Tips for Optimizing Twin-Screw Extruder Performance

Transcription

1 Tips for Optimizing Twin-Screw Extruder Performance Bert Elliott, Engineering Manager Leistritz 169 Meister Avenue Somerville, New Jersey Tel: (908)

2 Introduction There are many techniques known to operators and plant engineers for increasing the performance of an extruder. Because of their informal nature, however, most of these methods do not appear in any equipment manuals. Nor are they generally mentioned in textbooks and technical papers. But sometimes attention to several small details can add up to making a big difference in extruder performance. The tips described in this paper fall into three broad categories: A) Processing techniques B) Machine modifications C) Maintenance procedures Tip #1: Install a thermal insulator gasket after the feed barrel The feed barrel in almost all cases is water cooled. But because this barrel is bolted flange-to-flange up against barrel #2, which is heated, heat is continuously transferred from the hotter barrel to the cooler barrel. As a result, barrel #2 is often not able to maintain a high enough temperature. In a typical case, the operator has a setpoint of 180 C for barrel #2, but it can never get above 135 C because of heat loss to the feed barrel. The easiest way to solve this problem is to install an insulator gasket between the flanges of the feed barrel and barrel #2. These gaskets are typically 1mm thick, and it s recommended to stack two together for better insulating performance. There will still be some heat conduction through the bolts, and of course the screw shafts, but not nearly as much as without the insulation. New extruders are often supplied with the insulator in place, but many operators don t realize the importance of it and throw it away when disassembling the barrels. Also, after several years the insulator gasket should be replaced, as the material tends to degrade and crumble. Thermal insulator gasket

3 Tip #2: Set zones 1 and 2 to higher temperatures to decrease wear of plasticating screw elements In most plastic extrusion processes, the first task (after feeding) is to melt the material. This must be done relatively quickly, in a short amount of machine length, to leave adequate extruder length for following process tasks. In order to continuously plasticate cold materials which are introduced to the machine, a large amount of energy must be imparted to the material. This energy can come from only two sources: A) Mechanical energy (friction, shearing, kneading, squeezing, etc) or: B) Heat In a twin-screw extruder, both mechanical energy and heat are acting on the material. But many operators rely too much on mechanical brute force, and don t use heat effectively. Running the process this way causes the extruder to use too much torque just for melting, resulting in less torque available for other process tasks. Also, usually this causes rapid wear of the first kneader group, because it is being forced to do a disproportionate share of the melting work. Many people ask: why do screw elements wear, when the material isn t that abrasive? This can be understood from Newton s 3 rd law, which says that for every force, there is an equal force acting in the opposite direction. So if the kneader lobe is pressing against the cold pellets, the pellets are also pressing against the kneader lobe. Many customers set zones 1,2, and 3 at too low a temperature, which means the energy for melting must come primarily from mechanical work. If these zones are set to higher temperatures, it will lessen the workload on the first kneader group, thus reducing the wear rate. The reason most customers do not set the temperatures higher is because they don t want a high melt temperature. But in fact the melt temperature will not be any higher, as the material is moving through the plasticating zones in a matter of seconds. It is common to have a barrel setpoint of 250 C in order to transfer a lot of energy into the material to cause it to melt, but end up with a melt at 180 C.

4 Plasticating zones 1 thru 3 Tip #3: Side stuffing of powders Side stuffing is widely used with twin-screw extruders for feeding of various fillers. Many processors desire very high % loadings of fillers, many of which are low bulk density (fluffy) materials. The ultimate % which is attainable is usually limited by two parameters: A) Volumetric capacities of the side stuffer and main extruder screws B) Venting capacity, to allow air to escape the extruder The volumetric capacity is based on the free volume geometry of the side stuffer screws, as well as the main extruder screws, and of course the RPM of both sets of screws. Usually if a test is made running the material through only the side stuffer (with the stuffer unbolted from the extruder, discharging into a drum), it will easily feed a high rate of material. But when the stuffer is attached to the extruder, capacity is often limited by the amount of material the main extruder screws can accept. In terms of extruder screw design, it is best to have flighted elements with a long pitch (long flight advance) at the stuffer location, extending 2 to 4D downstream of the stuffer. This is to keep the melt material in the extruder moving rapidly forward, to allow the maximum free volume for the filler to enter. If the screw design causes any dam-up of material downstream of the stuffer, this will severely limit the amount of filler which can be fed.

