In 3D printing, print quality defects can go beyond aesthetic issues. They can compromise the role of functional parts, meaning that you’ll have to spend on resources and time to print the part again.
Two of the most common issues are 3D print zits and blobs. They appear as irregular ridges or bumps on the exterior of the print, compromising the quality of aesthetically oriented prints.
However, they can be just as bad for functional items, especially when printing parts that have to slot into one another.
Blobs and zits can occur for a variety of reasons ranging from extrusion and retraction issues to an incorrectly calibrated printer.
What Are 3D Printing Zits And How Do They Form?
Zits and blobs in 3D printing refer to bumps, ridges, or material accumulations on the exterior of the print.
They only affect the visible surface of the item, but can compromise the structural integrity of the object and lead to layer separation.
The ridges may sometimes appear as Z-banding; however, the inconsistency of the pattern differentiates zits and blobs from Z-banding.
Because zits and blobs occur in sporadic patterns, they are more likely the result of calibration issues. Here are the most common causes of 3D printing zits and blobs.
High Extruder Pressure
One of the main reasons your printer extrudes more filament than necessary is a too high extruder pressure.
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Because there are lots of factors that can cause a rise in pressure on the extruder end, you should run a recalibration before testing the printer.
Incorrect Feed Rate
The filament feed rate is determined by the volumetric extrusion and the printing speed.
Generally, the firmware calculates the volumetric extrusion automatically based on the filament diameter and type.
However, the G-code can’t actually determine whether you’re using the specified filament type, which means that it would still print the part (without reslicing) even if you’re using a larger diameter spool.
This would result in over-extrusion due to the higher volume of filament fed by the firmware, which could lead to anything from zits and blobs to banding.
Too High Printing Speed
Alongside volumetric extrusion problems, a too high printing speed can also result in over-extrusion.
That happens because, at higher speeds, the filament feed can increase the pressure in the hot end. Moreover, too fast speeds can prevent proper retraction.
The optimal filament printing speed can vary from one filament type to another.
Brands may also recommend specific speeds for their products, so you should follow the label and adjust by 5°C at a time until your prints are perfect.
Retraction problems can occur due to high speeds or incorrect calibration. However, slow retraction isn’t always the issue.
Sometimes, zits and blobs can occur due to too large or too frequent retractions. Once again, it is crucial to check your machine’s calibration and adjust accordingly.
Insufficient cooling can lead to over-extrusion by keeping the filament too hot. The melted plastic can lead to stringing, zits, blobs, and a variety of other defects.
Checking the printing temperature and adjusting the fan settings is crucial. However, you should also check the room temperature and avoid printing in spaces that are too warm.
Sometimes, zits and blobs could be the result of residual filament or build-up dirt blocking the nozzle. In this case, you may notice stringing and under-extrusion defects in addition to the zits and blobs.
Cleaning or changing the nozzle can solve the problem.
Lastly, wet filament is another potential culprit. When printing with wet filament, excess moisture evaporates when the material passes through the hot end.
Vapor can build up inside the hot end, increasing the pressure inside the extruder and leading to over-extrusion.
This is why you should always dry the filament before printing, especially if the material you’re using is highly hygroscopic.
7 Ways To Prevent Zits On 3D Prints
Now that you know what can cause zits on 3D prints, you should learn how to prevent them.
Follow the steps below to get rid of the issue.
1. Calibrate Volumetric Extrusion
The first thing to check when you’re dealing with zits and blobs is the volumetric extrusion.
As explained, this value is set automatically by the firmware based on the type of filament you’ve selected in the slicer.
If you haven’t sliced the model yourself – or if you sliced it a while ago and can’t remember the material used – check the settings and slice the model again if necessary.
When reslicing, make sure to select the correct diameter of the spool you’re using, based on the label.
2. Adjust Print Temperature & Speed
A too hot extruder or too fast printing speed are two other common culprits for over-extrusion.
These parameters are easy to check and adjust in your slicing software, or you could adjust them directly from the G-code.
3. Adjust Fan Settings
Also, check the fan settings and make sure they match the specifics of the material you’re using. The fan should be on, preferably at 100%, when you’re printing with PLA, PETG, Flexible, PP, or PVA.
However, some materials are better printed without cooling, such as ABS, ASA, or Nylon. In this case, you should check the room temperature and make sure it doesn’t exceed 77°F.
4. Calibrate Retraction
Unfortunately, any retraction issue can lead to zits and blobs. An easy way to avoid them is by adjusting your retraction settings (which could also solve other common problems, such as stringing).
