Additive manufacturing, also known as 3D printing, converts a computer-aided three-dimensional model (CAD) to a physical object without the need for moulds or tooling. It transforms the manufacturing method from analogical to digital [1
]. The manufacturing process is based on the addition of material combined with a contribution of energy to create a solid, layer by layer. Additive manufacturing technologies lead to new manufacturing paradigms such as decentralized manufacturing or mass customization. However, the transition from Rapid Prototyping (RP) to Additive Manufacturing (AM) entails new challenges in the mechanical and material fields [2
The HP Multi Jet Fusion™ technology, henceforth MJF, was designed to embrace the market niche for both functional prototyping and end-part manufacturing in the industrial field. MJF is able to achieve similar results to plastic transformation processes such as plastic injection moulding. It is considered a High-Speed Sintering (HSS) technology [3
], as it involves the sintering of 2D profiles of layers of powder, similar to Binder Jetting (BJ) technology, without the need for a laser. The MJF printhead prints functional agents in precise locations onto the material to define the geometry of the part and its properties. It is capable of printing thirty million drops per s across the width of the printing space [4
]. This leads to very accurate dimensional precision (±0.2%) compared with other technologies [5
]. The printing velocity, the dimensional precision, and the high quality in printed parts turn this novel technology into an interesting manufacturing system for AM solutions.
The aim of this study is to show how the MJF technology can be used to fabricate both functional prototypes and small batch series loaded with interior water pressure. Other additive manufacturing technologies do not achieve watertightness, due to poor mechanical properties or the porosity structure of the material. There is a need in the fluid handling industry to manufacture parts through AM technologies resistant to pressure and impermeable to leaks. As for the requirement of tightness under fluid pressure, the weak point is the porosity generated in the manufacturing process. Currently, coatings or infiltrations are required to seal the material and ensure no leakage through porosity [6
Due to the nature of the process, technologies such as Fused Filament Fabrication (FFF) or Selective Laser Sintering (SLS) [7
] create porosities placed at the interface between the layers [8
]. Moreover, the manufactured parts are not completely sealed which lead to alterations in their mechanical properties [7
] even as a function of time [12
]. Recent studies have tried to relate laser additive processing to functionality [13
]. Even though the resulting parts from SLS are not entirely dense, very low degrees of porosity can be obtained by adjusting the printing parameters [14
]. This porosity is created by defects in the sintering of the material, where some of the grains do not fuse, producing empty spaces in the subsequent cooling process [15
]. Two types of porosities are created: irregular pores caused by shrinkage and non-fused parts and spherical pores that come from trapped gases or evaporation of the material [16
On the other hand, technologies based on photosensitive resins such as Stereolithography (SLA) and Poly Jet (PJ) do not create porosities in their structure. The resins used are formed by acrylate groups, which crosslink quickly, and epoxy groups, which enhance the mechanical properties and reduce shrinkage and curling [16
]. The main problem with SLA for functional parts is not the liquid resin, but the poor mechanical properties of the crosslinked polymer. Namely, the parts are too brittle for most engineering applications. The degradation of the material caused by aging directly affects the mechanical properties [13
]. Photosensitive resins are not recommended as a final solution, although they are very useful in the design phase [15
]. In addition, these types of resins increase their volume in contact with water or moisture. For this reason, they cannot be used as the final product as they would lose all their manufacturing tolerances [18
]. However, we should differentiate the liquid resin (monomer) and solid crosslinked polymer for each study. Recent studies focus on parameters to obtain a uniform deposition of material [20
] with different additive manufacturing techniques.
Another issue is the sealing of the printed parts, which presents porosity in their structure. There are already existing solutions, where the choice of the solution depends on several factors, such as cost, machinery, and process time, among others. The selected solution has a direct impact on the time spent per part and, consequently, its cost. Therefore, when the part is analysed and requires a sealing treatment, it is replaced by the conventional plastic transformation processes due to the high industrial cost [21
]. The most used sealing methods in the additive manufacturing industry are [22
Painting and filling: When parts need only partially sealed surfaces, a few coats of paint and a little body putty can be an inexpensive option. Since this is a manual operation, the accuracy and quality of the product is influenced by the technician’s skill and care. The advantages of this option include low cost, short cycle time and ease of application. Its disadvantages are the lack of an airtight seal and inability to resist high temperatures and chemicals.
Smoothing station: This method seals the surfaces of a part by exposing them to a vaporized smoothing agent inside a chamber. The smoothing station is very easy to use and preserves dimensional integrity [23
]. Its use is limited to applications with pressures that do not exceed atmospheric pressure and temperatures equal to or below 100 °C. This technique only seals the surface holes but not the internal channels. If the sealing of the interior is required, it should be combined with a previous infiltration.
Solvent dipping: Dipping additive manufacturing parts in a solvent could be a substitute for the smoothing station, when it is unavailable or the part exceeds the chamber capacity. All the characteristics are the same as the smoothing station except the dimensional accuracy, which is lower. The solvent melting action is quick and aggressive, so dimensional accuracy is difficult to control. As with the smoothing station, the use of this method should be limited to low temperature and atmospheric pressure applications.
Thermal post-treatment: Typically between the Tg and Tm of the polymer used. Thermal post-treatment can also be used with one of the other mentioned processes (coatings or infiltration) to harden the infiltrant and increase the ceiling temperature of the end part.
