Calender Sheet Stocks

AppHcatioas of dibasic lead phthalate, coated grade, iaclude 90 and 105°C rated PVC electrical iasulation, plastisols, profile extmsions, calendered sheet, and molded products. The recommended range of usage ia viayl electrical iasulatioa is 5—7 parts pet huadred resia, depending oa the particular iasulatioa classiftcatioa to be met. Ia geaeral-purpose extmded and molded PVC stocks, approximately 3—6 phr of coated dibasic lead phthalate is suggested. 

Arrow-shaped markings on the surface of calendered sheet mbber due either to the use of insufficiently warmed stock or to too low temperature of the calender rolls. 

Calendering involves passing unformed or extruded rubber through a set or sets of rolls to form sheets or rolls of rubber product. The thickness of the material is controlled by the space between the rolls. The calender may also produce patterns, double the product thickness by combining sheets, or add a sheet of rubber to a textile material. The temperature of the calender rolls is controlled by water and steam. Products produced by this process include hospital sheeting and sheet stock for other product fabrication. 

While calendering the Neoprene stock it must be prewarmed uniformly before being fed into the calender rolls. Stock temperature will change as the quantity of the stock at the nip varies. This causes fluctuation in viscosity of the stock and in roll pressure leading to rough sheet with differing thickness. Roll temperature in calendering must be accurately... 

While sheeting in a calender, the stock must be soft so that they penetrate the interstices of the layers easily, and tacky so that ready adhesion between plies is ensured. Inevitably, such stocks can give rise to handling problems and call for compromise compounding . Natural rubber and Neoprene rubbers compounded with resins such as coumarone indene or petroleum resin produce good sheeting stocks. 

Calendered products include sheet stock, belting, and friction and coated fabrics. A large proportion of sheet stock is later diefinished products such as gaskets, pads, and diaphragms. 

Gut Rubber. To produce cut mbber thread, smoked mbber sheet or crepe mbber is milled with vulcanizing agents, stabilizers, and pigments. This milled stock is calendered into sheets 0.3—1.3 mm thickness, depending on the final size of the mbber thread desired. Multiple sheets are layered, heat-treated to vulcanize, then sHt into threads for textile uses (Fig. 2). Individual threads have either square or rectangular cross-sections. 

The processes used to produce the individual tire components usually involve similar steps. First, the raw stock is heated and subjected to a final mixing stage before going to a roller mill. The material is then peeled off rollers and continuously extruded into the final component shape. Tire beads are directly extruded onto the reinforcing wire used for the seal, and tire belt is produced by calendering rubber sheet onto the belt fabric. 

Tile is manufactured in several ways. In each method, a continuous sheet is formed gauge refinement and planishing are carried out in subsequent calendering steps. Stresses that could lead to poor dimensional stability are avoided. The efforts to prevent stresses are governed by formulation, stock and roll temperatures, conveyor speeds, etc. After the final calendering, a resin—polymer—wax finish is applied to the surface of the sheet which is then buffed before it moves to the punch press. Frame scrap and tile rejected because of defects are returned to the mixers and recycled. 

Air blisters generally occur due to trapped air during the processing of the rubber in the calender or extruder or during hand building up of the products. The remedial measures to be undertaken are 1) Modify process temperature or pressure, 2) Feed the calender or extruder with adequate rubber stock, 3) Prick out visible blisters when the compound is in unvulcanized state, and 4) Use bleeder cords between metal and the rubber sheets applied at the welded places in the rubber lining process. 

There is little work of a fundamental nature done that helps towards a better understanding or control of calendering of rubber. Most theoretical studies were concerned with the pressure developed when a stock of thick viscous rubber sheet material is reduced to a thinner sheet by passage between rolls. Calenders with 3, 4 or more rolls might be assembled to quite different configurations. Calenders in L shaped, F shaped, I shaped or Z shaped configurations with roll diameters of 168 mm, 250 mm and 350 mm are available on the market. 

The most successful method to provide an unplasticized PVC face to the corrosives was found to be a laminate, calendering a plasticized sheet to the underside (or back side) of the unplasticized sheet to provide a good surface for bonding to the substrate, leaving the rigid side exposed to the harsh chemicals. For many years, this laminate sheet was manufactured in Trenton, NJ, and readily available. Domestic manufacture was discontinued a decade ago, but currently, the material is available on import from Europe, and is stocked in northern New Jersey. 

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