Injection molders: what's your plasticating 'window'?

A simple procedure can help optimize settings for barrel-temperature profile, screw speed, and backpressure to obtain best melt quality.

Though it has been than three decades since the screw replaced the plunger in injection molding, the art and science of screw plastication is still relatively new to many molders. Some molders select barrel temperatures, screw speeds, and backpressures using rules of thumb that may not provide appropriate or optimal melt quality. Bubbles, streaks, splay, and burning can result. Such rule-of-thumb practices are changing as more applications require use of advanced engineering thermoplastics (ETPs) and careful consideration of how the melt is generated.

A procedure for setting up the extruder on an injection machine was developed by Dow Plastics in Midland, Mich., based on our laboratory molding experience and on our work with injection molding customers. This procedure helps you define what I call the "extrusion window" for a given resin--i.e. the range of settings for barrel temperatures, screw speed, and backpressure that provides the most efficient extruder operation at the desired melt temperature.

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Our customers frequently ask for the "standard" barrel-temperature recommendation for a particular resin. In fact, there is no standard temperature profile unless you are molding commodity resins in a high-volume, low-precision job. For ETPs the optimal barrel-temperature profile is highly dependent on the extruder design. Therefore, it must be identified for each material and machine combination.

For optimal performance it is sometimes necessary to use a so-called "reverse profile," in which the zone near the hopper is hotter than the nozzle. This reverse profile must be found through trial and error rather than using a general rule.

For some materials there are general guidelines that can be useful for setting the screw speed. For example, when molding ABS or an olefinic material with a conventional screw, the rpm should be set low enough that the screw reaches shot size just as the cooling timer runs out. This general approach, however, does not always work with ETPs or when using a well-designed mixing screw.

Some ETPs--polycarbonate in particular--require a minimal amount of "heat soak" for optimal plastication. You will want to set your screw speed a bit faster to leave some idle time during the cycle when the unmelted pellets for the next cycle can sit in the hot feed zone of the screw and "soak in" sufficient heat to provide good melting behavior. On the other had, a screw speed that is too fast can mechanically destroy the nonreturn valve when molding high-viscosity materials on a screw with a relatively long metering section.

Likewise, other parameters that work with a conventional screw cannot be applied to a well-designed mixing screw, which requires a minimal shear rate for efficient operation.

The last setting that makes up the extrusion window is backpressure, which varies significantly, depending on the machine, material, and mold used. A backpressure setting of 50 psi on one machine can have a completely different effect on the plastic in another machine, owing to differences in the intensification ratio of the screw or hydraulic-circuit design of the machine. Low-compression-rate or "zero-metering" screws typically require much more backpressure than a standard screw. However, increased backpressure is often an inappropriate remedy for any problems experienced with a well-designed mixing screw.

The common habit of using very low backpressure should yield to a better understanding of what setup conditions a given plastic "likes" and how the melt behaves during a specific processing run.

FINDING THE WINDOW

It is possible to identify a range of optimum extruder settings through the "extrusion window evaluation" procedure outlined on the previous page. Like any systematic optimization procedure, this evaluation does not yield exact "black-and-white" results. Rather, it helps the experienced molder determine the maximum and minimum values for key process variables in order of diminishing adverse effect on the process or part quality. The final machine settings should be in the middle of the operating window--not near the extremes.

The extrusion window evaluation requires time--24 hr on most medium-size to large machines. The evaluation is not required for every mold tryout or material sampling, but it is recommended when a new material or machine trial results in failure or necessitates a significant departure from familiar molding practices.

This evaluation assesses several barrel-temperature profiles and screw speeds in reciprocating- and fixed-(constant-L/D) screw modes.

The reciprocating-screw test is performed to account for the dependence of screw performance on heat soak. A waiting period between each test simulates the heat-soak time during the cycle and, by comparison with the fixed-extrusion data, shows the need for heat soak to improve solids conveyance.

The fixed-extrusion test uses backpressure to hold the screw forward during plastication. Start with a low backpressure and then increase it as necessary to prevent the screw from reciprocating at the selected screw speed. Since the screw will produce its highest discharge pressure at its optimal operating conditions, the backpressure requirement will increase dramatically as the screw performance is optimized during this procedure.

To begin the evaluation, the initial barrel-temperature profile should be flat and cooler than the intended melt temperature. The actual barrel temperatures, melt temperature, and torque (indicated by screw-motor hydraulic pressure) are observed throughout the evaluation as a basis for the next profile to be studied.

A barrel-temperature setting that causes all zones to override the set-point melt temperature and causes the torque to be near the machine maximum is too cool. In this case, too much heat is being generated through frictional and viscous heating.

