Additive manufacturing Voxeljet accelerates all kinds of casting processes

Author / Editor: Dr. Ingo Ederer / Alexander Stark

Whether sand molds for metal casting or plastic melt-off models for investment casting — foundries benefit in both cases from the many advantages offered by additive 3D printing processes.

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With an overall build space volume of 4 x 2 x 1 meters, the VX4000 is the largest industrial printer in the world. On the one hand, the huge construction space permits the fast production of extremely large individual molds, but it can also be used flexibly for the economical production of entire small series.
With an overall build space volume of 4 x 2 x 1 meters, the VX4000 is the largest industrial printer in the world. On the one hand, the huge construction space permits the fast production of extremely large individual molds, but it can also be used flexibly for the economical production of entire small series.
(Source: Voxeljet)

For many years now, a large number of foundries have regarded sand casting molds and cores produced with a 3D printer as the standard. This technology is well established in the field of iron and metal casting and is used wherever it pays off. While these applications primarily lie in the area of prototypes and small batches, the limits are increasingly shifting toward bigger and bigger volumes, as the performance of the 3D printing systems continues to improve.

Due to the elimination of tool costs, 3D printing up to a certain batch size is always more cost-effective than conventional printing methods. The smaller the batch size, the greater the cost advantages of Voxeljet technology. With complex geometries in particular, 3D printing is the most economical choice, even in batch sizes of several hundred units, although it cannot replace classic mold construction in large series. Other benefits range from shorter production times to less post-processing of the unfinished cast parts. The same essentially applies to investment casting: Here, 3D-printed plastic models replace the classic wax models, which are very time-consuming and expensive to produce with injection molds.

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Printed Sand Molds for Metal Casting

In contrast to the conventional production of molds, in which only the production of model plates or core boxes can take several weeks, with 3D printing even complex sand molds can usually be printed overnight or in a few days.

The molds are created without expensive mold set-ups, and are produced in a fully automated process purely based on CAD data in a layering process, in which 300-micrometer quartz sand layers are applied repeatedly and selectively glued together with a binder, using the system's print head. After the printing process is complete, the mold only has to be unpacked and cleaned of excess sand — that's all. Since the sand molds are produced directly on the basis of CAD data, they set the standard where rich detail and precision are concerned.

Along with a shorter production time, design freedom is also far less limited than in conventional manufacturing. Designs can be made true to their structure without having to watch for draft angles, separating lines or undercuts. Even molds that are modified during the testing phase can be printed immediately in accordance with the new CAD data, without time-consuming tool modifications. Moreover, gating systems can be customized individually to parameters such as casting pressure, thus avoiding turbulence and increasing quality.

Combination of 3D-Printed Cores and Classic Mold Construction

Innovative foundries today also use a combination of 3D-printed cores and conventionally produced molds whenever it pays off. This approach is suitable, for example, for the production of complex cores with undercuts, such as those required for impellers. The cores can be printed in the 3D printer and then integrated into the conventional mold. The advantages lie not only in the reduction of the number of parts and in the considerably minimized effort required for mold construction process, since the time-consuming conventional production and assembly of the complex cores is no longer necessary, but also the subsequent mechanical reworking is reduced.

Another interesting alternative, which experienced die casters are increasingly choosing, is the parallel production of the time-consuming molds and the 3D printing of the sand molds. Since the printed sand molds are immediately available, the first parts can be cast for test purposes in order to optimize the tools under construction. In many cases, this variant is faster and more cost-effective than classical mold making alone.

In terms of stability and strength of the molds, 3D printing and classic mold making are on the same level. With adjustable values between 220 and 380 N/cm2, the bending strength that can be achieved in the layering process is in the range of the strength of conventionally manufactured cores.

High-Precision Melt-Off Models for Investment Casting

In addition to printing sand molds, more and more foundries are also using models from the 3D printer for investment casting. This process enables the simple and uncomplicated production of wax models. The wax models are then no longer made of wax but of plastic, but this is irrelevant for the subsequent process steps. The production of PMMA models in 3D printers is very simple: they are printed exactly according to CAD data. In order to increase the quality of the melt-off process, the plastic models are infiltrated with wax, which gives them a particularly fine-pored and homogeneous surface.

