Barrel heater

All single-screw extruders have several common characteristics, as shown in Figs. 1.1 and 1.2. The main sections of the extruder include the barrel, a screw that fits inside the barrel, a motor-drive system for rotating the screw, and a control system for the barrel heaters and motor speed. Many innovations on the construction of these components have been developed by machine suppliers over the years. A hopper is attached to the barrel at the entrance end of the screw and the resin is either gravity-fed (flood-fed) into the feed section of the screw or metered (starve-fed) through the hopper to the screw flights. The resin can be in either a solid particle form or molten. If the resin feedstock is in the solid form, typically pellets (or powders), the extruder screw must first convey the pellets away from the feed opening, melt the resin, and then pump and pressurize it for a down-... 

Electrical problems related to barrel heaters, band heaters, and instruments occur constantly in most production lines. In most cases, the problem is easily identified and fixed. For example, most barrel heaters are configured with a current meter on the control panel. If the controller is calling for heat on the zone and the meter... 

Long lasting barrel heater/cooler elements that heat quickly. 

Table 5.2 Selection guide for barrel heater bands (courtesy of Spirex)...

Alternatively, for vessels up to 101, an electric heating band (barrel heater) may be used, having a power of approximately 30 W of reactor volume. In no case should heating rods or tubes inside the reactor be used, because localized high temperatures, aggravated by the low stirring rate, will cause unfavourable medium alterations and cell damage. 

Extrusion involves mixing and forcing molten material via a rotating screw, which is heated by external barrel heaters and via friction, through a die to impart final profile shape. The screw (shown in Figure 6.6) fits snugly (with sufficient clearance to allow rotation yet minimize back flow) into a cylindrical barrel. 

Another estimate of extruder capacity can be made by realizing that most of the energy needed to melt the thermoplastic stems fi-om the mechanical work, whereas the barrel heaters serve mainly to insulate the material. If we allow an efiiciency from drive to screw of about 80%, the capacity Q (Ib/h) can be approximately related to the power supplied Hp (horsepower), the heat capacity of the material Cp [Btu/lb °F], and the temperature rise fi-om feed to extrudate AT (°F) by... 

Barrel too hot] melt temperature > 271 °C/sensor error/faulty barrel heater control system/wom or incorrectly fitted screw and barrel configuration. [Contamination] dirty machine/dirty hopper/moist feed/too many volatiles in feed/[degradation] /lubricant or oil on mold/incorrect mold lubricant/feed contaminated during material handling/faulty raw material from supplier/poor shutdown procedures. 

For start up, the barrel heaters are critical because screw is not rotating. Major concerns about cold start. Rear barrel temperature usually remains important because it affects the bite or rate of solids conveyed in the feed. Barrel temp, must be set appropriately for polymer. Head and die temperatures = desired melt temp, (except where want gloss, flow distribution or pressure control). 

Perforated Barrel heater Control thermocouples Flight. ... 

Extrusion, autothermal (autogenous extrusion, adiabatic extrusion) n. An extrusion operation in which the entire increase in enthalpy of the plastic, from feed throat to die, or very nearly all of it, is generated by the frictional action of the screw. In such an operation, which most commercial single-screw extrusions approach closely, the functions of the barrel heaters are to preheat the machine at startup and, during steady operation, to prevent heat loss from the plastic through the barrel to the surroundings. 

The most common barrel heaters are electric resistance heaters. This is based on the principle that if a current is passed through a conductor, a certain amount of heat is generated, depending on the resistance of the conductor and the current passed through it. The amount of heat generated is ... 

The depth of heating reduces with frequency. At normal frequencies of 50 or 60 Hz, the depth is approximately 25 mm. This is similar to the thickness of a typical extruder barrel. The advantage of this system, therefore, is the much reduced temperature gradients in the extruder barrel because the heat is generated quite evenly throughout the depth of the barrel as opposed to resistance-type barrel heaters. 

The barrel temperature needs to be measured to provide information on the axial barrel temperature profile and to provide a signal for the controllers of the barrel heaters and cooling devices. The temperature should be measured as close as possible to the inner barrel surface, since the polymer temperature is the primary concern. The worst possible location of the temperature sensor would be in the barrel heater itself. However, there are some commercial extruders where the temperature sensor is placed in the barrel heater to reduce the thermal lag of the system. The major drawback of this approach is that one controls the heater temperature and not the temperature of the polymer in the extruder barrel. Some extruders are equipped with a combination of deep-well and shallow-well temperature sensors to improve... 

The power consumption of the barrel and die heaters can be determined by measuring voltage and current to the heater. This works well in current proportioned temperature control. It does not work well with on-off control or time-proportioning temperature control. In the latter case, a wattmeter should be used with a power integrating function. In this case, the integrated power over a certain time period can be measured so that the average power consumption of the barrel heater can be established. Commercial extruders generally do not have sufficient instrumentation... 

The dead time in extruders can range from about 1 to 5 minutes. This constitutes one of the main problems in temperature control, because this means that the effect of a change in power input level is not felt until after at least 1 minute. This thermal lag time is influenced by the depth of the temperature sensor, the thermal conductivity of the barrel, and the design of the barrel heaters. A typical time constant in an extruder can range from 30 to 120 minutes. This value depends on the heating capacity and the specific heat and mass of the extruder barrel. 

The heat supplied by the barrel heaters has to be conducted through the entire thickness of the barrel and through the entire thickness of the melt film before it can reach the solid bed. Problems with this energy transport are considerable heat losses by conduction, convection, and radiation. Another, probably more severe, problem is the low thermal conductivity of the polymer. The heat has to be transferred across the entire melt film thickness. Therefore, the conductive heat flux will be small, particularly when the melt film thickness is large. Increasing the barrel temperature can accelerate the heating process however, this temperature is limited by the possibility of degradation of the polymer. 

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