Calendering controlled nip pressure

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. 

Calendering in the manufacture and surface finishing of plastic products, such as URP sheets and films, plastic impregnated nonwovens, and woven fabrics sheets and films, requires roll systems to meet stringent control of their nip pressure requirements. In this respect, products of... 

For CD caliper control in on-line calenders, off-line calenders or supercalenders their nip pressure is zonal controlled by local change of the roll shell diameter or shape. 

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. 

The thickness of the calendered product must be uniform in both the machine and cross-machine directions. Any variation in gap size due to roll dimensions, setting, thermal effects, and roll distortion due to high pressures developing in the gap, will result in product nonuniformity in the cross-machine direction. Eccentricity of the roll with respect to the roll shaft, as well as roll vibration and feed uniformity, must be tightly controlled to avoid nonuniformity in the machine direction. A uniform empty gap size will be distorted in operation because of hydrodynamic forces, developed in the nip, which deflect the rolls. The resulting product from such a condition will be thick in the middle and thin at the edges, as shown in Fig. 15.2. 

The passage of hot melt between the rolls creates a pressure forcing them apart and the calculation of tolerances using a statistical model has been described (417). There is a rolling bank of feed material created in the gap or nip set between the first pair and also the second pair of rolls. Passage of the material is controlled by roll temperature, surface finish and the ratio of the roll speeds at the nip. The final calender nip controls film thickness. 

The development of the Roller Die (roller head) equipment made possible the production of wide thick sheets in a single operation, free from air inclusions and blisters. The extruder feeds a slab of material 30-60 mm thick directly into the calender nip, across the full width. The calendering operation is thus a gauge reduction no rolling bank forms, and the action may be regarded as an extension of the extrusion. The two parts of the plant are very close, the lips of the extruder die reaching deep into the calender nip. A pressure sensor is located in the die-lip, and this controls the calender speed to keep the stock pressure in the die constant. This ensures the matched running of the two halves of the plant. 

Zone-controlled calender rolls The roll shell is internally supported by an oil cushion or a series of oil hydraulic pistons which can be controlled individually. The number of CD control zones per roll varies, usually between 8 and 60, depending on the construction. Limiting factors are for example the number of pipes inside the roll, which are needed to allow individual oil pressures in the various zones. Additionally counter zones are frequently used. They are on the opposite side of the nip. The purpose is to get more degrees of freedom in the zonal nip adjustment. 

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