Calendering control

The winders roll a continuous film or sheet into certain weight (lb or kg/ton) or diameters on spools or reels. Material speeds are up to at least 2,200 ft/min (670 m/min) in cast-film lines and at least 1,000 ft/min (330 m) in blown-film lines. Blown-film lines may want to use reverse winding systems to allow coextruded films to be wound with a particular material as the inside or outside layer. Throughputs are over 2,200 Ib/h (1,000 kg/h). Transfers from one roll to another can take less than a second. See calendering, controlled nip pressure in extruder godet unit roll. 

Smooth surfaces are normally estabflshed by calendering, a process which subjects the fabric at the nip point(s) of two or more roUs to the influence of controlled time, temperature, and pressure. When calendering is used as a thermal-bonding process, the roUs are of the same dimension and composition and are independently driven. However, when calendering is used as a fabric finishing operation, the roUs are frequently of different dimensions and composition and are not always independently driven. 

Spray appHcations to the surface of the sheet have been useful, especially for appHcation of creping aids and release agents in towel and tissue miUs. The spray appHcation of functional chemicals has not been used widely. Generally, the uniformity of appHcation is mote difficult to control when functional chemicals are sprayed than when they are appHed by a size press. Eunctional chemicals also can be appHed to heavier grades of paper or paperboard at the calender stack. 

Curl Control. Many grades of paper tend to cud, especially as humidity varies, because of the stresses produced duting the dryiag process. This is especially troublesome when only one side of the paper receives a surface treatment. Judicious appHcation of water to the opposite side of the dry sheet followed by redryiag may correct the curling. Water may be appHed by surface appHcation at the size press, water box, or calender stack or it may be sprayed on. Small amounts of water can be appHed to the paper surface as a foam with exceUent results. 

In the case of unplasticised polymer the main concern is with control of gelation and ensuring processability and adequate stabilisation. The formulation selected thus depends almost as much on the actual processing equipment used as on the end-product. Formula 8 gives a typical rigid opaque formulation suitable for pipes and Formula 9 a transparent calendering compound. 

Calendering A process by which fabric or wire is passed through a pair of heavy rolls to reduce thickness, to flatten the intersections of the threads/wires and to control air permeability. Rolls are heated when calendering synthetic materials. Carbonized threads Nylon or polyester therads that have been treated to include varrying degrees of carbon. 

Calendering can achieve surprising accuracy on the thickness of a sheet. Typically the tolerance is 0.005 mm but to achieve this it is essential to have very close control over roll temperatures, speeds and proximity. In addition, the dimensions of the rolls must be very precise. The production of the rolls is akin to the manufacture of an injection moulding tool in the sense that very high machining skills are required. The particular features of a calender roll are a uniform specified surface finish, minimal eccentricity and a special barrel profile ( crown ) to compensate for roll deflection under the very high presurres developed between the rolls. 

Basically the calendering process is used in the production of plastic films and sheets. It converts plastic into a melt and then passes the pastelike mass through roll nips of a series of heated and rotating speed-controlled rolls into webs of specific thickness and width. The web may be polished or embossed, either rigid or flexible (9). One of its sheets major worldwide markets is in credit cards. At the low cost side these lines can start a million. A line, probably the largest in the world... 

The calender was developed over a century ago to produce natural rubber products. With the developments of TPs, these multimillion dollar extremely heavy calender lines started using TPs and more recently process principally much more TP materials. The calender consists essentially of a system of large diameter heated precision rolls whose function is to convert high viscosity plastic melt into film, sheet, or coating substrates. The equipment can be arranged in a number of ways with different combinations available to provide different specific advantages to meet different product requirements. Automatic web-thickness profile process control is used via computer, microprocessor control. 

Calender to be equipped with crowned rolls, external roll bending, and adjustable cross axes setting (controlled)... 

The process of controlled, gentle warming of unvulcanised calendered sheet rubber to reduce its anisotropy and prevent subsequent distortion of articles made from the sheet and cured by embedding in chalk in open steam. 

This basic operation utilises a two-bowl calender in horizontal or vertical configuration. The feed material, either in strip or pig form, is fed into one side of the nip and is squeezed by the bowls, thereby emerging as a sheet which is pulled from the bowl by some manual or mechanical means or supported by a liner cloth. Because of their versatility, three-bowl calenders are now more widely used for sheeting as well as other basic calendering operations. Thickness control is accomplished by use of the adjustable nips and may be further refined by automatic control systems using thickness sensors. It should be noted here that the force required in the nip to flatten the feed material causes deflection of the bowls, however slight. If some corrective steps are not taken, the product thickness will vary across the sheet, resulting in excessive variations of the product and possibly excessive use of expensive materials. In order to overcome these problems, three basic techniques are used to achieve uniform product thickness ... 

A more complex form of coating calender is the four-bowl Z or L arrangement. A four-bowl calender can simultaneously apply a rubber coating onto both sides of a fabric. In effect, the No. 1 and 2 bowls and the No. 3 and 4 bowls form pairs where two rubber sheets are produced. The sheets are then laminated to a substrate between the No. 2 and 3 bowls. Very sophisticated devices are usually incorporated into the calender design to control thickness and width of the individual sheets and the resulting laminate. 

Control of product gauge is of considerable importance for the majority of products produced from calenders and various means have been used to achieve this from the early days of the machine. Inaccuracies in gauge on calendered products can not only mean out of specification goods being produced, but also one can be giving away free of charge considerable quantities of expensive compounds (unless the product is sold by weight, of course). 

Early thickness control systems relied on the calender operator setting up the machine correctly by the use of hand-held thickness gauges, with the consequent slowness of response time during set-up and running before any desired modification to gauge could be achieved. It was usual to run the calender at a low speed to set-up the required dimensions for the product and then to speed up the... 

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. 

---