How to Polish 3D Printed Models to Get a Smooth Surface?

All manufacturing processes have certain limitations, as even machines cannot produce particularly smooth shapes. 3D printing, due to its own manufacturing principles, does not produce a very smooth part just after printing due to the layer thickness, so some post-processing is always required. Sanding is the most common post-processing technique in 3D printing and is also the most labor-intensive aspect. So how do you sand a printed part? We've compiled information about sanding parts, so let's read on!

    Grinding parts is always a time-consuming and labor-intensive step, and it's also the most "dissuasive". However, there are techniques that can help people sand faster and more professionally. Today, we will focus on sanding PLA as an example, and we will introduce the considerations and procedures to be followed when sanding parts to obtain a smooth surface.

To avoid a lot of subsequent sanding, the most effective way is to improve the precision of the part right at the time of printing the part, such as printing parts that are easy to sand and reducing the layer thickness. This can be set up in the slicer, and the smaller the layer thickness of the layup, the higher the accuracy and the less sanding.

Like most plastics, PLA is water absorbent, which means it absorbs moisture from the air. PLA that passes through the printer nozzle will leave a rough surface. Most new wire that has just been opened for use is vacuum-packed and desiccated, so using new wire can circumvent the effects of water absorption. However, only wire that has been in use for more than a few weeks should be stored in a dry place or dried before printing to minimize the effects of water absorption.

The way the 3D model is exported from the design software can also have an impact on the surface quality of the final printed part. The chord height is the maximum allowable deviation of the STL file from the source model. A lower chord height means that the model will have a higher polygon count, thus reducing the polyhedral angles seen on the surface. Keeping the chord height well below the print layer height when exporting the stl file will improve the surface quality. If you have downloaded an STL file from a shared site such as Thingiverse, you will not be able to change the chord height. Usually, however, STL files on repositories tend to have a good resolution, so you don't need to worry about this.

Once you have a smooth STL file, you can change the slicer settings, and there are a number of settings that can help reduce the amount of sanding required. A few settings are listed here for reference.

  1. Print orientation plays an important role in surface quality. Overhanging areas need to be supported, resulting in a very rough surface. If possible, choose the right print orientation to reduce the visible signs of support.
  2. Within reason, the layer height can be set to a small value. Very low layer heights add significant print time, but will produce a smoother finish. Shapes with vertical walls and flat tops can be smoother than angled walls or domed tops.
  3. Wall thickness is very important for sanding. You can set a little extra wall thickness before printing to prevent sanding too deep later. Otherwise, defects such as holes may be created. When sanding later, the thin walls will deflect and leave an imprint pattern in the filler. In addition, the part needs to be held firmly in place when printing to prevent breakage of fragile parts.
  4. A reasonable setting of the printing speed, too fast may cause the printer to vibrate and produce a rough avoidance. Therefore, please slow down the printing speed of the machine to reduce the vibration.

The unique slicing software feature also helps to achieve smoother 3D printing. Ultimaker Cura and PrusaSlicer offer a feature called "ironing" that can smooth out the top layer of the print.

The sanding process is indeed a time-consuming and labor-intensive one, so why exactly is it necessary? There are several reasons.

PLA is not very sharp, but can cause discomfort in wearable applications such as clothing or prosthetics. When sanding, sharp points can be removed first with coarse sandpaper or a file. It can be easier to power sand in subsequent steps without tearing the sandpaper.

  • Precision fit: If you are assembling mechanical parts, precision is important. Shafts and holes require a clearance, interference, or transition fit based on the precise clearance of their function, but depending on the accuracy of the printer, these fit sizes will require some adjustment. Test mating parts and mark areas to be removed before and during grinding, and be careful not to grind too deeply.

Regardless of the reason for sanding, you need to start with a quick removal of material, i.e., rough grinding. This will speed up the overall speed of sanding. Sandpaper is available in various types of grit (roughness). Coarse grit (60 or less) is used for fast material removal. Medium grit (60 to 80) is used for smoothing prior to priming. Fine grit (100 to 120). Very fine grit (150 to 220). Ultra-fine (400 or higher) is used for polishing and buffing after painting.

