The Art and Science of Plastic to Metal Replacement (Part 4)

By Michael Sepe, Materials Analyst

Will the material you’re looking at for your application stand up to the required conditions? Or maybe you don’t need such a robust (expensive) resin. Since data sheets don’t have all the answers, it’s best to conduct a full analysis.

We have spent the last three installments of this series taking a tour of the molecular structure of polymers and the influence that this structure has on the long-term behavior of these materials when they are placed under load. We emphasized the importance of considering temperature, time, and stress together and we demonstrated the utility of accelerated test methods to develop quantitative assessments of the long-term behavior of a material.

Now we are ready to look at case studies that illustrate the implementation of these methods. One case study presents a situation where a more expensive material was needed to ensure long-term success while in the other instance the intended material was found to be significantly overengineered and a less expensive resin was ultimately selected. In both cases, guesswork was not part of the process.

Before we dive into this segment of the story, it is important to point out that we are going to be applying concepts that do not get a lot of attention in the industry in general and we cannot recapitulate in full everything that we have laid as a foundation in the previous three articles. So if you find yourself getting a little lost, it may be a good time to put this issue down, retrace your steps to the first three articles, and get caught up on some important engineering concepts that arise out of the chemistry of these materials.

Finding data for the right time and temperature

The first case involves a medical part designed to perform over a five-year period at 90°C under maximum loads of 450 psi. The application involves constant exposure to water at this elevated temperature, so the first criterion is good resistance to hydrolysis. This eliminates material families such as polycarbonate, various nylons, and polyesters. The developer of the product had focused on two possible materials: modified PPO and a polysulfone variant known as polyethersulfone (PES). In order to boost the performance of the PPO, a grade with 20% glass was selected.

Finite-element analysis (FEA) had been conducted and created the usual set of colorful pictures showing the distribution of stresses and strains. The analysis that we are going to illustrate in this article is particularly useful in enhancing FEA. In the right hands, FEA is an excellent tool for examining a design and determining where problem hot spots might exist that could lead to early failure. Unfortunately, as in the case of the rotating fan in Part 2 of this series, you have to know what you are looking at. Stresses and strains are just numbers on a screen unless you can recognize when these numbers spell trouble.

The proprietary nature of the product prevents us from showing the part design, so you will have to use your imagination and envision a housing where the assembly points are under the aforementioned stresses of 450 psi.

The essential question here was, Can the more economical 20% glass-fiber-reinforced PPO be used in favor of the significantly more expensive PES? At this particular time the PES was approximately 3.5 times more expensive and the part weight was substantial, so the material selection was going to have a significant impact on cost.

Now, if you are one of those engineers who likes to surf the Net looking for data sheets, you no doubt are looking at this proposed problem and wondering where all the anxiety is coming from. A check of a data sheet for 20% glass-filled PPO shows a tensile strength of approximately 13,000 psi and the value for an unfilled PES is slightly lower at 12,000 psi. How can hundreds of pounds per square inch pose a problem?

Well, the data sheet quotes room-temperature properties. What happens when we raise the temperature to 90°C? We can answer this question by simply performing a temperature scan with dynamic mechanical analysis (DMA) and running some tensile tests on the two materials at 90°C. Figure 1 shows the DMA results and Figure 2 provides a comparative plot of the stress-strain behavior of the materials to a point a little beyond the yield point.

It may be surprising to some that, for starters, the tensile strength at yield of the PPO has declined to about 6000 psi. But that is still a long way from 450 psi. In addition, the DMA plot appears to give us some good news. At 90°C the PPO is stiffer than the PES, a fact verified by the slopes of the stress-strain curves in the initial stages of the curves.

But we have not yet factored in time. Five years is a long time and under load we can expect changes in the part. Will those changes lead to catastrophic failure? This part is a pressure vessel. If it cracks at the attachment points, the vessel will leak, and this is deemed a failure.

In order to determine the behavior of the two materials over the five-year period, we perform the accelerated creep characterization exercise. The creep master curves are shown in Figure 3. They clearly show that in the long term, the PES is a superior material. This arises directly from the higher glass transition temperature of the PES, which the DMA curves in Figure 1 show to be approximately 80 deg C higher than that of the PPO.

So PES or PPO?

While this study establishes that the PES is a more creep-resistant material, it does not determine that the PPO is not fit for use. To make that determination we need to crunch the numbers. At the five-year point the apparent modulus of the PES has declined from 380 kpsi to 150 kpsi, a drop of about 60%. Based on a constant stress of 450 psi, this means that the initial strain of 0.118% will have increased to 0.3% in five years. If we examine the stress-strain curve for the PES, we can see that this will not challenge the structural integrity of the polymer. A significant safety factor exists if this material is used.

