"Foam Extrusion In The Lab"
Foam Extrusion In The Lab by Keith Luker Randcastle Extrusion Systems, Inc. 74 Sand Park Rd. Cedar Grove, NJ 07009 1 I) Purpose: This purpose of this paper is to describe the typical lab extrusion equipment available for the various foaming processes. It will concentrate on the less well known aspects of flexibility available in the lab process for the researcher. II) Introduction: Laboratory extrusion equipment mirrors the production line in many respects. That is, the basic foaming processes used in production are duplicated in the lab. For example, various screw designs are often scaled down from production. Screws are available in an infinite variety of designs. They comprise various lengths for each of the individual sections of the screw (feed, compression, metering, and mixing); they are available in all varieties of compression ratios and with any number of mixing sections (including Dulmadge, carbide, pin, Saxon, and many others). All the various static, and active, mixers are applicable to the lab environment. Fitting particular mixers into a lab tool is only a matter of simple engineering and available funds. However, unlike the production environment, laboratory extrusion lines are not necessarily set up to do just one job. The nature of the lab environment requires more flexibility in the machinery than in production. By definition, you cannot know where lab experiments will lead. After all, you cannot know what the experimental results will be before you do the experiment. And so, knowing exactly what machinery is required is also impossible. As experiments progress, the machinery must be adaptable to the processing requirements. In turn, this means that the machinery must be initially designed for flexibility. It must be built so that it can be changed to suit unknown future constraints. The cost of such flexibility must be considered. If cost of this ‘designed in flexibility’ is greater than the cost of a new line, then there is no advantage. But, if the cost is simply to add components required by the nature of the new material to be processed, then the system is properly ‘flexible.’ III) Solids Conveying: Foam extrusion in the lab often starts with a single screw extruder. The first part of the screw is the solids conveying section of the screw and comprises the hopper, barrel feed section, and the screw. The purpose of this part of the extruder is to get the material from the hopper and into the screw channel. If the friction is sufficient, the material will be conveyed into the next part of the screw. Many extrusion problems—especially in the lab—are a function of the solids conveying zone. Unfortunately, we have not simple model to guide us. Solids conveying is not well understood. This area is still in the realm of the experimentalist rather than the theoretician. So, the tools that are available to enhance solids conveying become very important. Otherwise, the researcher will have nothing to experiment with. 2 In production, there are usually two different types of feed sections of the barrel: Smooth bore and grooved bore. Smooth bore feed sections are typically used with compressive type screws in the 2:1 to 5:1 apparent compression ratio (the channel depth of the feed divided by the channel depth in the meter) range. Screws used with grooved bore barrels tend to be less than 1.5:1 and can be fractional. Both types of barrel feed sections are available in the lab and all types of screws are available. However, in the lab, the feed stock may or may not be conventional pelletized feed stocks as in production. Many times, experimental materials are not available in free flowing regular geometry pellets. The feed stock may be combinations of granulations, powders, pellets or even strips. Even ‘pelletized’ lab feed stocks may have rough surfaces and be far more irregular than is likely in a production line. Therefore, the lab extruder must be capable of handling all sorts of feed stocks. The feeding requirements demand a laboratory tool with better general feeding than is available in production. This has lead to the design of the discharge driven extruder. The problem with simply scaling down a typical horizontal production extruder for the lab is that it loses flexibility. As the extruder sizes shrink to lab scale, the feed hole of the barrel becomes so small that many materials do not flow through it. Instead they arch or bridge over the opening. While this can secure the jobs of lab technicians constantly employed with ‘strirrer rods’ it is not efficient and it is dangerous. The discharge driven extruder screw is typically vertical. The screw usually extends through the barrel feed section and into the hopper. This greatly reduces arching in all but the most severe cases. As the material is put into the hopper, it falls into the screw channel without having to pass through an unnecessary hole. In extreme cases, such as feeding very difficult feed stocks (such as 100 percent granulation’s), stirrer rods of various designs are easily attached to the rotating screw. It is worth noting that this design is an inherently better feeder. A conventional production screw often feeds by means of archimedian transport. 