Optimizing barrel heat profiles: it shouldn't be a mystery.

The heat is on for injection molders in quality-conscious markets. Knowing how to control heat in your barrel can make or break your injection molding efforts. Here's a guide to the often misunderstood principles of setting heat profiles.

Though vital to product quality, barrel-temperature profiles are still one of the least understood factors in injection molding. A good heat profile with the right amount of heat from the proper source will help you mold parts that are free of burning, splay, or streaks and that deliver the intended physical properties. Mold with a bad heat profile, and you will likely wind up with rejects, low production rates, and premature wear of your barrels, screws, and check valves.

Fortunately, there is a relatively straightforward three-step method that can help you optimize the amount and type of heat required by your molding process. Here's how to do it.

1 Consider the heat source

The heat required to melt the plastic comes from two sources: the heater bands and shear induced by the screw. Conductive heat from the heater bands can be adjusted through the settings on the temperature controls. The heat generated from the shear of the plastic against itself, the screw, the barrel wall, and mixing devices is far more difficult to control. There are no settings for shear heat.

A common indication of excessive shear heating is a temperature override in a particular barrel zone while the heater bands serving that zone are not cycling. Under these conditions, shear is the entire heat source. To reduce this shear heat, you can try reducing the screw speed. Reducing rpm, however, provides only a limited solution because it may also extend the cycle time, resulting in a lower production rate.

A better way to compensate for the effects of shear heating is through somewhat counter-intuitive adjustments to the heater-band settings. Turning down the heater-band settings in the face of a temperature override would be the natural inclination of many molders. But this strategy will not drive the melt temperature down if all the heat is really coming from shear. Surprisingly, excessive shear heat often demands that you raise the heater-band settings.

As long as you increase the temperature of the heater bands in zones that are behind (upstream of) the zone that is overriding and allow time for all the heater bands to start cycling, the melt temperature will actually go down. Why? Because you've changed the source of the heat energy required to melt the plastic from uncontrollable shear heat to a balance of shear heat and controllable conductive heat.

As a secondary benefit of this approach, you may also find that the hydraulic pressure required by the screw drive motor during plastication will be less. You may even be able to reduce the backpressure for a better recovery rate. The result is improved product quality, fewer rejects, reduced energy consumption, and the potential for increased production.

2 Balance the heat sources

Once you've gotten all the heater bands to start cycling, it's time to optimize the balance between the two heat sources. This process is surprisingly simple if you evaluate the Screw Rotate Pressure (SRP), which is the hydraulic pressure required to turn the screw. Also referred to as extruder pressure, SRP is a measure of the degree of work required of the screw to melt and convey the plastic.

If your injection molding machine has the ability to measure SRP, the next time you have a temperature zone that is overriding, especially in the center zone, look at the SRP. It's a good bet that it will be registering above 1200 psi (assuming you have a 2000-psi hydraulic system). The reason is that the unmelted plastic is requiring more heat energy to cause it to melt, but the heater bands are not responding. In order for the screw to recover, greater shear heat must be substituted for the deficiency in conductive heat, causing the screw to work harder.

Our experience in a number of field cases, and under controlled conditions in a laboratory, supports the conclusion that heat developed from the two sources in approximately equal amount produces the best molding results. This balance of heat-energy sources is achieved by adjusting the heater-band settings so that the SRP measures between 800 and 1200 psi in a 2000psi system [ILLUSTRATION FOR FIGURE 1 OMITTED].

Readings of 800 to 1200 psi indicate that the heat produced from shear energy is between 40% and 60% of the total, a desirable range. Greater pressure indicates that the screw is working very hard, creating potentially excessive shear. Readings under 800 psi suggest that too little shear energy is being used and that you may be experiencing long residence times, long cycles, or abnormally high melt temperatures.

It is unfortunate that all injection molding machines do not have the ability to measure the SRP on a continuous basis. If your machines do not have this ability, it is very inexpensive to put a gauge on the hydraulic system to measure the SRP.

3 Select the appropriate heat profile

Once the SRP readings indicate a proper balance of heat sources, molders can choose from several types of barrel heat profiles [ILLUSTRATION FOR FIGURE 2 OMITTED] to achieve the target melt temperature. Picking a profile of temperature settings from zone to zone requires you to know both the rearzone temperature setting suggested by the resin manufacturer and the desired temperature of the melt being injected.

Although an "ascending" profile is recommended by many resin manufacturers and works successfully in some situations, our experience in the field and in the lab confirms that under most conditions, the use of a "hump" or "reverse" profile produces optimum melt quality at the lowest cost and with minimal component wear. We offer the following recommendations when selecting a heat profile:

* A flat profile can be used where the shot is in a range of 20% to 40% of barrel capacity and the resin is not reinforced or heavily filled.

* An ascending profile is acceptable when the shot size is less than 30% of barrel capacity and the calculated residence time is long - i.e., more than 4 minutes. This profile should not be used when processing reinforced or heavily filled materials or when short-to-medium residence times are involved.

* A hump profile is a very good solution for processing most nonreinforced materials where the shot size is in a range of 25% to 50% of barrel capacity and residence times are not extremely long or short - i.e., in the 2- to 4-minute range.

* A reverse profile is excellent for use with all reinforced or heavily filled materials. It is also recommended where screw recovery and/or residence times are short and where the shot size is at least 50% of the maximum. This is a very good profile for heat- or shear-sensitive materials.