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May 15, 2001
9 Min Read
This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure problem. Michael Sepe is our analyst and author. He is the technical director at Dickten & Masch Mfg., a molder of thermoset and thermoplastic materials in Nashotah, WI. Mike has provided analytical services to material suppliers, molders, and end users for 15-plus years.
Processors in these competitive times look for every advantage to improve productivity. One of these advantages can come in the form of a material that processes more easily. Normally we think of these types of materials in the commodity markets where high cavitation and thin walls can turn tenths of a second into big gains. Material suppliers have answered the demands of these markets with a host of compounds that flow more easily and set up faster in the mold.
However, anyone with experience in the business willing to do an honest appraisal will admit that productivity improvements that come from more easily processed resins usually involve a trade-off in properties. Higher flow typically involves lower-molecular-weight polymers, and we have covered the topic of the relationship between molecular weight and properties several times since this series began. Faster setup times are usually the focus of semicrystalline materials like polypropylenes and nylons and involve the science of manipulating crystallization rates through a process known as nucleation. Nucleated materials characteristically develop structures that are stiffer and stronger but also less impact resistant. In other words, there is no such thing as a free lunch.
In the high-performance-material market the attempts to ease the burden on the molder are more sophisticated. Certainly viscosity still gets attention in some product lines of amorphous materials where moldfilling can become a real challenge. But the feature that gets a lot of focus is the need for mold temperatures that exceed what is achievable with hot water. Many molders have an aversion to running mold temperatures higher than 100 to 105C (212 to 221F) because it requires the use of either electric or oil heat.
Employing different types of temperature control in a manufacturing facility adds to the complexity of the overall production landscape, and processors who have not worked with elevated mold temperatures are reluctant to do so for a variety of reasons. Electric heat is difficult to control uniformly, and using a heat transfer fluid that is pumping through lines at temperatures of 120 to 180C (248 to 356F) raises concerns about safety should a line or a fitting spring a leak. In addition, temperature control units that employ oil are much more expensive to purchase and operate than units that use water.
Material suppliers are therefore aware that when they introduce a material that requires elevated mold temperatures there is a significant portion of the market that will not consider the material as an option. Still, there are two important reasons for using elevated mold temperatures. It is recommended for some amorphous materials with high softening points because it enhances the moldfilling process and can reduce molded-in stress. Materials like polyetherimide therefore have mold temperature recommendations of 65 to 175C (150 to 350F). The need for electric or oil heat is somewhat negotiable in cases like this because the recommended range includes temperatures attainable with hot water. But there is a group of semicrystalline materials in which higher mold temperatures are critical to producing the desired properties. In these cases there is no negotiating with nature.
One of the first materials to be developed with these requirements was polyphenylene sulfide (PPS). But the list is growing and now includes some PET polyesters, polyphthalamides, and polyetheretherketone. In very thin sections, these elevated temperatures may even be required in acetal and syndiotactic polystyrene.
Figure 1. Effect of mold temperature on properties of PPA.
Figure 1 shows a modulus vs. temperature plot produced by dynamic mechanical analysis (DMA) that shows what happens to a material when the rules regarding mold temperature are not observed. The parts were produced in a polyphthalamide (PPA) with a glass content of 35 percent. One part was produced with a mold temperature of 90C (194F) while the other part was molded at 160C (320F).
Note first that the part produced in the hot mold has a higher modulus at room temperature. However, the most important difference occurs when the material reaches the glass transition. Near 130C both materials begin to lose modulus as the amorphous regions in the polymer structure become mobile. This modulus decline is much more severe in the part produced in the cooler mold because the polymer structure contains more amorphous material and therefore fewer crystalline regions to support the matrix. Even with a glass fiber content of 35 percent, the part molded at the lower mold temperature loses 70 percent of its room temperature modulus by the time the temperature reaches 170C (338F). At the same temperature the part molded at the higher temperature has only lost 50 percent of its original modulus and is almost twice as stiff.
