Processor Tips


How to Collect & Interpret Process Data in Extrusion: Part 1

By: Chris Rauwendaal 27. December 2016 23:13

Many extrusion lines sold nowadays are equipped with data- acquisition systems (DAS). These systems can present extruder operators and process engineers with large amounts of data. There are several critical issues in the proper use of a DAS, which are the focus of this article, part one in a three-part series: 

1. Are the correct process variables measured and monitored?
2. Are these process variables measured correctly?
3. Is the data-collection rate appropriate?
4. Are operating personnel capable of properly interpreting the data?

VITAL SIGNS OF THE
EXTRUSION PROCESS
The three most important extrusion process variables are melt pressure (P), melt temperature (T), and motor load (I). These three process variables represent the vital signs of the extruder. When the extruder is not functioning correctly one, two, or all three of these variables will show abnormal behavior. It is no coincidence that these three process variables are analogous to the vital signs of the human body: blood pressure, body temperature, and pulse. The human body functions because the heart pumps blood through our vascular system. Blood pressure and pulse are measures of how well our blood is being pumped. In extrusion P, T, and I are measures of the pumping efficiency of the extruder. 

One of the most important process measurements is the specific energy consumption (SEC). This is the motor power divided by the throughput. In the U.S. the SEC is typically expressed in units of hp.hr/lb; elsewhere it’s more commonly expressed in kW.hr/kg.

The SEC is a measurement of the amount of energy from the motor introduced into the polymer. As a result, the SEC is closely related to the increase in stock temperature in the extrusion process. Higher levels of SEC lead to higher melt temperatures.

The minimum SEC needed to extrude a polymer is determined by its thermal properties. It is the resin’s specific heat multiplied by the temperature rise in the extrusion process—the increase in enthalpy. For semi-crystalline polymers, the minimum SEC value is about 0.10 hp.hr/lb, or about 0.16 kW.hr/kg. It is good practice to monitor the SEC because it is a good measure of the energy efficiency of the extrusion process. If the SEC in a single-screw extrusion process is above 0.2 hp.hr/lb (0.32 kW.hr/kg), this indicates poor energy efficiency and will likely result in high melt temperatures.

MELT PRESSURE IS KING

The most important process variable is melt pressure. Die inlet pressure is a direct measure of the flow rate through the die. Variation of die inlet pressure causes variation in flow rate; this, in turn, will cause dimensional variation of the extruded product (see Fig. 1). A graphical display of die inlet pressure versus time provides a clear picture of the process stability—or lack thereof. A simple display of pressure (analog or digital) is much less useful. 

When an operator looks at a digital pressure readout with numbers typically changing 10 times or more per second, it is impossible to get a clear picture of the pattern. The human mind has very limited ability to analyze large numbers of numerical data. On the other hand, when the operator can look at a graph of pressure versus time, the trend of the pressure pattern becomes immediately obvious. The human mind has remarkable ability to interpret graphical information. For this reason trend plots are a very important element of a good DAS.

Pressure is measured with a pressure transducer. These have a thin diaphragm that contacts the molten plastic. Increasing melt pressure will deflect this diaphragm, and this deflection is a measure of the amount of pressure acting on the diaphragm. There are several different types of pressure transducers. The accompanying table lists several types with a comparison of their robustness, maximum temperature, dynamic response, and error.

Many strain-gage capillary pressure transducers are filled with mercury. Clearly, this is a safety concern. The diaphragm of a capillary pressure transducer is quite thin, usually from 0.100 to 0.125 mm—about the thickness of a 20 weight sheet of paper. As a result, the diaphragm is easily damaged. When the diaphragm of a mercury-filled pressure transducer ruptures, the liquid will enter into the workplace and contaminate the product. For that reason mercury-filled transducers should not be used for medical and food-packaging extrusion.

It is worth noting that the capillary pressure transducer is the most commonly used transducer in the extrusion industry. But from a technical perspective, the capillary pressure transducer, in my view, is not the best; in fact, it is not even second best. Considering that pressure measurement is the most important variable in the extrusion process, it is surprising that the most commonly used pressure transducer is one that has some clear deficiencies.

