Plasticator barrel temperature control

Time, pressure, and temperature controls indicate whether the performance requirements of a molded product are being met. The time factors include the rate of injection, duration of ram pressure, time of cooling, time of piastication, and screw RPM. Pressure requirement factors relate to injection high and low pressure cycles, back pressure on the extruder screw, and pressure loss before the plastic enters the cavity which can be caused by a variety of restrictions in the mold. The temperature control factors are in the mold (cavity and core), barrel, and nozzle, as well as the melt temperature from back pressure, screw speed, frictional heat, and so on in the plasticator. 

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 ... 

Cold feed extruders (Figure 14.25) have much larger length-to-diameter ratios because of the requirements of reducing green compound plasticity, heat buildup in the compound, and pressure buildup required to produce the extruded profile. The length-to-diameter ratio is typically 24 1. The modem cold feed extruder is also computer controlled, which enables adjustment of the compound temperature profile through the barrel, pressure control, flow rate, and feed rate. This provides accurate control over die swell, extrudate surface quality, and buildability of the extruded product. 

IMMs for processing TS resins and rubbers (that are TSs) control the barrel temperature most of the time indirectly with an external heat exchanger. It is operated by a liquid heat-transfer medium such as oil or brine. Curing of the plastic or rubber compound occurs in the mold cavity(s) by the application of higher heat than what exists in their barrel melt. A chemical crosslinking action occurs with the additional heat resulting in the solidification of the TS materials. 

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. 

Consider a barrel with a shallow well for its sensor. Assume a perfect temperature controller set at 400°F (204°C). There is a 75°F gradient from the outside to the inside of the barrel thus the actual temperature down near the plastic would be 325°F with the sensor set at 400°F. If the extruder started to generate too much heat, the temperature could reach 475°F before the sensor detected the increase. With this on-off control action, even with the controller set at 400°F the plastic temperature variation is 150°F. The result could be poor product performance and increased cost to process the plastic. 

The large number of variables summarized in Fig. 2-6 will cause part changes if not controlled properly. Basic settings for these variables are provided by the plastic producer the injection barrel temperature (Table... 

The feed zone has the most effects on solid conveying of the resin. Three coefficients of friction take place, namely friction between barrel and pellet, between pellet and pellet, and between root of the screw and pellet. Solid conveying is the process wherein the material must stick to the barrel and slip on the screw. Hence, cooling root of screw will reduce the coefficient of friction between the screw and the plastic pellets. Screw cooling should be installed in the core of the screw in the feed section to give another zone of temperature control on the extruder. 

Plastic Temperature. Correct temperature and uniformity are crucial for a consistent process. Monitoring of plastic temperature is difficult and rarely utilized in the industry. Control of temperature is done via thermocouples partially embedded into the barrel wall, usually three to four along the length of the barrel. The actual molten plastic is not monitored for temperature and >90% of the temperature values provided as data are barrel wall temperatures that can be off by 25°C (45°F). Temperature control is done via proportional-integral-derivative (PID) controllers or PID algorithms on computer controlled presses. Calibration of thermocouples is seldom done. PID temperature control of the nozzle tips is also important, yet 40% of the industry uses variacs. 

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