Evaluating the root causes of rubber molding defects through virtual molding

Transcription

1 Evaluating the root causes of rubber molding defects through virtual molding by Matt Proske and Harshal Bhogesra, Sigma Plastic Services Injection (and transfer) molding of elastomers is a complex operation. It may not seem very complicated, but when you look into the details, it really is. The combination of material, process, mold design and molding machine capabilities is among the primary ingredients for this stew of sorts. Every new mold, however, is like making a new stew; it might not turn out the way you planned. Then there is a big pot of bad stew that nobody wants, customers that are hungry, and you are back to the drawing board to figure out what did not work out as planned and how to fix it. Was it the carrots or the store I bought them from? Was it the temperature or the time? Let us face it, we have been making stew for 4,000 years and rubber parts for a little more than 100. We do not have 4,000 years to develop recipes for every possible combination of molded product. In this article, we explore a different way to evaluate molding problems to help unlock some of the mystery behind these secret recipes for quality molded elastomers. Virtual molding is a unique approach to molding simulation technology which combines the most relevant molding aspects so they can interact in a simulation together as they do in live production., time and shear rate dependent elastomer material properties, thermo-physical mold, insert and insulation properties, electrical heaters, wattage and thermocouple location(s) comprise the complete injection molding process. Fill speed, pressure limit, melt temperature, thermocouple set point temperatures, mold opening and closing times, and post curing processes are all critical pieces of information used by the simulation for the virtual molding production. Combining all of the inputs in one model allows for a comprehensive evaluation of potential production issues. The following questions are answered in this article: What is virtual molding? How does it work? What inputs are required? What do the outputs (results) look like? Which problems can be evaluated? The first important question is: Did the simulation results come from a part design evaluation (emulation), or did they come from a virtual molding simulation? Readers beware: A part design evaluation typically only consists of some basic material properties, a part geometry and a gate location. The validity of the results based on this approach in a production environment is skeptical at best. There is just not enough information for the results to reflect what happens in a molding machine. Virtual molding, on the other hand, is a fully coupled 3D heat and fluid flow program capable of simulating a real production trial, including how the mold is preheated and what process is used for the first 5, 10 or 100 production cycles. It requires runner geometry, mold components, BOM (bill of materials), electrical heater wattages, thermocouple locations, PI (proportional integral) controller or on/off settings, type and thickness of insulation plates, platen heater and the molding process, as previously described. Virtual molding will automatically calculate heat flow through all the mold components, and the heat lost from the cavities due to radiation, while the mold is open. It will also show why the mold will need to be shut down after 50 cycles because it is losing too much heat from insufficient heaters and insulation. This does not mean part design evaluations are not possible, or that they are not valuable. It simply means that in order to replicate what happens in production, more information is required. In order to know how much pressure is required to fill the cavity in a controlled way, temperature and shear rate dependent viscosity information, actual local mold temperature information, and a realistic 3D representation of the mold, part and runner are all required. The difference here is that virtual molding software is developed specifically for this approach, and traditional flow simulation software is not. For example, if a mold has two different mold inserts, one made of P20 steel and another made of MoldMax HH (ref. 1), they will both absorb and dissipate heat at different rates, which is calculated during the virtual molding simulation, as are the effects on the curing reaction. Not only one molding cycle, but multiple molding cycles, are calculated to match the real world molding. This distinction is very important. A virtual molding analysis either uses existing CAD for the mold/parts/runner, or they can be created inside the software if they are not available. Typically,.step assembly files are used, but other file types such as.stl or.sat are also possible, as are some native formats. Once CAD is imported, the entire assembly is automatically converted into millions of calculation points for the multi-physics software to compute the continuously changing environment. This process is often referred to as meshing (ref. 2) (figure 1). These millions of calculation points are populated throughout all the components of the mold -318 Figure 1 - geometry of the mold before meshing (l), and after meshing (r) (mm) (mm) Part Gate Movable mold Ejector pin 20 RUBBERWORLD.COM 12RW - 20,21,22,23,24,25,26.indd 20 3:36:13 PM

