Design of Paraffin 3D Printer and Its Application in Investment Casting Process
2026-04-15
3D printing technology originated in the 1980s and has now become a relatively mature technology, widely used in jewelry, footwear, industrial design, architecture, engineering and construction (AEC), automotive, aerospace, dental and medical industries, education, and geographic information systems. 3D printing can manufacture parts that cannot be processed in one go by many existing machine tools, and is known as a technology that may trigger the Fourth Industrial Revolution. Currently, mainstream 3D printers mainly print plastics such as polylactic acid (PLA) and acrylonitrile-butadiene-styrene (ABS), as well as photosensitive resin and metal powder. A small number of 3D printers for chocolate, industrial cement, ceramics and other materials have also gradually matured.
Lost-wax casting is an ancient Chinese technique for casting precision metal artifacts. The specific process is as follows: a casting pattern is made of beeswax, the inner core and outer mold are filled with refractory materials, the wax pattern is completely melted and drained off after heating, and then molten metal is poured into the mold to cast the artifact. Artifacts cast by the lost-wax method can be exquisitely intricate with hollowed-out structures, so it is still used for casting metal parts in modern times, known as investment casting. Nowadays, most wax patterns for investment casting are machined and then manually carved. Machining itself has a high upfront cost (requiring machining equipment), and manual carving is time-consuming and labor-intensive, which is not conducive to large-scale promotion.
Combining the above two technologies and using 3D printing technology to obtain patterns for investment casting is a very promising solution, and relevant technologies have indeed appeared on the market. However, at present, most of them use photopolymerization 3D printing technology, the raw material used is photosensitive resin, which costs thousands of yuan per kilogram on the current market, very expensive. In addition, this material generally has certain toxicity, which is also not conducive to large-scale promotion.
Since pure waxy materials cannot be printed by photopolymerization 3D printers in terms of curing method, and cannot be made into filaments for Fused Deposition Modeling (FDM) 3D printers in terms of hardness and flexibility, we propose a doped waxy material with properties that meet the requirements for use as filaments for FDM 3D printing. At the same time, we have improved and optimized the extruder of the existing FDM 3D printer to extrude wax filaments and complete printing.
3D Printer Construction
The mechanical structure of 3D printers mainly includes two types: delta (parallel arm) architecture and Cartesian XYZ architecture. For the delta architecture, only the print head moves while the base plate is fixed. For the Cartesian XYZ architecture, the print head moves on the YZ axis and the base plate moves on the X axis, or the print head moves on the XY axis and the base plate moves on the Z axis. Our first version of the print head (Figure 3) uses a lead screw to push molten paraffin to complete printing, and all paraffin material required for printing must be loaded in a syringe. Considering that this would make the entire print head heavy with a high center of gravity, moving the print head may reduce printing accuracy. Therefore, the architecture of this version of the printer has the base plate performing XYZ axis movement, and the print head is fixed at the upper middle position of the entire printer without movement. The architecture of the first version of the printer designed based on this is shown in Figure 1 (excluding the print head).
When the first version of the architecture design was completed, it was found that since the base plate needs to move on the XYZ axis, the floor space of this architecture is about four times that of the delta architecture and twice that of the standard Cartesian XYZ architecture under the same printable area. This did not meet our original vision of a desktop-level 3D printer. Therefore, after consideration, we abandoned this solution (the solution was discarded before the physical prototype was built, so there is no physical picture).
Due to the abandonment of the first version of the mechanical architecture, the design of the first version of the print head could not be further used. Since our innovation is not in the mechanical architecture of the 3D printer, we analyzed several existing printers on the market and finally decided to adopt the Cartesian XYZ architecture, with the print head moving on the YZ axis and the base plate moving on the X axis. Based on this, we designed the second version of the 3D printer architecture and produced a physical prototype (Figure 2, including the second version of the print head, this figure is the final version). It has been verified that this design can work perfectly.
The first version of the print head was designed to use a screw-driven syringe method (Figure 3), considering that paraffin material is difficult to be made into filaments and cannot be fed by traditional gear extrusion. Considering that the overall weight and height of the print head were relatively large, the motor was originally designed to be placed on the top of the print head, but later when drawing the design drawing, the motor was placed in the middle of the print head to reduce the overall center of gravity. In principle, this version of the print head should also be able to work, but finally, due to the abandonment of the mechanical architecture of the first version of the 3D printer, this version of the print head only stayed in the design stage and no physical prototype was produced.
When designing the second version of the print head, since the mechanical architecture of the 3D printer was changed to the Cartesian XYZ type, our print head should also adopt the traditional FDM printer print head, that is, the gear extrusion filament method. For this, we studied the formula of the waxy filament (hereinafter referred to as "wax filament") while researching the new print head structure. The flexibility and hardness of the wax filament are not suitable for the traditional gear extrusion structure (drive gear + idler gear). The traditional print head structure has an idler gear, and the filament is sandwiched between the drive gear and the idler gear. If the clamping is too tight, the wax filament is easy to break; if the clamping is too loose, the wax filament is forced on one side and slips on the other, and the extrusion direction of the wax filament will change, making it impossible to complete feeding. Therefore, we designed a print head with a dual-drive gear extrusion structure to ensure the stable extrusion direction of the filament and produced a physical prototype (Figure 4).
We modified the open-source 3D printer firmware Marlin as the basic architecture, including motor model, lead screw Z-axis layer height, temperature sensor parameters and motion mode. Since the firmware is the printer code, it cannot be displayed as a physical object or picture, so no further explanation is given here.