Advantages of 3D Printing for Investment Casting

In addition to tool-free and cost-effective model production, the process also offers impressive time savings. The Voxeljet Service Center can provide models up to 1,000 x 600 x 500 millimeters within a few working days. Of course, the large Job Box can also be used to print complete small batches in a single print job. The subsequent steps in the investment casting process are identical both for the use of classic waxed models and for the 3D-printed models.

Case Studies

How powerful the Voxeljet printers are in practice can be seen in a wide variety of applications, such as the rapid manufacturing of two-stroke motors for chainsaws. A motor is just 78 x 76 x 59 millimeters in size. In order to produce the models as quickly and cost-effectively as possible, Voxeljet has combined 780 of these motors into a single print job in a build space measuring 1,000 x 600 x 500 millimeters. With the VX1000 high-performance printer, printing was completed in 23 hours, which corresponds to a printing time of only 1.8 minutes per motor.

The traditional production of prototypes can take months. 3D printing is quite different: as soon as the CAD draft is ready, the data is sent by e-mail to Voxeljet's order processing department. Depending on the size of the workpiece, printing takes one to two days. Since the core is printed "in one piece", there is no need for the time-consuming final assembly of individual components.

One of the companies convinced of the advantages of Voxeljet technology is the pump manufacturer Nijhuis. At its Winterswijk site, the Dutch company casts both pump housings and impellers weighing up to 800 kg. The company has already successfully completed several projects with Voxeljet. "Especially a company like ours, which produces many prototypes and small batches, benefits a lot from the possibilities of 3D printing in terms of time and quality," says Nijhuis development engineer Luke Vrielink.

Cylinder Head Reconstruction for Vintage Cars

3D printing technology can also play out its strengths in the spare parts sector. Often only small quantities are required or individual parts have to be reconstructed, as in the case of cylinder head parts for a Porsche engine. In the event of damage, only customized parts reconstruction helped — 3D printing turned out to be the most economic option.

The reconstruction of a Carrera aluminum cylinder head began with the measurement and scanning of the defective component. Valve guides, seat rings, camshaft bearing, intake and exhaust ducts, cylinder head screws etc. had to be set up as 3D base bodies in a meticulous detailed process. The next step was the transfer to superordinate functional models and the adding of design features from casting technology like site measuring, bevels, and fillets.

The geometric reconstruction was followed by the production of the sand cores. The implementation of the project with conventional cores on based on core-making tools was impossible for cost reasons alone. The only alternative was to produce the cores using a 3D printer.

The printing of the complete core package with a total of eleven cores was carried out by the Voxeljet service center. Thanks to the outstanding print quality of the Voxeljet printers, it was also possible to print the extremely thin-walled cooling rib measuring two millimeters without additional support structures in the inner and outer cores.

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Component Optimization with Computer Simulation

By combining topology optimization, casting simulation, service life analysis and 3D printing, the stability and weight of components can be optimized. By sing software solutions the surface of a cast aluminum wheel carrier could be optimized, its manufacturability analyzed, and its service life verified. Voxeljet produced the molds required for the component using a 3D printing process. The optimal shaping through structural optimization and the design freedom in mold construction offered by 3D printing, significantly improved performance characteristics. The result is promising: The stiffness of the newly designed wheel trunk could be increased by 3 to 5 times, without the need for additional material. A further advantage is that the production process is already well established and certified in many industries. It is also suitable for series production.

The number of practical examples could be continued at will. Each individual case confirms how 3D printing technology expands the tool range of foundries considerably— without replacing traditional mold making. Whether in the spare parts business, in small and special series or in prototype construction — whether in the automotive industry, the aerospace industry or in classic mechanical engineering — additive manufacturing can help foundries to achieve significant competitive advantages in all areas.

This article was first published by Schweizer MaschinenMarkt.