Haul-off

In addition to flattening the tube, the collapsing frame also helps eliminate wrinkles in the final product. These devices are generally adjustable for both height and entry angle. Proper positioning of these two adjustments is often used to correct wrinkling problems. 

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. 

While one of the rollers is located in a fixed position, the other uses pneumatics to be moved laterally into the closed or open position. This allows for the line to be strung between the rollers for start-up. 

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. 

The size of the nip rolls and all downstream idle rollers, known as the roll face width, determines the maximum layflat width that the system is capable of producing. The layflat width is related to the bubble diameter by the following equation BD = 2 LF/tr (where BD = bubble diameter and LF = layflat). 

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In the tubular process a thin tube is extruded (usually in a vertically upward direction) and by blowing air through the die head the tube is inflated into a thin bubble. This is cooled, flattened out and wound up. The ratio of bubble diameter to die diameter is known as the blow-up ratio, the ratio of the haul-off rate to the natural extrusion rate is referred to as the draw-down ratio and the distance between the die and the frost line (when the extrudate becomes solidified and which can often be seen by the appearance of haziness), the freeze-line distance. 

A lace of polyethylene is extruded with a diameter of 3 mm and a temperature of 190°C. If its centre-line must be cooled to 70°C before it can be granulated effectively, calculate the required length of the water bath if the water temperature is 20°C. The haul-off speed is 0.4 tn/s and it may be assumed that the heat transfer from the plastic to the water is by conduction only. 

One technique used in a number of facilities that utilize molten salt for metal surface treatment prior to pickling is to take advantage of the alkaline values generated in the molten salt bath in treating other wastes generated in the plant. When the bath is determined to be spent, it is in many instances manifested, hauled off-site, and land disposed. One technique is to take the solidified spent molten salt (molten salt is sold at ambient temperatures) and circulate acidic wastes generated in the facility over the material prior to entry into the waste treatment system. This in effect neutralizes the acid wastes and eliminates the requirements of manifesting and land disposal. 

Conveyor systems are applied in a number of areas in the rubber industry. The types used can range from simple canvas belt conveyors used for haul-off from conventional extruders, to systems used for transport and cooling of profile products, both in and emerging from continuous vulcanisation units. The latter types have to be resistant to the temperatures used in such systems and are variously constructed from glass fibre-reinforced polytetrafluoroethylene or a silicone rubber covered belt. 

Fig. 8 Blown film line for PE-LD. 1 Extruders, 2 die head with air cooling ring, 3 sizing unit, 4 thickness measuring unit, 5 haul-off unit and collapsing boards, 6 reversing unit, 7 winder. (Reproduced with permission from Reifenhauser)...

The drive unit usually is based on an electric motor and turns the screw at a suitable speed set in advance the electric elements in the cylinder are connected to temperature controllers that maintain the levels required. While screw speed and temperature are of critical importance the ability to extrude a given material continually to a satisfactory standard depends also on more fundamental considerations—like the suitability in design and construction of an entire line, including the operation of the die and of any haul-off or other technique applied after material passes the die. 

Downstream of the die, the extrudate (melt) is calibrated, cooled and packaged by an array of ancillary auxiliary devices including vacuum calibrators, water tanks, cooling rolls, haul-offs, cutters, and winders (Chapter 18). Upstream of the die, a melt pump may be interposed between the extruder and the die to produce a more uniform extrudate. The exact selection and arrangement of these component parts of an extrusion system will depend on the end product and tolerance requirements that have to be met.25 27> 33> 143 196>476... 

This operation requires the personal skill of the startup person. That person is required to integrate/interrelate extruder and down-stream equipment. Extruder screw speeds and haul-off rates may then be increased. Downstream equipment is adjusted to meet their maximum operating performance, such as having the vacuum tank water operate with its proper level and vacuum applied. The extruder can be fine-tuned to obtain the final required setting for meeting the desired output rate and product size. 

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. 

Typically, the expansion ratio between die and blown tube of film would be 1.5 to 4 times the die diameter. The drawdown between the melt wall thickness and the cooled film thickness occurs in both radial and longitudinal directions and is easily controlled by changing the volume of air inside the bubble and by altering the haul off speed. This gives blown film a better balance of properties than traditional cast or extruded film, which is drawn down along the extrusion direction only. 

The production of rigid PVC profiles is described with reference to state-of-the-art twin screw profile extruders and downstream equipment from Battenfeld. Features of the machinery are described, and particular details are given of the profile dies, calibration block, calibration table, haul off and profile saw. 

Saints croaked. You back again I hauled off my hood. 

Vasco s face said it had barely begun. Vasco will die happy if he can just see me hauled off to the galleys, but burning at the stake would be much nicer. 

Many of the disadvantages of ram extrusion are eliminated in hydrostatic extruaon, where the piston is replaced by high pressure fluid. As shown in Fig. 15, the billet stands clear of the cylinder walls, so that this element of friction is eliminated. The pressure transmitting fluid also lubricates the interface between the die and the billet, so that the deformation is essentially plug-flow (i.e. an extensional deformation, identical to drawing polymer with a free surface). Homogeneous oriented sections are therefore produced. A small haul-off force is applied to provide control of the extrusion process, and this also serves to ensure that the extrudates are straight. 

Acid consumed in the process is also in need of definition. Hydrofluoric acid is literally consumed it reacts to form heavy polymers and ultimately is pumped from the system to a combustion chamber or to a neutralization stage where the products of neutralization are hauled off to a dump or used as landfill. Sulfuric acid is consumed in a similar manner but only as a small fraction of that charged to the process, The primary effect upon sulfuric acid is that of dilution, rather than actual consumption. The fresh sulfuric acid catalyst is typically 98, 0-... 

During stretching flow, material is drawn from one cross-sectional area to another. This type of flow dominates fibre, film, blow molding, and vacuum forming processes (57). Let us assume that a filament of a molten polymer is hauled off under a force F (see Fig. 13.40). Taking the die of the extruder as the origin of the reference frame, the cross-sectional area A of the filament... 

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