2 in seconds. Each calculation point is contained by an element which contains initial information, such as specific volume, material type and starting temperature (depending on what geometric domain it is part of). Prior to starting the simulation, the molding process must be described. It is broken into two distinct sections, including preheating and production. The preheating phase starts the mold at room temperature and activates the electrical heaters based on the thermocouple locations. Each heating zone is controlled independently. Once the integrated PI controller finds a solution for how much wattage each heater will require to maintain the set point temperature, the simulation can proceed to the production phase. This phase consists of previously described process information, as well as any post molding curing process information (if applicable). The same molding process can be repeated over and over again, or the production cycles can be interrupted, the process changed and further production cycles calculated. These two steps assure that the thermal gradient of the mold is accurately established. If there is a problem getting the mold up to the desired temperature or keeping it there, the heaters will run at full power and the mold temperature will decline, just as it can in reality. The result will be a mold running cold and uncured parts when the mold opens. It is also possible to prescribe the best locations for thermocouples and to identify where higher or lower wattage heaters are needed. Different thickness insulation plates or blankets can also be evaluated. It is clear that mold temperature is not only critical for curing, but also for filling. Therefore, the questions arise: Is the mold temperature under control, or does it just exist and we find a way to make production despite this lack of control? Once the mold is up to temperature, the cold rubber flows into it. The rubber temperature rises due to its interaction with the higher temperature mold and shear heating. Shear heating is a phenomenon where internal friction within the rubber, while it is flowing, creates heat and locally reduces the melt viscosity. This can affect how much pressure is required to fill the runner and cavity, and can create unexpected filling patterns. If the rubber temperature rises too much, either due to shear heating or mold temperature, it can begin to cure during cavity filling. This creates problems with mechanical strength because crosslinking progressed too far for the material to bond sufficiently with other merging melt fronts. This is typically referred to as scorch. If the rubber cures too much during filling, the viscosity will rise and some areas may not fill properly. This can also be related to trapped air in the cavity and poor venting. Trapped air can produce bubbles, burn marks, non-fill or poor mechanical properties. Venting must be properly located and properly sized. When air is trapped, the air volume decreases due to encroaching higher pressure rubber. As the air pocket volume decreases, its temperature rises. If too much air was trapped, the air pressure will become so high that the air temperature will rise above the burning temperature of the rubber, and voila, a burn mark is created. Filling pattern and melt pressure are critical to a robust process. We must have enough machine pressure to fill the cavity, but enough clamp force to keep the mold closed at the same time. If not, the fill pressure must be reduced so the mold does not flash (as much). In some cases, the filling is not balanced in the cavity, which means certain areas fill much earlier than others. When this happens, the local pressure of the early filled areas goes too high and the mold flashes anyway. This typically results in a process which requires the machine to slow the injection speed as the cavity fills to avoid generating this high local pressure. This can make it difficult to fill the other areas of the cavity. Accurate mold temperature plays a significant role in the ability to properly understand what will happen. Will the rest of the part fill and without scorch? This is why virtual molding considers the complete mold and multiple consecutive molding cycles. The mold temperature is always changing, and mold temperature during the first cycle is quite different from the mold temperature after the 100th cycle. Also, if the mold cavity temperatures are evaluated at the 100th cycle, it will not be the same everywhere. Certain areas will be cold at the same time other areas are hot. So in every cycle, heat is exchanged between the mold, parts, heaters and other components. All of these heat flow calculations are occurring simultaneously, and these hot and cold spots inside the mold will lead to uneven or uncontrolled curing of the rubber part. Some areas will cure faster, while other areas will cure slower, depending on the local temperature of the mold surface in contact with each area of the part. The late areas of curing will increase the cycle time, reduce the quality of the parts or even require a post molding curing process to cure completely. As one can see, injection molding elastomers is actually a very complex process. While the virtual molding simulation is in process, various graphs like mold temperature versus time, or heater power versus time, are available (figures 2a and 2b). This information (C) Power (watts) Figure 2a - mold temperature versus time Figure 2b - heater power versus time 2,000 1,600 1, ,000 2,000 4,000 4,000 6,000 Relative power 6,000 8,000 Time (seconds) 8,000 Time (seconds) 10,000 10,000 12,000 12,000 14,000 14,000 FOLLOW US ON 21 12RW - 20,21,22,23,24,25,26.indd 21 3:36:13 PM

