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Polyolefins 2002 |
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Levi Kishbaugh Polyolefins in general, and HDPE in particular, have a history of use in foam processes, the primary process being structural foam molding. The MuCell® process is a proprietary process for producing foamed injection molded parts in applications and markets where traditional foam processes have not been used. In particular, the process is focused on the production of parts at wall thicknesses of less than 3.0 mm and as thin as 0.25 mm. The nature of the process is such that the advantages of traditional foaming techniques can now be applied to parts not previously considered for foaming. In addition, the cell structure achieved with the MuCell process is significantly smaller and more uniform. This presentation will address the use of MuCell foaming technology in PP applications for various Automotive and Business Machine injection molded products. Introduction High density polyethylene (HDPE), and to a lesser extent polypropylene (PP), have been used for many years with both chemical and physical blowing agents to produce foamed structures. Traditionally, these have been structural foam applications which can be described as parts with wall thicknesses in excess of 5 mm, and in some instances as much as 25 mm, the basic idea being to produce thick wall parts without heavy sink marks and part distortion. The MuCell process provides injection molders with the ability to apply the advantages of a foaming process to parts that would not traditionally be candidates for standard foaming technologies. To date, the process has been applied to parts as thin as 0.3mm while still achieving a cellular structure. There is, however, no indication that this is the lower limit on part wall thickness.As wall thickness continues to decrease, the cell size will be smaller and the weight reductions less. The MuCell process allows this to happen due to the manner in which the foaming process is controlled. The MuCell process uses physical blowing agents, usually either nitrogen or carbon dioxide, in a supercritical state as the foaming agent. This foaming agent is then introduced into the barrel of the injection molding machine during screw rotation to form a single-phase solution. The single-phase solution, a homogeneous dispersion of blowing agent dissolved into the polymer melt, is the precursor to a microcellular foam structure. On injection of this single-phase solution, the cell nuclei form, then the cell structure develops. A true microcellular structure results in an average cell size of less than 100 microns. Return to TopThe MuCell Process The MuCell process can be broken into four stages: creation of a single-phase solution, homogeneous nucleation, cell growth, and part formation. A single-phase solution as defined by Trexel is molten polymer into which a physical blowing agent (typically either nitrogen or carbon dioxide) in a supercritical state is uniformly dispersed and dissolved. During the injection molding process, this single-phase solution is created by injecting the supercritical fluid (SCF) into the barrel of the injection molding machine during the screw recovery step. As the mixture of SCF and molten polymer is conveyed down the screw, the SCF dissolves into the molten polymer creating the single-phase solution. Once created, it is maintained by keeping the solution under pressure. The single-phase solutions creates the conditions under which homogeneous nucleation can occur. From a theoretical perspective, homogeneous nucleation occurs when the single-phase solution undergoes a thermodynamic instability large enough to simultaneously nucleate millions of cell nuclei. With respect to the injection molding process, this instability occurs when the molten polymer under pressure in the barrel of the injection molding machine is injected into the cavity of a mold which is at a much lower pressure. While it is not possible to instantaneously fill the cavity, injection time is usually sufficiently short to create uniform cell nucleation. Once nucleation is completed, cell growth starts. The cells will grow until the material completely fills the mold, the material develops enough rigidity to withstand the pressure of the gas in the cells, or there is no longer sufficient gas to continue to expand the cells. Cell growth is a function of the restraining forces on the expansion of the cells and the gas pressure in the cells. The final phase of the process is part shaping. In extrusion, this is more difficult as the material is expanding in the open atmosphere. During the injection molding process, the mold provides the means for shaping the part.
