Manufacture by 3D printing of metallic pieces in lost wax vs DMLS

The world of manufacturing prototypes in metal alloys and small series manufacturing has advanced a lot thanks to 3D printing technology. However, there are some traditional techniques that have not disappeared, but evolved.

This is the case of the manufacture of lost wax, a millenary technique that evolved from having to make an original wax in a more or less handmade way, to make light wax molds and to be able to manufacture short series, until finally being able to manufacture pieces in 3D resin printers, such as Form 2 and the Castable resin, or FDM 3D printers printers with specialized materials for lost wax (MoldLay) that have revolutionized the market like PolyCast, thanks to its high precision and excellent surface finish.

Video 1: Manufacture of pieces by lost wax using PolyCast. Source: Polymaker

Returning to the beginnings of 3D printing, we find the PLA, a cheap material used by almost all 3D printers, including domestic printers, capable of manufacturing the original piece that is then inserted into a mass of sand and binder to form a mold when heated in the oven.

Some years ago (2015), when this technique was not yet in common use, ennomotive, an innovation platform with a global community of engineers, asked them, at the request of a client, to propose an innovative, faster and cheaper, to make their prototypes in titanium alloys for the world of aviation, and if it was possible, also manufacture short series, fully functional, without the economic cost and time involved in the traditional process of manufacturing metal molds for injection or casting of pieces.

At that time, one of the engineers of the community, from the Gaza Strip, who had developed his engineering studies in the USA and whose final project had been on this technique of 3D printing in PLA and manufacturing with the method of the lost wax proposed it as a solution. He knew his virtues very well.

The client, incredulous before a solution that saved him months and many thousands of euros, and that was not used or proposed by any of the thousands of engineers that the company had, commented: "The piece thus manufactured meets the strict standards of the air regulations? "The response was devastating:"Yes, and there are also those who are starting to use it, and we can put you in contact with who can make it, it will take 3 more days to transport".

At that time 3D printers that could make metal parts, they still did with only a few alloys, they did not get the necessary finishes, nor the same technical specifications of a piece made by molding (and centrifugation, or vacuum if necessary), apart to be slow and expensive.

But the technology continues its evolution, and the technical properties of the manufactured parts have improved, as well as their cost and the number of alloys that are available. However, the technology of the lost wax is still fully in force and evolving based on 3D printing FDM, so although the DMLS technology has advanced a lot, what would now be the recommendation of the engineers of the community if they were to ask again on ennomotive.com?

Well the answer is that varies, and much, according to the piece, technical requirements, dimensional, materials etc .., and it could well happen that the evolution of manufacturing methods, some of the engineers recommend: "Modify the design" o "Change materials", as they did the first time, asking why this piece was made in a titanium alloy and not in some type of resin that could meet the technical specifications demanded.

That is why there are a number of recommendations when choosing manufacturing technology, but it is the expert eye of the designer engineer, who knows these techniques well (and eye because they improve every day) what can be achieved with the best combination of quality / cost.

• At this time the two techniques get very similar surface finishes, highlighting one or the other depending on the quality of the process and the material. For example there are DMLS printers that offer great surface quality but there are also filaments for 3D FDM printing (PolyCast) that get a completely smooth finish.
• As with the surface finish, with the geometric precision the debate is fairly even, it being true that with the DMLS technology, final pieces with a greater accuracy of 0.1mm±0.3% are achieved.
• As for sizes, large pieces can be printed on the lost wax, of 2 m for example, while with DMLS they are usually smaller, although they can later be joined, reaching the necessary size.
• Where the lost wax takes advantage, it is possible to choose the material that is desired, without limits, while the catalog of materials for DMLS, although it is increasing, is limited.
• In contrast, DMLS technology allows more complex geometric shapes (for example, internal structures) and thinner walls, which is not allowed by the lost wax technology, which allows complex geometries but with a limit.
• The DMLS allows to print different components at the same time, integrated in the same piece (although this was not the case).
• But if you have at all speed and cost, which is fundamental for the industry, at the moment it is favorable to lost wax, which is why it is still widely used in the aeronautical industry; Although the rapid progress of the DMLS technology makes it closer and that every day is more used in the industry.

So in the piece mentioned above, since its geometry was not too complicated, and the material was a special alloy, for the production of short series, it would probably opt again for the lost wax, especially now that there are materials for 3D printing FDM as the PolyCast, a filament developed exclusively for the manufacture by lost wax. Although by the time you read this article, the tables may have changed. Will we ask the engineering community again? Or will we be aware of the latest in 3D printing?

The development of materials and technologies is so fast that they make a solution accepted as valid today, be out of date in less than 5 years. The frenetic pace of advances in additive manufacturing is unstoppable.

Article in collaboration with ennomotive.