Pull-extrusion process

Similar to the described combination of pultrusion and extrusion is the so-called pull-extrusion process, which is illustrated in Fig. 8.15. It enables the production of products which are reinforced with both continuous and discontinuous fibers in one profile. A thermoplastic polymer is molten in an extrusion unit. In a first step, the extrudate is combined with a continuous fiber strand. In the second step the remaining polymer is added with... 

Pultrusion is also used to make structural shapes from composite materials. The incoming material is generally unidirectional and must be pulled through the pultrusion die because the uncured composite material is entirely too flexible to push (as in extrusion processes). The incoming material can be preshaped by various guides and rollers as in Figure 1-18. 

The formula mixture is finally forced (extruded) through a set of dies where it is drawn into long, thin strands.These strands are pulled through contact cooling water for a few seconds, which causes the resin to set before it enters a pelletizer. The pelletizer is a sharp fan or chopper that rotates at a specific speed coordinated with the strand feed rate to slice the strands into pellets of the desired size. The pellets are then automatically screened, and pellet product of the correct specification size range is dropped into the desired packing container. Rejected pellets and particle product are returned as feedstock to the front of the extrusion process for reprocessing. 

Pipe haul-off units form a key part of the extrusion process since they have the function of exerting a steady, uniform pull back through the cooling and calibration zones to supplement the extrusion pressure at the die head. The haul-off unit effectively creates a tensile stress in the solid material that balances the hydrostatic and viscous forces in the liquid state of the extrudate and overcomes the resistance of external friction. Haul-off units may be belt driven or multi-pad caterpillar systems. Large diameter pipes require multiple caterpillar units and thin wall pipe also require more units to avoid distortion. 

The use of a lubricant delivered separately from the polymer melt stream in a polymeric fiber extrusion process can provide a number of potential advantages. For example, the use of separately-delivered lubricant can provide for oriented polymeric fibers in the absence of pulling, i.e., in some embodiments it may not be necessary to pull or stretch the fiber after it exits the die to obtain an oriented polymeric fiber. 

Figure 9.10 (a) shows an extrusion process while Fig. 9.10 (Z>) shows a drawing process. In extrusion, the billet is pushed through the die by a ram, while in drawing, the exiting product is gripped and pulled through the die. Drawing is limited to relatively small reductions in area because otherwise the product may fail in tension. The reduction in area in extrusion may be very much greater than in drawing. Drawing is normally done on a cold billet, but extrusion may be done either hot or cold.

Pulltrusion processing is common in metals and polymer composites. The extrusion process used in this laboratory has a possibility of extruding the polymer under a combined push force and pull load by attaching weights to the emerging extrudate. The effect of pull load on the extrusion of Alathon 7050 split billets has been studied (11). Figure 7 shows the marked increase in extrusion rate produced by adding a pull load to the extrudate. The effects of push force and pull force on extrusion rate are found to be superimposable. 

In conventional extrusion the maximum pressure that can be applied without stick-slip is around 0.23 GPa and so this provides a "process dependent" restriction on extrusion rate and the maximum draw ratio available. In push-pull extrusion, forces up to the tensile fracture of the material can be applied so the true maximum draw ratio for the material at a given temperature can be obtained. The limit of temperature and draw ratio detected in these studies has been added to the extrudability map (Figure 6) showing the maximum regions of the temperature/draw ratio map that may be studied. Clearly the push-pull process greatly extends the range of the extrusion technique. A series of samples prepared at the same draw ratio and temperature, but with different combinations of push and pull, have the same modulus, i.e. mechanical properties are independent of the applied pressure. 

The most prevalent form of plastics processing (by volume of material converted) uses a pull-push strategy to move raw materials and convert them into finished shapes. The extrusion processes pull material through a heating mechanism and then push it through a die to produce two-dimensional profiles or sheets of material. 

Extrusion. In general, extmsion is the process of forcing a polymer melt through a die (104,105). Typical extmsion appHcations include initial resin pelletization after manufacture and production of film, sheet, pipe, tubing, and insulated wire. The HDPE extmsion temperature is around 150°C, the pressure 40—50 MPa (5800—7250 psi). An extmsion production line usually consists of an extmder (mono- or twin-screw) with a die at the end, a cooling and shaping device, a pulling device (a roUer), and a cutter. 

Polycarbonate melts adhere strongly to metals and if allowed to cool in an injection cylinder or extrusion barrel may, on shrinkage, pull pieces of metal away from the wall. It is therefore necessary to purge all equipment free of the resin, with a polymer such as polyethylene, after processing. 

In a final RTD experiment, a sheet of dye was frozen as before and positioned in the feed channel perpendicular to the flight tip. The sheet positioned the dye evenly across the entire cross section. After the dye thawed, the extruder was operated at five rpm in extrusion mode. The experimental and numerical RTDs for this experiment are shown in Fig. 8.12, and they show the characteristic residence-time distribution for a single-screw extruder. The long peak indicates that most of the dye exits at one time. The shallow decay function indicates wall effects pulling the fluid back up the channel of the extruder, while the extended tail describes dye trapped in the Moffat eddies that greatly impede the down-channel movement of the dye at the flight corners. Moffat eddies will be discussed more next. Due to the physical limitations of the process, sampling was stopped before the tail had completely decreased to zero concentration. 

In contrast to extrusion, in this process a combination of liquid plastic and continuous fibers (or combined with short fibers) is pulled continuously through a heated die of the shape required for continuous profiles. Glass content typically ranges from 25 to 75wt% for sheet and shapes, and at least 75% for rods. RP shapes include I-beams, L-channels, tubes, angles, rods, sheets, etc. 

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,... 

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