The optimal retraction settings vary from material to material, while a variety of other factors could also influence it. In broad lines, for instance, the optimal retraction settings for PLA are:
- 2mm retraction distance
- 0.6mm minimum travel distance
- 40mm/s retraction speed
If you’re using any other material, check out the label or indications from the manufacturer, and adjust the printer in small increments at a time until you get it right.
5. Adjust Coasting & Wiping Settings
Coasting is a 3D printing feature that stops extrusion right before a layer is finished.
The pressure build-up in the extruder will cause the filament to ooze out; this is enough material to finish your layer without having to worry about over-extrusion.
Wiping has the same goal as coasting, but instead of extruding on an empty spot, it adds to the layer that was already extruded.
Both coasting and wiping can be adjusted to control the volume of filament being coasted or wiped. These functions can also help you fix issues like under-extrusion at end points.
6. Keep The Nozzle Clean
Keeping the nozzle clean is crucial to prevent all sorts of 3D printing problems, including zits and blobs. There are various ways to clean the hot end, including:
- Wiping the nozzle with a damp cloth
- Scrub off excess filament with a wire brush
- Use the needle in your nozzle or printer kit
- Use cleaning filament
- Perform a cold pull
- Perform an atomic pull
- Use solvents
- Remove excess filament with a heat gun
- Blow out debris with an air compressor
- Use an ultrasonic cleaner
7. Dry The Filament
As a rule of thumb, you should always dry the filament before printing, unless you’re using a new spool that comes from a freshly opened package.
If you’re using hygroscopic materials, also pay attention to storage to prevent over-soaking the material.
Troubleshooting Common 3D Printing Zit Issues
The tips above can help you get rid of zits and blobs in general. However, mapping the issue or issues related to specific scenarios could help you find the culprit faster.
Blobs On Surface
Too much pressure is usually the culprit of blobs occurring randomly on the print surface.
Adjust the feed rate and extrusion temperature. Make sure the filament is dry and that the nozzle is clean. Coasting and wiping could also help fix the problem.
Zits On Curves
Over-retraction is one of the most common causes of zits on curves. Decrease retraction settings in small increments (0.5mm) at a time and test after each adjustment. Decreasing the inner wall speed when printing round or curved objects could also fix the problem.
Blobbing On First Layer
If blobbing only affects the first layer or layers, the most likely culprit is over-extrusion or incorrect nozzle height. The successive layers squish too much into the first layer, causing blobs.
A clogged nozzle that extrudes material at inconsistent rates can also lead to large blobs on 3D prints, especially on first layers.
Over-extrusion can also be the cause of 3D printer blobs on corners.
How To Remove Zits From 3D Printed Parts
The steps above can help you avoid zits and blobs. But what if you’ve already printed the item?
If the part is not functional, you might still save it with the methods below.
Sanding the surface is one of the easiest methods to remove blobs and zits from a printed part. Before choosing it, keep in mind that it will dull off the item – so, if you’ve used a glossy material, you’ll end up with a matt part.
Choose ultra-fine-grit sandpaper and sand down the imperfections. It is recommended to smooth out the part manually, but you could use a palm sander set on the lowest speed if the part is large.
Wipe with a damp cloth to remove dust when you’re done.
Filing works more or less like sanding, but you can use a file instead of sandpaper. This method can increase precision.
Acetone smoothing is a simple method that allows you to remove small imperfections without altering the finish.
This method only works with high-purity acetone (99% to 100%), so nail polish won’t do.
Choose a large container in which you can position the part without it touching the walls. Coat the container with paper towels and pour acetone over them so that they are soaked.
Place the piece you want to smooth in the center of the container and close the lid. Make sure the acetone-soaked paper doesn’t touch the piece.
Now, all you have to do is sit and wait for the acetone vapors to smooth all bumps and ridges on your piece.
The actual time can vary based on piece size and room temperature, but it usually takes up to two hours for small to mid-sized items.
This method heats the outer layer of the print just enough to melt the material on the surface and smooth out the ridges.
Place the item on a turntable and set the heat gun to the lowest setting. Keep it at a distance from the piece to prevent excessive melting and heat the material as you’re slowly turning it.
Once the bumps are gone, let it cool off completely before manipulating it.
Zits and blobs are common 3D printing issues caused by over-extrusion or retraction problems. Each of these problems can happen for a variety of reasons, such as too much pressure in the hot end, incorrect calibration, or even wet filament or a dirty nozzle.
Identifying the underlying issue and fixing it is the key to getting rid of zits and blobs. If you want to save the model, you might be able to smooth it with acetone, a heat gun, or sandpaper.