Fiber composite materials are well known for use in lightweight construction. Conventional shaping for the molds means working with milling machines to obtain the desired shape. The level of complexity is therefore limited. This article presents a method where the laminate structure is built on a core which is generated by 3D printing. The base material of this core is not harmful for the environment and can be easily dissolved by water. Simple process steps make it possible to produce a complex body within several days. This indicates a considerable potential for producing prototypes and small series using the 3D printing technology.
The production method for lightweight composite structures is characterized by several steps. First, a tool has to be generated, which defines the shape of the product. This tool can represent the outer or inner surface of the body. Depending on the complexity of the part, the tool has to be divided into sub-tools, which are combined to a bigger structure. As an alternative, the various sections of the tool can be laminated, and the sub-molds can then be glued into the finished mold in a subsequent production step. Common tools could be considered to work in two different ways: As a lost mold and as tool for many production cycles.
A significant reduction of formwork can be reached using lost cores, if bodies with a shape like a tubing network are desired. These shapes are very common in the field of lightweight engineering. As cores usually bound, sand or other formable material is used. This material is shot into shape in a process similar to the injection molding process. In certain cases this mold is then coated with a sealing material to prepare it for the next steps, followed by a laminating procedure.
Depending on the desired surface quality, the core is coated with a gel coat. Now different lamination technologies can be used. On the one hand there are pre-pregs. These tissues, containing glass fibers or carbon fibers, are prepared with a polymer which sticks onto the surface and can be hardened to a high strength level at higher temperatures. The temperature level for common material is up to 180°C. A weaker material will be the result of a cold laminating process. A common method here is to bring on a polyester resin by brushing or spraying, partly hardening the resin and adhering the fiber materials onto the sticky polymer by a roller. Compared to the pre-preg technology, this method makes it possible to achieve a higher level of complexity.
The named technology is based on a tool to produce the core. This dependency means a reduced capability for producing smaller series. The manufacture of the product takes too long, and the production of prototypes is not financially feasible. Here a lack of consistency in the product development process can be found. Printing cores using 3D printing technology can close this gap. In the following the production process of a motorcycle part is shown step by step in order to present the possibilities of this process.
3D printing of cores:
voxeljet uses the powder-based 3D printing process. This process is characterized by a stepwise production of the part directly from CAD data. During the preparation process, the data is sliced into bitmap pictures that can be processed by the printer.
In the first step, a specific amount of powder is applied to a building platform inside a building box. The powder spreading unit is guided over the build space and levels out the powder. After this step, an ink-jet print head prints an activating liquid on the powder. This activates the binder in the powder layer, and it glues together the particles. In order to build the body of the core, the building platform is lowered one layer and the process starts again. This cycle is repeated until the part is completed in the powder.
After the printing process the part can be unpacked from the loose powder. In order to clean it properly, it is blasted by air or treated with powder blasting. A short hardening phase in a convection oven completes the process.
The voxeljet IOB material is used as the base material for the cores. This material mixture consists mainly of silica sand. In addition, the powder material also contains a binder that is activated by the application of a water-based liquid with the print head. All components of the material system can be considered environmentally-friendly.
The 3D printing process generates parts with a certain porosity. This porosity has to be closed for safe processing during the lamination process.
In order to reach an acceptable result, a two-step process is chosen. The first step involves conventional sealing with a slurry normally used in the foundry industry. Here the material Zirkofluid 6672 from Hüttenes-Albertus is used. This slurry is based on alcohol as a dispersion agent, so there is no effect to the printed core. The coating is applied by quickly dipping the part in a bath of Zirkofluid. After dipping, the part has to be inspected for overspill and droplets. These are removed with a wipe. The slurry coating process is completed by drying the parts at 60-80°C in a convection oven.
The second step of the coating process involves the application of a fine layer that completely closes the pores. This can be achieved by using the product Aquaseal by Aeroconsult from Switzerland. Aquaseal is a water-based agent which can be brushed on the surface or applied to the surface using an airbrush. Dipping is not recommended in order to avoid the dissolving of the water-soluble part.
The part is subsequently dried in the convection oven at 60-80°C. Once it is completely dry, the coating can be repeated several times in order to get a securely sealed surface. The bluish color indicator in the Aquaseal material is a useful tool for evaluating the quality level that has been attained.
The prepared core is now ready for the application of the laminate. In this case a polyester material is used. This polymer is reinforced by glass fiber tissue.
A quickly hardening material with an open time of approximately five minutes is used, so this step can be quickly repeated without significantly hindering the work process. The mixture of the two material components is brushed on the core. If the surface becomes sticky due to the polymerization, the glass fiber tissue is applied and smoothed out with a brush. These steps are repeated until a material wall thickness of one millimeter is reached.
Core removal and finishing:
In order to remove the core, the freshly laminated structure is opened by drilling holes in the polymer material. These holes are placed at locations at which the metal inserts are applied during a subsequent work step.
The core is dissolved in warm water. To reach a good dissolution rate, the entire part is dipped into a water bath. Moving the core promotes convection in the bath and thus accelerates the dissolving process. The part remains in a bath of about ten liters for two hours.
In some situations, it is necessary to produce the required convection in the long channels of the laminate body by using a tube and syringe. All material must be safely removed to optimize the weight of the part.
By its nature, the part will feature a rough surface and various imperfections such as entrapped air or misaligned fiber material after this process. Hence a step of filling and grinding should be added to achieve a smooth surface. The part can be painted after that step.
This article describes the use of cores produced with the 3D printing method in connection with composite fiber material. Thanks to the rapid manufacture of the laminated body, this method is ideally suited for the production of prototypes and small series parts for lightweight construction. This creates a closed process chain for the use and investigation of possible application areas.