Thursday 31 January 2019

Metal-2-Plastic in Automotive: How to do it. (incl. Youtube Training Video)

Hello and welcome to this blog post where I show you how effective metal replacement with polymeric materials in automotive is done. A successful metal to plastic conversion can significantly reduce weight and CO2 emissions and increase mileage.

The training consists of 5 parts:
1. Update on the current challenges in the automotive industry
2. Clarification on why aluminum die cast replacement is beneficial
3. Deep dive on how metal replacement can look like including which high performance plastics can be used.
4. Explanation of metal replacement with a commercial example
5. Closure with key takeaways

Watch below the video on my Youtube channel:

1. Update on the current challenges in the automotive industry

Modern cars are gaining in weight and size. In the past 50 years, the weight increase was in average 10 kg per year. Globally, legislations are pushing Original Equipment Manufacturers (OEM’s) to design for better fuel economy, lower emissions and improved safety. In parallel, consumers are seeking a better performance of their cars at lower costs. And in all this discussions comes now the central question: why is applying light weighting beneficial?
A suitable light weighting strategy in place allows OEM’s to fulfill regulations and consumer wishes such as active safety, customer experience, new energy vehicles and reduction emissions.
In Figure 1 the current regulation situation is shown. In the European Union (EU) only 95 g/km CO2 will be allowed by 2020. The EU has the strictest of all CO2 emissions limits. OEM’s have to pay high fees for their cars if those are not fulfilled. Currently, the new Worldwide Harmonized Light Vehicle Test Procedure (WLTP) pushes OEM’s even faster to improve their fuel economy. It is expected that China is moving fast in such a direction too.

Figure 1: Overview on the worldwide CO2 regulations for automotive industry.

The statement of Goldman Sachs sums it up in a concise way: “We expect a 15% (200 kg) drop in the average car body weight by 2025 as automaker race to meet CO2 rules”. The modern cars need to loose around 200 kg in weight. One way is to switch to aluminum which has 1/3 of the weight of steel. Another way is to increase plastics in the body in white structures (Figure 2).

Figure 2: Results of Goldman Sachs study.
Currently around 10-12 % of a modern passenger car is made out of plastics. Comparing the different structural materials used in cars, advantages of polymeric materials can immediately be seen. Steel has a density of 7.8 g/cm3. Aluminum has 1/3 the density of steel and plastics have half the density of aluminum, leading to a 50% reduction.

2. Why is aluminum die cast replacement beneficial?

In this section I show you why metal replacement with polymeric materials is beneficial for consumers and OEM’s. On one hand, aluminum prices increased 33% in the past three years (Figure 3). On the other hand, due to the significantly lower density of polymers in comparison with metals major weight reduction can be achieved.

Figure 3: Volatile material prices of LME aluminum.

Some of the technical advantages of replacing metals with plastics are for example:
1) Possibility of consolidating several metal parts into one plastic part.
2) Better resistance to corrosion or chemical attacks
3) Better acoustics (particularly relevant in electric vehicles)
4) Improved friction and wear properties of parts

So far, we made a quality assessment of why metal replacements in automotive are beneficial. Now, we make a direct quantitative comparison with a split into process, part and additional advantages (Figure 4). As an aluminum representative I have selected the alloy A380 and compared it to high performance plastics such as polyphtalamides (PPA), polyarylamide (PARA), and polyphenylene sulfide (PPS). I highlighted the major differences in orange. Manufacturing process for aluminum parts is in most cases a die-casting process which requires high operating temperatures. Aluminum allows parts with high dimensional tolerances and the casting is close to net shape. For plastics injection moulding can be used to ensure a high volume mass production at high production efficiency. As a result, besides allowing 50% lighter parts than aluminum, plastics may allow for cost reductions in the order of 10 to 30%. On the top of these advantages is the design freedom and linear performance up to 120-150 °C e.g. semi-aromatic nylons and PPS.

Figure 4: direct comparison of aluminum A380 and high performance plastics.

3. How a metal replacement can look using high performance plastics
Here, we discuss the plastic material selection for metal replacement. There are over 60 thermoplastic resins and over 100 additives you can add to your base resin, ending up with thousands of potential compounds. Therefore, you might ask: which of those compounds is the optimal material for my metal replacement? Basically, a selection method consisting out of three steps will help you to find the right polymeric material. In a first step, we will select the resin morphology, deciding if we need an amorphous or semi-crystalline morphology. Then, in the second step we apply thermal, mechanical and cost requirements. Finally, in the last step the manufacturing process is considered and a final review is made (Figure 5).

Figure 5: material selection steps for metal replacement.

Amorphous and semi-crystalline polymers have their strength in different areas. Amorphous resins have excellent transparency combined with low shrinkage; low warpage and tight part tolerances can be achieved. Semi-crystalline resins have an easy processing behavior combined with chemical resistance, mechanical strength and wear resistance. To be suitable for metal replacement in automotive a plastic parts has to exhibit certain mechanical strength and wear resistance and has to be able to withstand oils and fuels. Therefore, semi-crystalline morphology will be the clear choice. For most metal replacements at least the temperature stability and mechanical strength level of an engineering plastic is required. Figure 6 shows you the major commodity, engineering, and high performance plastics.

Figure 6: Overview of major commodity, engineering, and high performance plastics.

In the second step, the thermal, mechanical, and cost requirements are evaluated. An important criterion to rank materials in relation to each other is the strength to weight ratio (Figure 7). Comparing the aluminum grade A380 with the engineering and high performance plastics, it can be seen that Nylon 6.6 and PPS are in a similar strength to weight range. PARA as well as PPA with 60% glass fiber are even outperforming aluminum.

Figure 7: Strength to weight ratio of different metals and plastics.

So far we just considered values of our materials at room temperature. How PPA and PARA behave at higher temperatures? In Figure 8 you can see that PARA is not a high temperature polymer since it has a glass transition of 85°C. PPA’s, however, have higher glass transitions which can reach up to 135°C and higher. For standard operating environments, both materials are suitable for an aluminum die cast replacement.

Figure 8: Tensile strength of semi-aromatic Nylons and aluminum.

In the last step of the material selection we have to check the processing methods of the selected materials. Most parts are produced in injection moulding which comes with several benefits such as reduced post processing costs and moulded in threads if needed. Apart of material selection we have to consider some more things for our project management to be successful in metal replacement. One point is the definition of the primary aims: which functions need to be fulfilled by the system and which function does each single component need to fulfill. Another step is the product conceptualization: here it is important to look for design alternatives, feasibility studies, potential savings, and test programs with prototype parts. Design in plastics is not difficult, it is just different. Design checks are needed to find out if there are no critical weldline situations. Part optimization by using e.g. moldflow studies combining with practical filling studies can be beneficial too. Finally, evaluation of the material suitability through test specimens and component testing should be done. After this your new plastic part can be ready for a pre-series test – congratulations when you reach this phase.

4. Explaining metal replacement with a commercial example
Now let’s close the theoretical part and have a look at an industrial example. Engine support mounts are a good example for metal replacement. In this case a PA6.6 with 60 % glass fiber load was selected (Figure 9). It is a high strength type of Nylon and having a specific strength in the range of steel which makes it a good candidate for replacement.

Figure 9: Engine support mounts as a metal replacement example.

5. Key take aways
In Figure 10 I summarized what we have discussed in this post:

Figure 10: key takeaways of the metal to plastic training.

Thanks for reading this training and if you have questions or need help with your metal replacement, then please let me know.

Greetings & till next time!
Herwig Juster

New to my Find Out About Plastics Blog – check out the start here section

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