Water-Injection Technology is Key to Lightweighting of Automotive Door Panel

By on May 25, 2019 in IN THE NEWS

Renault turns to thermoplastics.

For several decades, thermoplastic and thermoset materials have been used in the automotive industry, with steady improvement in thermal and mechanical characteristics, processing behavior and end-product quality.

At the same time, manufacturers have been responding to customer demand for connectivity, autonomous operation and comfort features that add to the overall driving experience, while also protecting the environment and reducing the use of fossil fuels. When it comes to reducing emissions, automakers have taken advantage of the following:

• Optimized engine designs and other new technologies
• Reduced mechanical friction within the engine
• Lightweight construction and improved aerodynamics

Renault’s EOLAB
Recently, Renault introduced a lightweight-construction prototype automobile called EOLAB. Among the objectives of the project was the investigation of materials and production methods that could be applied to exterior bodywork. The aim was to meet current and future requirements and regulations, while also saving vehicle weight and delivering new features and functions to the customer. Of course, the increased use of thermoplastic materials would be one of the solutions evaluated, and converting the side-door modules from metal-frame construction to synthetic materials became one of the projects.

Among the objectives of the project was the investigation of materials and production methods that could be applied to exterior bodywork.

Using the technical specifications of the doors on Renault’s popular Megane Coupe production vehicle, the challenge was to develop a replacement for the door’s central structural layer using 40 percent-glass-filled polypropylene (PP LGF40). In the end, the door-frame solution would meet several critical objectives:

• Design freedom, including the functional integration of features without assembly or welding
• Lower weight … the thermoplastic structure was almost 6 kg (13 pounds) less than the steel design
• Low total cost of production with an impressive 90-second cycle

Yet, getting to that end result would not be easy.

The engineers wanted to use many of the same door components—seals, windows, locking systems, handles— as were used in the original metal design. This meant that the glass-filled PP door would need to have stiffness and other mechanical properties similar to the metal structure. Designers decided to include a large cross-section channel around the outside of the door to meet these requirements. This channel would form the upper and side frame for the window and also connect the upper and low portions of the door.

Gas Injection Falls Short
In order to keep weight to a minimum, Renault designers planned to use gas injection to hollow out this critical channel. However, the results of initial molding trials were disappointing since both costs and cycle times exceeded project targets by a substantial margin. So, they approached PME fluidtech (Ettenheim, Germany) to see if water-injection technology (WIT) could be used to achieve better results.

Water injection, of course, requires different mold-design features than gas injection for several reasons. First, gas injection uses an overflow chamber to receive the resin displaced by gas. PME fluidtec, on the other hand, planned to use a technique that uses water to drive displaced material through hot-runner valves “back-to-barrel.”

This decision necessitated redesigning the existing mold, with special attention to the hot-runner system. Hot-runner design normally would be straightforward for a typical injection-molded part, but there were no established principles for a part that includes a large cross-section channel as a structural element.

It was difficult to determine the number and location of the hot-runner nozzles, and how the flow paths should be configured, in a part with wall thicknesses that range from 2.7 mm in most of the part and 30 mm in the channel sections. That’s mainly because the pressure needed to fill the large-diameter sections is significantly less than what is required to fill the thinner areas.

This large pressure delta meant that as melt flows into the channel during the injection phase, filling stops at all other gate points or does not start at all. Therefore, the location of the gates and sequence of filling had to be carefully organized to achieve proper filling and avoid weld lines.

In addition, the large cross-section structural channel itself had to be reconfigured slightly so that melt could flow in at one end and water at the other, pushing material back to the barrel at the injection site. Also, the water had to drain by gravity out of the channel after doing its work. This meant the water injector had to be located at a low point in the tool. In the end, the mold would be mounted in the machine at an angle (rather than square to the platens) to achieve the correct orientation.

Renault engineers conducted computer-aided mold-filling studies with the original mold design, anticipating gas injection, but there was not much time to restudy the new tool design. Consequently, the automaker had to rely on the experience of PME fluidtech technicians and a certain amount of trial and error to get the design right.

Water or Gas Injection?
One reason that Renault decided to switch from gas injection to water was the greater flexibility and stability of water, due to its incompressibility. Thus, pressure and volume can be managed and controlled more effectively during the cycle.

The engineers wanted to use many of the same door components—seals, windows, locking systems, handles—as were used in the original metal design.

WIT also makes it possible for the channel to end at the place where it is required. The water pushes material back into the barrel until the hollowed-out area extends as far as it needs to and no farther. In fact, the water volume and pressure can be controlled so precisely that the process is perfectly repeatable. The WIT system also controls the valve gates of the hot-runner system, as well as other mold or machine movements so that all functions are perfectly synchronized. The process is precisely reproducible, from cycle to cycle and from one production run to the next.

Another advantage for using WIT is the cooling effect of the water inside the channel. Water can cool the polymer 40 times better than gas so the dimensional stability and resistance to deformation required for demolding is achieved very quickly. In fact, water injection can begin up to 10 seconds earlier than with gas, without the risk of delamination or penetration of the fluid into adjacent thin wall areas. Thus, cycle time reductions of 15 to 25 seconds can be achieved without adversely affecting product quality.

Excellent Results
In the end, the result of the prototype door project exceeded expectations.

• Compared to the steel version, the lightweight door module reduced weight by 9 kg (20 pounds).
• The new door passed all essential validation checks and could be assembled now in production vehicles.
• Cycle time is around 90 seconds, which was longer than the original target, but acceptable.
• Stiffness and other mechanical properties are about 5 percent better than predicted.

Automobile manufacturers like Renault and Nissan are already using plastics in components like tailgates in series production, proving the potential to reduce weight even in complex designs. The Renault door project has demonstrated that is practical to also use thermoplastics even for large components like doors and body panels.

It is a very good example of a lightweight component used in the automotive industry. However, similar results could be achieved in such components as:
• Front ends
• Car seats
• Backrests
• Battery cases
• Wheel wells

Thermoset and thermoplastics modules will certainly be a part of the global lightweight construction solutions.

As noted, lightweight construction is an important pillar in the automotive industry’s effort to reduce exhaust gas emissions. Thermoset and thermoplastics modules will certainly be a part of the global lightweightconstruction solutions. Likewise, water-injection technology will be equally important. Creating cavities that result in higher component stiffness opens up new applications, particularly for modules made from thermoplastics.

 

Authored by Friedrich Westphal, founder & CEO, PME fluidtec GmbH, Ettenheim, Germany

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