At a very minimum, pressure should be measured at two locations: before and after the breaker plate. Usually a screen pack is placed against the breaker plate to capture contaminants and to make sure that these contaminants do not end up in the extruded product. As contamination builds up on the screen pack, resistance to flow increases and extruder output will drop. This, of course, will reduce the extrudate dimensions. To counteract this problem a pressure-feed- back control is typically used. In this control, the pressure after the screen pack and breaker plate is measured and controlled by the screw speed (see Fig. 2).

When contamination builds up on the screen pack, the pressure after the breaker plate will reduce. With pressure-feedback control, the screw speed is adjusted so that the pressure after the breaker plate (die inlet pressure) remains constant. Pressure-feedback control has been available in extruders for decades. It is also used when the extruder is equipped with a gear pump—in this case, the inlet pressure of the gear pump is controlled by the screw speed.

One limitation of pressure-feedback control is that it can only act on very slow changes in pressure—changes occurring over 10 min or longer. Pressure-feedback control cannot reduce short-term pressure variations because the control loop is too slow. However, there are several other methods that can be used to reduce short-term pressure variation.

With pressure-feedback control there is a gradual increase in screw speed and the pressure just before the screen pack and breaker plate. This pressure is often referred to as the barrel pressure. When the screw speed increases, the viscous heating in the extruder increases and will generally result in an increase in melt temperature. It is important to understand that pressure-feedback control will result in changes in the extrusion process—in particular, melt temperature. Changes in melt temperature can affect the properties of the extruded product even if product dimensions do not change. 

When contamination on the screen pack builds up rapidly, this can affect the extrusion process significantly. This problem can be avoided to a large extent by using a continuous screen changer.

It is also important to have an overpressure shutdown, as shown in Fig. 2. This is a critical safety feature because under certain circumstances pressures can increase very rapidly—much faster than an extruder operator can act to shut down the extruder. Excessive pressures can cause the die to blow off the extruder in an explosive manner. Clearly, this is a problem one should avoid. For this reason, over- pressure shutdown is an important safety feature. Another important safety feature is a rupture disk. This is typically placed in the extruder barrel just before the breaker plate and screen pack, where the highest pressure is likely to occur.

For proper process analysis and troubleshooting, it is very helpful to have pressure transducers along the length of the extruder. Unfortunately, this is rare in the extrusion industry. Without pressure transducers along the barrel, the ability to troubleshoot extrusion problems effectively is substantially diminished.

When the extruder is teamed with a gear pump, it is very helpful when two pressure transducers are located downstream of the pump some distance apart. With this setup the melt viscosity in the polymer line can be easily determined. The gear-pump speed is a direct measure of the volumetric flow rate. The shear rate in the line is determined from the flow rate and the geometry of the flow channel. The shear stress can be determined from the difference in pressure between the two pressure transducers. The melt shear viscosity is simply the ratio of shear stress and shear rate. Such a setup provides real-time monitoring of the melt viscosity at minimal cost.

PRESSURES INSIDE
THE EXTRUDER
The pressures inside the extruder reflect the flow that occurs in the extruder. Figure 3 shows the flow in a cross-section of the screw channel in the metering section of the extruder. At the top of the channel there is a drag flow toward the pushing flight flank, and the pressure increases from P4 at the trailing flight flank to P1 at the pushing flight flank. At the pushing flight flank, material will move toward the root of the screw by pressure flow. As a result, pressure P1 will be greater than pressure P2 at bottom left of the screw channel in Fig. 3. At the lower portion of the screw channel, material will move to the trailing flank of the flight by pressure flow because pressure P2 is greater than pressure P3. There is pressure flow along the trailing flank of the flight because pressure P3 is greater than pressure P4.

The highest pressure in the channel is pressure P1 and the lowest pressure is pressure P4. This is the pressure drop across the flight. This pressure drop ΔPx (P1-P4) can be expressed as follows for a Newtonian fluid:

∆Px = 6μvbWxcosφ H2

In this expression, μ is the melt viscosity, vb is the barrel velocity, Wx is the width of the channel, φ is the flight helix angle, and H is the depth of the channel. The pressure drop ΔPx depends on the viscosity, screw geometry, and screw speed. When the screw geometry and screw speed are known, the viscosity can be determined from the pressure drop ΔPx.

