True-to-CAD, High- Fidelity, Surface Texture, Entrapped Cavities, and Printed Assemblies
The second installment of 3D Systems’ MJP Best Practices for the ProJet MJP 2500 series continues, and this time focuses on True-to-CAD, High-Fidelity, Surface Texture, Entrapped Cavities, and Printed Assemblies.
There are numerous 3D printing technologies – each with unique attributes and limitations. There are likewise numerous end-use applications each with unique requirements. Nearly all applications can benefit from a technology that is able to accurately reproduce part geometry, achieve smooth surfaces, create entrapped cavities, and enable high feature fidelity, sharp corners, and extremely small separation distances between articulating part surfaces. These capabilities can only expand the functionality of the printer as it enables more application breadth to serve a broader customer need. This breadth includes things like prototyping plastic injection molded parts, creating jigs and fixtures, making optically clear parts and flow visualization, painting/dyeing, indirect manufacturing and numerous functional requirements like snap fits and complex printed assemblies. Unlike any other technology currently available, the 3D Systems MultiJet Printing (MJP) process provides these properties through its use of high-resolution inkjet technology combined with a melt-away support material system.
The MJP printing process generates a part with over 1.1 billion drops per cubic inch of part volume allowing the creation of even the smallest details. The support wax technology instantly solidifies in place during part formation and greatly contributes to sharp features and smooth surfaces. The wax also allows for simple and hands-free post-processing removal of support from even the most complex geometry and entrapped cavity without scarring of the part surfaces.
True-to-CAD and Reproduction of Fine Features
There is nothing that typically needs to be done to accurately match your part design dimensions with an MJP printer. For most geometries, the part should come out true-to-CAD the very first time printed and every time printed. The printer was designed with a scale factors for each material that is built into the 3D Sprint software. These scale factors automatically correct for the material you wish to print. You can also adjust the scale factors if desired for your specific needs. Part accuracy is independent of orientation for even the smallest and sharpest of features so you do not have to worry about numerous special part design rules, part orientation and placement issues or printer setups.
Surface roughness and 3D surface texture are important for many applications including painted and dyed surfaces, reproducing injection-molded textures, making molds for casting and forming, or for any need that requires a professional-looking part. MJP technology creates a smooth surface in all orientations. It is also possible to utilize the technology to simulate surface textures for engineering, aesthetic or scientific (flow chamber or flow visualization) needs.
Smallest Feature Resolution
For some application requirements, it is often desirable to know parametrically the reproduction capability of the printer. One simple and common method of quantifying part fidelity is to print geometric features of variable dimension and characterize those that the printer is able to reproduce. Such measurements are often made for holes and slots, as well as for protruding features such as shafts and beams.
This type of pattern can be positive (protrusions) or negative (holes). The following figure shows such a pattern for a traditional process using a competitive process compare to 3D Systems MultiJet Printing process.
Patterns such as this can also be printed with variable heights as the support material can often completely plug such holes in a manner that is impractical if not impossible to remove. For example, this can affect articulating joints and bearing surfaces between moving parts which can be different depths depending on the specific design.
Therefore, the 3D Systems’ MJP technology is able to resolve positive protrusions and holes as small as about 100-200mm (0.004”-0.008”) in size and the melt away supports allow for very deep features or flow structures.
Rapid prototyping opens up the traditionally strict manufacturing rules for legacy manufacturing processes, like casting and injection molding. One particularly useful capability is the ability to print slip fits for functional assemblies. A slip fit is composed of a circular overhang requiring support material for its construction. The support material must be removed for proper function. Slip fits such as this can be quantified with a simple diagnostic part with varying gaps and is often printed in multiple directions.
The gap for a functional turning slip fit using this diagnostic is shown below for both a competitive material jetting process and 3D Systems MJP technology.
The 3D Systems MJP technology is able to create a slip fit turning shaft with as little as about 100mm (0.004”). Slip fits with this size gap typically turn easily and smoothly after post processing. Tight fitting gaps such as this reduce the amount of slip-fit slop and allow for better functionality.
Very tight slip fits may take some amount of rotational torque to free the rotation after post processing. Carefully turning the part multiple times often will free up the rotation of two mating parts. Also, larger contact areas and/or smaller gaps between the sliding surfaces will both tend to increase the torque required to free the part. Therefore, a larger gap may be needed if the bearing surface is wide or the shaft size is small such that the torque required to free rotational motion will not break the part. Typically, 200mm will work in all cases; smaller gaps may work, but only for specific geometry.
