The Extruder

Extrusion is one of the most popular processes for fabricating parts from fluoropolymers. Fiber and filament account for a relatively small share of all fluoropolymers extrusion. The principal end products are wire insulation, tubing, film, and sheet. Fluo-ropolymer films are widely used in release, surface protection, and packaging. Sheet and profile extrusions account for a small share of fluoropol5uners consumption. 

The products of degradation of molten fluoropolymers are highly corrosive, often containing hydrofluoric acid. It is important that the surfaces of machines that come in contact with molten fluoropolymers are constructed from special grades of corrosion-resistant metals. These metals are significantly more expensive than lower grades of steel. Corrosion of process surfaces can result in the contamination of the finished product and deterioration of its physical properties. 

Extrusion is a continuous process that involves forming a product in two dimensions. These x-y di- 

The output rate of the extruder is a function of screw speed, screw geometry, and melt viscosity. The pressure developed in the extruder system is a function of melt viscosity, screw design, and barrel and die resistance. Extrusion pressures are lower than those encountered in injection molding, and are typically less than 35 MPa. 

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When we speak of the solidification of the extruded polymer, we use the term in the broadest sense It includes crystallization, vitrification, or both. The extent of the drawing of the fibers and the rate and temperature of the drawing affect the mechanical properties of the fiber produced. This conclusion should be evident from a variety of ideas presented in the last three chapters ... 

Specimen Location Inner ring of cooling system following the extruder... 

Failures of the inner ring of the extruder cooling system were occurring with increasing frequency. Six failures had occurred over a 2-month period. 

Once the required sizes of the conductor and the enclosure are established, one can choose the nearest economical sizes of the extruded sections available in the market, or have them specially manufactured if possible. 

In a molded polymer blend, the surface morphology results from variations in composition between the surface and the bulk. Static SIMS was used to semiquan-titatively provide information on the surface chemistry on a polycarbonate (PC)/polybutylene terephthalate (PBT) blend. Samples of pure PC, pure PBT, and PC/PBT blends of known composition were prepared and analyzed using static SIMS. Fn ment peaks characteristic of the PC and PBT materials were identified. By measuring the SIMS intensities of these characteristic peaks from the PC/PBT blends, a typical working curve between secondary ion intensity and polymer blend composition was determined. A static SIMS analysis of the extruded surface of a blended polymer was performed. The peak intensities could then be compared with the known samples in the working curve to provide information about the relative amounts of PC and PBT on the actual surface. 

Many articles, bottles and containers in particular, are made by blow moulding techniques of which there are many variations. In one typical process a hollow tube is extruded vertically downwards on to a spigot. Two mould halves close on to the extrudate (known in this context as the parison ) and air is blown through the spigot to inflate the parison so that it takes up the shape of the mould. As in injection moulding, polymers of low, intermediate and high density each find use according to the flexibility required of the finished product. 

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. 

That the compound is not allowed to stagnate in heated zones of the extruder and thus decompose. The life of many compounds at processing temperatures is little more than the normal residence time of the material in the extruder. 

In a typical process a preform billet is produced by compacting a mixture of 83 parts PTFE dispersion polymer and 17 parts of petroleum ether (100-120°C fraction). This is then extmded using a vertical ram extruder. The extrudate is subsequently heated in an oven at about 105°C to remove the lubricant, this being followed by sintering at about 380°C. By this process it is possible to produce thin-walled tube with excellent flexing fatigue resistance and to coat wire with very thin coatings or polymer. 

Tape may be made by a similar process. In this case the lubricant selected is a non-volatile oil. The preform is placed in the extruder and a rod extruded. The rod is then passed between a pair of calender rolls at about 60-80°C. The unsintered tape is often used for lapping wire and for making lapped tube. Sintering is carried out after fabrication. The current most important application of unsintered tape is in pipe-thread sealing. 

Optical properties of the blends are somewhat dependent on the molecular weight of the polystyrene, presence of additives such as lubricant in the polystyrene, ratio of polystyrene to SBS, processing conditions and mixing effectiveness of the extruder. It is stated that the optical properties of the sheets are similar whether linear or radial type stereoblock polymers are used. 