When pressure is measured in this part of the extruder, it will have a saw-tooth pattern as shown in Fig. 4. The time between peaks equals the time for one screw revolution for a single-flight screw. This means that the screw speed can be determined from the pressure trend. A double-flighted screw will exhibit two pressure peaks for each revolution of the screw.

The pressure pattern in the melting zone of the extruder will be different because the screw channel is not only filled with molten polymer but also unmelted polymer. In a single-screw extruder, melting usually occurs by contiguous solids melting or CSM.

In contiguous solids melting, the solid particles are compressed together in a solid bed (see Fig. 5). The solid bed forms a helical solid ribbon that wraps around the screw. As melting progresses, the size of the solid bed reduces, while, at the same time, the size of the melt pool increases. There is a thin melt film between the solid bed and the barrel. Most melting takes place at the interface between the solid bed and the melt film.

If pressures are measured in the melting zone, the pattern will be different from the melt-conveying zone. The pressure gradient in the melt film will be different from the pressure gradient in the melt pool, as shown in Fig. 5. Therefore, when pressure is measured midway along the barrel, the pressure pattern reflects the location of the solid bed, the melt pool, and the flight. By locating several pressure transducers along the length of the barrel, the progress of melting along the length of the extruder can be analyzed.

ABOUT THE AUTHOR: Dr. Chris Rauwendaal is a well-known author, lecturer, researcher, entrepreneur, and consultant in the field of extrusion. He holds numerous patents and has written more than 200 articles and seven books related to extrusion, mixing, injection molding, and statistical process control. A Fellow of the Society of Plastics Engineers (SPE), he is the developer of the CRD, VIP, and ASM mixing technologies that utilize strong elongational flow to improve mixing in extrusion and molding. Rauwendaal also developed the HHT (high heat transfer) extruder screw designed to improve cooling in foam tandem and other extrusion operations. In 1990 he founded and is still president of Rauwendaal Extrusion Engineering. Contact: (530) 269-1082; chris@rauwendaal.com; rauwendaal.com.

A Molder’s Plea: Let’s Standardize Controllers

By: Robert Gattshall 27. September 2016 18:08

When choosing a new press molders must take into account several factors, including training from the supplier. Choosing a machine that your technical staff is familiar with is obviously important, because the learning curve on a new controller can sometimes be significant. But why is that, exactly? When you rent a car you don’t need to spend much time getting familiar with it. P,R,N,D is a standard for any automatic-transmission vehicle, regardless of who makes it. It isn’t so simple when it comes to injection molding machines. There seems to be no standardization from manufacturer to manufacturer with regards to controller layout, icons, or even terminology. (John Bozzelli wrote a great article a few years back that points out the multiple phrases used to describe “cushion” and it is worth a read. See May’12 Injection Molding Know How; short.ptonline.com/Bozzelli.) 

SCRAMBLING THE KEYBOARD
One of my favorite analogies to show how subtle changes in layout and icons can have a big impact is the standard QWERTY computer keyboard, shown in Fig. 1 (p. 54). We are all very familiar with this layout (some of us learned it on typewriters), and many of us are extremely efficient using it, even your hunt-and-peck, two-finger typists.

Imagine if you bought a new laptop, and when you opened it up, you had the layout in Fig. 2 in front of you. Not a big change from what you know, as far as changes go. Just a single row of keys is reversed, although many would point out that the alteration affects the most important row of keys. Frankly, it would take a lot of time to get accustomed to this new layout, and we would make many mistakes trying to figure it out, even those of us who are 60+ words/minute typists on the original QWERTY keyboard. 

Now, what if the change was a bit more complex? What would our learning curve look like with the kind of alteration in Fig. 3? Keep in mind that none of the key symbols has changed, only the order in which they appear. I don’t know about you, but my emails would be a nightmare.

So, I hope we can all agree that most of us would make some mistakes while trying figure out these new layouts, even if they could be easily corrected with a “DELETE” or “BACKSPACE” key.

Well, injection molding machine controllers don’t allow us to fix mistakes with a keystroke. Pressing the mold-close button, because you confused it with the ejector-forward button could cost you a mold. Think about that—something as simple as confusing an icon can cost your company hundreds of thousands of dollars! 

