Nip roll

Tubular film may be biaxially oriented by passing eooled film through a pair of nip rolls, reheating it to a uniform temperature and then passing it between a further pair of rolls. Between the two pairs of rolls a mandrel of air is introduced in order to expand the film. The proeess is similar to that shown for poly(vinylidene ehloride) in Figure 17.4. 

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. 

The prepreg is prepared as the carbon paper from a roll is run at approximately 3 mmin-1 through a bath containing the resin solution. Then the impregnated paper is run through a pair of driven stainless steel nip rolls set at a gap of from 0.5 mm to remove excess resin solution and achieve the desired pick-up of 78-80% by weight of the resin solution. 

Problems of this nature may be reduced or eliminated by ensuring that during treatment the substrate film is flat against the base roll—which can be achieved by good control of the web, and by means of a nip roll located immediately before the discharge electrode. 

Blown film is usually extruded vertically upward through a circular die. This forms a tube that is then blown into a bubble that thins or draws down to the required final gauge. Orientation takes place in two directions horizontally (transverse direction/TV) as the bubble is formed, and in the machine direction (MD) as controlled by adjustable-speed haul-off nip rolls. 

The bubble diameter is normally always much greater than the die diameter. This bubble diameter divided by the die orifice diameter is called the blow-up ratio (BUR). The BUR is usually 1.5 to 4.0, depending on the plastic being processed and the thickness required. The bubble diameter must not be confused with the width of the flattened double layer of film between the nip rolls. The width of this double layer is 1.57 times the bubble diameter and is called the blown-film width (BFW). 

Figure 9.1 Example of the sheet or film passing through nip rolls to decrease thickness...

Dancer Roll These can be used as a tension-sensing device in film, sheet, and coating (wire, film, etc.) lines. They provide an even controlled rate of material movement. Type roll can have an influence on the roll s performance. As an example, chrome plated steel casting drums would seem to be very durable dancers. If used in the absence of a nip roll, should last many years. However, these rolls are in fact very soft due to the annealing which good rolls receive for stress relieving the steel. 

Some devices employ impact (hammers) and others employ crushing by nipping (rolls or jaws). Within limits, kinetic energy and dimensions of crushing elements can be selected to give a desired reduction ratio. Because of the deformability of solid materials, however, a theoretical limit does exist to the size of particles that can be crushed. These limits are 1 fttn for quartz and 3-5 fim for limestone. The products of crushing these sizes, of course, can be very much smaller, so that really there is no practical lower limit to grinding. 

Sufficient vacuum deaeration must be effectively distributed prior to the nip roll region, particularly for low bulk density powder feed stock. 

Pressing. The press seetion removes water and affeets eonsolidation of the sheet. The wet-web is lifted from the wire and transferred to porous earrier felts before passing between a series of roll presses or press nips. As the wet-web is pressed, first air and then water is expressed from the web. The water is absorbed by eapillary aetion in the pores of the earrier felts. However as the web exits the roll press it is deeompressed and some of the expressed water is sueked baek into the web. The effeetive press time is no more than 5 milliseeonds. At the first nip roll the speed of flow of water from the wet-web is rate-limiting, while at the last nip roll the pressure that ean be applied without erushing the sheet is rate-limiting. Traditionally moisture is squeezed out until the mat is down to 50-60% water (50-40% dry solids). Thereafter physical dewatering ceases to be efficient. 

A coating may be achieved by passing the ply between a coating roller and a back-up roll (usually rubber) or by kiss coating whereby the tensioned web makes contact with a coating roller. With certain more difficult materials a nip roll system (hot or... 

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