Article written by ADMASYS CZ (formerly 3DWiser)
Modern desktop 3D printers have surprisingly versatile uses. This is demonstrated by Martin Tobiáš's project – hybrid casting of complex engine parts. He was able to significantly shorten the process from 3D scanning and CAD design to the final casting. In practice, it shows that thanks to 3D printing from polymers, it is not necessary to produce a very expensive permanent mold with complex shapes and cores. Every single casting can be original. This is very advantageous, for example, when developing prototypes of complex machine parts. In addition, the prepared 3D model of the casting is printed directly, including the inlet system, without the need to prepare drawing documentation. This results in a significant acceleration and cost reduction of the production process, especially in the case of piece and small-batch production of complicated castings.
At his alma mater – Brno University of Technology – Martin Tobiáš has long been dedicated to design, reverse engineering, rapid prototyping, measurement and testing in the automotive industry. It uses 3D printing on the UltiMaker S3 (and S5) devices supplied by 3Dwiser and the subsequent technology of so-called hybrid casting. Martin's hobby and work became the production of prototype engines for motorsport and the production of increasingly sought-after replicas of historic vehicle parts, without which many veterans would no longer drive. In addition to fidelity and functionality, the production price plays a significant role here.
The project of development and production of a prototype racing motorcycle engine would not have been created without the support of C.S.O. dedicated to the precise production of components for demanding industries. Foundry technology was provided by the Foundry Department of BUT and Alucast s.r.o. The experience of the practical use of hybrid casting led Martin Tobiáš to found the MTA engineering company, which will further specialize in this technology in cooperation with the aforementioned entities.
3D printing of polymers (FFF) on dual-material UltiMaker desktop printers was already used in the original project during the design phase, when selected components were printed to verify correct installation. Finally, the entire engine was also printed to verify its placement in the chassis frame. As part of the development, special cylinders were also printed to measure the efficiency and flow function doc. Karl Jaroš. It was already necessary to print such parts using temporary water-soluble supports, and this method of dual-material 3D printing then advantageously led to the production of castings - so-called hybrid casting.
The cylinder of a modern water-cooled two-stroke motorcycle is complex in shape. In addition, it requires high manufacturing precision for optimal function and contains complicated water jacket cavities. The most suitable method of production is therefore casting.
"The hybrid casting method we use in ceramic molds makes it possible to move from the phase of a 3D CAD model to the phase of a dimensionally accurate product relatively quickly and cheaply. It allows the designer to realize complicated shapes and choose from many different materials. In combination with 3D printing of flushable supports, it is also possible to prepare structures with complicated cavities - while maintaining a high-quality surface," mentions Martin Tobiáš.
Direct printing from metal materials is often demanding and expensive, requiring specialized technologies. The production of permanent molds for castings is also very expensive, as are any later changes to the molds, if they are even possible.
“Our method requires printing a separate model for each individual casting, so it's easy to make each piece unique. In combination with casting simulations, we are able to design the technological process in such a way that we reliably achieve flawless results. In the case of the production of prototypes and functional samples of components, where later changes in the design can be assumed, these do not lead to high additional costs," reminds Martin Tobiáš.
The same approach has also proven itself for other castings produced with the help of 3D printing. For the engine head, it was, among other things, that the resulting surface faithfully imitated the shape of the original part, or that it was cast in sand. When casting in sand molds, it is necessary to create a permanent foundry model facility. It is traditionally made by hand from wood based on drawing documentation. In the case of castings with internal cavities, it is also necessary to produce cores for the production of sand cores. But again, this is time-consuming.
Today, this method of production is uneconomical in many cases under domestic conditions. However, it is possible to help yourself by using modern 3D technologies. The original part can be reconstructed thanks to 3D scanning, the necessary technological modifications can be made on it, and it can then be printed. In simpler cases, the printout can be used directly to create a sand mold. In more demanding cases, it is used to create a negative for casting the model from a special, mechanically resistant foundry resin that can be molded into sand. In this case, it is also necessary to take into account the double shrinkage of the material.