3 Figure 3 - jetting inside part cavities ShinEtsu optical lense Filling, temperature 1.028s, 13.01% provides important insights to process engineers about potential problems related to temperature control of the mold. Knowing if there are enough heaters, proper thermocouple locations or appropriate insulation for the mold is a powerful position to be in before the mold is even built. Once the mold is brought up to temperature, the production cycles begin. If the mold continues to lose heat during the production cycles, even when all the heaters are operating at maximum power, it is a clear sign the mold needs more (or higher wattage) heaters or insulation. From these graphs, we can clearly visualize this would be a production issue. Solving this issue in a real mold is an expensive ordeal. A new heating system must be designed in an existing mold. Retrofitting molds like this is never optimum because there are already so many constraints regarding the existing design. This may be combined with more effective insulation around the mold, but this can also be accomplished virtually. There can also be a wide variety of filling related issues commonly found in injection molded elastomers. Each image contains a color scale (user scale) at the right side of the molded product. Values always decrease from top (red) to bottom (blue). Colors in the molded product correspond to values in the user scale. Figure 3 shows the filling of a thick-walled cylindrical part with two small gates on the top surface (left), and a thin-walled part gated with an optical grade LSR (right). Both scales represent temperature to show heat exchange differences between thick and thin walled parts. No real heat exchange occurs in the thick part because the rubber does not yet have contact to the walls compared to the thin part where the thermal gradient is stronger. Melt jets into the cavity because the melt does not engage with the surface of either the mold or the insert, so it does not slow down. Rather, it enters the cavity and travels through the part to the opposite side. This behavior can also be attributed to high velocity or low viscosity (or a combination) of the flowing polymer. Jetting can create trapped air, but the most common problem is that it leads to uncontrolled filling susceptible to small process or material changes. Trapped air also creates certain challenges. Initially, it ends up in the final part, which is, of course, the problem. The challenge is finding out where it came from and how to get rid of it. Figure 4 shows where contamination from air is located in the part. The user scale displays the percent concentration of air. Red areas have higher concentrations. Was the air trapped in the nearby rib because the melt flowed too quickly below the rib and encapsulated the air? Or maybe the venting was not sufficient, and the air pressure in the cavity was just too high? Actually, it was not either of these. Figure 4 - contamination from air inside the part entrapment (%) RUBBERWORLD.COM 12RW - 20,21,22,23,24,25,26.indd 22 3:36:16 PM

4 Figure 5 - progression of melt with trapped air inside the part The second series of images (figure 5) shows where it came from. Providing clarity of the root cause of molding issues makes virtual molding not only a predictive tool, but a teaching tool. The highest concentration of air (shown in red) was initially trapped inside the ribs at the gated side of the part. Now, virtual molding becomes a communication tool to show others exactly what they need to see. This elastomer part has adequate venting around the parting line. But the image shows that sometimes, even when we have adequate venting, it may not be sufficient to allow all of the air to escape through the vents, depending on the filling pattern. In this example, air contaminates the rubber and becomes trapped inside the part due to its filling pattern. It does not even reach the parting line where the vents are located. In this case, the part design or the gate location should be changed, if possible, in order to get all the air out of the cavity. Another option might be to include vented ejector pins at the base of the ribs. Virtual molding allows such variants to be modeled and compared. Filling imbalances in a multi-cavity runner system can be tricky to nail down (figure 6). The user scale represents velocity to identify which areas are moving faster than others. Near the end of the fill, the red areas indicate highest velocity towards the unfilled cavities. Once the