This study investigated the viability of laminated molds made with an FDM 3D printer as molds for autoclave molding of GFRP. Our results suggest that PC molds are effective as male molds for autoclave molding when a fluorine-coated tape is used for surface treatment.
4 − 1 Male molds for autoclave molding by 3D printing
This study created the upper core and midsole with a hexagonal star structure inside a 3D printed mold. The print time for each mold was about 18 hours for the upper core and 8 hours for the midsole of one foot. It is expected to take more than 24 hours for a mold made by the cutting process. Therefore, the molding in this study contributes to shortening the manufacturing time and reducing the mold's weight due to its hollow structure. A previous study of 3D printing compared honeycomb and solid structures and reported that the honeycomb structure reduced the molding time by about half and the material consumption by about 80%[7]. Despite the difference in the material used, the result from the present study generally agreed with the previous study, and it is likely that a structure with vacant space is appropriate to reduce molding time and material consumption. On the other hand, it should be noted that it is necessary to carefully consider the conditions for printing, such as the internal shape of the molded product, in high temperature and pressure conditions.
The PC additive manufacturing used in this study did not show any significant deformation even after more than 15 autoclave molding cycles. Compared to ULTEM 1010, which was confirmed to be a valid material for autoclave molding[7], PC is low-cost but has a lower heat resistance and impact resistance. Therefore, PC is a great material to be used to create a mold for autoclave molding, and the use of general materials such as PC will have a positive impact on projects that involve small lot production and many shapes and specification changes such as tailor-made equipment. However, even with the same structure, the success or failure of molding differs depending on the surface treatment method, suggesting that appropriate surface treatment is necessary as described in the following sections.
4 − 2 Surface treatment
The surface roughness of the part is a defect in the FDM additive manufacturing product. This is caused by the side-by-side line or staircase effect that results from layer-by-layer deposition, which results in a rough surface, and is most noticeable on inclined or curved surfaces[11–14]. In general, these defects can be minimized or eliminated by chemical treatment, laser treatment, heat treatment, and ultrasound treatment[15]. On the other hand, each process requires expensive equipment and specialized techniques, and another potential issue is that such processes might decrease the strength of the additive manufacturing product. Therefore, in the present study, three types of surface treatment methods were tested to investigate a low-cost and simple treatment method.
Among the surface treatment methods tested in the present study, the fluorine tape was the easiest for demolding. As for the clamping process, there is a possibility that additional finer polishing would have improved demolding. However, the polishing process requires great skill to erase the stacking marks in the FDM method completely, and excessive polishing causes a change in the shape of the mold. Therefore, it can be concluded that the use of fluorine tape as a surface treatment method is an easy and practical way that enables the use of a laminated PC product as a mold for autoclaving molding.
The use of aluminum tape deformed the mold significantly. Since the aluminum tape is a metal, it has high thermal conductivity. Therefore, the large deformation with the aluminum tape was likely caused by the excessive temperature rise that went beyond the thermal deformation temperature of the PC as well as the exposure to high pressure. On the other hand, there was little deformation with the fluorine tape, and it might be the case that the tape served as insulation. Even though a surface treatment with tape might increasing the size of the mold, the thickness of the tape is collapsed under high pressure, and the risk is likely minimal.
Our results suggest that fluorinated tapes are a convenient and effective surface treatment. However, if the overlapping of the tapes is insufficient, there is also the issue of resin entering through the gaps between the tapes. In this study, the weight of the upper core mold increased by 4g, and the weight of the midsole mold increased by 2g before and after molding, suggesting some amount of resin being stuck on the mold surface. Therefore, the mold must be polished to remove the resin before it is used again.