5 Side stuffing of powders Important factors: Back-venting: The object of venting is to allow air to escape easily, while preventing large amounts of filler being lost out the vent. The best configuration for this is to have a top vent in the barrel immediately upstream of the side stuffing barrel. Sometimes a small ½-slot vent insert can also be used in the top of the side stuffing barrel. Feeder drop height: Ideally the feeder should be positioned as close as possible above the side stuffer, to minimize the drop height. If a fluffy material is allowed to drop through air, it becomes aerated to the point where the bulk density is significantly reduced. This can have a net effect of limiting the throughput rate of the entire line. Feeder agitation type: Make sure the agitator in the filler feeder isn t aerating the material, reducing the bulk density. Many feeder manufacturers have special agitator designs for powders, to preserve bulk density. Make sure stuffer hopper is vented: Along with the filler, the stuffer also introduces a lot of air into the extruder. This air has to come from somewhere. If you have an open top on the side stuffer chute, then venting is already taken care of. If you have a solid cover on the chute with a round stubup and flex connector to the feeder, it s important to also have a vent opening. Ground all hoppers/chutes to drain static electricity: Some materials generate static electricity from friction. If static is present, it can cause powder to cling to the inside surfaces of hoppers and chutes, leading to problems such as caking. If you think this could be happening, an easy fix is to run a ground wire (10 gauge wire is recommended) from the chute to a known good ground on the machine frame.

6 Compressed air blaster: If caking persists, sometimes a special solution is needed. Hopper vibrators can be used, but are tricky to size and mount. Another device which can be used is a blaster. This consists of air jet nozzles strategically placed within the wall of the chute, directed to break up any cakes before they get too large. The air jets get connected to a solenoid valve using poly tubing, and the solenoid valve is actuated by a repeat cycle timer. This way you can set both the blast period and dwell time in between blasts. Just upstream of the solenoid valve it s best to have a small air accumulator tank, to provide a sharp pulse of air. Drop chute with air jet nozzles Tip #4: Insulate melt pipes and dies Very few processors insulate their melt pipe adapters and dies. The reason is because the machine didn t come from the factory with insulation, it takes some extra effort, and it is difficult to see the benefit. But if your control system has temperature trending capability, the benefit can easily be demonstrated. As an example, take a heat zone actual temperature which is cycling up and down all the time. The trend looks like a sine wave. Now if you wrap the melt pipe section with insulation, you ll find the temperature trend is much more like a straight line. The reason is because you ve greatly slowed heat transfer from the melt pipe to the environment, so the temperature stays the same all the time. If the temperature doesn t fall over time from heat loss, the heater contactor doesn t have to come on. If you can prevent the melt pipe from cooling and then needing heat cyclically, the temperature trend is going to be much smoother. An exposed, heated steel part like a melt pipe loses a large quantity of heat to the surrounding environment via radiation and convection. This can easily be demonstrated by standing 12 or so away from a heated melt pipe or die. You can feel heat emanating from the melt pipe, even though you are not touching it.

7 There are some other benefits to insulating melt pipes. Because it slows heat loss, insulation will tend to make all portions of the melt pipe the same temperature. This has the effect of eliminating cold spots from uneven heater coverage. Another benefit is faster initial heat-up times. Another important benefit is that insulation makes the equipment safer for operators. Burns are the most frequent injury around plastics equipment. If an operator accidentally leans against a melt pipe which is wrapped with insulation, he or she will probably not be injured. Melt pipe wrapped with insulation Tip #5: Die lip buildup Die lip buildup is a common problem. With a strand die, lip buildup may be only a minor annoyance, but in film and sheet extrusion it is a major problem. Ways to minimize buildup: Keep die exit and outer face ground smooth. If the die has baked-on layers of old polymers and pigments, this will encourage new material to adhere. Remove the die from the machine, and take it to a machine shop that has a surface grinder. Have all the outer surfaces near the material exit ground. This will remove all the old baked-on material deposits, as well as clean up nicks and gouges in the steel. After grinding it should look like a new steel surface. Coat outer lip surfaces with mold release. Mold release is designed to prevent plastic materials from adhering to steel. It does a good job, but doesn t last long on the surface. Operators have to get in the habit of reapplying it often.

