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AUTOMOTIVE LIGHTWEIGHTING Lightweighting in automotive: A systematic approach for metal to plastic conversion

May 19, 2021

Amol Mangalmurti, General Manager - Application Development, Autotech Sirmax India - Lightweighting is a familiar term in the automotive industry and has been quite successfully implemented in the passenger car segment to improve fuel efficiency and reduce emissions to meet the stringent BS-VI emission norms by all OEMs across the globe. For any lightweighting programme to be successful, innovation in polymer compounds, part design integration and adaptable product design are required. The objective of the article is to focus on the Systematic Design thinking approach of explore, create and implement, which will provide a new roadmap to approach a new product development programme in an organisation, leading to the ultimate lightweighting solution.

When it comes to lightweighting, the first thing to strike the mind is polymer compounds as they are strong and lightest in weight amongst all the materials. In the recent developments in the automobile industry, the usage of polymers has surged significantly, as polymer compounding companies felt the need to develop innovative materials which are lighter in weight and better in strength than existing polymers. Automotive companies have benefitted from enhanced fuel efficiency and fulfilling emissions targets and this has emerged as one of the key factors in increasing the usage of polymers.

Any material substitution comes with various challenges, especially when a high strength material, like a metal, has to be replaced with a polymer composite. We analysed various successful metal to plastic conversions in the last two decades and evolved a common methodology or approach to be followed. Below are some application examples commercially implemented in the four-wheeler industry –

  1. Front-end module

  2. Air Intake manifold

  3. Door modules

  4. Engine covers

  5. Gears/pulleys

  6. Oil sumps

Systematic Design Thinking approach

Explore phase

In a metal to plastic conversion project, selection of a component is an important first step towards the successful implementation of the material conversion project.

Identify material conversion opportunity

There are some important aspects to be considered before selecting the component for replacing the material into a lightweight plastic material:

  • The manufacturing process and the part cost – What type of material (steel, aluminium, MS, rubber, glass, etc) is used in the present component? What is the manufacturing process (diecasting, forging, sintering, machining, etc) in use? Are there any post treatments required on the component (anti-corrosive coatings, paintings, etc).

  • The key driver for changing from metal to plastic – Is the cost or is it the improvement in the performance? Is it just lightweighting?

  • The part’s function in the assembly – Is it aesthetic, structural, semi-structural? Can we integrate some other parts in the assembly?

  • The surrounding environment – Is there any exposure to aggressive chemicals, oils, gases, greases, etc?

  • The operating temperature and peak temperature – Is the part subjected to any heat radiation from the adjoining systems?

  • The tolerances required in the final part – Some polymers are hygroscopic and absorb moisture. The dimensions change during its service life.

Nylon 6 absorbs more moisture than Nylon 66 – the dimensional change between dry-as-molded and equilibrium at 50% RH, substantially less for Nylon 6. This implies that in a majority of cases, Nylon 6 would be more dimensionally stable when moisture absorption is the primary concern. This result does not consider factors, such as temperature variations, etc. Finally, these moisture absorption characteristics need to be considered when designing and building moulds to produce parts using any nylon material.

Define material specifications based on end product functionality

Once the part/assembly is selected for a lightweighting project, the next step is to select the compatible and appropriate material of choice which meets the functional requirements.

  • The components subjected to repetitive force/pressure loading and unloading undergoes fatigue and after a certain number of cycles, it fails without any warning at a much lower stress than the typical yield stress. Typically represented as S-N curve, it stands for stress versus number of cycles of loading.

  • A component under constant force will undergo creep (change in dimensions) over its lifecycle

  • In case the temperature exceeds 100 °C, the strain is higher than at room temperature, resulting in higher deformation. The reduction in tensile strength, tensile modulus & impact strength at high temperatures should be considered along with stress strain curve of the polymer at higher temperatures.

Prepare techno-commercial value proposition of the opportunity

Estimate the cost economics by understanding the manufacturing process of the metal component. Numerous times, the post-process is cumbersome and expensive.