1 However, in the discharge driven design, the screw acts more like a crammer feeder since the screw’s helix constantly pushes forward. This is why it is an inherently better feeder. This constant push is particularly helpful in keeping chemical blowing agents moving down the screw rather than letting the gas escape out the hopper. Lab extruders have developed several different smooth bore feed sections to further enhance feeding. Again, this arena belongs to the experimentalist. We can say that differently shaped feed sections have evolved and are known to improve feeding. Yet, we do not have a predictive model of what will work. 1 Polymer Extrusion, Rauwendaal, page.... 3 There are four common smooth bore feed 2 sections available as well as grooved bore feed sections available in the lab. However, since the development of smooth bore feed is unique to the lab, we will concentrate on their use and application. Discharge driven extruders allow the quick interchange of the various feed sections. Removal of just four screws allows the exchange in less than a minute. In a scaled down production extruder, the screw would first have to be cleaned and removed, then the hot barrel removed, and finally the exchange would be made. Including reassembly, this is not likely to take less than an hour. Experimental data showing the interplay of feed section to material is shown below. Pressure stability is used as a common reference to determine the consistency of feeding. These results were obtained from a 5/8 inch Randcastle 24:1 extruder. Specific smooth bore barrel feed section designs are proprietary but will be referred to here as "Standard, Classic, and Aggressive." A) HDPE: The first material that was processed was HDPE from Federal Plastics #F15896. This was an underwater cut pellet. Barrel conditions for all trials were zone one 360F, Zone two 370F, Zone three 380F, and the die at 380F. The extruder used was a standard Randcastle 24/1 working L/D 5/8 inch Microtruder. The results were: HDPE 1600 Standard 1400 Classic Aggressive 1200 Pressure 1000 Variation In 800 Pounds Per 600 Square Inch 400 200 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM The "Standard" feed section was not stable but that both the "Classic" and "Aggressive" smooth bore feed sections produced very good average fluctuations during the test. Averaged pressure fluctuations for the "Classic" feed section were plus or minus 23 PSI and for the "Aggressive" feed section plus or minus 22 pounds. Average output for the "Classic" barrel feed section was 0.30 grams per revolution while the "Aggressive" feed section yielded 0.32 grams per revolution. 2 Since the roller feed sections pertain only to strip feeds such as silicon and speciality created lab materials, we will not discuss this feed section further at this time. 4 The output for the HDPE was: HDPE 35 Standard 30 Classic Aggressive 25 Output In 20 Grams Per 15 Minute 10 5 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM The output from both the "Classic" and "Aggressive" feed throats are much higher than the "Standard" feed throat and both produced stable pressures. This implies that the "Standard" feed throat supplied too little material to the metering section and it was consequently starved and surged. B) LLDPE: LLDPE was processed at barrel zone temperatures of 385, 390, 400, and 400 F from the feed to the die. The pressure variation follows: LLDPE 200 Standard Pressure Classic Variation 150 Aggressive In 100 Pounds Per 50 Square 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM The output graph for the LLDPE follows: 5 LLDPE 35 Standard 30 Classic Aggressive 25 Output In 20 Grams Per 15 Minute 10 5 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM As in the case of the HDPE, the output is consistently higher when changing from the "Standard" to the "Classic" to the "Aggressive" feed throats. Unlike the HDPE trial, the output fluctuation for the "Classic" feed throat is probably not because the metering section is starved. After all, the average output values are lower for the "Standard" feed throat (compared to the "Classic" feed throat) but higher for the "Aggressive" feed throat. It seems more likely that some other aspect of the process is causing the instability. C) LDPE: Federal LDPE #Nat:F13600 was processed at temperatures starting at the hopper and moving progressively down the die from 300, 325, 350 and 350F. The output pressures were: LDPE Standard Classic 90 Aggressive 80 70 Pressure 60 Variation In 50 Pounds Per 40 Square Inch 30 20 10 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM The pressure for the LDPE tested was: 6 LDPE 45 Standard 40 Classic 35 Aggressive 30 Output In 25 Grams Per 20 Minute 15 10 5 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM Apparently, in this case, the "Classic" feed section fed much better than either of the other two feed sections. Apparently, it fed too well and as a result overwhelmed (at this set of process conditions) the screw's metering section making the pressure unstable. Additional evidence may be seen in the motor amps shown below: LDPE 5 Standard 4.5 Classic 4 Aggressive 3.5 DC Motor 3 Amps 2.5 2 1.5 1 0.5 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM D) Flexible PVC: The last material tested was flexible PVC from Federal Plastics. It was a clear flexible underwater cut feed stock #F15763 and was processed with profile of 350 at the hopper, 345 at zone 2, 340 in zone 3 and 335 at the die. Pressure stability was: 7 FLEXIBLE PVC Standard 80 Classic 70 Aggressive 60 Pressure 50 Variation In 40 Pounds Per 30 Square Inch 20 10 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM In this case, the "Standard" feed