8 Install a vacuum box to suction away vapors. Some materials cause a lot of smoke and solvent vapors to be released at the die exit. If these vapors aren t taken away rapidly, they will leave a sticky residue on the die face, which is the beginning phase of die lip buildup. The way to take vapors away quickly is to position a hood or vacuum ductwork over the die. The suction duct should be positioned as close as possible to the die exit, and should be designed with enough CFM airflow to cause vapors to be vacuumed away quickly. Reduce die/melt temperature. With most materials, high melt temperatures make a buildup problem worse, and cooler temperatures make it better. A cooler melt is generally less sticky and volatile, which causes less buildup. Try various additives. Materials suppliers which sell additives sometimes have a special package to aid in preventing die lip buildup. Stearates have been known to be especially effective. Die lip buildup Die lip buildup Tip #6 Change to a high pressure pump for more efficient barrel cooling In the study of fluid dynamics, it s well known that turbulent flow in a pipe will induce far greater heat transfer from the pipe wall, as compared to laminar flow. Laminar flow is present in the case of low fluid velocities, caused by low pressure delivery. In a laminar flow situation, a stable boundary layer develops, which acts just like an insulator between the main fluid flow and the pipe wall. Layers of fluid slide over adjacent layers, without crossing over each other. What this means is that the water flows all the way through the barrel without removing much heat. With turbulent flow caused by high pressure delivery, there is a high degree of transverse momentum exchange, which breaks up the boundary layer. As a result, the violent fluid motion causes much greater heat transfer from the pipe wall to the fluid.

9 Given the fixed geometry of the extruder s cooling system, the easiest way to induce turbulent flow in the barrel cooling bores is to increase the delivery pressure. There are a wide variety of cooling recirculation systems used for extruders, with supply pressures typically ranging from 20 psi to 60 psi. To achieve turbulent flow, a pressure of approximately 120 psi is desired. This can be attained relatively easily by changing the pump in the recirculation system to a high pressure type. Almost all extruder cooling system components (hoses and valves) are rated for at least 150 psi, so 120 psi still provides some factor of safety. The advantage will be immediately apparent with a process which is highly exothermic. Zone overrides can often be greatly reduced or even eliminated. A side benefit of turbulent flow is that it inhibits fouling of the cooling bores from scale build-up. Laminar and turbulent flow through a pipe Tip #7: Set barrel cooling water temperature at 120 F Many people tend to think because the function of the water is to cool the barrels, the water should be as cold as possible. But to an extruder barrel at 420 F, even water at 180 F is cool, and water at 55 F is absolutely frigid! The problem with running very cold process water is that it can cause overcooling, which frequently shows up as zones cycling up and down, in a sawtooth pattern. The reason: the temperature in the barrel rises normally until the controller calls for cooling, opening the solenoid valve. When the valve opens, even for only a short duration, the cold water removes too much heat. This causes the temperature to plummet below setpoint, which turns on the heater again.

10 For most plastics processes a water temperature of approximately 120 F seems to work well. With a higher water temperature the cooling rate will be slower, and less likely to overcool. The result will be smoother temperature control. Heat zone trend chart showing overcooling Tip #8: Acid flushing of barrel cooling bores Water-cooled extruder barrels are heat exchangers, and like all other heat exchangers the coolant bores are subject to fouling from scale buildup. Most operators notice that the cooling performance of a new extruder is much better than an extruder after 3 or 4 years of running. This is because the new extruder barrel has smooth, shiny fresh-drilled cooling bores. The older machine has a layer of crusty mineral deposits lining the bores, that effectively act like an insulator. If left unchecked, scale build-up can lead to a much more serious problem than reduced cooling. Eventually the cooling bores can become completely blocked, and zero water flow means a lack of cooling ability. If this happens, the only solution is to remove the barrels from the extruder and drill out all the cooling bores a time consuming procedure.