To avoid this, some molders decide to stick with one machine manufacturer across the board. There are pros and cons to this approach, and I think the benefits are obvious, but what about the drawbacks? What keeps your machine manufacturer competitive from both a cost and technology standpoint if it knows it has no competitors for your business? What if your current preferred machine supplier doesn’t offer the technology you need for newly awarded business? There is a very plausible scenario in which you would have no choice but to introduce a new controller into your facility, even if you use a single source for machines. 

Standards for machine controller layouts would ensure that when your single machine source decides to upgrade its controller, the layout, icons, and terminology don’t get scrambled. This is not a “What if?” circumstance—that very sort of scramble has happened on almost every machine on the market. Upgrades are expected—technology keeps changing—but that doesn’t mean that we can’t maintain standards.

This also doesn’t mean that machine manufacturers can’t set themselves apart with additional options or technologies. There will obviously still be unique features that are proprietary to their brand. With today’s technology and the operating systems that the machine manufacturers use, a standard screen layout could be agreed upon and made available. This doesn’t mean that this is the only interface option the manufacturers would offer. It just means that there would be a standard available if the end user wishes to use it. There are machines on the market right now that allow the end user to set up custom screen layouts; why not have a standard layout be one of the options as well? 

There are many examples that demonstrate why a standard layout would be a good idea. There are machine controllers, for example, that depict screen-access buttons for mold close and open, but the action buttons (which actually move the mold) are depicted in the opposite order. Not a big deal, right? They have arrows on them, so they should be perfectly clear to the operator, right?

Not necessarily; especially in the 5S manufacturing world we live in today. How easy would it be to hit that mold-close button thinking you were hitting the mold-open button? Especially considering you might have just pressed the button to access the mold-open screen. The correct layout for both sets of buttons should mimic the movement of the mold from the operator side of the machine. Keeping a controller well organized is important: a place for everything, and everything in its place. 

STANDARDIZED LAYOUT, ICONS & TERMINOLOGY
Screen layout is important, and so are the icons that machine manufacturers use. It’s hard to understand why we don’t have a stan- dard layout, icons, and terminology for basic functions of an injection molding machine. It has been 144 years since John Wesley Hyatt invented the first injection molding machine in 1872, and we still can’t agree on how to label fill time.

That’s not the only term without a consensus. If you look at four different machine manufacturers, you’re going to find four different terms to describe ejectors, fill time, fill speed, cushion, decompression, screw rotation, and so on. Those of us that have been in the industry long enough are more than likely familiar with most of these, but even today I discover new ways machine manufacturers label the basic functions and outputs of an injection machine. How much time and money would the industry save us all if standards were required from machine manufacturers for the basic functions of the machines we run? In an industry where the tooling costs can often reach hundreds of thousands of dollars, standardization could represent a significant savings or cost avoidance. A technician simply confusing an icon or the order of its placement could be catastrophic to a program and your customers.

Once again, this does not hinder the machine manufacturers from standing out and offering options that other machine manufacturers don’t have. It just provides the end user with a consistent platform that would reduce the learning curve on new equipment. It would also ensure that updates or upgrades to controllers retain the standard as well.

Maybe machinery manufacturers are avoiding standards in an attempt to lock a customer into using their equipment to save training time. But machine manufacturers need to stand out from competitors on the basis of their machine performance and customer service. They should set themselves apart with their will- ingness to stay competitive via new technologies, repeatability, and economy. It should not be dependent on the time it will take to train your staff to use their controls.

Remember—our job as processers is to reduce the effects of variation on our process. Variation comes from all angles of an injection molding process: the material, the mold, the machine, and the processor. Standardized machine controls would be one way to reduce the effects of human error and variation coming from the processors themselves.

ANOTHER THING: INJECTION STAGES
For the last several years, machine manufacturers have been touting the number of injection or hold stages they offer. One manufacturer offers the option to set 10 stages, and the next manufacturer outdoes them with 12. I am not going to argue whether or not we should ever be using a 12-stage injection profile; I am just going to ask that the machine manufacturers give me the option to set one stage.

