Temperature Settings
IMM 03/02

I'm sure everyone's familiar with the three zones of the plasticating screw: feed, transition, and metering. In the past several days I've spoken to four molders who all have told me to keep the feed zone set at the same temperature as the transition zone, but they couldn't explain why. I was under the impression that the metering zone should be set the same as the plastic temperature. Why would the rear temperatures (feed zone) be set the same as the compression zone? Can anyone shed some light on this and correct my assumptions?

When setting the temperature profile of the barrel, there are four possible profiles: ascending profile (the feed zone is lowest and the front zone is highest), a flat profile (all zones are the same), a reverse profile (the feed zone is highest and the front zone lowest), and a hump profile (the center zone is the highest and the feed zone usually lowest). Ascending and reverse profiles are probably the most common.

You are correct in setting the front zone the same as the actual plastic temperature as the zone should neither add nor subtract from the melt temperature. The feed and rear zone setpoints are influenced more by the type of material, the residence time, the screw design, and the percentage of barrel capacity you are using. These factors are often overlooked and will affect melt and part quality, energy usage, recovery time, and screw and barrel wear.

Here are some suggestions on setting the rear and center zones. Heat-sensitive resins, long residence times, or a barrel capacity of less than 20 to 30 percent tend to favor lower rear and center zone temperature settings. Filled material, short residence time, hard-to-melt materials, highly viscous materials, and the use of a high percentage of the barrel capacity (70 percent or more) favor higher rear and center zone temperatures. In these cases, extra heat from the heater bands is needed to efficiently convey and melt the material.

Don't forget about you feedthroat temperature. Running this too cold can cause moisture condensation in the feedthroat and greater energy use in the rear zone. I recommend between 125 and 175F, depending on the material.

Westland Corp. publishes a cylinder and screw handbook that includes useful recommendation on setting barrel temperatures. —B. Tutmark, GM Nameplate, Beaverton, OR, (503) 350-2984, bradt@gmnameplate.com.

The objective of heating plastic before injection molding is to achieve a homogeneous melt at a minimum temperature. A typical starting point would be to set the barrel heats across the board at a setpoint that is equal to the melt temperature that you want to achieve. This is called a flat profile. Why use a minimum temperature? Thermoplastics react unfavorably to being melted. If you leave the materials in the barrel too long at heat, the plastic starts to degrade. Secondly, melted material set at its minimum acceptable temperature cools more quickly in the mold, allowing faster cycle times.

So why adjust barrel heats to different setpoints? Some plastics, especially nylons, feed better at a specific setting. Too high (or too low) and your screw feed time might be too long, causing longer cycles while the machine waits for the next shot to be picked up.

Most of the melting in the barrel occurs in the middle (transition) zone(s). Often, the actual temperature of the melt is higher than what the barrel heat settings indicate. The screw is working the material and creating shear heat in this area. An indicator that your heats are set too low is when your temperature zones, again usually the center zones, override the setpoints and the barrel heaters never come on when the machine in running. We'll usually go to the reverse profile, described below, when this happens.

Another method is to set the feed zone at or just below the desired melt temperature. This allows the materials to start to melt as it moves up the screw; the transition zone(s) can do the rest of the work. The transition zone is often set higher than the feed zone. Finally, the metering zone, where your plastic has now melted to just about where you want it to be, is set at the desired melt temperature. This profile is known as a hump profile. All temperatures can be adjusted as needed.

The last heat setting is a reverse profile. In this, your feed zone is the highest, with a lower temperature setpoint in the transition zone, lower still in the metering zone. With this setting, you are melting the materials as soon as possible, and then keeping the melt temperature low as it is prepared to be injected. Shear heat is kept to a minimum. This setting would be more common with low-flow materials such as PC and PVC.

Finally, nozzles (other than mixing nozzles or other special purpose ones) have a sole function: to transfer the material from the barrel to the mold. The nozzle should be as short as possible, as wide open as possible, and the temperature should be set at the lowest setpoint that allows free flow into the mold without freezing off between shots. —M. Frey, Integrity Plastics, Denver, PA, (717) 336-1200, ext. 22, mfrey@integrityplastics.com.

Conversely, if the barrel contains a large number of shots of material (eight or more), the rear / feed zone might be set below that of the compression zone and the rpm of the screw might be lowered to 40 or less in an attempt to be thermally gentle with the materials. This is the art of the mold technician, and not fixed. The development of the process for a new part and a new mold in a press leads a talented mold technician to set the optimum barrel heats and all other settings to maximize part quality at a reasonable cycle with all press parameters within normal limits. Adjustments to the mold may also be indicated, since runner size, gate size, cooling channels, and vents are seldom optimal on new molds. After one or two trials with a new mold, the skilled mold technician is often able to establish a robust process, which produces a consistent part to print at a reasonable cycle. Hopefully, this cycle will be equal to or slightly lower than the cycle that was estimated.—W. Foster and T. Cholette, Polymer Technology, Elbridge, NY, (315) 689-2077, ext 112, bfoster@tessy.com

It was a good idea to consult other knowledgeable molders to try to find this explanation. However, the answers are commonly vague and most often only opinions, not science. There are three main reasons to keep the feed section at a lower temp: degradation of materials, feeding problems, and energy savings.