"When producing replicas of historic vehicle parts, it can be advantageous for a number of reasons to approach production in the classic way - sand casting. After all, 3D printing enables us to effectively realize the production of model equipment," mentions Martin Tobiáš. "The process of producing replicas usually begins with the reconstruction of the original part from a three-dimensional scan. The original drawing documentation is no longer available, moreover, we can preserve the appearance of the original production by observing inaccuracies that do not affect the function."
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Printing complex shaped models with temporary supports (from water-soluble materials) would not be possible without solutions with high reliability and precision. The affordable and universal desktop 3D printers UltiMaker S3 and S5 have proven themselves in various projects, and they excel in this area - and that is why they have been seen in the automotive industry for years with the most famous European car manufacturers.
But back to Martin Tobiáš, who is thinking about increasing the printing capacity by adding more UltiMaker printers.
"Cohesion of the layers, dimensional stability and overall reliability of the printing process are important to me. For example, 3D printing one piece of a two-stroke cylinder prototype takes three to six days, depending on the surface quality chosen. Any printing failure will have a very negative impact on the schedule of downstream production operations. Furthermore, the service and support of operating printers, as well as the supplier of original filaments, are also important to me. Consultations regarding news in the field of 3D technology are also advantageous," says Martin Tobiáš. "We use 3Dwiser's support as needed due to the operation of the printers, and in the future also with regard to plans to expand our printing capacities."
As far as materials are concerned, mounting fixtures, holders for measuring devices on vehicles and similar parts are most often printed from PLA, PETG, ABS and Nylon strings. For these purposes, Martin Tobiáš was also interested in the printing technology of very durable composites with continuous fiber from Markforged. Polymaker PolyCast on UltiMaker PVA supports turned out to be the best for making casts.
If there is a CAD model of the part, it must be adjusted from the point of view of foundry technology, i.e. allowances for machining, shrinkage, the design of the inlet system, etc. must be taken into account. This is why 3D scanned objects must first be reconstructed into a CAD model.
In hybrid casting in a ceramic mold, the model and sprue system are first printed from the 3D model; followed by their assembly, if they are composed of several copies. Then comes encasing the model with a shell, firing the model from the shell, gravity casting, controlled cooling, finally removing the shell. At the same time, the production of sand molds takes place using standard, generally known procedures, as well as the subsequent casting.
Finishing work plays an important role, especially heat treatment, sandblasting, shot peening, precision machining at the partner company C.S.O. In the case of the cylinder, it was also treatment of the working surface with Nikasil technology.
"Examples of parts produced in this way for demanding applications are pistons, cylinders, heads and casings of internal combustion engines. We are not limited to light alloys, it is also possible to choose from different types of steel and cast iron. In terms of size, we focus on small to medium-sized parts. In principle, this is the so-called gravity casting, with which we achieve excellent results thanks to the possibilities of simulations and testing," reminds Martin Tobiáš. "It is worth noting that without additional finishing operations, the resulting cast surface is only as good as the model print."
Where are the main advantages of hybrid casting production using 3D printing compared to traditional methods? Above all, in the speed and cost of the entire process, in eliminating the need for construction and production of permanent molds. This difference multiplies with the shape complexity of the products, especially in the case of their piece and small series production.
“The traditional method of shell casting, where a fusible model is produced by injecting wax into metal molds, is used in mass production. A 3D printer, however, makes it possible to produce a burnable model without the need to produce a permanent (mother) metal mold. The production method used by us allows us to solve the wrapping of the model manually so that it is not necessary to use ceramic cores to form complex shaped cavities. With the standard injection technology of wax models, flushable or ceramic cores must be used, which makes production significantly more expensive," summarizes Martin Tobiáš, founder of MTA engineering.
"For parts with very complex shapes, where it is not possible to produce a demountable mother mold, production using hybrid technologies may be the only possible solution. In the case of the production of replicas cast in sand, the procedure with the use of 3D printing is significantly accelerated, also in proportion to the complexity of the manufactured part."
An example of the financial advantage of the mentioned procedure is the example with the above-mentioned prototype cylinder of a large-volume two-stroke engine, when the price of the product was reduced by an order of magnitude - from approx. using 3D printing). And this can be a crucial argument not only for replica parts for veterans.
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