INJECTION MOLDING: Get Control Over Barrel Zone Temperature Override
Molding plastic into an acceptable part is a complicated process with hundreds of variables. Some would like a nice ISO 9000 procedure to deal with them. Trouble is, does anyone know them all? New ones always seem to keep popping up for me. But one variable often overlooked that will foul up your process is “zone-temperature override.” What is it and what can a processor do to control it?
Zone-temperature override occurs when one of the temperature zones on the barrel (not nozzle) is consistently hotter than the setpoint by more than 5o C (10o F). Zone-temperature override can happen even when the zone-temperature controller never calls for power to turn on the heater band. Temperature controllers should cycle on and off to control the zone temperature. Not all zones will have the same cycle, as each barrel zone has different functions. For example, the rear zone may cycle on more often or longer than the center zone, as it is near the cooler feed throat.
The bottom line is that each zone needs to control temperature for the screw and process to work consistently and properly. If the barrel zone is too hot and the controller is not cycling, then all the heat in this zone is being generated by screw rotation or recovery. Some may think this is OK or even a good deal, as it saves electricity. Early in my career, my mentor—whom I typically refer to as “Crusty Senior”—showed me how he could turn off the power to the zone heaters and still keep the machine running. As I recall it, he was really proud of himself.
Unfortunately, this is not the way to process. If the heater band is constantly on or off, it is not controlling the temperature. This is poor temperature control and will not provide a consistent melt stream or process.
So how do we get a melt stream uniform in temperature? The screw, barrel, and zone-temperature controllers must perform properly to get melt uniformity. This is a complicated balancing act, which involves screw design, resin, temperature settings, barrel wall design, coefficient of friction of pellet on barrel wall and, well, the omnipresent Murphy’s Law. Melt consistency is further complicated in injection molding because the screw does not rotate continuously, it is reciprocating. In addition, don’t forget that we typically cannot measure actual melt temperature as we are molding. I am not sure I understand all of these interactions, but I do have some tips on what causes zone-temperature override and what to do to gain control.
To understand the causes of temperature override, you focus on how the screw melts plastic. Figure 1 is a basic diagram of a general-purpose (GP) screw; our focus is the transition (compression) zone as this is the area where the plastic pellets melt. It’s generally accepted that about 80% of the energy needed to melt the plastic comes from the friction and compression developed from screw rotation. The rest comes from the heater bands. The energy necessary to melt the plastic comes from two main sources as the screw rotates:
1. Friction of the pellets between the barrel wall and screw flights.
2. Compression developed in the transition zone.
Note in Fig. 1 how the root diameter is thicker in the transition section. This is where large compression pressures are developed. It is my understanding that these pressures are in the thousands of psi (tens of megapascals). Further, these forces can be concentrated in small areas, between flight and barrel wall, to develop hot spots or areas where the temperatures spike. For evidence of this, note what a new screw looks like—bright and shiny. Then note the color of the metal after use; the feed section looks fine but the transition section is now dark. Now look closely at the darkened areas and you will see not only black, but also bluish black, browns, and sometime iridescent colors.
Next, find a handbook on metals and look up the colors that metals turn when subjected to heat. You will find the blues and other iridescent colors indicate the metal experienced temperatures far exceeding your melt temperatures and much higher than you set on the temperature controllers.
This is all proof of hot spots, and the barrel has to dissipate this heat before it degrades the plastic. This explains why you should have gaps between the heater bands and not use barrel blankets. It’s true that insulation blankets save energy, but they can also foul up melt-temperature control, so insulate the individual heater bands, not the entire barrel. Higher-end extruders and some molding machines have cooling between the heater bands to help control temperature. The bottom line is that there are times when the screw-recovery melting process generates too much energy and it shows up as zone-temperature override. So how to you deal with it?
Here are some possible solutions:
• Remove barrel blankets and insulate the individual heater bands for energy savings.
• Lower the screw rpm so that screw recovery takes all but 1-2 sec of the set cooling time. Try not to extend the cycle. Do not have recovery time too close to the cooling time, as recovery time varies during production. Do not lower backpressure unless it is excessively high.
• Replace the screw with one that provides better melt uniformity. Avoid mixing screws; do not use a stock GP screw, what fellow columnist Jim Frankland calls the “No Purpose” screw. Screw design is critical for melt uniformity, payback is significant and real. I have documented more than $250,000 in savings in one year from changing to more appropriate screw design on seven machines (400 to 1200 tons).
• Add cooling fans between the heater bands.
• Raise the zone temperature and that of the zone before it. It sounds crazy but this will lower the friction of the plastic against the barrel wall, which in turn lowers the energy generated and improves temperature control.
• Monitor on/off cycling of the temperature zones during production.
Let me know if you have any further ideas. Maybe zone-temperature override is the variable feeding Murphy the ammunition to ruin your day and pay. It doesn’t have to be.
ABOUT THE AUTHOR
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