Extruder barrel adapter

Against the forces exerted by the melt flow, the screen packs are backed by a thick, densely perforated steel disk called a breaker plate. The outer rims of the breaker plate and of the screen pack fit into a round recess in the end of the extruder barrel and are clamped in place by the adapter flange of the adjoining piece of equipment, usually that of the extrusion die. To change a clogged screen pack, the die adapter flange has to be removed, the old pack taken out and replaced with a new one, and the equipment reassembled. 

The usual die used is a crosshead with a 90° angle between the wire line and the extruder body axis (Chapter 17). With this setup, the entire length of the extruder projects sideways from the coating lines. To help melt flow from developing dead spots in the melt channels, 30° or 45° crossheads can be used. They provide a more streamlined interior and the extruder location is better adapted to some plant layouts. They are sometimes preferred when processing PVC because of the streamlining and better control of the melts heat profile. Most dies are subjected to very high internal pressures since the uncommon pressure in the extruder barrel of over 5,000 psi (35 MPa) is required. 

These on-off controllers are unsatisfactory for a loading having a long time constant, such as an extruder barrel, a die adapter, a die, and so forth. The temperature will oscillate violently at an amplitude that is set not by the characteristic of the controller, but by the delay in the load, as reviewed in Fig. 1-7. To reduce this variation, a proportional control was developed. It is similar to the on-off, but operates in between full on and off, with its output proportional to the deviation of temperature from the set point value (Fig. 1-8). Variations still exist with this system, but they are less than those of the on-off control. 

The breaker plate (Fig. 5.14a) acts as a seal between the extruder barrel and the die adapter, thus preventing leakage of the melt. The... 

Injection molding barrels are shorter than extruder barrels. The typical L to D ratio is 22 1 to 26 1 for fast-running machines and 28 1 for vented barrel injection machines. Unlike extruder barrels, the feed port is cut through the barrel and connects with water-cooled feed housing. The discharge end of the barrel fastens directly to the end cap or nozzle adapter, the counter bore at the end of the barrel centers the end cap. 

Because Teflon PFA melt is corrosive to most metals, special corrosion-resistant alloys must be used for the extrusion equipment, such as Hastelloy C, Monel 400, and Xaloy 306. Barrels, liners, screws, adapters, breaker plates, and dies are made of corrosion-resistant metals (39). Corrosion is promoted by resin degradation and high processing temperatin-es, long residence times, or dead spots. Extruders used with Teflon FEP are also suitable for PFA resins. Heaters and controllers capable of accurate operation in the range of 330-425°C are required. Extruder barrels should have three or foin- independently controlled heating zones, each equipped with its own thermocouple and temperature-indicating control. 

The die is attached either directly to the end of the extruder or with a transfer pipe or adapter that connects the die to the extruder. Polymer melt temperature in the adapter must be maintained. Transfer pipes like extruder barrels should have healer bands covering as much surface area as possible to minimize the possibility of hot or cold spots. Remember two of the extruder objectives are to provide uniform melt temperature and the proper melt temperature. Transfer pipes normally have smaller diameters than the extruder barrel with a converging hole as material... 

It is common for an extrusion line to be separated into several temperature control zones. The number of zones depends on the length of the barrel, the type of adapter or transfer line to the die, and the size and complexity of the die. An extruder may have as few as three or well over ten zones. Each zone, or circuit, contains up to four of the following components temperature controller, temperature sensor, heating unit, and cooling unit. Usually only barrel temperature zones utilize a cooling unit. 

Each heating circuit (zone) of an extruder contains multiple heat bands, cooling fan(s), a temperature sensor, and a temperature controller. While the number of heating zones depends on the extruder s LID, each zone contains 3500 to 4000 W of power. In contrast to extruders, dies and die adapters have no cooling circuits and the number of heating zones depends on the die geometry. Temperature sensors are usually thermocouples located two-thirds into the barrel or die thickness these deep-well sensors provide accurate temperature readings and are relatively insulated from air currents. 

The experiments were conducted on a 2.5 24 L/D Davis-Standard (D-S) water-cooled single-screw extruder. The extruder was instrumented with pressure transducers along the barrel. It contained a 1/2 test head with a valved adapter at the die plate. Both a conventional metering screw of 24 L/D and a barrier screw of 30 L/D were tested. For the 30 L/D barrier screw trials, a 6 L/D barrel extension was added to the end of the barrel. The metering screw had an overall compression ratio of 3.64, while the barrier screw had a compression ratio of 2.53. A rod die was used of 0.5 diameter and 8 length with a... 

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