You can offer 12, that’s fine, but let me be able to select a single stage. One through 12 should be my options, not two through 12. It seems like a simple thing, but trust me, requiring two stages can increase the risk of a technician making a mistake. If I need to change my fill time from 2.0 to 1.95 seconds to better center it within my upper and lower control limits, I have to change two setpoints on my machine, rather than one. So in order for me to use a single stage of injection, I have to set two setpoints at the exact same number. I don’t care if you want to “upgrade” to 50 stages, allow the molder to set them from one through 50. 

ABOUT THE AUTHOR: Robert P. Gattshall is currently engineering manager at the Richmond, Mo., Adhesive Technologies operation of German conglomerate Henkel AG. He has worked 20 years in automotive and medical injection molding, including 17 years in process engineering and process development. Certified in John Bozzelli’s Scientific Injection Molding, Gattshall has developed more than 1000 processes using its principles. Contact: 262-909-5648; rgattshall@gmail.com. 

Know Your Mold-Building Terminology

By: Robert Beard, P.E., Honored Fellow SPE 12. August 2013 21:53

For many years I held a seminar called Purchasing & Quoting of Plastic Parts aimed at OEM purchasing and molding personnel.  Attendance has waned during the recession, but the need for knowledge is still there, especially where conformal-cooled molds are concerned. 

I have seen many purchase orders over the years worded only  “Build a 4-cavity mold to produce ABC,” and nothing else. So it continues with conformal-cooled molds:  “Build a 4 cavity conformal cooled mold to produce ABC”.  People who purchase conformal-cooled molds need to understand the technology so that they know how to specify a conformal-mold in a P.O.

So what questions should OEMS or injection molders be asking their moldmaker? My list:

1. Who is designing the mold?  

2. What analyses will be done?  (You’re paying for these; put it in the P.O.)

3. How much experience do they have with each of the software packages?

4. Who is building the conformal inserts and what are they responsible for?

5. Who has the responsibility for the whole mold?

You have to have an understanding of the technology in order to write a good P.O., whether you are an OEM or a molder.

About the Author

Robert A. Beard is president of Robert A. Beard & Associates, Inc. which was formed in 1984. He has more than 40 years of experience in plastics. He presents seminars nationally and internationally entitled: Purchasing & Quoting of Plastic Parts, and Virtual Workshop On Trouble Shooting The Injection Molding Process. He has been elected to the prestigious grade of Fellow and is a Honored Service Member in the Society of Plastics Engineers, and has served as the Chairman of the Fellows Selection Committee. Contact: (262) 658-1778; email: rabeard@plastic-solvers.com; website: plastic-solvers.com

Busting the Conformal Cooling Myths

By: Robert Beard, P.E., Honored Fellow SPE 8. August 2013 16:43

Conformal cooling is opening up new ways of doing things with new tools to solve problems. As cooling lines get smaller and closer to the core and cavity wall, and take a torturous path through the mold, the hydraulic resistance is increasing for each channel. There is a myth, at the floor level, that if the main water inlet to the mold is connected to say a 12-port manifold, that the manifold splits the water into 12 equal parts. This is not true. Hydraulic resistance determines that. The higher the hydraulic resistance is in each channel, the less water that goes into the channel.

In a conformal mold, it is important to measure the flow rate of each cooling line and calculate the Reynolds number to see that it is above 5000 for turbulent flow. This can be done by installing a flow meter with a metering valve on each cooling line on the return manifold so that each cooling line can be manually balanced.

If we continue to do what we have always done, we deserve to get what we’ve always gotten.

For more on the myths of conformal cooling, check out an upcoming new FastTrack training program on September 4th and 5th, near Toledo, OH, sponsored by Plastic Technologies, Inc. (PTI), which will feature two modules— Conformal Cooling for Injection Molding (September 4th) and Medical Plastics Design and Processing (September 5th).

Conformal Cooling Seminar Outline


1.) Understanding heat management

2.) How resin selection affects heat management.
How resins can be modified to cycle faster.

3.) Choosing the right mold metal.

4.) Understanding how Fluid Dynamics impacts Dynamic Heat Transfer.

5.) Alternative cooling technologies to be used with conformal cooling.

