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Achieving automotive lightweight is vital for boosting fuel economy for internal combustion engine vehicles and extending the range of electric vehicles (EVs). Reducing the vehicle weight by 10% can improve fuel economy by 6—8%, as well as cut down carbon dioxide emissions ^[1]. The reduction in fuel use also means a reduction in emissions, which is great for environmental sustainability.

Beyond fuel economy and range improvements, making automobiles lightweight can significantly improve performance, including braking, handling, and acceleration. Also, lightweight manufacturing lowers the strain on suspension, brakes, and tires, which lowers wear and tear. The long-term outcome is that these parts will last longer and need fewer maintenance.

So, how can manufacturers make automotive parts lighter? Can this be achieved by simply swapping the traditional materials with lightweight alternatives, or does this goal require the rethinking of how automotive parts are designed?

Debunking the “Novice Myth” on Lightweight Manufacturing

There are lots of misconceptions associated with making lightweight automotive parts. One school of thought says lightweight can be achieved through material choice. In other words, an automobile can be made lighter simply by switching to lighter materials.

Based on this misconception, they view manufacturing processes like automotive injection molding, tooling, and CNC machining as merely playing the role of “laborers” who follow a laid-down blueprint. The second school of thought is that using lighter materials compromises safety. Neither of these schools of thought on lightweight manufacturing of vehicles is true.

In fact, modern composites have been shown to offer better crashworthiness ^[2]. They are more effective in absorbing impact energy compared to metals used in traditional automobiles.

Automotive Lightweight Manufacturing Using Injection Molding

Without a doubt, the choice of material plays an important role in lightweighting. However, optimal strength and form are almost entirely achieved without increasing the weight through design optimization and innovative manufacturing practices as follows:

1. Hollowing of Sections to Cut Material Use

Bulky parts can be created to have hollow interior sections. This void is usually achieved through gas-assisted injection molding or foaming. For example, in physical foaming, nitrogen gas or carbon dioxide is injected into molten plastic. The gas causes the expansion of the molten plastic in the mold. The molten plastic will trap the gas bubbles, creating an internal structure that is porous and resembles a foam.

Chemical foaming is also often used, and involves the addition of a chemical blowing agent (CBA) like Azodicarbonamide (ADC) and sodium bicarbonate or citric acid, to the resin. Upon heating, the CBA decomposes and releases gas to create the same effect as experienced in physical foaming. Foaming creates a solid outer skin and a foam-like core. This lowers material usage and helps to keep the product lightweight, without undermining dimensional stability.

2. Using Ribs to Complement Thin-walled Structures

Another important practice in automotive lightweight manufacturing is the use of advanced manufacturing techniques (like thin-wall injection molding and vacuum forming) to create parts with thinner walls (<1mm thickness), while preserving the structural integrity of the part.

This injection molding technique uses high pressure, speeds (>1000mm/s), and advanced machinery to ensure proper filling of the thin cavities. Thin walls are usually supported using ribs and gussets to provide rigidity and strength where these properties are required. Ribs can also prevent defects like sink marks.

3. Consolidation of Multiple Parts

When an automotive part is made up of multiple components, each of the different components must be welded or fastened together. The welding or fastening agent eventually adds to the weight of the finished part. In lightweight manufacturing, parts that are too complex are redesigned to make it easier to produce them using a single injection molding process.

Consolidating multiple parts into a single, molded unit eliminates the need for secondary fasteners, such as rivets and bolts, which reduces the part’s weight. However, molds for creating snap-fit designs that require no extra fasteners during assembly may need the addition of lifters or sliders, which will potentially increase their cost. Other benefits of parts consolidation to automotive lightweight include:

• Creating structures with a single continuous part usually has higher structural integrity compared to multiple pieces joined together, which may introduce weak points at joints.
• Consolidation allows manufacturers to create more parts using less labor and at a reduced cost.

You may also be interested in “Difference Between ICE And EV Parts Manufacturing“.

Automotive Lightweight Manufacturing Using CNC Machining

Computer Numerical Control Machining, or CNC Machining, is one of the common subtractive manufacturing methods. In this manufacturing process, a pre-programmed software controls a machine tool to precisely cut a block of material (wood, plastic, or metal) into a desired part or product.