prematurely filled cavities are full, all of the incoming melt is directed towards the unfilled cavities, resulting in higher velocity. This image was taken when the cavity is 96% filled, and we observe that the center four cavities are completely filled, while the remaining four cavities are still unfilled, even when the flow length for all of them is the same. The mold is hotter at the center, so the steel swells more and the runners are bigger (unlikely), or the mold temperature is higher and it affects the viscosity (getting warmer); actually, the viscosity is affected by the local shear rate the rubber experiences. When rubber flows faster at one specific location than the location immediately next to it (figure 7), there is a shear rate difference. The higher the shear rate, the greater the frictional heat and the larger the effect on viscosity. The issue is that the viscosity is not affected uniformly everywhere; it is only affecting the material at the higher shear rate. This creates a problem that lower viscosity material flows FOLLOW US ON 23 12RW - 20,21,22,23,24,25,26.indd 23 2/22/2017 3:36:18 PM

5 Figure 6 - filling Imbalances inside an eight-cavity tool Figure 8 - areas with higher and lower scorch inside the part Unbalanced velocity Absolute velocity cm/s e-007 Scorch (-) e-007 more easily under pressure, so the low viscosity areas flow faster and filling imbalances are created. If the imbalance is big enough, some cavities will fill too early, resulting in high pressure and potential flashing at those cavities. Again, the process would require slowing the injection speed towards the end of filling, possibly resulting in unfilled cavities. These issues can only be fixed if they are quantifiably understood. I would not attempt to trial and error my way through this kind of issue. I did once, many years ago, and that left a terrible taste in my mouth; worst stew ever. Scorch results show how much material is cured during filling. In the rubber molding industry, sometimes the gate dimensions are reduced in order to initiate high shear during filling. Due to increased shearing, the polymer temperature will rise, which will ultimately jumpstart the curing of the material during the filling process. It is easier to identify the areas with higher and lower scorch using virtual molding, as shown in figure 8. The user scale for scorch value is set to 2%, meaning the scale result of 1 shows the curing degree of 2% or more is fully achieved. ellow areas indicate a higher degree of scorch (or premature crosslinking). Scorch values can be lower if the mold temperatures are reduced, or if the material is sheared less. Weld lines are created where the two melt flow fronts meet inside the part. Computer-generated tracer particles are automatically deposited as each weld line is formed (figure 9). The user scale displays temperature to convey the melt front temperatures when they engage. These tracer particles aid in visualizing what is happening behind the flow front or beneath the surface skin. They are used during the filling, packing/holding and curing phase to evaluate the molding conditions, such as mixing polymer, stagnation or re-direction of the polymer flow Figure 7 - differences in temperature and viscosity inside the runner system High Low High Viscosity Low Dyn. visc. (Pa s) 1.767c ,143 11,786 11,429 11,071 10,714 10,357 10,000 9,643 9,285 8,929 8,571 8,214 7,857 7,500 2, RUBBERWORLD.COM 12RW - 20,21,22,23,24,25,26.indd 24 3:36:19 PM

6 Figure 9 - tracer particles deposited at the weld line Figure 11 - non-uniform temperature gradient in the metal insert Insert temperature rise through the weld line regions. Tracer particles not only show the location of the weld lines, but also provide specific information to determine if they will be comparatively strong or weak. Important parameters such as the temperature, curing degree, pressure, contact angle and velocity at which the flow fronts meet are considered by the weld line strength results. This information is used to determine how effectively the flow fronts will fuse together during the molding process. Figure 10 displays the curing degree inside the part when the mold opens at the end of the molding cycle. The user scale represents curing degree in percent. Orange and red areas have achieved a higher degree of cure (90-95%) compared to the blue areas (25%). The parts are sliced to visualize the inside core of the wall Figure 10 - curing degree inside the part at ejection Percent cured (%) thickness. From the outside surface, the parts are fully cured, but inside the core of the wall thickness, they are still uncured. In order to cure these parts completely, they might need to be placed inside an oven after molding. Secondary curing is also fully coupled to the production phase and simulated in virtual molding. Decisions can be made very early in the design about whether or not we prefer to have a longer cure cycle, higher mold temperature or a post ejection curing process. The virtual molding oven is also calculating the curing progression with respect to time and temperature. The curing state is monitored to calculate production rate and energy cost prior to