Process Effects A number of changes in the molding process occur due to the use of the MuCell process, the first being that SCF is dissolved into the molten polymer during screw recovery. The introduction of the SCF is integrated into the controller of the injection molding machine. When processing with polypropylene, injection speed tends to be faster than for conventional molding. The increased injection speeds result in higher cell nucleation density, which yields improved cell structure across the part. This affects part performance as a uniformly small cell structure results in better retention of properties. The traditional packing phase of the molding process is eliminated due to cell growth. During standard injection molding, hold pressure is applied to complete the filling of the mold as well as compensate for shrinkage until the gate freezes. This has the affect of increasing cavity pressure, which results in higher clamp tonnage requirements. It also introduces residual stress in the part and differential part shrinkage. The growth of the cell structure in the MuCell molded part replaces the traditional packing phase. This cell growth occurs at a low pressure, 200 to 500-psi, and the expanding force is evenly distributed throughout the mold cavity. Viscosity Effects The affect of SCF on the polymer viscosity will depend somewhat on the material type, filler type and filler level. Filler type and level become important in that SCF does not dissolve into the filler and can not change the viscosity of the filler. Therefore, the apparent change in viscosity for a filled material is not as large as for an unfilled material. In addition, the viscosity benefit for polyolefins is much less evident in thicker wall (greater than 1.5 mm) parts. One example of the viscosity reducing effects of the MuCell process is a thin wall food container. The food container had a wall thickness of 0.22 mm and a flow length to part thickness ratio of 280:1. Under equivalent material and mold temperatures as well as injection velocity, the hydraulic pressure required to fill the mold went from 1873-psi in solid molding to 1640-psi using 2% nitrogen and 1500-psi with 5% carbon dioxide. Surface Appearance The surface of a part molded with the MuCell process will have some splay. The level to which this occurs and the uniformity of the surface appearance can be controlled through processing conditions. However, even under the best circumstances, the MuCell process will not produce parts with a high quality Class A surface. Due to this, the process is best suited for non-appearance parts, either structural or as the rigid material behind a decorative fabric or film. Return to TopShrinkage Characteristics The shrinkage properties of all materials but particularly crystalline materials are highly dependent on wall thickness and packing pressure. If packing pressure varies, the shrinkage in the part will also vary. The advantage of the MuCell process is that the packing pressure equals the pressure of the gas in the cells, and this is essential constant throughout the part. For this reason, shrinkage is more uniform which, along with a reduction in molded in stress, results in a significant reduction in part warpage. Note that the key less a change in shrinkage as much as it is a more uniform shrinkage. The graph below shows shinkage values from ISO tensile bars for 4 different grades of PP, an unfilled copolymer, a 20% talc-filled copolymer, a 30% glass fiber-filled copolymer and a 40% glass fiber-filled copolymer. The shrinkage values for the glass fiber-filled materials and the unfilled material do not show a change. There is, however, a slight change in the talc-filled material as the part weight is reduced, although the difference of 0.002 mm/mm of length is within the normal window provided by the material suppliers.  Figure 2: Shrinkage Data Properties Clearly, part performance is critical, as all parts must meet some set of performance criteria. It should also be clear that as plastic is replaced by cells, there will be changes in the performance characteristics of the molded part. While part performance is usually not directly related to test specimen data, this information is an indicator of the performance trends. The graphs below show ISO property data for four different polypropylene materials: an unfilled impact copolymer, a 20% talc-filled copolymer, a 20% glass fiber-filled copolymer and a 40% glass fiber-filled copolymer. Each was evaluated using standard ISO bar test specimens. Under these conditions, it can be seen that flexural properties are relatively unaffected by changes in weight reduction. This relative insensitivity of flexural modulus to weight reduction is a primary reason why the process is effective for structural applications. The other benefit in structural parts is relatively small changes in flexural modulus have little affect on overall part performance.
Tensile properties show a different behavior. Typically there is an initial drop at 5% weight reduction, then a leveling off of material performance. This initial drop in tensile strength is quite significant, 20% or more. This is not observed in the flexural properties because those are much less dependent on cell structure and more dependent on the skin to core ratio, which tends to be unaffected by weight reduction.
