(4) Development and Refinement of High Cycle CFRP Molding Processes
- The MOMA Concept Car and High Cycle CFRP Molding Processes
Automotive Materials Vol.10 No.2 December 2013 Issue #29
- ※Photographs and diagrams have been omitted for copyright reasons.
CFRP molding materials and processes can be grouped according to the performance, quality, utility for mass production, and formativeness of thermoset CFRP and carbon fiber reinforced thermoplastics (CFRTP), as shown in Figure 5.
Previously the most common molding method was the autoclave process mentioned above, which achieved outstanding strength by hardening the material for a long time at low pressure. However, since molding was time-consuming and required large equipment, this method was well-suited to high-added-value CFRP products but unsuitable for mass production.
In recent years, attention has turned to high cycle CFRP molding processes, and Resin Transfer Molding (RTM) has established a strong presence, especially in Europe.
Meanwhile, Mitsubishi Rayon developed the Prepreg Compression Molding (PCM) process, suited to medium-sized production runs of up to 3,000 units per month. This is a high cycle CFRP molding process based on high-pressure press molding techniques (with press pressure of 30-100 kg/cm2) and press molding of quick-hardening (2-5 minutes) prepreg consisting of carbon fiber impregnated with thermoset high-cycle plastic.
These high cycle CFRP molding processes are set out in Figure 6, and Chart 2 compares their characteristics. The chart shows that the properties of CFRP molded using PCM processes, especially quick-hardening prepreg for press molding, are equivalent to those of CFRP molded using the autoclave method. Thus, compared to RTM, PCM has the advantage of enabling the use of low-cost autoclave molds for trial and error at the prototype stage of development, before switching to PCM processes, which use high-cost molds, for mass production.
PCM processes can be broadly divided into two stages: preforming, which includes pattern-cutting and laminating as well as preforming, and high-pressure, high-temperature press molding using high-output hydraulic presses. To optimize productivity, preforming processes need to be matched to the cycle time for press-molding processes (Figure 7). PCM is a high cycle CFRP molding process enabling use of not only quick-hardening prepreg for target materials and parts, but also carbon fiber sheet molding compound (CF-SMC) and hybrid composites (Figure 8).
As an example of how PCM processing can be used for exterior panels, combining unidirectional (UD) prepreg carbon fiber, which is suited to applications requiring rigidity, with high-pressure press molding technology achieved the goal of devising a technique for mass production of CFRP trunk lids that are 40% lighter than aluminum units while maintaining equivalent performance. This technique has the following features (Figure 9):
- （1）Achieves A-Class appearance required for external panels on luxury cars.
- （2）Achieves void-free formation of molded parts (for process stability and reproducibility).
- （3）Eliminates paint pinholes arising from CFRP parts.
- （4）High-Tg properties achieve equivalent paintability to metal exterior panels.
To devise molding processes suited to structural parts, Mitsubishi Rayon also developed hybrid molding methods enabling simultaneous molding of CF-SMC and quick-hardening prepreg, and P core molding methods for shaping complex hollow parts. Hybrid molding processes have the following features:
- （1）Achieve void-free formation of molded parts (for process stability and reproducibility).
- （2）Simultaneous molding of prepreg and SMC achieves adhesive properties superior to those of adhesives.
- （3）Handle complex shapes (ribs, bosses, metal inserts, etc. [Figure 10]).
Previously, skilled operators carried out the time-consuming task of preforming complex three-dimensional shapes (especially complicated deep-drawn shapes) by hand, but Challenge succeeded in developing high cycle preforming technology that maintains tension on prepreg while affixing it to the face of the mold for shaping in the press. For example, this high cycle preform technology reduced the time required to preform a half-size seat back (Figure 11) from more than an hour to less than 10 minutes, and this automation achieved the goal of enabling mass production.
The trend towards full-scale use of CFRP components is gathering speed around the world, and PCM processing has gained a level of awareness matching that of RTM processing as a method of molding thermoset plastics for medium-sized mass production runs.
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- Photographs and diagrams have been omitted for copyright reasons.