Band heaters

PBT resins are very fluid in the melt and sometimes drooling from the injection no22le can be a problem in machines that do not have melt decompression. A simple free-flow no22le can be used satisfactorily to minimise drooling if its temperature can be adjusted by a separate heater band. PBT does not drool as badly as nylons. 

The heater bands are expected to supply this power. 

A term applied to a variety of vessels used in vulcanising mbber articles. See Autoclave and Dry Heat Cure. Also refers to heater bands used on extruder barrels and similar machines. 

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... 

Fig. 14.23 Typical blow molding die A, choke adjusting nut B, mandrel adjustment C, feed throat D, choke screw E, die head F, plastic melt G, die barrel H, heater band /, choke ring J, centering screw K, clamp ring L, die heater M, die N, mandrel. [Reprinted by permission from J. D. Frankland, A High Speed Blow Molding Process, Trans. Soc. Rheol., 19, 371 (1975).]...

Four separately controlled heated zones each with 2/1.5 kW heater bands. 

Melting the Plastic Material. The plastic pellets are fed through a hopper into an extruder screw, typically with an L/D ratio of 20/1. The first half of the screw compresses the pellets and squeezes out air. The third quarter of the screw melts the pellets, 70 percent by friction, 30 percent by conduction from heater bands on the extruder barrel. This is called preplastica-tion. The fourth quarter of the screw pumps the melt forward to the front of the extruder, where it goes through a one-way valve to prevent backflow. As the molten plastic accumulates at... 

There are still many machines in service without soak timers. Try to compare the machine with other similar machines that do have a timer. Remember the soak time starts after the injection unit reaches the set point temperature. You know the injection unit is at set point by watching the cycling of the heater bands. Add your soak time at this point, and maintain records for your future use. Remember, you can apply the other methods stated above in General Thoughts. 

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

Heater band closely fitted to the housing (and for larger pipes to the exposed portion of the die pin) to ensure that the die is held at a temperature close to the required temperature of the melt. 

Fig. 7 U-Profile stack die exploded view (top) section view (lower left) and end view (lower right) (1) extruder mounting plate (2) die adapter plate (3) transition plate (4) preland plate (5) die land plate (6) die bolt hole (7) alignment dowel pin hole (8) thermocouple well (9) pressure transducer port (10) heater band (11) breaker plate recess. Detail (lower right) (A) die exit profile and (B) product profile.

Welding - A method used for joining thermoplastic parts without an adhesive. Heat, pressure, and time are variables used to weld together plastic parts. A simple method consists of clamping the parts together and heating the joint with a heater band. Also see Heated Tool Welding. 

Electrical shocks fiom unguarded heater bands. 

Control of machine faulty] incorrect screw stop action/inconsistent screw speed/ malfunction of nonreturn valve/wom nonreturn valve/uneven control of back pres-sure/faulty temperature sensor/heater band faulty/control system fault or poorly tuned/machine has inadequate plastidzing capacity/inconsistent control of cycle. [Cooling cycle too long] [melt too hot] /[mold too hot] /inadequate cooling in local heavy sections. 

Melt too cold at the nozzle or hot tip] nozzle too cold/temperature sensor error/ too few heater bands/heater bands to far from nozzle tip/hot tip heat source too far from orifice or faulty/sharp corners near the gate. 

Nozzle too hot] sensor error/control error/temperature setpoint at nozzle too hot/localized heater bands on the nozzle instead of being spread along the nozzle. [Operator issues] slow setup of mold/need to trim flashing /poor monitoring of cycle times/excessive machine dead time. 

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. 

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