The PPO has an initial apparent modulus at 90°C of 475 kpsi, which means that at the moment when the stress of the assembly is first applied, the initial strain will actually be less than that of the PES, approximately 0.095%. However, at the five-year point, the apparent modulus has dropped to 25 kpsi, almost a 20-fold decline. This means that the strain at this point will be 20 times higher based on linear considerations alone. This would put us at 1.9%.

But if we factor in the nonlinear behavior exhibited by the stress-strain curves, we can see that the material cannot sustain a predicted linear equivalent strain this high. The applied stress must be reduced to slightly below 300 psi in order for the material to sustain the five-year performance, and even then we would have no safety factor. The maximum working stress for the PES turns out to be 3150 psi, greater than an order of magnitude higher than the PPO.

This shows how drastically the assessment of fitness-for-use changes as we consider the full scope of the application environment. In this case the different long-term capability of the two amorphous materials centers on the difference between the application temperature and the respective glass transition temperatures. This provides us with a very useful tool for quickly assessing whether properties will improve or worsen as we change from one amorphous resin to another. For amorphous materials, lower glass transitions generally mean shorter lifetimes under load at a given temperature.

It is also important to note that if we were to change the time scale of the application, we would come up with a different conclusion about the utility of the PPO. If the product was only expected to last one year, for example, the maximum working stress of the PPO would be about 750 psi and we would have a margin of safety to work with. The outlook for the PES would also improve, being able to operate at stresses of up to 4500 psi, but at this point we might decide that this material was overengineered and we would opt for the less expensive resin. This illustrates the importance of the time element in assessing the capability of a material.

Of course, design changes could also be instituted to further reduce the stress at the attachment points. There is any number of decisions that can be made at this stage in product development. But notice how far we have come in our decision-making process from the traditional comparing of data sheets.

And PA or PP?

This type of analysis does not always result in the selection of a more expensive resin. Our second case study involves a part that was originally developed in a 15% glass-filled nylon. Chemical resistance concerns led to a focus on polypropylene.

Conversions of existing products from nylon to polypropylene are a popular and often successful cost-reduction strategy. But the behavior of the two materials with temperature is very different and this has implications for the way they will respond in an application where creep is a concern.

To be on the safe side, the polypropylene first selected was reinforced with 40% glass fiber to more closely match the initial properties of the glass-filled nylon. However, the part design had features that created concern over warpage. One of the easiest ways to reduce warpage in a fiber-reinforced semicrystalline material is to replace some or all of the fiber with a particulate filler such as a mineral or glass bead. This replacement of fiber with filler also reduces cost.

Here again, FEA had been performed, so the stresses were well understood and the resulting strains could be readily calculated. The task was made simpler by the fact that this part would live at room temperature for its entire life, which is projected to be 10 years. Therefore, room-temperature stress-strain curves can be used in the analysis. While the FEA gives us the needed information about initial strains, it does not tell us about the condition of the material 10 years in the future. Here again, we do not need to wait through 10 years of testing before launching the product. Instead, we use the same techniques already discussed to accelerate the creep modeling process.

Again it starts with an understanding of temperature-dependent behavior. Figure 4 shows the modulus vs. temperature behavior for the 40% glass fiber and a 20%-glass-fiber and 20%-talc-loaded material. This provides an excellent illustration of the positive and negative aspects of the fiber replacement strategy.

The overall shape of the modulus-temperature curve does not change, but there is an across-the-board decline in the stiffness of the compound as a result of the removal of the fiber in favor of the filler. This would be reflected in the strength and modulus numbers on the data sheet, and the heat deflection temperature of the talc and glass-filled compound will likely be lower because this material will reach the critical modulus associated with the HDT at a lower temperature.

Because the modulus at any given temperature is lower in the glass- and talc-filled material, it may also require a slightly longer cycle time to mold the part. However, as mentioned already, the part will be less prone to warpage and the raw material will be less expensive and produce less wear on the mold and the processing equipment.

Figure 5 shows the creep master curves for the two materials, extended to 100,000 hours based on tests that took just days to conduct. With the appropriate stresses plugged in and taking into account the onset of nonlinear behavior, Figure 6 shows the strain vs. time curves. Clearly, the glass-filled material is the better compound when it comes to creep resistance. But in this case, the results for the lower-cost glass and talc combination still only produce a predicted strain of just under 0.8% after 100,000 hours. This is a sustainable level of deformation and shows that in this case, the initial material selection was dramatically overengineered and that a much lower-cost resin could be used.

The common denominator in these two exercises is that knowledge of material properties at application conditions is critical to understanding fitness for use. This forces us to go beyond the usual tools in making good decisions about what will and will not work. Once the hard work has been done, this understanding also provides guidelines for evaluating lower-cost materials that might come under consideration later in the life of the product.

As the database fills up with real understanding of polymer performance, not only will product development take place more rapidly and with greater assurance of success, but also we in the plastics industry will be able to call on the users of metals with much greater confidence that we actually know what we are talking about.

March, 2008 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.