In the low-pressure systems a shot of material is injected into the mould which, if it did not expand, would give a short shot. However, the expanding gas causes the polymer to fill the mould cavity. One important form of the low-pressure process is the Union Carbide process in which the polymer is fed to and melted in an extruder. It is blended with nitrogen which is fed directly into the extruder. The extruder then feeds the polymer melt into an accumulator which holds it under pressure (14-35 MPa) to prevent premature expansion until a predetermined shot builds up. When this has been obtained a valve opens and the accumulator plunger rams the melt into the mould. At this point the mould is only partially filled but the pressurised gas within the melt allows it to expand. 

The particular features of the nylons should also be taken into account in extrusion. Dry granules must be used unless a devolatilising extruder is employed. Because of the sharp melting point it is found appropriate to use a screw with a very short compression zone. Polymers of the lowest melt viscosity are to be avoided since they are difficult to handle. Provision should be made to initiate cooling immediately the extrudate leaves the die. 

In the first place, although the moisture pick-up of the resin is small it is sufficient to cause problems in processing. In the extruder or injection moulding... 

The extrudate is cut up into appropriate lengths and cooled by plunging into cold water. Subsequent operations depend on the end-product required. 

When rods are required they are placed in wooden trays in a formolising bath. If the requirement is for a disc or blank such as used by the button trade the extrudate is cut up by an automatic guillotine and the blanks are immersed in the formalin solution. For manufacture of sheets the rods are placed in moulds and pressed into sheets before formolising. Many attractive patterns may be made by pressing different coloured rods into grooves set on the bias to the rods, thus forming new multi-coloured rods. This operation may be repeated a number of times in order to produce complex patterns. 

USATHAMA) completed a trial burn of explosive, contaminated soil in a rotary kiln (Noland, 1984). Soil contaminated from red and pink water lagoons was successfully burned. A transportable rotary kiln yrstem was set up. The technology by Therm-All, Inc., had been used in industry for destruction of solid wastes. The normal screw feed system was not used, due to fear of a soil explosion during the extruded plug feed process. Therefore, the soil was placed in combustible buckets and individually fed by a ram into the incinerator. The feed rate was 300 to 400 Ib/hr and the operational temperature was 1200° to 1600°F in the kiln and 1600° to 2000°F in the secondary chamber. 

Figure 1. The wire is drawn through a cleaning solution bath, which removes the copper dust from the surface of the extruded wire. The wash liquor is slightly acidic and contains a detergent so that the wire surface is clean. From this point on the wire is spooled, and then sent to another part of the operation which manufactures multi-strand telecommunications cable.

Another feature of an extruder is the presence of a gauze filter after the screw and before the die. This effectively filters out any inhomogeneous material which might otherwise clog the die. These screen packs as they are called, will normally filter the melt to 120-150 fim. However, there is conclusive evidence to show that even smaller particles than this can initiate cracks in plastics extrudates e.g. polyethylene pressure pipes. In such cases it has been found that fine melt filtration ( 45 p.m) can significantly improve the performance of the extrudate. 

The thermal state of the melt in the extruder is frequently compared with two ideal thermodynamic states. One is where the process may be regarded as... 

As discussed in the previous section, it is convenient to consider the output from the extruder as consisting of three components - drag flow, pressure flow and leakage. The derivation of the equation for output assumes that in the metering zone the melt has a constant viscosity and its flow is isothermal in a wide shallow channel. These conditions are most likely to be approached in the metering zone. 

This may be compared to the situation in the extruder where the fluid is being dragged along by the relative movement of the screw and barrel. Fig. 4.8 shows the position of the element of fluid and (4.2) may be modified to include terms relevant to the extruder dimensions. 

From equation (4.9) it may be seen that there are two interesting situations to consider. One is the case of free discharge where there is no pressure build up at the end of the extruder so... 

The other case is where the pressure at the end of the extruder is large enough to stop the output. From (4.9) with <2 = 0 and ignoring the leakage flow... 

In Fig. 4.12 these points are shown as the limits of the screw characteristic. It is interesting to note that when a die is coupled to the extruder their requirements... 

Equation (4.12) enables the die characteristics to be plotted on Fig. 4.12 and the intersection of the two characteristics is the operating point of the extruder. This plot is useful in that it shows the effect which changes in various parameters will have on output. For example, increasing screw speed, N, will move the extruder characteristic upward. Similarly an increase in the die radius, R, would increase the slope of the die characteristic and in both cases the extruder output would increase. 

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