HP-RTM Technology For Lightweight Automotive Design

 

The aviation industry is currently using resin transfer molding (RTM) to produce large, high-strength structural parts. However, its production efficiency is very low and sometimes requires a lot of manual work. In the manufacturing process of automobiles, RTM is used to produce parts mainly for better visual effects and is only used in small batches of top-class cars. If a reliable automated RTM line can be developed for mass production, fiber reinforced plastics (FRP) will be more widely used in the production of high-strength load-bearing automotive structural parts.

 

German processing machinery experts Dieffenbacher and KraussMaffei have cooperated in the field of RTM technology and responded to the above market needs by developing an automated production line for high-pressure resin transfer molding (HP-RTM). The production system includes preforming, pressing and finishing processes. Compared with the traditional RTM process, the HP-RTM process reduces the number of resin injections, improves the impregnation quality of the preforms, and shortens the molding cycle.

 

Preforming

 

For the production of HP-RTM components, a preform made of textile reinforcement is produced. Such preforms are manufactured at the Preforming Center in Dieffenbacher, usually using a fully automated process.

 

 

The soft fiber fabric or mat made of carbon or glass fibers is unwound from a reel and fed into a cutting machine. Using CNC cutting technology, the fiber plies are cut to the required dimensions for the component. This is done by a cutting program derived from the existing CAD part parameters. The cut fiber plies are laminated together and then placed in the forming unit.

 

Robots can be used to reliably handle the cut fabric, fiber mats, and preforms. The preform forming center can be operated as a separate unit or integrated into the production line with the pressing process.

 

Pressing process

 

The preform processing is followed by the pressing process. This process includes the impregnation of the preform with an epoxy resin system and its curing process.

 

After the preform is placed into the RTM mold by the robot, the actual pressing process is carried out using a Dieffenbacher hydraulic press with a pressure of up to 36,000 kN (3,600 t), depending on the mold pressure and the size and complexity of the component. The press achieves a closing speed of 450 mm/s and a pressing speed of 40 mm/s with a very short pressure build-up time. The Dieffenbacher short stroke system ensures that the press is very energy efficient, especially with short forming times and large slide strokes. Compared with conventional press forming technology, it can improve energy efficiency by 50%.

 

 

The press control system allows for different operating procedures and shorter production cycles. The mold carrier enters and leaves the press cyclically, requiring only short changeover times, loading and unloading processes, and cleaning of the lower mold half off the line.

 

After the high-pressure injection process, improved part quality can be achieved by repressurizing.

 

Injection process

 

The low-viscosity reactive mixture is injected into the closed mold to impregnate the preform. Using KraussMaffei technology, injection speeds of 10-200 g/s can be achieved, depending on the resin system as well as the size and process design of the part. In a closed-loop process, resin and hardener are precisely metered and mixed under high pressure to produce the reactive mixture.

 

High-pressure metering allows for shorter injection times and improved preform impregnation, which allows the resin system to be processed with shorter curing times. This results in shorter production cycles and greater cost-effectiveness. In addition, this provides additional benefits, such as lower porosity while maintaining excellent surface quality. Precise temperature control allows for further shorter processing cycles and benefits from special RTM resin systems.

 

 

KraussMaffei high-pressure mixing heads eliminate costly downtimes and the special materials required for cleaning conventional low-pressure mixing heads. The mixing heads are self-cleaning and therefore offer outstanding energy efficiency advantages in large-scale production, which has been proven in a variety of mass production processes.

 

The release agent feed block allows the release agent required for the process to be introduced directly into the compact, self-cleaning KraussMaffei high-pressure mixing heads with high accuracy and process reliability, even at the lowest dosages (e.g. 0.1 g/sec). Highly precise and reproducible metering of release agents is essential for the reliable implementation of downstream processes.

 

Finishing

 

Finishing is one of the last steps in the process chain. It involves trimming the outer contours of the component and adding mounting holes and insert cutouts. Finishing with a milling cutter is a customized solution developed by KraussMaffei. It can be done with an automated cutting table or a portable cutter. The choice of tool depends primarily on the size and complexity of the component. Robots are used to handle the parts between process steps.