11 Barrel having clogged cooling bores drilled out There is a simple preventive maintenance procedure to avoid these problems and keep the extruder cooling efficiency like new. The recommended procedure is to circulate a scale removal chemical through the barrels periodically. With production extruders, this is recommended to be done every 3 to 4 months. On a lab extruder seeing much less use, probably once every 2 years is sufficient. Many of the tower and chiller manufacturers make a small, inexpensive recirculating de-scaler unit on a roll-around cart, with an integral reservoir and pump. This unit is designed specifically for flushing out mold cooling passages, and is also ideal for extruder barrels. Tip #9: Laser barrel alignment In the old days, extruder barrels were aligned simply with a machinst s level. Or, as in the case of single screw extruders, an optical borescope. But unlike a single screw machine, a twin-screw extruder doesn t have an open bore through the gearbox, so a borescope can t be used. The most accurate way to align a twin-screw extruder is with a laser. The laser is mounted concentrically to one of the gearbox output shafts in a holding collar, and the laser light is projected all the way down the barrel to a target mounted on the discharge flange. The end target is made from clear Lexan, and has vertical and horizontal crosshairs etched into it. When the extruder is aligned, the laser dot will appear in the exact intersection of the crosshairs. If the dot is not in the center, the barrel supports are adjusted to bring it into alignment. Extruders don t need to be checked for alignment on a periodic basis, but there are some specific situations when it s advisable:

12 If the extruder has been moved to a new location If the concrete floor is settling, causing uneven support of the extruder If screws or barrels are wearing at an abnormally high rate, or wearing in an abnormal pattern If shafts keep breaking for unexplained reasons If the machine vibrates more than normal If the screws are difficult to slide in/out of couplings If somebody changed the barrel support adjusters If the machine has been hit with a forklift (yes, it happens!) Tip #10: Switch to synthetic gear oil Barrel laser alignment The gearbox is the heart of a twin-screw extruder. If it fails, it s likely to be expensive to repair, and it may take quite awhile. There is one thing everyone can do, easily, to avoid gearbox problems: switch to synthetic gear oil. Synthetic oil is a huge advancement in lubrication technology, offering the following benefits: Synthetic oil is slipperyer, causing less friction Gears, bearings, and seals last longer Gearbox runs cooler and quieter Synthetic oil doesn t lose viscosity from mechanical shearing Maintains higher viscosity at high temperatures Improves overall efficiency of the gearbox

13 Reduction gearbox with damaged gears Tip #11: Anti-Seize Most people in the plastics industry are aware of the need to use anti-seize. There are several different types of anti-seize, Copper-based, Moly, Lithium, etc. For something like a bolt thread probably any type will work. But there is one application within the extruder where the choice of anti-seize is much more critical: the spline shafts. Screw elements fit onto the spline shafts with a very small gap tolerance, plus low viscosity polymers tend to creep down between adjacent elements and get into this gap. With time and heat, this polymer forms a high strength adhesive just like an epoxy. If screws are left together in an extruder without being taken apart periodically, they can be extremely difficult to remove from the shafts. This is why the choice of anti-seize is so important. Many types of anti-seize are acceptable for a short period of time, but over long periods at high temperature, they degrade into a fine powder. This makes them a poor choice for spline shafts, where you need the anti-seize to perform when you re ready to disassemble the screws, months or years after applying the antiseize. After testing all the anti-seize brands on the market over a period of many years, Leistritz has found one which is ideal for this application: Fuchs Gleitmo 820, manufactured by Fuchs Lubricants. Gleitmo 820 is a white grease containing high solids, with very slippery qualities. It s rated for 1,150 C. What makes it perfect for spline shafts is that it doesn t degrade into a powder. When the screw elements are removed after months of use, the Gleitmo 820 is still a slippery grease.

14 Fuchs Lubricants is a German-based company, but they have distributors in the U.S. and a website with technical information. Tip #12: Purging techniques Anti-seize on spline shafts Purging is a technique widely used as an easy way to clean out materials from the extruder and die, to avoid a time-consuming stripdown cleaning There are many types of materials used for purging, with both commercial purge products and home engineered purges being used extensively. Purge materials generally fall into two categories: chemical purges, and mechanical abrasive purges. Many materials combine both chemical and abrasive actions. Chemical purges work by attacking the plastic residue like a solvent. Many of them also incorporate a foaming action, and are intended to be run to fill the machine, and then the extruder drive is shut off for 5 minutes, to allow the chemical action to work. The the drive is then restarted to run the purge out of the front end. Mechanical abrasive purges work by scouring and scrubbing material off the metal surfaces, using friction and shear. Most processors do a lot of trial and error experimentation to find the right purge material and technique. Since there is such a wide variety of plastic and filler materials run, no magic purge material is going to work best for all materials. Cost is also a factor. Commercial purge products are generally more expensive per pound than home engineered purges. But of course if you can use less material with the commercial purge, or get the job done quicker, maybe the total cost is actually less.