6.) Conforming Cooling Technologies, including a European technology
presentation not seen in North America.

7.) Examples why Moldflow and Computational Fluid Dynamics (CFD) are important,
if not necessary, tools for designing conformal cooling channels

 

_______________________________________________________________________________________

About the Author

 

Robert A. Beard is president of Robert A. Beard & Associates, Inc. which was formed in 1984.  He has more than 40 years of experience in plastics. He has been president of the Chicago and Philadelphia sections of the Society of Plastics Engineers, and has served as National Councilman for the Chicago Section.  He presents seminars nationally and internationally entitled: Purchasing & Quoting of Plastic Parts, and Virtual Workshop On Trouble Shooting The Injection Molding Process.  He has been elected to the prestigious grade of Fellow and is a Honored Service Member in the Society of Plastics Engineers, and has served as the Chairman of the Fellows Selection Committee. Contact: (262) 658-1778; email: rabeard@plastic-solvers.com; website: plastic-solvers.com

Calibrate Those Instruments

By: Timothy Womer 28. June 2013 09:37

I was recently asked to visit a sheet processor to determine the cause of a major screw design problem. So, as always, I started at the beginning to gather all of the technical information to determine the root cause. This facility had 5 large extrusion sheet lines, and they were issues with all 5 extruders.

With the extruder at room temperature, I set up three dial indicators on the discharge flange of the barrel in the X,Y and Z axis.  Then I turned on the barrel heaters to the standard zone setting to make sure that the barrel thermally expanded in the Z-axis direct as much as it should theoretically, and that the X and Y indicators move minimally.

 

The simple equation to determine the amount of expansion that a barrel should grow is:

 

ΔL=0.00000633 X ΔT X L

where:

                                             ΔL = The change in length

                                             ΔT = The change in temperature, in this case from room

                                                       temperature to the barrel zone setting

                                                     

                                               L =  The heated length of the barrel

 

Amazingly the barrel grew within about  0.030-in. of the theoretical change in length, which in this case was approximately 0.750 in.

 

Then I measured the flight OD on several of the screws for various designs to determine if there was a consistent wear pattern. There was, so that was noted.

 

Then I gathered all of the process data.  This is a very important part of doing a “CSI” on screws.  This is where you collect the given throughput rate at a given screw speed against the headpressure during that timed rate check, motor load and melt temperature.

 

The motor load reading is taken from ammeter on the control panel; the screw speed is taken from the tachometer.  If at all possible, it is best to have the customer’s plant manager to check the motor load with a hand held meter to verify that the ammeter on the control panel is reading correctly.  As for checking the screw speed, this typically can be done by using a stop watch and counting the rotation of the drive quill at the back of the gearbox.

 

In this case the control panel ammeter was reading correctly, but the screw speed was not.  The customer’s setup sheet showed that their standard setup was to have the extruder operating at 70 rpm, but when I counted the revolutions of the drive quill, I was getting 92 rpm.  This is an error of 24%! 

 

I then checked the tachometer on the line next to the one that I was gathering the process data from and the tachometer on it read 86 rpm but when I did the count, it was only rotating at 70 rpm. This meter was mis-calibrated by 23%!!!

 

So, the moral of the story is, the only thing worse than no data is BAD data.  In this case, the customer immediately had their maintenance people re-calibrate all of their control instruments.

 

NOTE: Sometimes the screw rotation is faster than what a person is able to visually observe. In these cases, I take the advice given to me when I was a kid by an old mechanic mentor of mine (who only had a 4th grade education)...I  “count the clicks.” I had no idea what he meant until he showed me.

Howard took this machinist scale (a pencil or pen will work) and turned on the chuck of the engine lathe in his shop, then took the scale and let it rub against the chuck. On an extruder it can be a small bolt in the back of the rotating drive quill or the drive key on the shank of the screw.  Then with your stopwatch in one hand the “clicker” in the other, you can count the number of times that bolt or key hits the end of the scale, pencil or pen...or the number of clicks.  “Count the clicks.”  Very simple but very effective.

 

Just make sure that your instruments are calibrated on a regular bases and also do a check and balance when gathering data.  Never trust what you think  you see the first time.

__________________________________________________________________________________


Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com
 




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