You can use 3D printing to create casting molds, but there are several limitations inherent with the printers. However, alternatives are emerging, such as the use of paper for some casting applications.
By Dr. Conor MacCormack, CEO, Mcor Technologies
Traditional methods of producing castings with patterns that are CNC machined, hand sculpted and silicon molded can be expensive, time consuming, labor intensive and sometimes geometry prohibitive. These constraints often inhibit the production of metal prototypes during the product development stage and small-batch or one-off part manufacture.
3D printing molds for casting has been considered an alternative, but foundries haven’t generally gravitated to 3D printers as a popular alternative to conventional casting methods because most 3D printers are limited to just one type of casting approach and, even more importantly, producing castings using 3D printed molds is still often cost prohibitive, due to the large amount of print material needed to produce a mold and the large print size required (since molds are much bigger than the cast part needed). Even 3D printing patterns are usually cost prohibitive due to expensive print material costs and the fact that many 3D print materials don’t successfully burn out from investment cast shells; instead they expand and crack the mold.
Mcor Technologies, based in Ireland, uniquely uses paper as the build medium for the Matrix 300+ and IRIS line of 3D printers. As such, paper 3D printing technology provides a distinctive versatility in the industry to produce both investment and sand casting patterns, and at a fraction of the cost of conventional methods and other 3D printing technologies. Specific cost savings will vary according to the size and complexity of the part geometry, but can be substantial.
The process still maintains the traditional casting method of pouring metals into molds created using patterns normally made from wood, metal, plastics or other materials. Using Mcor technology, the casting pattern is 3D printed out of paper directly from a PC on a Mcor 3D printer and enables you to directly print complex casting patterns affordably and faster than conventional complex methods. The 3D printed parts closely resemble wood, feel very smooth and are surprisingly robust.
Using traditional casting methods, molds would need to be created by first producing a pattern, or pattern set, which would then be used to produce the molds. In the case of sanding casting, a set of patterns is used to create the impressions in the sand, and for investment casting, wax patterns would be used to subsequently create a ceramic or Plaster of Paris mold. Mcor makes the patterns for sand castings and investment castings in the form of 3D printed parts.
The patterns produced on a Mcor 3D printer are made of standard business letter or office A4 paper that can be purchased inexpensively from any office supply store around the world. As such, they are suitable for casting low temperature metals, such as gold, silver, zinc and magnesium, to very high temperature metals, such as aluminum and steel. Any metal traditionally cast can be used because the mold is still a traditional casting mold; it’s the pattern making that is optimized using Mcor 3D printing.
How to produce castings with paper 3D printing
Investment casting: To produce investment castings of low temperature metals using Plaster of Paris, you first create a 3D printed version of the part to be cast. The 3D printed part is then dipped into cyanoacrylate (CA) to seal, and once dried (5 minutes), it is placed into silicon to produce a silicon mold for creating a wax pattern. The wax pattern is then encapsulated in Plaster of Paris. After the Plaster of Paris dries, the wax is melted out of the plaster mold and metal is poured into the mold. The advantage of this method is that the original pattern is 3D printed rather than hand sculpted or CNC machined.
If you need to produce investment castings using ceramic shell casting for high temperature metals, the steps are similar, but no wax is needed. The 3D printed pattern can be sealed with CA or paraffin wax and then repeatedly coated in liquid ceramic slurry, dried and placed in a furnace. This is slowly heated to 1,600-1,950° F. The paper prototype will burn out, leaving a hollow cavity ceramic shell mold, which must be flushed with water and compressed air to remove any ash. Then you pour in the molten metal. Once the casting has cooled, the ceramic will easily break away from the metal cast part. Multiple parts can be investment cast by creating a tree of parts using traditional methods.
Sand casting: To produce basic sand castings using paper 3D printed parts, first 3D print in two pieces a prototype of the part to be cast. Taking each half of the 3D printed part, the two parts of the sand casting flask (cope and drag) can be made using traditional methods which involve compacting the sand around the part and calculating the ideal locations for the sprue and risers. When the cope and drag are separated, the 3D paper part can be easily removed to reveal the negative impression of the shape to be made. The cope and drag are then joined together, allowing the molten metal to be poured. When cooled, the metal part can be removed. With more complex sand castings, many of the complex pattern pieces can be made with an Mcor 3D printer, saving days or weeks of manufacturing time.
Pros and cons
The cast metal parts produced with paper 3D printed molds have identical metallurgical characteristics as metal parts made using conventional casting methods, and can therefore be machined. A leading UK foundry, in addition to Mcor customers, have produced parts using this process and yielded results that mimic traditional sand and investment casting finishes and tolerances.
Mcor’s paper 3D printed molds work for basic, solid industrial geometries, such as an engine block, door hardware, statues and walking stick handles. It is not well suited for complex geometries, such as small, detailed jewelry.
Paper 3D printing is a useful, versatile and low cost addition to your toolbox for many applications, including concept modeling, prototyping, investment casting and sand casting. MPF