Many processes must be in place to achieve truly efficient extrusion.
To get high quality, consistent production, you need excellent people to run the line and manage operations, equipment must be in good condition, a comprehensive preventative maintenance program should be in place, and equipment should be designed to allow for efficient operation.
Efficient extrusion requires good industrial discipline, human attention to detail and a constant effort to achieve process improvements. Successful extrusion isn't getting two or three things right -- it's getting hundreds of things right.
Do three "M"
Efficient extrusion requires proper instrumentation. Here, the most critical process variables are the "three M's": melt pressure, melt temperature, and motor load (melt pressure, melt temperature, motor load). They are vital signs of the extrusion process. They must be continuously measured and monitored. The following parameters should also be measured and monitored:
●Barrel temperature,
● screw speed,
● power consumption per heating or cooling zone,
● ambient temperature,
● relative humidity,
● The temperature at which the raw material enters the extruder,
● the moisture content of the raw material entering the extruder (if moisture absorption is required),
● cooling water flow,
● inlet water temperature,
● outlet water temperature,
● Vacuum at the exhaust port (if applicable).
Retrieve data
Data Acquisition (DA) capabilities are critical to developing a robust extrusion process, maintaining process consistency, optimizing the process and troubleshooting efficiently. Fortunately, with today's inexpensive computers and widely available DA software, it is possible to install a powerful DA system on an existing extrusion line. Unfortunately (and bafflingly), very few processors take advantage of this feature effectively.
The author worked with a processor to install a computer-based DA system on a relatively old extrusion line. A DA system costs less than $20,000. In about three months, the line's scrap rate dropped from about 15 percent to 5 percent. As a result, the DA system paid for itself in a matter of months and now helps the converter run consistently with significantly lower scrap rates on every line.
Scrap rates are reduced because processes can be optimized like never before with the DA system. In addition, problems that may lead to non-conforming products can be identified and corrected immediately before they are actually produced. This is not aerospace technology! It's just simple and effective using off-the-shelf tools. Even old extrusion equipment can produce quality products as long as they are well maintained and have good instrumentation and DA functionality.
Feed selection
The consistency of the feed and the flow of the feed to the extruder are critical to achieving process stability. Changes in the feed (for example, different degrees of regrinding) always result in changes in the extrusion process. Even seemingly trivial issues such as particle size distribution can affect the process. In general, a narrower particle size distribution increases the stability of the extrusion process.
Figure 1. The overflow feed completely fills the screw channel and utilizes the full length of the screw. It does not require additional feeding equipment, but reduces process control.
There are two basic feeding methods: flood feeding and starvation feeding. In overflow feeding, the feed hopper is filled to a certain level and the material flows in mass flow (most of the time) to the extruder, which sucks in as much material as possible. The screw channels are filled almost immediately (see Figure 1). The result is that, in an overflow feed, the effective length of the screw is more or less the same as the flight length of the screw.
Figure 2 Starvation feeding requires a feeder, but provides better process control and tends to reduce melt temperature.
In starvation feed, the polymer is metered into the extruder through a feed device (Figure 2). There is no buildup in the hopper; instead, the material falls directly into the screw channel, which is only partially filled at the feed opening. When the material is conveyed forward, the screw channel will be completely filled at a certain distance downstream of the feed.
In starvation feeding, the effective length of the screw is less than the flight length of the screw. An important advantage is that the effective screw length can be adjusted while the extruder is running. This allows for wider process control than flood feeds where the effective length is not adjustable. Starvation feeding is only useful if the extruder is long enough to achieve complete melting and effective mixing. Therefore, starvation feeding will generally not improve the process on short (25D long) extruders. Starvation feeding requires a feeder, but it reduces motor load, melt temperature and the potential for lumps, bridging and separation in the hopper.
Figure 3 This example from a pipe extrusion operation shows that some degree of starvation feeding results in a more uniform wall thickness. Flood feeding at 100% fill; anything less is starvation feeding.
Starvation feeding allows a level of process optimization that cannot be achieved with overflow feeding. Figure 3 shows an example of a pipe extrusion operation where the wall thickness variation was measured at several fill levels. One hundred percent fill indicates flood feeding; less than this indicates starvation feeding. The fill percentage is the actual feedrate relative to the overflow feedrate.