  1. Any additional investment required

  2. The cost of scrap and wastage

  3. The cost of tooling and assembly jigs & fixtures

  4. Any post processes

  5. Metal inserts for strengthening

The scrap produced in a blow moulding process is particularly very high. For a part weighing around one kg, the scrap produced is 40-70% of the actual part weight. Higher the parts produced per year, lower will be the part cost and, break-even in the additional investment can be achieved earlier.

After completion of all the three steps, we can assess the actual cost of producing the part and the Return on Investment, in case there is higher investment required on the project.

Create phase

This is the creative phase in which the primary product requirements/specifications are documented and solution concepts are evolved. It starts with identifying primary, secondary and hidden requirements of the identified opportunity by listing them priority wise. These requirements are further sub-divided and solution concepts are created.

For the product concept design, benchmarking of product specifications and prototyping with this solutions template, we use the Systematic Inventive Thinking (SIT) templates on the final solution concept. The closed world condition is crucial for SIT’s methodology. The first step in using SIT is to define the problem world. Once defined, the problem solver knows that all the building blocks for the solution are right there in front of him/her and that the solution simply requires the reorganisation of the existing objects. This adds great focus and power to the method. It can also turn every real problem into an amusing puzzle.

The closed-world condition deals with the resemblance between the problem world and the solution world. The condition stipulates that in the development of a new product – or when addressing a problem – one must utilise only elements already existing in the product/problem or in the immediate environment. This condition forces us to rely on resources already at our disposal rather than ‘importing’ new external resources for the solution. Thus, the closed-world condition sets us on a collision course with our fixedness, allowing us to arrive at solutions which are both innovative (different from the usual) and simple (since based on existing and known elements).

Based on the above, we arrive at a final solution concept, Design of a Door module, which fulfils all the functional requirements. From here on, we move towards the prototyping stage. Conventionally, prototyping of a new product design was done after the CAE design validation stage, which fulfilled all the product specifications.

Rather than starting out with a detailed analysis of technical & user requirements and development of specifications, a typical process involves developing at quite an early stage in the process, a wide range of low-fidelity prototypes from which to learn. In this phase, the designers try to find multiple solutions to the same problem & go on improvising by making low fidelity prototyping solutions during the initial development stage and use them as learning tools to quickly test the key features of a product.

Basically, the product development happens by making and testing smaller versions of the bigger product by deploying only key features in the quickly made prototype. These are made from inexpensive and accessible materials — such as paper or card, foam core or wood models or cost-effective 3D Printing or even soft tools. Through trial and error, we evolve and refine ideas to integrate multiple elements of design by gradually moving towards more real & expensive prototypes.

Implement phase

The last and final step is the ‘Detailed Part Design’, Accurate Tool design with proper gate location and cooling layout with aid of a flow simulation software, CAE and endurance testing in the above application conversions.

  1. This design needs to be validated structurally by simulating the pressure, forces etc, acting on the part by conducting CAE simulation to verify and optimise its strength.

  2. Part design modifications: Thickness optimisation, ribs, bosses, weldlines control at critical locations.

  3. Part and system level endurance testing which is the responsibility of the OEM & Tier-1s and is extremely critical to establish the functionality, fitment, durability and reliability of the part.

For any part to be converted from metal to plastic, the part has to be redesigned for equivalent strength and this can be achieved by using high strength polymer composites with geometrical modifications to achieve the similar strength as the original metal part.

Effective implementation of Systematic Design Thinking approach

To summarise, the Systematic Design Thinking approach has led to many breakthrough developments of applications in the field of polymers and has given us so many new innovative designs. In the future, the success of a new concept in polymers will depend upon how effectively the design thinking approach is implemented to create new innovation in material and application.

Image Gallery

  • The components, when subjected to repetitive force/pressure loading and unloading, undergo fatigue, and after certain number of cycles, they fail without any warning at a much lower stress than the typical yield stress. Typically represented as S-N curve, it stands for stress versus number of cycles of loading.

  • Amol Mangalmurti

    General Manager - Application Development

    Autotech Sirmax India

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