15 Probably the most important habit to get into is to purge often. The object is to prevent old materials from being baked on to the metal surfaces. Once this happens, purging will generally not be able to remove the material, and a manual stripdown cleaning will be the only remedy. One mistake many processors make is to run the purge only at a fixed screw RPM. It s much more effective to vary the screw RPM, alternating from low to medium to high RPM s, running for perhaps 30 seconds at each speed. This induces different shear rates against the metal surfaces, helping to dislodge old material from the walls. Screw set being pulled from extruder after purging Tip #13: Home-brewed extruder/die cleanout methods Processors have run all sorts of materials through extruders, in the quest for a cheap and effective purge material. For instance, in the Midwest USA many operators use cow corn, because it s readily available and does a fairly good job of scouring the machine out. There are two of these homemade purges which are particularly good: Mix #1: 50% poly, 50% Diatomaceous Earth Diatomaceous Earth is actually finely milled fossilized shells of tiny sea organisms. It is used in gardening among other things, so it is readily available in garden supply stores. Diatomaceous Earth looks like a fine powder, but on a microscopic scale the particles have very sharp, angular edges, which produces a scouring action. Processors who run sheet and film use this purge, as it can sometimes get rid of die lines, saving a time-consuming die teardown. Mix #2: Poly with Kitchen Cleanser and water

16 Mix a slurry of water and 3 or 4 cans of Comet or Ajax kitchen cleanser in a bucket. Feed poly at a normal purging rate and temperatures, and slowly pour the slurry into the main feed port. (Safety note: Warn any operators nearby that steam may be discharged from extruder openings. Do not look into die or vents unless wearing a full polycarbonate face shield and long sleeved shirt) The water in the slurry will flash to steam, which gives a very good cleaning effect combined with the abrasive in the kitchen cleanser. Tip #14: Efficient color/product changeovers Color or product changeovers tend to create problems. Extrusion people would ideally like to always have nice long runs. But in the real world, demands from customers mean frequent changeovers to make 80 lbs. of Product A, followed by 200 lbs. of Product B, etc. So it is important to approach changeovers in a way to minimize operator effort and problems. Know how critical cleanliness is to the next product to be run. There are anecdotes around any extrusion shop of somebody purging for 16 hours, to avoid pulling the screws to brush them off. The point is, if you know the next product requires a thorough manual brush-cleaning, don t waste time and material trying to get by with purging. If possible, plan runs in order of decreasing cleanliness requirements. In terms of colors, this means from lightest to darkest. This should make it so you only face one difficult changeover, when going from the dark color back to a light color. Have written purging or strip-down procedures to run each product. This helps operators plan their time, and eliminates discussions later on about X lbs. of product has to be scrapped because the operator didn t clean the machine properly. Use the right size extruder for the lot size. This may seem obvious, but many processors don t plan runs for the right machine, and end up running a 500 lb. lot on a 75mm extruder, producing 300 lbs. of scrap while getting it to run correctly. Help operators by organizing the necessary tools, cleaning supplies, screens, die plates, etc. out near the extruder. If frequent changeovers are required, set up a workstation to have everything the operator needs right by his side. Study raw material/end product flow in and out of the process area. Try to arrange it to minimize operator effort. The less operators have to hoist drums and gaylords around the more they can concentrate on maximizing running time. Tip #15: Warm-up/cool down procedures to avoid degradation Warm-up: Extrusion people tend to keep the machinery hot, all the time. Operators typically turn the heat zones on the minute they come in the plant at 7:00am. If you re really going to start the extruder at 8:00am, there s nothing wrong with this. But