The first variable to understand is that as the plastic progresses downstream in the injection unit, it is melted by two energy sources: the conductive heat generated by the warm barrel / screw and the frictional heat generated by shear with the plastic molecules. For most plastic materials that are processed on screw / barrel combinations designed to handle them, a generally increasing temperature profile is used. This means the feed section is cooler than the transition section, which is cooler than he metering section. A typical profile for nylon 6/6 would be 510F, 520F, 530F, with a nozzle temperature of 540F. However, there are hundreds of reasons to deviate from this rule of thumb.

• Degradation.  In a theoretically perfect melting process, plastic pellets enter the feedthroat of the barrel and are fed by the feed section of the screw with virtually no melting. As plastic is an insulator of energy, it actually resists the increase in temperature. The material is conveyed through the feed section of the screw and as it reaches the transition section, the barrel and screw heat begins to melt the pellets. As the plastic is compressed and sheared in the transition section, the pellets gain energy (heat) and melt. It is critical not to overheat the material in this section by having temperatures that are too high. If a high amount of shear develops and then combines with excessive heat energy, material degradation will occur.
  
  In the metering section, the screw is designed to produce the final homogeneous mixture at the precise temperature at which you wish to inject it into the mold. While the plastic flows downstream, it absorbs energy. If the plastic absorbs the energy too quickly, this can cause material degradation. If the absorption of heat is too slow, a non-homogeneous mixture with unmelted pellets can be injected into the mold. In order to make certain that the material is brought to temperature as gently as possible, a fine balance of shear and temperature is required. This ensures that all the properties inherent in the original pellets are contained in the final product.
   
• Feeding efficiency. If the feed section of the barrel is too hot, the plastic pellets melt prematurely. As the pellets melt, they cling to the barrel and screw and resist feeding. Anyone who has ever had the cooling lines of a feedthroat fail knows this. Within minutes, the plastic pellets that were falling directly down into the crew begin sticking to the sidewalls of the barrel and form a blockage. As this blockage grows, the material melts and forms a solid bridge, preventing all other material in the hopper from entering the barrel. This starves the machine of material, eventually shutting it down. Depending on the amount of time that passes before help arrives, many other serious and potentially dangerous situations can occur.
   
• Energy savings. Because the heated feed section is located directly next to the cooled feedthroat housing, there is a constant battle to keep one side hot and the other cool. As stated earlier, most melting should occur in the transition section of the screw. Therefore, the feed section of the barrel is primarily used as a thermal barrier between transition section heating and the necessary cooling of the feedthroat. If the temperature of the feed section is excessive, the heat energy is simply wasted by the cooling action of the feedthroat housing.
   

These are the logical reasons for using an increasing temperature profile. While I would like to tell you this is the absolute answer, there are too many other variables to take into account. I hope this helps. If you have any further questions, please let me know.—K. Begue, Plastic Services & Equipment, Fort Wayne, IN, (800) 627-1033, kurtbegue@plasticservices.com.

My experiences have varied with press condition (injection unit condition and plastics pressure capabilities), shot size, runner, gate and part details, type of material being molded, and cycle time. I base my need to alter barrel temperatures in the three zones on the following factors (assuming all equipment is functioning properly and mold details and nozzle are properly sized):

• Type of material being molded (PVC or acetal vs. PP): For a smaller shot I'd be less likely to use the reverse profile, especially on the more heat-sensitive PVC or acetal. Olefins are more tolerant of higher processing temperatures, while ABS, nylons and most engineering resins allow some above-normal temperatures.
• Shot size vs. press barrel capacity: For a small shot I'd be less likely to reverse profile the temperatures than on a shot using 80 to 90 percent of the press's capacity.
• Cycle time: For a smaller shot in a large-capacity barrel, I'd be less likely to reverse profile than on a shot using 80 to 90 percent of the barrel capacity.
   

It is my feeling that this profiling game is played to overcome the following:

• Defects in the molded part (i.e., burns, wrinkles, flow lines, and so forth).
• Cavity filling problems, possibly caused by trapped air or improperly sized sprues, gates, and / or runners.
• Malfunctioning machine, including worn barrel and screw, bad check ring, blown fuses, burned out heaters, and malfunctioning timers. There are many people who have many ideas on how to solve a variety of molding problems and usually something will work.

It is very important that all people responsible for processing really understand how the molding machine and mold are functioning. The majority of the time I use lower temperatures in the rear zone than in the front zone.—D. Zvobada, Venture Plastics Inc., Newton Falls, OH, daves@ventureplastics.com.