Rubber Production and Manufacture

Mixing.

The mixing operation is the most important of all through which the rubber compositions pass in the fabrication of goods. In fact, proper running in all subsequent operations de pends upon the mixing operation having been properly performed. Mixing mills vary in size from 24 in. to 84 in. in width. The in. mills are the largest in common use. They consist of two parallel, horizontal rolls set close together side by side and re volving in opposite directions. The back roll is fixed in posi tion and geared directly to a drive shaft; the front roll is float ing. The clearance between the rolls is adjustable by means of set screws near the ends of the front roll. Rolls are made of cast iron with chilled surface. The rolls of 84 in. mills are in. or 26 in. in diameter, cast hollow and fitted for service with internal perforated pipes for the introduction of cooling water. A new type of roll is cooled by water running through numerous tubular channels close to the surface and connecting with headers at each end. Large quantities of heat are generated in the milling of rubber materials and this must be rapidly carried off to pre vent vulcanization during the mixing operation (scorching). Cooling water from 50° to 55° F. is desirable.

The methods of mixing are complicated and dependent upon the materials to be blended. In general, the rubber materials are softened on the mill rolls first and the softeners and powdered ingredients added only after the rubber is plasticized to a smooth sheet on the roll. The procedure for batches containing crude rubber is somewhat different from those using reclaimed rubber only. It is essential, however, in all cases, that all parts of the batch be uniformly blended. This blending is secured by cutting the sheet on the mill roll with a sharp knife, beginning at one end and rolling it on itself until the sheet has been cut almost entirely across. The roll thus formed is permitted to sheet out again on the mill and the process repeated in the opposite direction. This is repeated six or seven times to secure uni form mixtures. Mills of 84 in. face will mix batches from 150 to 300 lb. in 25 to 4o minutes. Several types of internal mixers have been introduced as improvements over mills for mixing pur poses. Machines of 600 to 1,000 lb. capacity are in common use. Batches mixed in internal mixers run hotter than on mills and it is often necessary to add the sulphur separately on mills to avoid danger of scorching.

Calendering.

Calenders of various types and sizes are used in the rubber industry. The usual type is the three-roll calender, consisting of three rolls of equal diameter mounted vertically one above the other. The centre roll is direct-driven and the top and bottom rolls may be run at the same speed or at slower speeds than the middle roll. At one side of the calender the three rolls are geared together with pinions of the same diameter; at the opposite end of the rolls the pinion on the centre roll is smaller than those on the top or bottom rolls. Even or odd

speed is arranged by keying the proper centre pinion to its shaft. Calender rolls are bored and ground true on their bearings to ensure uniform wall thickness to resist the mechanical pressures and the heating and cooling to which they are subject in service. The middle roll is usually a true cylinder, the upper roll convex and the bottom roll either straight or concave. The "crown" of the upper roll varies from .005 in. to .008 in. depending upon the service, and the concavity of the lower roll from .002 in. to .004 inch. The distance between rolls is adjustable to suit various operating demands. Three kinds of operations are performed on calenders—sheeting, frictioning, and coating.

Sheeting.—Rubber is sheeted on calenders by feeding between the upper two rolls material previously plasticized on a mill. The rubber sheet passes half way around the middle roll and is removed and wound in rolls with cloth (liners) to prevent the layers from adhering. All but extremely thin sheets are ordinarily run in plies. One ply is sheeted and wrapped in a liner. To add a second ply the first ply with its liner passes between the lower two calender rolls, meeting and adhering to the second ply as it runs from the calender. This process may be repeated so that the final sheet consists of three, four or even more layers. The top roll may run at the same speed as the middle roll or slower, depending upon the quality of the rubber stock or the quality demanded of the product. Sheeting calenders are run from ro to 3o yd. per minute, driven by variable speed, direct current motors. Calenders ioo in. wide with rolls 3o in. in di ameter are the largest used in the industry. Sheets varying in thickness from point to point and sheets with specially embossed surfaces are also run on calenders equipped with special rolls. Treads for pneumatic tires and soles and uppers for footwear are examples of such sheets. Footwear calenders are four-roll machines. The engraved or embossed rolls are so mounted that they are quickly replaceable. As many as 5o rolls bearing differ ent designs are used in a day's run of shoe stocks.

Frictioning.—Frictioning consists in squeezing rubber into cloth as it passes between the lower two calender rolls carefully adjusted to permit the cloth to pass between them without ad ditional clearance. The rubber is fed between the upper two rolls, passes in a thin sheet around the middle roll, meeting the fabric as it passes between the lower two. The rubber is thus forced into the meshes of the cloth, but, because of its high viscosity, little penetration into the threads and around the fibres is secured. The frictioning process may be repeated on the op posite side of the fabric. In the frictioning operation the lower roll is always run slower than the middle roll. Before friction ing, the fabric is dried and delivered hot to the calender. Fric tioning speeds vary from 7 to 4o yd. per minute.