Another unique requirement for many customer applications is the ability to print entrapped cavities. Each additive manufacturing technology utilizes a specific method to support overhangs. For example, Selective Laser Sintering (SLS®) and Color Jet Printing (CJP®) utilize the powdered build materials themselves, which naturally surround the part. SLA parts are built in a liquid bath and this technology must create very small structures to support overhangs, which are typically easily removed but leave behind scars on the part. Heat is used to remove MJP supports allowing for hands free processing for even the most complex parts. This capability is useful for very small structures like capillary flow visualization, but also useful for larger structures like making a simple pump with an entrapped cavity. With MJP technology, any entrapped cavity can be created as long as the cavity has a single small hole for the melted support wax to be removed. The fluid simply drains out.
Entrapped Support Marking
It is possible to entrap the support within a part with MJP technology. Basically any cut or hollow cavity within the part without a drain hole will be filled with support material by the printer. Features such as this are very easy to create in CAD. This is useful for engineering or fixture needs for adding numeric marking to knobs or assembly features or for pure aesthetic purposes.
The high-resolution capability of the MJP process is particularly useful for this of capability because it allows for very small marking and/or features. This capability can also be used to add white colorization to parts.
While this capability can be produced in CAD, 3D Sprint also has a built-in feature. The engrave tool can make both text and drawing based markings on a file either protruding out from the surface, cut into the surface (inward or outward engrave) or entrapped with a given height (into the part) and depth from the surface.
Few technologies have this type of capability and none in the same price range. One unique aspect of the ProJet 2500 is that three different clear materials are available for the printer that span a wide range of material properties from rigid (polycarbonate-like), to stiff and tough (ABS-like), and also flexible and extremely tough (polypropylene-like). Entrapped support marking can be used for all of these clear materials.
Printed Assemblies and Mechanical Design Elements
There are a number of key 3D printer attributes that are important in the design and production of printed assemblies including surface roughness, accuracy, concentricity of shafts and holes, smallest feature resolution, slip fit capability, and entrapped cavity creation. The capabilities of MJP technology are ideal for this need and can be used to make all the foundational mechanical design elements like beam members, shafts and frames, rotating elements, gear designs, spring designs, ball bearings, and wheels designs, etc.
There are numerous applications for such printed assemblies. For example, this can be useful in a medical model to simulate a printed bone structure like a spine or joint. An actual rotational hinge can replace a living hinge in applications such as connectors. Complex kinematic requirements for a fixture can be printed as an assembly for part fit checks, or go/no-go gauges for part assembly lines in manufacturing, etc. One interesting component of a printed assembly is that of a printed gear assembly.
Each member of a gear train assembly must be carefully positioned during design such that there is no overlap between the meshing teeth. If the gear teeth overlap, the system will become locked together during printing. Also, in this type of component, it is often impossible to keep the part separation distance above the 100-200um requirement to avoid the parts sticking together during printing because of the extremely tight gear mesh that can be required to achieve appropriate part size, gear ratio, and teeth engagement. In this case, it is possible to print the gear teeth mesh with a gap smaller than that possible to create two completely separate parts. There will be a gap in the actual CAD design, but it will be so small that there will be some slight joining of the two parts at the tangent points of the gear mesh where the separation distance is the smallest. The trick for such tight tolerances is to break the weld after printing by turning the gears slowly. Start by rocking the gears back and forth lightly and proceed to turning the gear or the shaft multiple manual rotations in order to smooth out the break points and allow for a functional rotational part.
Shells and Infill Capability
The high fidelity and melt away support system allows a part to be hollowed and filled with a complex lattice pattern if needed. The function works by creating a shell of any desired thickness and then adding an infill within the shell with a given density. 3D Sprint software comes with tools to add the shell, a drain hole to the shell, and to filled the internal cavity with either support or numerous different lattice structures. This capability is common in extrusion based rapid prototyping technologies to reduce cost and/or weight of printed parts, typically with no impact on the visual or dimensional accuracy of the part.
The support material costs less than the build material and so using the shells and infill capability in 3D Sprint allows for a substantial cost reduction. The exact amount saved is dependent on the shell thickness and the percent acrylate in the infill pattern you chose and is typically in the range of 40-70% weight reduction and 20-35% cost reduction.
Feature Fidelity, Entrapped Cavities, and Printed Assembly Examples
Do you have any questions on concept modeling, rapid prototyping, or indirect manufacturing? Reach out to one of our experts today! Stay tuned for the third part in the ProJet MJP 2500 Best Practices Series brought to you by 3D Systems.