The high level of precision of the machining tool makes this technique useful for the creation of complex designs. Also, the high level of automation eliminates human errors and interventions, which can help manufacturers save labor costs. Common automotive lightweight components created using this technique include:

• Electric vehicle motor components and cooling systems
• Chassis and suspensions, including control arms and brackets
• Parts of the engine, including the engine block, piston, cylinder heads, and crankshafts

One of the reasons why CNC machining is crucial for lightweight manufacturing is because of its material versatility. It can be used for creating parts from different materials, including aluminum, carbon fiber, titanium alloys, magnesium, and other specialized plastics. These materials are often chosen for their strength-to-weight ratio.

It is crucial to maintain high-dimensional accuracy when creating optimized lightweight components. Inaccuracies in dimensions can compromise the performance, functionality, or structural integrity of the product or part. Modern multi-axis CNC machining, like the 5-axis machines, can create complex multi-dimensional parts. Advanced lightweight manufacturing design modifications that can be achieved using CNC machining include:

• Complex hollow or internal channels: In the design of hollow sections of automotive parts like engine components and cooling plates, CNC machining is used to precisely remove materials from the internal components in a way that is practically impossible to achieve manually. To create automotive lightweight parts, this technique can be used to hollow out sections where strength is not necessary, thereby cutting down the weight of the part.
• Creating parts with tight tolerance: CNC machining can be used to achieve an extreme level of precision (around ±0.01mm), accuracy, and consistency. This increased level of precision ensures every part fits perfectly, which can increase safety by using the lowest possible material thickness.

The high precision of CNC machining optimizes production in a way that cuts down material waste compared to other traditional methods. This is particularly useful for automotive lightweight manufacturing using high-performance, expensive materials.

Automotive Lightweight Using Hybrid Manufacturing

Hybrid manufacturing is a term that describes the combination of different manufacturing techniques to create lightweight parts. For example, CNC machining (a subtractive manufacturing process) is paired with 3D printing (an additive manufacturing process) to create complex, lightweight parts with tight tolerances that would be harder to achieve using either of the methods.

Hybrid Manufacturing Using 3D Printing and CNC Machining

Hybrid manufacturing leverages the complementary strengths of the individual techniques in material efficiency, design, and finishing. A common hybrid lightweight manufacturing combines the powers of 3D printing and CNC machining.

3D printing is used to create highly complex internal geometries like hollow channels or lattices. Hybrid manufacturing unlocks a level of design freedom that is unmatched by other methods. This additive manufacturing process’s greatest strength is in creating these sorts of complex internal geometries without compromising structural integrity. However, it performs poorly in tolerance and finishing.

Therefore, after 3D printing the hollow part using a lightweight material, CNC machining is used in post-processing to achieve the desired tolerance and extreme precision (±0.002 mm) in the interior structure and smooth surface finish on the exterior (Ra0.4μm). Other benefits of using a hybrid lightweight manufacturing process involving 3D printing and CNC machining include:

• Greater reduction in material waste: 3D printing is first used to create the hollow shape, and CNC machining only has to remove minimal material, cutting waste and costs.
• Faster production cycles: Since 3D printing and CNC machining can be automated, combining both eliminates manual movement of parts, which can slow down the manufacturing process.
• Streamlining production process: An integrated software manages both processes, which helps to eliminate inefficiencies and errors.

Hybrid Lightweight Manufacturing Using 3D Printing and Injection Molding

3D printing is often combined with injection molding, especially in the Voxelfill process ^[3]. The process was developed and patented by AIM3D. The Voxelfill process uses a 2-step manufacturing process to overcome the weakness associated with the Z-axis of layer-by-layer 3D printed parts as follows:

• The first step is the creation of the lattice structure using 3D printing: The structure that resembles a honeycomb is 3D-printed using a composite extrusion modeling system.
• The second step is the filling of the lattice or Voxel filling: An extruder is used to inject thermoplastic material into the lattice’s internal cavities. The filling material can be foams and is intended to increase the stiffness and strength without increasing the weight.

The future of lightweight manufacturing revolves around multi-material design (MMD). Instead of a blanket substitution of material, MMD strategically places the best material for a specific requirement in the right location. For example, high-strength steel can be used in areas that require high crashworthiness, while aluminum is used in outer panels where the priority is weight reduction.

References

[1] U.S. Department of Energy. (n.d.). Lightweight materials for cars and trucks. Office of Energy Efficiency & Renewable Energy.

[2] University of Tennessee Knoxville. (2023, February 27). PhD student tests composite crashworthiness in unprecedented depth. Department of Civil and Environmental Engineering.

[3] Engineering.com. (2022, October 24). What is the Voxelfill process? Engineering.com.