Adhesives coatings and infiltrations: These are substances based on epoxy formulation with different viscosities. Adhesives of high viscosity should be applied with a surface coating. In contrast, low viscosity infiltrations can be performed in vacuum chambers in order to ensure the adhesive enters to the interior of the part by controlling the necessary process time [24
]. By applying adhesives to parts manufactured with SLS and FFF technologies, resistance to water pressure can only reach a value of 0.45 MPa [25
The results reached in this work are important for the industry, because they show manufacturers the possibility of considering additive manufacturing as an alternative to traditional manufacturing methods, especially for the production of spare parts and small batches including mass customization for pressurized components. This paper is a novel approach to designing and manufacturing fluid handling components through additive manufacturing Multi Jet Fusion technology while avoiding the problems of water leakage. This problem is common in most of the additive manufacturing technologies and is caused by porosity structures in the material.
2. Materials and Methods
The fabrication of a valve through MJF technology with PA12 is presented in this study. PA12 powder was provided by 3D HP Jet Fusion 4200 (Hewlett Packard, Barcelona, Spain) with the following specifications (Table 1
Specifically, a ball valve designed to isolate a liquid belonging to a conducting fluid system following the standard EN ISO 16135:2007 [26
] and the directive 97/23/EC [27
]. The valve is catalogued as PN10 (1 MPa). Nominal pressure (PN) is used as a reference for its mechanical resistance and corresponds to the maximum allowed water pressure at 20 °C.
For the sake of simplicity, only the main parts of the valve (shell and union nuts) were considered to be printed with MJF (11 and 3, respectively, in Figure 1
). These exterior coverage parts support two pressure origins: the pressure from the inner parts and the hygroscopic pressure.
The printed parts were evaluated with the same quality tests applied to an industrial production valve. It must satisfy ISO 9393-1 [28
]. This standard describes the method to verify the shell resistance under water pressure and the inner parts package effort. Together with the union nuts, they must withstand the tensile strain caused by the water pressure inside the valve and comply with ISO 228-1 [29
The methodology used was divided into three stages. In the first stage, referred to as the fabrication process, the three-dimensional model was analysed. It was treated in the printing software. The thickness of the layer, the material used and the orientation of the parts in the space were set up. Before the printing process, the material was subjected to a tensile test to correctly characterize its mechanical properties. Also, a fractography analysis was done to characterize the porous media. Furthermore, a leaks study was implemented by using a flowmeter, given different printing orientations and wall thickness for a range of pressure values. In the s stage, the physical dimensions of the parts were verified by using a three-dimensional scan. This compares their dimensions with the tolerances allowed in the production of the valves. In cases where tolerances are not satisfied, machining of the parts must be performed. The full valve assembly will be checked with a leak test using air as a fluid. The final stage is product validation. This determines whether the complete valve satisfies the quality standard for valves made of thermoplastic material.
A 3D HP Jet Fusion 4200 (Hewlett Packard, Barcelona, Spain) was used to manufacture the parts of the valve. The printing parameters used were: 0.08 mm of layer height in the Z axis with a standard resolution of ±0.2%. A balanced print mode was set up: One rolling step and two injection passes spending 10.5 s per layer. Due to the shape of the parts, the cylindrical sections were oriented in the XY plain, where the resolution is highest. In the XY plane orientation, the step effect caused by an angular gradient lower than 30° in the Z axis is avoided.
Once manufactured, the parts were dimensionally analysed using a 3D scan (ATOS Scanbox 4105, Leuven, Belgium). Then the set was assembled in order to validate the product. The next stage was identifying whether there were any leakage points throughout the leak tester.
In this case, the shell test and the seat and packing test were studied. These tests provide information about the resistance and watertightness of the material. To carry out the tests, there are some necessary restrictions, as shown below:
Pressure appliances, as specified in ISO 1167-1, have to be able to connect the sample and progressively apply water pressure following the standard of the product. It has to maintain a constant pressure between +2% and −1% for the time specified in ISO 9393-2 [30
], maintaining the temperature indicated in the product standard.
Pressure calibrated sensors must be able to verify the test specified pressure without polluting the product.
Thermometers must be able to verify the specified temperature in the assay.
Timers have to be able to record the duration of the pressure application until the fail momentum during the trial time.
The procedure established for the shell test is as follows:
In the first place, the sample must be filled with water and conditioned for at least 1 h at a temperature that does not deviate by more than ±2 °C from the specified trial temperature.
Place the test sample in a mode where the entire valve body is under the trial pressure.
Make sure that the water temperature in the test tube is adjusted to the specific trial temperature.
Release any trapped air inside the trial sample.
Raise the pressure progressively until the trial pressure specified in ISO 9393-2 [30
] is reached; this should be done as fast as possible, but not in less than 30 s.
Maintain the pressure and temperature for the duration specified in the standard ISO 9393-2 [30
Diminish the pressure until atmospheric pressure is reached.
The ISO 9393-2 [30
] parameters of the trial according to the fabrication thermoplastic material are specified in detail. Since PA 12 is not included in the standard, the test parameters for the PVC-U, which is the original material of nut unions and shells, as well as being the most restrictive case, were used. The shell test parameters are shown in Table 2
The procedure established for the seat and packing test is specified below:
Firstly, the sample must be filled with water and conditioned for at least 1 h at a specified temperature, which does not deviate more than ±2 °C.
The procedure determined for the seat and packing test and the corresponding parameters are specified in detail in the ISO 9393-2 [30
], and they are shown in Table 3