section seems to have performed most reliably. The output was: FLEXIBLE PVC 70 Standard 60 Classic Aggressive 50 Output In 40 Grams Per 30 Minute 20 10 0 20 RPM 40 RPM 60 RPM 80 RPM 100 RPM All these outputs seem strikingly high compared to the previous trials even given flexible PVC's high specific gravity. E) Conclusions: It’s obvious that pressure stability is important to foaming in the lab. Clearly some feed sections work well and some do not for specific materials. As it is not perfectly clear what material will work in what barrel feed section, the lab researcher should make sure various tools are available. IV) Screw Strength: Conventional production extruders can be scaled down to only about 1 inch screw diameter before screw strength becomes a limiting factor. In a typical production extruder, the entire load of the extruder is transmitted through the root diameter of the screw under the hopper. This is usually where the root diameter of the screw is smallest and consequently weakest. 8 The formulae for allowable stress for main power-transmitting shafts (using 4,000 pounds per square inch) can be used as a simple approximation of the screw root diameter: D3 N P 80 where: P= Power transmitted in horsepower D= Diameter of the shaft in inches N= Angular velocity of shaft in revolutions per minute Using a three-quarter inch diameter screw having a channel depth of 0.180 inches as an example, the root diameter of the solid conveying region would be about 0.390 inches. At 80 revolutions per minute, 0.390 x 0.030 x 0.390= 0.059. In a discharge driven design, the entire load of the extruder is transmitted through the metering section root diameter. Typically, this root diameter is significantly bigger. Using a typical 3:1 apparent compression ratio for the meter channel depth and the same feed channel depth of 0.180 inches, the meter channel depth would be 0.060 inches. The root diameter for the meter would then be about 0.63 inches. At 80 revolutions per minute, 0.63 3 equals 0.25. Dividing, 0.25/0.059 = 4.23. So, the same screw driven from the discharge end of the screw is about four times stronger than a conventional screw. This is not completely correct of course since some of the load is transmitted through the tapering root diameter of the melting zone. Nevertheless, discharge driven screws are substantially stronger than conventionally driven screws. In practice, discharge driven extruders are now built as small as 0.25 inch in screw diameter and 0.500 inch diameter for conventional 1/8 inch pellets. Thus lab extruders for foam can be made much smaller than in scaled down production extruders. This affects several key issues with respect to foamed lab extrusion lines: A) Output: Since the output of an extruder is a function of the square of the diameter, the smaller lab extruders consume much less material. Experiments are routinely carried out with grams—rather than pounds—of feed stocks. B) Size: Because of the vertical discharge driven design of lab extruder, the size of the entire extrusion line can be minimized. Given the cost and availability of lab space, this becomes an important consideration. Designs are often table top even for tandem lines. Consider a tandem foaming lab line for conventional pelletized feed stock. The primary extruder for gas injection can be a 1/2 inch extruder and a second ‘larger’ 3/4 inch extruder can be used for cooling. This tandem system will fit on a 30 inch bench. 9 C) Metering Section Water Cooling: Foam applications often require the removal of heat to lower the extrudate temperatures. In scaled down conventional extruders, it becomes rather difficult to bore the metering section for cooling as the piping must come through the small diameter root of the solids conveying section. However, in discharge driven designs, this is not the case. The metering section can be cooled because of the larger diameters. V) Types of Lab Foaming Processes: Lab foaming can be done with chemical blowing agents, liquid or direct gas injection. Lines using chemical blowing agents usually have one extruder and the other’s multiple extruders. Lab coextrusion lines are also available for multiple layer work. For flat lines making higher density foams, lines up to seven layers are considered routine and additional layers are available if necessary. The forward engineering allows the lab extruders to be added to the first extruder as necessary using modular feed block elements. Lines for coextrusion rod (usually a coating over a foamed core) are available using two to three extruders . Tandem lines are made with up to four extruders though more are possible. This allows the researcher to combine mixing applications with foaming applications. Single screw extruders are excellent distributive mixers. Tandem applications also allow downstream solids addition to prevent degradation and they enhance mixing. This occurs without the problems associated with vent flooding of one piece two and three stage screws. Of course, the are also used as cooling extruders. Tandem lab lines are forward engineered so that extruders can be added as necessary to meet the on going needs of the researcher. VI) Conclusions: Foaming in the lab is more, not less, demanding than in production owing to the nature of research. Lab extrusion equipment has been specifically designed with the researcher in mind. Substantial tools—with built in flexibility for the future—are available to speed your research. 10