Conversely, a barrel-temperature setting that results in very low torque requirements and still produces melted plastic is too hot. To avoid these extremes, target a screw torque that represents approximately 50% of the machine's maximum capability.

A reverse temperature profile should be tried when a test--in either reciprocating- or fixed-screw mode--indicates that the screw performance relies on heat soak. It should also be used if a test reveals a barrel-temperature override of 25 [degrees] F in the middle barrel zone(s), an uncontrollable melt temperature, or a high torque requirement.

In any of these cases, set the nozzle zone to the intended melt temperature and the rear zone to the observed override temperature. Then profile the rest of the zones downward so that the forward zone (barrel adapter) is lower than the intended melt temperature. If the screw output increases, the torque requirement drops, and little or no override is observed, then these settings are approaching the desired level of control. If the temperature override and torque are lower but the output rate decreases, the rear zone may be too hot. If no effect is noticed, a hotter rear zone may be tried.

A reverse barrel profile will not work in all cases that require a heat soak. But it is preferable to the alternative of lengthening the molding cycle by extending cooling or mold-open time.

OPTIMIZING BACKPRESSURE

In the final step of the extrusion window evaluation, screw speed and temperature profile are selected by reviewing graphs such as those shown in Figs. 1, 2 and 3. Start out using the lowest controllable backpressure (50 psi on most machines) while the machine is molding parts. Increase backpressure in small increments to eliminate bubbles and splay and to achieve a repeatable recovery time. Once the machine has been molding parts for at least an hour, cushion variation can be monitored as another indication of insufficient backpressure (unless variation is due to a leaking check valve).

The backpressure derived from this evaluation can be considered the minimum required for production. Carefully increase it from there, remembering that the full effect of the change may be seen only after several cycles (roughly the number of shots in the barrel). Monitor the part quality, actual melt temperature, and any indication of melt-temperature change (i.e., changes in hydraulic and cavity pressure) as backpressure is increased. The upper limit of the backpressure window is found when further increases do not significantly improve the process.

EXTRUSION WINDOW EVALUATION

Reciprocating Extrusion Test

1. Set up the machine.

a. Retract the carriage and go into manual control. b. Turn beckpressure off or set it very low. c. Set a ballpark shot size.

2. Select your first temperature profile. For a new resin or machine, start with a flat profile with all barrel zones set equal to the intended melt temperature.

3. Select a reasonable injection rate--i.e., as close to realistic as your machine will permit in manual mode.

4. Turn the screw at the lowest possible rpm until it reaches shot size.

5. Immediately inject (air purge).

6. Collect all the plastic (both extruded and injected) and record data.

a. Melt temperature (using a manual probe).

b. Peak screw-motor pressure while screw is turning.

c. Weight of the purged and extruded plastic.

d. Time required to recover screw shot size (together with "c" gives output in lb/hr).

7.Repeat the above for at least three higher rpm settings, with three repetitions at each setting, allowing a pause between each shot equal to the estimated cycle time. (The pause helps you get a feel for heat-soak requirements. If the output is higher than for the fixed-screw test below, heat soak is important for your process and you probably should try a reverse temperature profile.)

Fixed Extrusion (Constant L/D Test)

With the temperature profile and rpm used in the reciprocating test above:

1. Find the backpressure that prevents the screw from reciprocating during rotation at the lowest feasible rpm. (This is a measure of the pressure the screw is generating at this condition. The screw will produce more pressure in the optimal operating range.)

2. Turn the screw at the test rpm for 15 sec. Collect the purge patty. Note the peak screw-motor pressure.

3. Collect all the plastic (both extruded and injected) and record data.

a. Melt temperature. b. Peak screw-motor pressure while the screw is turning. c. Weight of the extruded plastic.

4. Repeat for at least three higher rpm settings, making three shots at each setting and allowing a pause between each shot equal to the estimated cycle time. (The pause again helps you get a feel for heat-soak requirements. If the output is lower for the second and third sample of the set or lower than in the reciprocating-extrusion test, then heat soak is important to your process.)

Identity the Window

1. From these data, plot separate graphs of screw-motor pressure, screw output, and melt temperature versus rpm, as in Figs. 1-3. Find the barrel-temperature profile and screw speed that are in the center of the extrusion window.

2. Find the optimal backpressure while molding parts.

a. Start at the lowest controllable pressure, then increase in small increments until bubbles, splay, and cushion variation are eliminated. That's the low limit of the extrusion window for this parameter.

b. Continue increasing beckpressure gradually, waiting for several shots (a full barrel's worth) between changes. The last pressure increment that significantly changes part quality, melt temperature, hydraulic pressure, or cavity pressure on injection is the upper limit of the window.