Notched Izod impact performance shows the same trend as tensile properties but the initial drop with the unfilled material is larger. This 40% drop in impact for unfilled polypropylene is relatively common for ductile materials. In filled materials, the characteristics of the filler tend to control the impact performance of the product, and the MuCell process does not have an affect on the fillers. Because of this, and the fact that the materials tend to be more brittle, impact property retention is better. Given the nature of these changes in tensile and impact properties, it is recommended for most applications that weight reductions of no more than 10% to 15% be used when running with polypropylene materials.  Figure 5: Notched Izod Impact Weight Reduction and Cycle Time Questions about achievable reductions in cycle time and weight reduction are usually part of any discussion on the MuCell process. Weight reduction is driven by one primary factor: the ratio of flow length to part thickness. It generally can be assumed that for an application with a flow length to part thickness ratio of 100:1, weight reductions will not exceed 15%, and as that ratio approaches 150:1, 10% is a more realistic number. Other factors can affect this result, including the balance of the part, the overall wall thickness, and the filler type and level, but these are reasonable estimates.  Figure 6: Weight Reduction Estimates Cycle time is more complicated since in most molds, there is not a uniform cooling rate. Therefore, the hottest location in the part will dictate cycle time. As a general rule, parts thicker than 2.5 mm will not result in faster cycle times when running the MuCell process. Also, cycle time is usually not reduced when working with unfilled polypropylene or talc-filled polypropylene, the exception being when the cycle time is artificially lengthened to achieve dimensional compliance. Glass fiber-reinforced materials do offer the potential for cycle time savings over solid molding at thinner wall sections. This occurs primarily due to the elimination of hold time. While the hold phase of the injection molding process is eliminated with the MuCell process, some of the hold time normally must be added back to the cooling time. As such, 10% to 15% cycle time reductions would be possible for a well-cooled mold in a glass fiber-reinforced polypropylene for wall thicknesses of 2.5 mm or less. Return to TopMarkets and Applications Given the cost of polyolefins, weight savings is rarely sufficient to drive investment in the MuCell process. Those applications which can benefit most from applying the MuCell process to polyolefins are those where quality issues can be resolved, substantial cycle time can be eliminated, or when a higher cost material can be converted from a to a polyolefin. Quality issues can fall into a number of categories, perhaps the most common being associated with sink marks and dimensional stability. Dimensional stability is also the primary driver behind excessively long cycle times and the use of higher cost materials. Dimensional compliance is a recognized problem with polypropylene. This is particularly true in long, unsupported walls and flatness. While this situation often can be improved with the use of fillers, completely eliminating warpage is very difficult.  Figure 7: Automotive Turn Signal Housing
 Warpage can also be eliminated by using a higher cost material, typically an amorphous resin, either filled or unfilled. In some instances, this is done for performance as well as dimensional compliance, and in those instances material substitution may not be possible. However, where dimensional compliance is the sole selection criteria, considerable cost savings can be gained simply by switching to a polypropylene material and applying the MuCell process. One example of this is printer spittoons. In this application, side wall bow drives material selection. Applying the MuCell process to this application with talc polypropylene produced a part with no side wall bow. The final application, which is also in the quality area, has to do with molding behind decorative films and fabrics. These processes allow a decorated component such as A pillars to be produced without a secondary operation to adhere the fabric to the plastic component. However, the temperatures and pressures of standard injection molding limit the types of fabrics and cover stocks that can be used. The MuCell process allows for a wider range of fabrics and less expensive fabrics since processing temperatures can be easily reduced by 20oC to 30oC and cavity pressures by 75% to 80%. Summary The MuCell process is a proprietary foaming technology that allows the benefits of tradition foaming processes to be applied to the conventional molding markets. However, due to the cost of polyolefins and the material characteristics, weight reduction and cycle time are usually not sufficient economic justification for the process. Instead, the process must be applied to solve a quality issue through the benefits of reduced cavity pressure, reduced molded in stress and lower material viscosity. The most common of these quality problems are warpage in long, unsupported wall sections and in flat parts. In these instances, the improvements in quality provide the justification. Given the material performance changes with weight reduction, filled materials are also preferred to unfilled materials. Typically, 20% talc or glass fiber-reinforced materials provide better property retention and better potential for cycle-time savings. Return to Top | Return to Technical Papers & Presentations |
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One way to achieve this is to extend cycle times beyond what is necessary to cool the part by allowing the mold to serve as a shrink fixture. In this situation, the mold is used for a function it can do rather than for what it was designed. The mold was designed to produce parts and is a rather expensive check fixture. The MuCell process, by significantly reducing molded in stress as well as uneven pack pressures, allows parts to be produced with improved dimension while eliminating extended cooling time. This can be seen in parts such as HVAC blend doors, turn signal housings and battery cases.