Usually, cheaper sandpaper uses a poorer binder and the sand will come off from the paper. Worse still is the inconsistent grit of sandpaper. A single coarse grit on a fine paper can leave deep scratches and "undo" the sanding process in between. For this reason, it is important to clean the part to remove any remaining coarse particles before changing to finer sandpaper.

The geometry of some parts may require specialized tools to sand specific surfaces or hard-to-reach areas. Fortunately, this does not have to be expensive, as there are many DIY solutions. For example, a sanding block can be created by wrapping sandpaper around a piece of hard material (usually wood). A foam, cloth or rubber pad between the sandpaper and sanding block helps the sandpaper fit the shape of the part. This sanding pad is suitable for softening hard edges and removing stl file artifacts. Thicker pads will allow the sandpaper to conform to the shape of the part. Note, however, that sanding sharp corners with a thick soft pad can cause the paper to tear if too much force is applied. Deep grooves and corners can usually be handled well by folding just one sheet of sandpaper. Roll the sandpaper around the pins to enlarge or smooth out the holes. Of course, you can also print your own sanding tool.

Sanding tools are also very useful for areas that are hard to reach by hand. PLA does not wear out the file, but can clog the teeth, so using a fine sanding tool is a good solution. Another great item to keep handy is a tack cloth for cleaning objects before priming or painting to get all the sanding dust off!

  • Power tools

Power tools are great for removing material quickly over large areas. This can save a lot of time, but be aware that if the pressure or speed is too high, especially if the walls are thin or the part is small, the frictional action can soften or melt the PLA after a few seconds.

It is recommended that you start with a less critical sanding area, understand and adapt to its rate of heating up, and be careful not to sand one area for a long time. If the part becomes less smooth as it is sanded, then it will become too hot. This can be solved by using some kind of heat sink, such as a fan.

  • Other materials

Fillers and primers are essential to hide deep lines, gaps and other low-lying defects. Larger gaps can be repaired with wood putty or similar fillers, while primer is a porous coating that adheres well to the substrate.

Sandable primers are suitable for smooth surfaces. Brush-on primers are much less expensive and are suitable for on-site filling or very large projects. Spray primers are easy to apply and do not leave brush marks.

The printed surface of the part has peaks and valleys. Wave peaks consist of print layers, support materials, printer artifacts, or STL nodes. The troughs are spaces and gaps between layers caused by interrupted nozzle flow. To make the surface smooth, the crests are flattened by removing material and the troughs are filled with filler or primer. The sanding process once again starts with a single pass with coarse sandpaper to remove almost all of the peaks. Then, a finer grit sandpaper is used to further remove the scratches from the previous step. Wet sanding can be used for finer grit. Using a little water helps remove debris and prevents the paper from sticking, as primer and filler can stick to the paper and clog the grit. But most importantly: remember to always use a dust mask and goggles when sanding!

Once you have mastered sanding PLA, you can try other materials. Be aware that variations in hardness, texture and softening temperature can affect the way parts are sanded.

  • ABS and PETG are popular for printed filaments and can be steam smoothed so that the surface material is temporarily dissolved using a solvent.
  • Rubber materials tend to stain if too much force is applied. Fast, gentle sanding works best. Use wet sanding on sticky materials to avoid catching material and clogging. If the rubber wants to catch the grit, use finer sandpaper.
  • Fibrous materials such as wood and composite filaments should be sanded in the direction of the grain. Remember to wear gloves when sanding or some fibers will scratch your fingers. Wet sanding will prevent the composite fibers from becoming airborne, but you still need to wear a dust mask at all times.
  • Metals take longer to sand, but can achieve a high gloss. Over time, oxidation can dull some metals such as aluminum and copper. Add a clear coat to prevent oxidation or polish to restore luster.
  • High density polyurethane foam is easily sanded with medium grit. Keep a vacuum running as sanding dust will continually float in the air. Polystyrene foam can be melted by paint, so test with scrap before you put too much time on the part.
  • Glass and ceramics are harder than the grit in some sandpaper. Special sanding stones, wooden sticks or mud are required.

Sanding a 3D printed part is not an easy task, it directly determines the surface quality of the part, so after reading the sanding methods we've put together, will you be inspired in some way? Give it a try!