Everyone knows that rapid prototyping is one of the most popular applications for 3D printing. For product design and product development, it has become standard practice to use a 3D printer for prototyping on almost a day basis.
For the past six years, I've worked in product design and development. I know how great it is to have 3D printed parts to play around with when you develop a new product.
One of my biggest challenges is communicating my design ideas and solutions to a client. No matter how hard I tried to explain, or how many hours I spend on rendering pretty images, creating fancy presentations, etc., the clients were still had trouble envisioning my designs.
That's normal, though. Not all people can see, or are trained to see, the big picture from CAD screenshots, to fancy renders, to the product itself.
But presenting your design through a 3D printed model is a different story. When clients see the 3D printed product for the first time, they get the “aha” moment, and put a happy smile on their faces, sometimes even acting like a little kid who got his long wanted toy.
Now you probably know that 3D printed parts can sometimes be rough and a bit chunky. Before I showed my clients their products, I prepared models to look just like real products made by traditional methods. There were several projects where we showed a product made entirely by 3D printing to the prospective customers before spending a penny injection mold tools. The response was great, users loved the product, and clients got feedback before investing a lot of money into first product run.
Here are my practical methods that I have used in my work for post-processing and painting 3D printed parts.
3D model preparation
The orientation matters!
How you position your model in the 3D printer will have an impact on layer visibility, surface finish, and can greatly reduce or increase your post processing time. Depending on part geometry, you should position your model to minimize layer visibility, especially on curved surfaces of the model. In simple terms, this means that curved surfaces must be oriented perpendicular to the printing direction where layers will be created.
Also, pay attention which surface will be facing 3D printer base and where support material will be added. The side where the 3D printer starts to build the model, and the surfaces where there are support materials, will have a lower surface finish quality.
You should take note, however, that orienting parts for better surface quality can increase 3D printing costs.
Another important aspect of model preparation is STL file quality (unless you’re using GrabCAD Print and don’t need to use STL). When exporting your files to STL, pay attention to use high-quality mesh refinement. Most CAD software has great export tools which will allow you to see a preview of the model mesh and refine it if necessary by increasing options like the number of triangles or mesh quality. Models with high mesh quality will have smoother surfaces, especially if the geometry is curved.
Additional post-processing depends on several factors. First, which 3D printing technology was used to create the part? If the parts were built using SLA or PolyJet, then you have less post-processing work ahead of you. On the other hand, the if parts were created using FDM or SLS, then more post-processing needs to be done. Depending what level of surface roughness you want to achieve, there are several post-processing methods that can smooth the surfaces of the part.
To remove the rough surface after 3D printing with FDM, SLS, it’s best to use a bead blasting process. For FDM parts, it's recommended to use plastic media. And for SLS parts, it’s recommended to use glass beads. The surface after treatment will be smoother, but nevertheless layer marks are mostly visible.
The most efficient method for surface finishing is manual processing. You can achieve almost every level of surface finish using just sand paper and your hands. But then again, the final result depends on your skills and patience. It is also recommended to use fillers that will help seal the gaps in layers and smooth the surface.
If parts aren’t bead blasted after 3D printing, it's best to start with 400 grade wet sandpaper. Continue to smooth it with 600 grade sandpaper, and then super finish with 2000 grade sandpaper. My secret tip for smoothing the geometry in parts that are hard to get by hand is to use a nail file/emery board.
For a glossy surface finish, filler must be applied on the surface of the part. I have used filler for scale model kits to fill layers on the printed model. If you use a flat plastic spoon, you can quickly spread a thick layer of filler over the surfaces. After the filler has dried, use wet sandpaper to smooth the surface. The full glossy finish can be achieved with color or lacquer, so the goal with sanding is only to have a smooth surface.
Both methods involve manually sanding the surface with or without filler and will result in smooth surfaces without any layer marks or errors.
Note that even if it’s tempting to use hand grinders it’s not recommended because it will most certainly leave marks on the surface. Then you will have scratches all over the part, which will not look good at all.
Parts made with SLA or PolyJet need little to no additional post-processing. For these parts, it's best to use manual processing with wet sandpaper to smooth surfaces which were laying on 3D printer base or curved geometry where layer marks can be slightly visible.
Painting and coloring
Painting is the fun part, but at the same time it is the most challenging part of the process. For a proper color surface finish, it’s important that you prepare the surface of the part very well. Surfaces free from greases and dust are crucial to ensuring effective paint adhesion. Cleaning the parts with warm soapy water prior to painting will do the job.
The simplest way to paint is to use spray paint – either in colors that can be found in regular color shops or special spray paint colors like the ones used for scale model kits. The downside to using spray paint is that it’s not possible to control the fineness of the color spray, which will result in grainy surfaces on the part.
For best results, I prefer to use an airbrush that has fine nozzles. That way I can control air pressure when painting. For basic painting with regular colors such as white, black, red, etc., I use scale model kits paints. But if I need specific color from RAL color palettes then I’m using custom mixed acrylic paints. This allows me to have real colors on the prototype that correspond to the final production product.
Don’t forget that the paint must be thinned with solvent so it can be easily sprayed on the surface. Note that mixing paint and solvent must be done in an appropriate ratio. Otherwise the paint can be too thin or too thick. Thick paint will result in a grainy surface and thin paint will result in watery color effect. To achieve the best results it is most common to apply ratio which is recommended by paint supplier.
Painting process begins with applying plastic primer in several thin layers spraying on part surfaces. Primer ensures better adhesion of paint to the surfaces. After the primer has dried, you can begin with color painting. Color painting with an airbrush is a bit of an experiment and requires some practicing.
When using an airbrush, you must have a steady hand. Move the airbrush with uniform movements in the same direction until paint covers the entire surface area. Don't stay too long on the same spot because paint drips will ruin your otherwise perfect part
After you finish painting, the surface can be additionally sanded by hand using highest grade wet sandpaper you can find. This will help you achieve the high glossy surface finish.
And now for the last tip for the end of my first post here on GrabCAD: you can achieve similar surface finish by VDI standard for injection mold parts just by controlling paint thickness and air pressure in the airbrush. This, however, needs a lot of practice and experimentation.
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