Nip roller

During blown film processing, orientation is imparted to the film by both pulling the melt into the nip rollers and by increasing the circumference of the bubble with air. By managing... 

Figure 19,7 Machine directional orientation using pairs of nip rollers...

Consider the procedure for immobilizing an enzyme using polyurethane technology. A solution of the enzyme is produced in water. The solution is then emulsified with a hydrophilic polyurethane prepolymer. The emulsion is applied to the structural members of a reticulated foam by means of nip rollers. After curing... 

The chemistry function is produced by the prepolymer technique. Examples will follow in later chapters. The prepolymer is mixed with a reactant (polyol and/or water) and immediately pressed into a reticulated foam using a nip roller arrangement as shown in Figure 2.16. 

FIGURE 2.16 Nip rollers for forcing prepolymer emulsion into reticulated polyurethane foam web. 

Composite boards are made by bonding decorative laminate of thickness normally 0.4 to 1.5 mm to one or both faces of a substrate, using a suitable adhesive. The most common method of manufacture is flat pressing, which (depending upon the type of adhesive) may be carried out hot or cold. If the adhesive is of the spray-coated contact type, mechanical nip rollers may be employed instead of flat pressing. 

Blown film extrusion is perhaps the most widely used extrusion technique, by production volume. Billions of pounds of polyethylene are processed annually by this method to make products such as grocery sacks and trash can liners. In a blown film system (Figure 14-30), the melt is generally extruded vertically upward through an annular die. The thin tube is filled with air as it travels up to a collapsing frame that flattens it before it enters the nip rollers, which pull the film away from the die. The flattened tube then travels over a series of idle rollers to a slitter,... 

Fig. 3 Schematic of spiral mandrel blown film die operation (1) ring-shaped melt distribution (2) die body (3) spiral flow mandrel (4) sizing ring (5) spreader (6) film bubble (7) frost line (8) solidified film (9) bubble collapsing rollers (10) nip rollers (11) external bubble cooling air (12) internal bubble cooling air inlet (13) internal bubble cooling pipe and (14) heated internal bubble air return.

In machines with fixed rolls the optimum compaction force is obtained by adjusting the feed rate and density to the nip. Roller diameter, speed, and gap, as well as characteristics of the raw feed such as friction between material and rolls and particulate size, shape, and brittleness, all greatly influence the respective operating conditions. 

The most common procedure for mechanically dewatering in continuous processes is the squeezing using nip rollers followed by vacuum extraction. In processing batch materials, centrifuging is commonly used. 

A procedure for the production of a film has been reported as follows [10] In NMP, 0.85 mol 2-chloro-p-phenylenediamine, and 0.15 mol 4,4 -diaminodiphenyl ether are dissolved. 0.985 mol 2-cldoroterephthaloyl chloride is added. After 2 h, the polymerization is complete. The mixture is then neutralized with lithium hydroxide. The polymer solution is filtered and cast onto an endless belt The solvent is evaporated at 160 °C. A film with a polymer content of 45% can be continuously peeled off from the belt Then the film was guided into a bath of NMP and water to extract the residual solvent, the inorganic salt, and impurities. The film is stretched between nip rollers in the longitudinal direction at a stretching ratio of 1.20. 

Antiblocking agents are additives that inhibit the stickiness (blocking) of polymer surfaces. Blocking is most apparent in films, particularly blown films. After a blown film bubble has passed through the nip roller and the two halves have been pressed together, they may tend to block. Antiblocks help the film to be easily pulled apart. Diatomaceous earth is a commonly used antiblock additive. 

As the bubble moves upward and approaches the nip rollers, it is preflattened by the collapsing frame (Fig. 3.17). This device provides a smooth transition from a round tube shape to a flattened tube shape. Collapsing frames utilize wooden slats, metal rollers. Teflon-coated rollers, or an air cushion to perform the shape transition. 

A pair of nip rollers (the haul-off device) is located at the top of the cooling tower (Fig. 3.18). Their purpose is to pull the film up from the die. Also, the nip serves as an air seal for the top end of the bubble, so, at least one of the rolls is usually rubber-covered. 

Figure 3.18 A pair of nip rollers with a motor-driven chrome roll and a rubber

Increasing the cooling air speed causes faster heat removal from the bubble. Because the film reaches solidification temperature sooner, the primary effect is a lowering of the frost line. As a result, the bubble diameter decreases from the constant internal air volume being distributed over a greater distance from frost line to nip rollers. Because a lower bubble diameter means the film is not stretched as much in TD, the film thickness increases. 

Blocking is the term used for the tendency of two pieces of film to stick together, such as after the two sides of a blown film bubble are pressed together through the nip rollers. Blocking in films can lead to difficulties in handling and conversion or can simply bean inconvenience to customers. Many processors use an antiblock additive during blown film production to minimize this tendency. 

A final cause for TD thickness variation is nonuniform exit velocity around the die gap circumference. If the melt exits the die slower than average at one position, the final film will tend to be thinner at the corresponding position in the web. This is because the pull of the nip rollers is the same at all positions. Some die designs, particularly older ones, do not lead to a uniform die exit velocity. Also, a die that contains material buildup in the lips may cause nonuniformity. 

After the reader has started the extruder screw with a valid temperature profile, extrudate begins to flow from the top of the die (Fig. A.6). The molten, high melt-strength polymer can then be dragged up the tower to the nip rollers. Dropping the melt onto the nip rollers will string the bubble, if the nip rollers have been turned on and set to a speed... 

Return the screw speed to 20 RPM. Set the nip roller speed to 35 feet/minute and the blower speed to 110 cubic feet/minute. String the bubble onto the winder. Establish an 8.0 in layflat. 

An important point to remember is that carbon fiber is electrically conductive and great care must be taken to exclude carbon fiber from all electrical equipment and this aspect is covered in the chapter on operation of plant and safety, as are other safety matters such as guarding the nip rollers and dealing with plant emissions. 

Figure 10.4 shows a number of drive systems, which can be used. A rubber covered nip roller (75-80 Shore A hardness) is pushed into contact with the inlet steel roller. The rollers should preferably be chrome plated carbon steel and polished to 0.2 pm finish. A better arrangement is the use of ceramic coated steel rollers, which although more expensive, resists damage from the knives used to cut wraps from the rollers. 

The importance of guard systems for nip rollers has already been emphasized. The sets of nip rollers must close exactly parallel and can be initially checked by trapping a thin piece of paper (e.g. a paper used for rolling cigarettes) to ensure that it is firmly held in the closed nips across the entire width. Rollers not closing properly will permit slippage. 

Since the polymer is molten when drawn into the nip, the surface of the coating conforms to that of the chill roll and both matt and polished rolls are used depending on the properties required of the final coating. The coated web passes around the drum and is peeled away by a secondary nip roller, edge trimmed and electronically treated if required for further processes. 

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