Zits or blobs can appear on your prints for a multitude of reasons. The cause of them in all cases is due to the printer pausing for a split second that results in filament oozing from the nozzle. The core reason is due to intricate moves that cause the printer to slow down.
When you are slicing a model your slicer looks at all the "faces" of the model and generates moves based on them. The issue will present itself more when you are printing objects of organic or complex nature. You can get around this by having your slicer merge these moves into simpler ones that result in less tiny moves from the printer. Technically this will reduce the resolution of the print but you are not going to notice it.
This is relevant for SD or USB printing. If using an SD card try a new one from a name brand before going through all these changes. A bad SD card can cause all kinds of issues with your printer.
We have not had this issue with Simplify3D and it does not have a setting to change the resolution of the sliced model. There is an option to discard extrusions below a certain length that can improve print times.
Here is a video from CNC kitchen that goes over this issue with Cura and PrusaSlicer in detail.
If you are using OctoPrint the issue can be a multitude of issues. But the core issue is the buffer running out for some reason and the printer pausing as it waits for the next command. We did a detailed video on it here if you want to watch. Doing the above resolution changes with your slicer will also help with the serial data transmission pauses with OctoPrint as well since the printer is receiving larger moves instead of smaller moves.
Things to check and do (or not do) if you are using OctoPrint to help fix zits or blobs on your prints:
- TH3D Firmware - Make sure you are using a firmware that is the latest for your board/printer if its one that we support with our TH3D firmware releases
- Marlin Firmware - Increase the buffer sizes in your printer firmware if you are using a custom machine or standard Marlin
- Power Supply - Make sure you are using a 3 Amp power supply to power your Pi. This means a real power supply NOT a phone charger or other USB charger. A charger is not a power supply and will cause issues with a Raspberry Pi
- USB Cable - Make sure the USB cable you are using to connect between the printer and the Pi board is a high-quality cable, shorter is better.
- SD Card Issues - Make sure you are using a working MicroSD card that is a Class 10 or better card from a reputable brand.
- Reinstallation - Flash a fresh OctoPrint installation and do NOT install any plugins if you have ruled out all hardware issues with your OctoPrint setup (PSU, SD Card, Cooling)
- Plugins - Plugins are awesome for OctoPrint but they can cause issues if they are spiking the CPU. Reinstall OctoPrint from a fresh image and then reinstall your plugins one by one until you find the one (or combination of plugins) that is causing the CPU spikes.
- CPU Power - You must use a Raspberry Pi 3B or higher board. Anything lower than this (Pi Zero Pi 1, Pi 2, etc) will not work well and cause issues as they are underpowered.
- Patience - Stop refreshing and pulling up the OctoPrint UI unless you need to. This can spike the CPU and cause slowdowns