building the mold. Part of a comprehensive virtual molding environment includes the ability to consider over-molded inserts placed into the cavity during each cycle. These inserts are ejected with the part and often used to provide higher mechanical properties. The metal inserts can be preheated or placed into the mold at ambient temperature. Once placed, the insert exchanges heat with the mold at a rate defined by their different temperatures, thermo physical properties and amount of surface contact. However, the heat exchange is not uniform (figure 11). The user scale displays temperature and is used to convey the message that the insert is not always the same temperature everywhere due to thermal exchange that occurs between the insert and the mold/part. On the surfaces in contact, there is heat transfer from conduction, but the exposed surfaces exchange heat with the environment through radiation. This non-uniform temperature also produces non-uniform expansion, and it can lead to potential shut-off or other dimensional issues. Over-molded inserts can distort, depending on pressure during filling or due to a nonuniform shrinkage of the rubber that occurs during the curing and cooling process (figure 12). The user scale shows displacement from the original insert shape in mm. Original shape appears transparent for reference. FOLLOW US ON 25 12RW - 20,21,22,23,24,25,26.indd 25 3:36:20 PM

7 Figure 12 - distortion of the metal insert Total displacement (mm) It also results from the insert design not having enough strength to withstand this pressure from the rubber part. Time in contact also plays an important role in the thermal exchange between insert(s) and mold, especially if multiple inserts need to be placed, one at a time. This produces significant variation and should be avoided. Deflection of core pins during filling due to high pressure imbalances are also difficult to quantifiably prove and fix. When the melt pressure on one side of a core pin is higher than the other side, there is a potential for the pin to bend. If it will, it is because the pin does not have the mechanical strength to resist the net pressure. Having a material database of temperature dependent thermo-physical and mechanical properties of a wide variety of mold materials makes these calculations possible. If the pin will bend, the filling pattern will need to change to reduce the pressure imbalance (figure 13). The user scales represent melt pressure (1) during filling and resulting core pin deflection (r) in mm. The deflection is magnified for clarity. Material data for the mold, inserts, insulation, heaters, and a variety of polymers and elastomers, are already present in the virtual molding database. However, many, if not most, elastomers are custom materials whose properties can be measured through a material testing laboratory., pressure, shear rate and time dependent properties are required for the virtual molding simulation, and these properties include thermal conductivity, specific heat capacity, rheology, pressure volume temperature (PVT) curves and curing kinetics. All types of elastomers are possible, including natural rubber (NR), nitrile rubber (NBR), hydrogenated nitrile butadiene rubber (HNBR), fluoroelastomer (FKM), ethylene propylene diene monomer rubber (EPDM), styrene-butadiene rubber (SBR) or liquid silicone rubber (LSR). It is imperative to have accurate material data in order to achieve accurate results. Figure 13 - part at 50% filled (l); core pin deflection during filling (r) Pressure (bar) Displacement (mm) While measuring these material properties, it is also important to capture the data to cover the entire processing range for the particular elastomer. If the material will be processed at 120 F, then rheology should be measured at 100, 120 and 140 F to cover material behavior at, above and below the initial temperature. If the mold will operate at 350 F, then curing degree curves should be measured at 330, 350 and 370 F. Various models are provided to fit the material data inside the virtual molding database. Rheology models include Cross- Arrhenius, Carreau WLF, Carreau asuda WLF and interpolated viscosity. For reaction kinetics, common models are Kamal and Deng-Isayev. Similarly, various models are included for fitting the PVT, for curing shrinkage calculations (ref. 3), and reactive viscosity data. Overall, virtual molding is a completely different approach to understanding injection molded elastomers because the entire mold, material properties and process are fully coupled and calculated over multiple consecutive molding cycles to match the real world production environment. Automatic meshing and process specific user interfaces support such comprehensive calculations. Various tools like x-ray, clipping, scale, zoom, rotate, etc., can be used to visualize, evaluate and communicate the molding issues and the root causes for quantifiable solutions. The examples shown clearly identify multiple potential elastomer molding issues and some potential solutions. Virtual molding is a unique approach which makes production visible. References 1. Virtual Molding: RUBBERWORLD.COM 12RW - 20,21,22,23,24,25,26.indd 26 3:36:21 PM