17 many times people get distracted with maintenance chores, and the extruder sits idle, heated to full operating temperature for hours at a time. The reason this is a bad practice, is because it allows polymer to bake onto the screws and barrels. Once baked on, the material will continue to degrade and carbonize. Purging will not remove this material. Even worse, as the extruder is running product, black specks will periodically flake off the screws and barrels, contaminating the product and causing customer complaints. The best way to handle warm-up is to plan the time you really want to start running the extruder. Then start heating up only as far ahead of this time to allow for complete warm-up plus adequate soak time. Cooldown: There are preferred shutdown procedures also, to avoid problems with black specks. Before shutting the extruder down, it s good practice to fill the extruder and die to the maximum degree of fill with some inert polymer such as HDPE, to seal the machine. The HDPE displaces oxygen, and coats the metal surfaces to greatly slow down oxidation and formation of carbon. After the extruder has been sealed, it is better to crash cool the machine, rather than let it cool from ambient air over the next 18 hours. A crash cool involves turning all the temperature setpoints down to zero, to force the cooling solenoid valves to open. With the coolant pump running, this will force cool water through all the barrels, bringing the temperature down quickly. A quick cooldown also doesn t give the polymer a chance to degrade and carbonize. Screw set showing baked on degraded polymer

18 Tip #16: Key process indicators Most extruder operators tend to be like the old time pilots, preferring to fly by the seat of their pants. And many of them have excellent instincts for knowing when the process is running right, and when it is not. But as products get to be more complex, with tighter processing windows, it is much better to have some kind of quantitative way of assessing how the machine is running. A common example of this is when the operator is convinced something is different about the material. The line is just not running the same. If the material supplier is contacted, they re probably going to say their QC records show that the material is the same as its always been. Without some real numbers, how can you prove to the material supplier, as well as yourself, that the material is indeed different? Indicator #1: Specific Throughput: kg/hr. 4RPM = kg. per hour per RPM Specific Throughput gives you a number which is proportional to the degree of fill. Degree of fill is useful especially if records are kept over a period of time for many different products, as it can help you predict how best to run a new product. It also helps you plan machine hours, as a product which needs to run at a low degree of fill will take longer to run a certain lot size. Finally, degree of fill is helpful in scaling up (or down) runs on different size extruders. Indicator #2: Specific Energy This has to be calculated in 2 steps: Kw (applied) = Kw (motor rating) X % torque X RPM running X 0.97 Max. RPM Specific Energy = Kw (applied) Kg. per hour Specific Energy defines how much power is being expended to process each Kg. of material. It is a measure of how much mechanical work is being performed on the material. Again, it is useful to keep records of this figure for products over a long period of time. After awhile, operators will get to know which products are energy hogs, and which aren t. This will help in planning runs for new products, and estimating the production capacity of any given line. Specific Energy is also very helpful in pinpointing problems. If a product is known to always run with a S.E. around 0.25, and one day it s only running 0.16, if all other conditions are the same this would be a reason to suspect the material.

19 Tip #17: Finding the Optimum Degree of Fill Most products will run acceptably on a twin-screw extruder under a wide variety of conditions. For instance, on a 50mm extruder, a given formulation might be able to run at 400 lbs./hour using any RPM from 200 to 600. To the average operator, there may seem little or no difference in the mixing or material quality. But from theory we know there has to be a difference, because the shear imparted to the material has to be different from low to high degrees of fill. The way to answer this question is by running a test grid, which is a basic SPC (Statistical Process Control) technique. The grid should be set up to test combinations of low, medium, and high values for RPM and throughput rate, within a realistic range. The grid also needs to have columns for key physical properties and quality parameters. For example: RPM Rate Tensile strength Impact Color Surface Degree of fill Spec. energy The chart clearly shows the best combination of properties (in bold), running 500 lbs./hour at 400 RPM. But if nobody takes the time to do the tests, the product could be run in production for years under conditions which produce less than optimum properties. Summary: An extruder seems like a very simple machine at first glance, but there are many interactions of components and sub-systems involved. If the operator has an awareness of how these details affect the overall process performance, he or she will have the capability to tune the factors to optimize the process.

20 References: 1. David B. Todd, Plastics Compounding Equipment and Processing. Hanser, New York, W.F. Stoecker, Design of Thermal Systems. McGraw-Hill, New York, Charlie Martin, Extrusion Technology and Troubleshooting. Chapter 2, Society of Plastics Engineers, Brookfield, CT Bert Elliott, Guidelines for Installation, Operation, and Maintenance of Twinscrew extruders. Leistritz, Somerville, NJ Victor L. Streeter and E. Benjamin Wylie, Fluid Mechanics. McGraw-Hill, New York, 1975.