and air circulator. Some large companies are using long conveyors which deliver cooled sheets a t their final destination. A few concerns employ special blending units mounted on sheeters which permit one man to operate two and three mills. These blending devices are commercially available. Where mill blending is not considered necessary, the stock passes through roll8 once and proceeds full width through the cooler, and is finally cut by flying shear and then handstacked, Another large concern has an automatic stacker which loads approximately 3000 pounds of sheet stock on a single skid.
Calenders and Tubers
Pellets Passin throu h First Set of Sprays (above) and through geeond %et (center); Close=upView of Rubber Pellets (below)
In practically all plants the mill is still used for warming and feed to calender or tuber, and its size is governed by the size of tuber or calender it must feed. The corrugated back roll has reduced, by almost half, the time required for warming on two smooth rolls and uses less power. Strip feed direct from the feed mill is now almost universal. Change in stock temperature before passage through rolls invariably alters the gage. Stripfeed mechanisms are cheap investments with huge dividend returns. Tubing machines have undergone considerable improvement. The undercut feed hopper permitted automatic feed, and new designs of screws and more effective cooling have allowed higher speeds and, consequently, higher production. Several large concerns have standardized on the roller feed type, which compacts the stock, expels the entrapped air in the entering thread, and obviates the necessity for gradual decrease in screw pitch. During the past few years many companies have adopted the single-thread rather than the double-thread screw. An astonishing drop of horsepower input has resulted with no drop in production. The need for balanced tires, requiring uniform-weight treads, has extended the use of tubing machines in place of tread calenders. Tubers are being used in pairs to extrude dual stocks from the same die. They are extensively utilized on tubes, treads, and some mechanical goods. Many plants are receiving dividends from potentiometer control on their tubers. The thermocouple located in the head and directly in the rear of the die indicates excessive temperatures and directs immediate attention to plasticity of stock feed. Probably the most scientific take-away and shrinkage conveyor is composed of a series of power-driven rollers over which the extruded stock passes. The rollers are successively smaller in diameter and, be-
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ing driven a t the same r. p. m., their peripheral speed decreases; the linear speed of the stock is thus reduced and its natural shrinkage hastened before it is cut into tread lengths. Probably the greatest advance in calender design can be found in sizes and roll combinations. The four-roll calender came into use principally to apply a coat simultaneously on both sides of tire fabric in a single pass.
Lubrication Overheated bearings cause periodic shutdowns. Practically all equipment today obtains its oil from sight-feed oilers or mechanically operated lubricators. Too much reliance is placed on oil’s reaching or spreading over the requisite surface. If the feed is excessive, the oil overflows the bearings, and good housekeeping plus economy dictate regulation until the bearing is starved. I n other words, our present-day oiling systems demand a difficult adjustment between overflow or starvation and bearing seizure. Flood lubrication of bearings is the engineer’s answer to this problem. Bearings are sealed a t both ends by special glands which hug the journals; then oil can be delivered to the bearings in generous quantity and gravitate back to be strained and recirculated.
