Nip speed

The strong interdependence of process variables is another aspect of the process that requires a high level of operator skill and has led to extensive advancements in measurement and control techniques. There are many process variables - screw speed, nip speed, internal bubble air volume, and cooling rate (frost-line height) - that influence bubble geometry and, as a result, film properties. An adjustment to any one... 

The fixed-position roll is motor-driven to establish the line speed. Since the line speed (nip speed) is a primary control determining film thickness, bubble diameter, and frost-line height, fluctuations in the motor speed should be minimized, generally less than 1% full scale. 

Table 4.1 defines the response that increasing each of the four main process variables (nip speed, screw speed, cooling speed, and bubble volume) has on each of the three main bubble geometric variables (film thickness, bubble diameter, and frost-line height). An asterisk identifies the primary response to each increase. 

The TUR is the ratio of him velocity (Vf) to melt velocity (Vj ), i.e., TUR = V IV. This quantity provides an indication of the amount of stretching, hence molecular orientation, in MD. The him velocity is the upward speed of the him above the frost line and is established by the control system. It is equivalent to the nip speed. The melt velocity is the upward speed of the molten polymer as it exits from the die lips. It is related to, but is not equal to, the screw speed. The melt velocity can be determined experimentally by marking the him and tracking the mark, but an easier method is to employ the principle of conservation of mass. 

Since the nip speed is always greater than the melt speed, the TUR is always greater than one. 

Each of the three geometric characteristics depends differently on each of the four control variables. For example, bubble diameter increases at a low rate with increases in nip speed, but at a much higher rate with increases in internal bubble air volume. By tracking values in the measurements panel with changes in the control variables, the various geometric dependencies can be identified. 

Change any controls to produce film with the following specifications film thickness = 1 ( 0.1) mil, bubble diameter = 14 ( 0.1) in, frost-line height = 7 ( 0.1) in. For these conditions, record the nip speed, screw speed, and blower motor speed. 

Ko//M //s. When dispersion is requited ia exceedingly viscous materials, the large surface area and small mixing volume of roU mills allow maximum shear to be maintained as the thin layer of material passiag through the nip is continuously cooled. The roUs rotate at different speeds and temperatures to generate the shear force with preferential adhesion to the warmer roU. 

At high speeds, the pond of the flooded nip size press becomes turbulent. RoU maintenance is a problem with the gate-roU size press. To avoid these problems, the blade/rod metering size press was developed. Short-dweU coater heads are used to apply a precisely controUed quantity of chemicals to the size press roUs. This quantity is controUed with either a metering blade or a metering rod. Blade or rod metering eliminates the pond, and does not increase the number of roUs required for surface chemical appHcation. 

A web of molten plastic is pulled from the die into the nip between the top and middle roUs. At the nip, there is a very small rolling bank of melt. Pressure between the roUs is adjusted to produce sheet of the proper thickness and surface appearance. The necessary amount of pressure depends on the viscosity. For a given width, thickness depends on the balance between extmder output rate and the take-off rate of the pull roUs. A change in either the extmder screw speed or the puU-roU speed affects thickness. A constant thickness across the sheet requires a constant thickness of melt from the die. The die is equipped with bolts for adjusting the die-gap opening and with an adjustable choker bar or dam located inside the die a few centimeters behind the die opening. The choker bar restricts flow in the center of the die, helping to maintain a uniform flow rate across the entire die width. 

Thermoplastic urethane adhesives may be processed into an adhesive film. I,amination of two substrates can, in theory, be done immediately, but the film is often extruded onto one substrate, covered by a release liner, and allowed to cool. Crystallization follows to create a non-tacky film that may be cut into specific shapes. The release liner is then removed, and the shaped adhesive can be heat-activated on one substrate, using infrared lamps. The second substrate is then nipped under pressure, followed by a cooling press to speed crystallization. Once the backbone has crystallized, the bond should be strong. 

The major advantage of film blowing is the ease with which biaxial orientation can be introduced into the film. The pressure of the air in the bubble determines the blow-up and this controls the circumferential orientation. In addition, axial orientation may be introduced by increasing the nip roll speed relative to the linear velocity of the bubble. This is referred to as draw-down. 

A calender having rolls of diameter 0.3 m produces plastic sheet 1 m wide at the rate of 2(XX) kg/hour. If the roll speed is 5 rev/minute and the nip between the rolls is 4.5 mm, estimate... 

Since quite a bit of difference exists between raw materials, the recipe, and the equipment, the processing procedure and conditions vary a lot. Also, the processing procedures of commercial products are usually not available to the public. Thus, much work needs to be done to find the best procedure and condition for each individual system. In general, a good procedure is a combination of optimal processing time, temperature, and rotating speed of the screw (in the case of extruder use) or the roll nip (in the case of calender use). 

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 purpose of this section is to explain in a qualitative way how the product quality is related to the calendering parameters. Therefore a simplified calender model is presented. The model describes the pressure buildup in the calender nip region as a function of compound viscosity, clearance, calender line speed, rolling bank height, as well as geometrical data. The general layout of a typical steel and fabric cord calender is explained by means of the result of the presented calender model. 

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