Barrel heater zone

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... 

High-quality barrel and die temperature controllers are desirable to minimize fluctuations in machine temperatures. Heater zones with cast-in channels are desirable for water cooling of the barrel. 

The temperatures ofthe eight zones were between 160°Cand 190°C. The screw rpm was between 30 and 66. The side-stufifer speed was 30 rpm. The water bath was warmed to 45°C with a barrel heater. The PHA extrudate was cut into pellets at the take-up roU. 

A schematic view of an extruder is shown in figure 1. The extruder barrel is essentially a ferrous alloy cylinder, with aluminum block heaters attached to the outside. There are several temperature control zones along the length of the extruder. Measurement thermocouples are installed in the extruder barrel itself. Barrel temperature is used to control the temperature of the polymer melt. Energy from the heaters is conducted both radially and axially in the barrel. Below, figure 2 shows a sketch of the extruder barrel, with the heaters and the temperature measurement points used in this paper marked. 

As the polymer granules enter the melting zone (which we also refer to as the transition zone), their temperature rises and they begin to melt, The heat required to melt the polymer comes from two sources, external heaters and mechanical work from the action of the screw. When we extrude viscous polymers at high rates we may need to cool the barrel to remove some of the heat induced by working the molten polymer. 

An extrusion trial was performed at the processor s plant using a 38.1 mm diameter production extruder, a proprietary screw design, and resin that had previously exhibited flow surging and reduced rate. The extruder was equipped with three barrel zone heaters with control thermocouples (labeled Tl, T2, and T3) and two pressure sensors. One pressure sensor was located in the midsection (zone 2) of the barrel (P2) and the other at the end of the barrel near the tip of the screw (P3). Both transducers were positioned over the top of the screw such that a pressure variation due to screw rotation would be observed. 

In setting up the barrel temperature profile start with the front to rear zones (die end to feed section). The heat controllers are set slightly above the plastic melting point prior to turning on the heaters. Heat-up should be gradual from the ends to the center of the barrel to prevent pressure buildup from possible melt degradation. With this startup follow through with ... 

Heat is usually applied in various amounts and in different locations, whether in a metal plasticating barrel (extrusion, injection molding, etc.) or in a metal mold/die (compression, injection, thermoforming, extrusion, etc.). With barrels a thermocouple is usually embedded in the metal to send a signal to a temperature controller. In turn, it controls the electric power output device regulating the power to the heater bands in different zones of the barrel. The placement of the thermocouple temperature sensor is extremely important. The heat flow in any medium sets up a temperature gradient in that medium, just as the flow of water in a pipe sets up a pressure drop, and the flow of electricity in a wire causes a voltage drop. 

Barrels are made of steel, which is not a particularly good conductor of heat (being ten times worse than copper). Thus there is a gradient in the steel barrel from the outside of the barrel to the inside next to the plastic. In 31 2 in. (88.9 mm) and 4 in. (114.3 mm) extruder barrels, these gradients or differences in temperature can routinely be 75 to 100°F (23.9 to 32.8°C) or more, as the zone heaters pump in heat or zone coolers take excess heat out. Yet, for years users routinely accepted extruders with sensors mounted in very shallow wells, or, even worse, mounted in the heating/cooling jacket. 

Held in place by the head assembly is an adapter, which guides the melt stream from the barrel exit into the die entrance. There may be a fixed nozzle in the head, which can be fitted with a custom adapter for each die employed. For the best temperature control, the nozzle and adapter should each have their own heater band. Usually a single temperature control circuit is used for the nozzle/adapter zone, but for a long transfer pipe to the die, multiple circuits maybe necessary. 

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