The Mill Room of the Future The foregoing remarks give a brief account of forty years of progress. We have seen a great increase in output per worker. As years pass, larger tools are placed a t his command. Instead of walking, he holds the reins to tremendous horsepower. Banbury mixers, driven by 700-horsepower motors with push-button control, mix 800- and 900-pound batches in 10 or 12 minutes. Gordon plasticators with less than half the attention of one man are breaking down rubber at about 9000 pounds per hour and are consuming 600 to 800 horsepower. Automatic scales, fully housed, weigh pigments silently and accurately with no one in attendance. We now turn our attention to material handling and a more uniform product. Why does an occasional tire give out a t 4000 or 5000 miles? Why do some travel 50,000 or 60,000 miles? Type of usage is only part of the answer. We have spent huge sums in product control through the plant, but the unknown qualitative question stilI remains-rubber. Our blending methods are crude and most inadequate. We stagger bales from different estates behind the bale cutter, hoping for an averaging out or blend. From the plasticator come cut sheets of rubber. A man hangs them dripping wet on a conveyor, and another removes and piles them in a compound room. Here a third man with butcher knife and scale carves the sheets t o correct weight, and then carries them to batch pan or Banbury feed belt. Stock from Banbury drops on sheeting mill, following which the slab again is hung on rack or conveyor, only to be removed by another and later carried to the warm-up mill. Additions to mill rooms are necessary for capacity increase because an average of over half this valuable floor space is occupied by skids piled high with stock. Suction fans and sweepers are continually busy in an effort to keep mill rooms clean. Some progressive concerns are meeting this challenge by reducing rubber to dusted pellet form. Pellets can be quickly cooled and automatically conveyed to storage bins. These bins, holding a t least 3000 or 4000 pounds, are equipped with slow-moving agitators that blend and keep the pellets in freeflowing condition. From here they can flow into automatic scales (or tramcar scale) and thence into the Banbury hopper. The remaining handicap to automatic Banbury operation is herewith eliminated because the floating weight can no longer
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seize on a large chunk of rubber. Pellet rubber will probably speed the mixing (some already testify t o this), because of its high ratio of area to weight. Poured into a Banbury simultaneously with carbon black, for instance, there is an immediate commingling. The initial circumstances are more favorable t o quick dispersion than where large chunks of rubber must be penetrated. Designers are working on a plasticator-pelletizer, which will work the discharge from a Banbury in a way similar to a sheeting mill. Experiments point t o success, and thus we eliminate the sheeter, again pelletize the mixed stock, quickly cool, pass through control laboratory and thence t o bins where this mixed stock is again blended. A steady stream is conveyed from here t o warm-up mills and by cross-feed ribbons advances to the tuber or calender feed mill. In this mill room of the future we replace the bale cutter with two crackers. The first cracker has large rolls with special spiral corrugations. It runs slowly but will easily take entire frozen bales and reduce them to thick sheets. These then pass to the second large cracker with closer set rolls which further warm and soften the rubber and make it adaptable for automatic feeding into the plasticator. Rubber then leaves the plasticator in pellet form, and the cycle is complete.
Control Board In the mill room of the future a single operator watches a panel board having all necessary controls for weighing the various ingredients, including pelletized rubber, reclaim, and master batches. A continuous inventory of these ingredients is in full view. A vertical row of four lights indicates when the stock level in each particular bin is empty, one-fourth, one-half, and three-quarters full. There is a vertical row of such lights for each bin of raw material. Then a t the bottom of the same control panel is a similar array of inventory lights indicating stock level of finished mixed stock ready for warming mills. Lowering levels of finished stock immediately prescribe the schedule. Formula charts are provided. By turning a hand wheel, stock x is brought to view. The weights of the component parts appear below each respective bin of the required raw material. Remote scale controls are also located below each bin, and here we have an interesting scientific contributionthe Selsyn generator and the Selsyn motor. These can be any distance apart. The panel dial can be rotated any required number of full turns, plus a slight fraction of another turn, and the Selsyn motor will respond by rotating likewise. The latter is located a t the scale and moves the beam poise to its exact position. Another ingenious device is a counter, permitting the operator to set all this machinery in motion and repeat a given number of times. In other words, each scale can be set and the number of weighings will repeat six, eight, or as many times as there are batches required. On the formula chart are sequence numbers for each ingredient. Buttons are pressed in the order indicated. This compels the scales t o weigh and discharge on the Banbury feed belt in proper sequence and a t proper time intervals. The Banbury operation cycles are interlocked with this sequence and do not require separate remote-control settings. Therefore, science and engineering ingenuity have combined to make simple the one-man control of compounding and mixing. A prominent scale manufacturer is prepared to furnish the automatic scales and panel outlined. To many, the one-man mill room may seem far away but it is not so distant as are today’s machines viewed from a few years back, Remember the one-man automobile top? True, it took several to raise it a t first, but now the modern convertible has only a push button.