It is clear that the optimum process conditions to minimize wall thickness variation are about 98% fill. The variation in wall thickness at optimum starvation was about half that of overflow feeding. This means that under optimum conditions, less material can be used because the wall thickness of the pipe can be closer to the minimum. In this case, the material savings alone are approximately $100,000/year.
The environment within the factory also plays an important role in the extrusion process. Changes in room temperature and relative humidity can affect the process, as can airflow: opening a door or window can change heat transfer conditions around the extruder and cause a process shift, and turning a fan near the extruder on or off can cause similar process changes . Since events of this nature typically do not show up on dashboards, it may not be easy to find the source of this process change.
Screw speed and barrel settings
An extruder operates most efficiently when the screw provides approximately 80% to 90% of the energy required to heat and melt the plastic. In this case, the barrel heater provides 10 to 20 percent additional heat. Sometimes the screw provides more than 100% of the energy needed to heat and melt the plastic. We might call this an "overactive" screw. Barrel cooling is needed here to control the temperature.
Figure 4 Each resin has a specific heating and melting energy consumption (SEC). Ideally, the screw provides 80% to 90% of the energy. If it provides more than the SEC, it needs to be cooled to remove the excess heat, which is inefficient and has the potential to degrade the plastic. The self-extrusion point is where the screw delivers precise 100% SEC to the plastic. Anything above that indicates an "overactive" screw.
Barrel cooling wastes energy, and energy, obviously, is not free. Figure 4 shows how specific energy consumption (SEC) in kWh/kg varies with screw speed. Each plastic has specific heating and melting energy requirements. For semi-crystalline plastics this value is approximately 0.15 kWh/kg and for amorphous polymers this value is approximately 0.10 kWh/kg.
The curves in Figure 4 represent the combination of frictional and viscous heat generated by the screw. This is often referred to as shear heat, although this term is not strictly correct. At low screw speed, the heat generated by the screw is very low, and the effect of the barrel heater is great. At higher screw speeds, most of the heat (80% to 90%) is generated by the screw – this is the preferred operating range.
When the screw speed is increased further, it crosses the horizontal line indicating the SEC requirements of the plastic. This point of intersection is called an autogenous extrusion point. At this time, all the heat is generated by the screw, and the barrel heater does not need to provide heat. Beyond this autogenous point, the screw generates more heat than necessary - it becomes overactive. When the screw speed increases beyond the crossover point, the barrel needs to be cooled to remove the excess heat provided by the screw.
As the barrel cools, the melt temperature in the extruder will be higher than the barrel temperature setpoint as heat flows from the inside of the barrel to the outside. When a small amount of cooling is performed, the melt temperature may be 10°F to 50°F higher than the set point. With moderate cooling, the melt temperature may be 50°F to 100°F above set point. When cooling is at full speed throughout, the actual melt temperature may be 100°F to 150°F higher than the set point, or even higher. Since the melt temperature at this point cannot usually be measured, most processors are unaware that this condition can be detrimental.
It is important to understand that the need for cooling means that the plastic is overheating. This increases the risk of the plastic degrading and developing black spots, gels and discolourations. It also reduces the melt strength at the die exit and makes it more difficult for the extrudate to cool. Running an extruder with barrel cooling is like driving a car with the brakes on - it wastes energy and causes excessive wear.
Efficient extrusion requires careful optimization of barrel temperature. Many companies do not pay enough attention to the barrel temperature profile. There are several ways to set the barrel temperature. One of the effective methods is dynamic optimization. This method involves making large changes to the set point and tracking how the actual temperature and pressure change over time. Figure 5 shows how pressure varies with temperature as the set point is decreased from 390°F to 300°F.
In the case shown above, the optimum temperature set point for Barrel Zone 1 is approximately 330°F. This method of finding the optimal set point is faster than making real-time changes to the set point and waiting for the extruder to react to the change. For large extruders, it may take 30 minutes or more for the machine to react to a change in setpoint. If you make six changes, the extruder can easily take three hours or more to react to those changes.