But it gets better. Note what happens above 170C to the part run in the cooler mold. No, your eyes are not deceiving you; the material is becoming stiffer as the temperature rises. Why? Because new crystals are forming in the material while it is in the solid state. This phenomenon is known as cold crystallization and it is the reason that annealing is sometimes employed to accomplish what the molding process cannot. Most molders understand that when a material crystallizes it also shrinks. So whether this solid-state crystallization occurs in the annealing oven or out in the field, it is difficult to control and may lead to secondary effects such as warpage or elevated stress levels in the part.
Any attempts to detect this diminished capacity to perform at elevated temperatures through measurements of heat deflection temperature (HDT) will be in vain. If you followed the series on HDT measurements a few months back you already know why. The HDT in a highly filled semicrystalline material is closely associated with the crystalline melting point of the polymer. If we draw a horizontal line at .8 GPa (116,000 psi), which is the modulus associated with the HDT at 264 psi, we can see that the two materials appear to be equivalent; the modulus line crosses both curves at the same temperature. As the full plot shows, nothing could be further from the truth. Ironically, the new method C point prescribed by ISO 75 would detect a problem because the critical modulus value for this test method is 3.5 GPa (500,000 psi). Notthat at this level the test would capture a reduction in the HDT of approximately 60C (108F).
So how do we deal with this problem? Material suppliers are not enthused about the prospect of molders running their materials at conditions that make field failure more likely. It makes them look bad and erodes confidence in the material even though it is not inherently at fault. And a material that processes with a high degree of difficulty will send many molders looking for an alternative.
Figure 2. DSC comparison of fast and slow crystallizing PPA in cool mold.
Enter the easy processing variety of these high-performance materials. Through modifications in chemistry some of these high-performance materials can be induced to crystallize at a faster rate. This translates to lower mold temperatures. Figure 2 shows DSC results for a part molded in a traditional polyphthalamide and a fast-crystallizing variety using a mold temperature of 90C (194F). As the standard material is heated it reaches a point where a very strong, sharp exotherm occurs. This thermal event corresponds to the modulus recovery that we saw on the DMA in Figure 1. When the material reaches a certain temperature, uncrystallized regions that should have formed crystals during the molding process begin to rearrange to finish the process. But if we look at the faster crystallizing material we can see that this exotherm is not present. This tells us that the chemists have been successful and this new material does, in fact, crystallize in the cooler mold.
If we follow the DSC scan all the way to the high-temperature end, we also see that this has been accomplished without reducing the melting point of the material; in fact, it's a few degrees higher. So we can expect the same elevated temperature performanceâ€”the HDT assures us of this.
Or does it? It is a fundamental rule of polymer structure that the opportunity for crystal formation exists when the material is below its melting point but above its glass transition. We will come back and treat this principle in more detail in subsequent articles. For the moment, however, let's assume that this is true and consider what has been done to accomplish this faster crystallization. The reason that some materials require higher mold temperatures is that they have high glass transition temperatures. This is part of what gives them the high-performance profile.
If we want to permit crystallization to occur at lower temperatures, one of the ways we can accomplish this is by lowering the glass transition temperature; this opens the window of opportunity for crystal formation. In Figure 1 we have seen the effect the glass transition has on even a properly crystallized part. If we reduce the threshold for crystallization by some 60 deg C, then it is probable that we have also reduced the glass transition temperature by the same amount.
Figure 3. Comparative properties of slow and fast crystallizing PPA.
Figure 3 shows a modulus vs. temperature comparison for properly crystallized parts from the traditional and the easy molding material. The difference is striking and has serious implications for a lot of applications where the end user may be counting on a material with a stable modulus up to an elevated temperature. While both materials begin and end at approximately the same place, the process by which they get there is quite different. This is the price to be paid for the easier processing, and it is largely hidden because short-term property measurements do not capture the differences. Next month we will begin a series on the subject of crystallization and highlight some of the unexpected headaches that occur in the real world when things do not turn out as planned.
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