Wax Filament Formula and Properties
Wax patterns usually used for investment casting are made of paraffin wax or casting wax, but their properties are too soft to be made into wax filaments at all. After investigation, we found that paraffin wax, casting wax, ethylene-vinyl acetate copolymer (EVA) and stearic acid can be doped to produce wax filaments with good properties. Among them, paraffin wax has a low melting point and good fluidity after melting, and is the main raw material for wax filaments. Casting wax has high hardness, high toughness and good stability, which can improve the hardness and toughness of the wax filament and help maintain the shape of the wax filament; it has a low shrinkage rate, so the printed object has small shrinkage and higher precision. Ethylene-vinyl acetate copolymer has good moisture and water resistance, corrosion resistance and excellent low-temperature performance, which can ensure the long-term storage of the wax filament; it has high resilience and tensile strength, and high toughness, which is also conducive to maintaining the shape of the wax filament. Stearic acid has certain lubricity, which can ensure the uniform and stable discharge of the wax filament; at the same time, it can also be used as a wax pattern in precision casting.
According to the formula of 60-70 parts of paraffin wax, 20-30 parts of casting wax, 7-9 parts of EVA, and 5-7 parts of stearic acid, wax filaments with low melting point, high toughness, high stability, low shrinkage, moderate hardness and uniform and smooth discharge can be obtained through dissolution, mixing, kneading and extrusion.
To verify that the produced wax filaments are suitable for investment casting, we conducted a set of comparative experiments between the wax filaments and PLA, testing the viscosity of the two materials at different temperatures with a rheometer. The lower the viscosity at the same temperature, the better the fluidity of the material, and the more suitable it is for investment casting. The data shown in the figure below (Figure 5) is obtained. Since the viscosity of PLA rises sharply below 200°C, while the viscosity of the wax filament is extremely low, which cannot be represented in one figure, only the data of PLA above 200°C is given. The data proves that our wax filaments are suitable for making wax patterns for investment casting.
Wax Filament Printing Conditions, Precision and Printed Products
To explore the suitable printing conditions and printing accuracy of the wax filament, we designed three types of models: cylinder, cone and concave shape for printing tests. We printed one sample every 10°C from 130°C to 200°C, with three top solid layers, and obtained the models shown in the figure below (Figure 6).
It can be seen from the figure that the wax filament can print models in the range of 130-200°C, which indicates that the wax filament has a wide printable temperature range. No matter in the south or north, summer or winter, the change of the external environment has little impact on the discharge of the wax filament, and it can be printed smoothly. Comparing the wax patterns at different temperatures in detail, the results are shown in Figure 7 (only the cylinder is shown).
The printing results show that the wax patterns printed by the wax filament at 130-160°C are of good quality, with smooth edges and top surfaces, and little difference, all as shown in the 130°C wax pattern in Figure 7. At 170°C, the edge of the wax pattern will bend to form a wavy shape, but the top surface is still relatively smooth. At 200°C, not only the edge of the wax pattern is bent, but also the top surface cannot be completely sealed. This is because the viscosity of the wax filament is too low at high temperature, and it is difficult to form a continuous filament on the hollow surface, resulting in incomplete top sealing. This situation can be better improved by increasing the infill density of the model and the number of top solid layers, but this will also increase the power consumption of the machine and the amount of wax filament used. Therefore, the temperature of the samples we printed later was set to 150°C. At the same time, to ensure a smooth top surface, the number of top solid layers was set to 5.
The printing accuracy of the cylindrical and concave wax patterns was tested, and the results are shown in Figure 8. The designed diameter of the cylindrical model is 20mm, and the actual diameter is 19.980mm; the designed width of the concave model is 10mm, and the actual width is 9.962mm. The accuracy error is within 0.5%, which indicates that the printer and wax filament have excellent performance.
The wax parts for investment casting were printed with the parameters of 150°C and 5 top solid layers, and the samples shown in Figure 9 were obtained. Since investment casting is a relatively mature process, we directly sent the wax parts out for external processing to obtain castings.
Investment Casting Finished Products
The model of the copper parts cast with the wax parts we printed is shown in Figure 10. The casting method is as follows: first fix the wax parts on the wax tree, then pour the plaster and vacuum degas to evacuate the air in the plaster. Since the infill density of our sample is only 15%, most of the area of the bottom/top surface of the model is suspended, and its surface is relatively soft. After the air inside the wax mold is evacuated by vacuum degassing, the plaster exerts pressure on the surface of the wax part, resulting in partial depression of the surface where there is no support contact, and finally causing pits on the surface of the cast copper part. This situation can be effectively improved by increasing the number of top layers and infill density.
Conclusion
The 3D printer and wax filament designed in this project have achieved the originally expected performance, and can print wax parts for investment casting. At the same time, the wax parts printed under suitable printing conditions have high precision and smooth surface. However, the 3D printer also has certain shortcomings. For example, when printing support for suspended structures, the surface roughness of the contact surface between the suspended structure and the support is relatively large, which needs to be polished before being used for investment casting or after casting to achieve a good surface smoothness. At the same time, when designing the 3D printer, to facilitate production and reduce the R&D cycle and cost, the frame parts of the printer are all made of standard aluminum profiles, which results in the disadvantage of relatively large mass and volume of the 3D printer. However, we consider that we can redesign and manufacture a new frame with sheet metal parts later, which can effectively make up for this deficiency.
In general, the application of 3D printers in investment casting is a very promising project. This project will continue to optimize the design of our 3D printer, reduce the manufacturing cost of wax patterns while improving printing accuracy, and strive to gain a foothold in the fiercely competitive market.
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