Blobs

BLOBS are Small lumps on the printing surface.

Blobs are mostly noticeable over the whole object, and each blob belongs to one layer. A blob arises when the filament is still under pressure in the print head when a layer is finished. This results in a short burst of over extrusion: a blob, (also known as Z-scaring or Z-seam). It can also occur, at the beginning of or within a layer. If the retraction is not set optimally, the nozzle oozes slightly during travel moves and can leave  blobs. Offcourse, it is difficult to join two pieces of filament together without leaving any mark, but there are several tools in Simplify3D that can be used to minimize the appearance of these flaws.

Retraction and coasting settings to prevent blobs

Check the first layer since a poor quality of the first layer is the most common source of extruder blobs. Always make sure that the first layer is adhered properly to bed. Watch the progress closely for the first 5-10 minutes. As soon as you notice small blemishes on the surface of the object, try to diagnose the cause. Does the defect appear the moment the extruder starts printing the perimeter? Or does it only appear later when the perimeter is completed, and the extruder is coming to a stop?

Retraction

When the flaw appears right at the beginning of the loop then it’s possible the retraction settings need a slight adjustment. Click in Simplify3D on “Edit Process Settings” and go to the Extruders tab. Right below the retraction distance, there is a setting labeled “Extra Restart Distance.” This option determines the difference between the retraction distance when the extruder is stopping and the priming distance that is used when the extruder is restarting.

If the surface flaw is right at the beginning of the perimeter, the extruder is likely priming too much filament. You can reduce the priming distance by entering a negative value for the extra restart distance. For example, if your retraction distance is 1.0mm, and the extra restart distance is -0.2mm (note the negative sign), then each time your extruder stops, it will retract 1.0mm of plastic. However, each time the extruder has to start extruding again, it will only push 0.8mm of plastic back into the nozzle. Adjust this setting until the flaw no longer appears when the extruder initially begins printing the perimeter.

Coasting

If the defect occurs at the end of the perimeter where the extruder stops, there is a different setting to adjust: coasting. To be found right below the retraction settings on the Extruder tab. Coasting will turn off your extruder a short distance before the end of the perimeter to relieve the pressure that is built up within the nozzle. Enable this option and increase the value until you no longer notice a flaw appearing at the end of each perimeter when the extruder stops. Typically, a coasting distance between 0.2-0.5mm is enough to have a noticeable impact.

Be aware of unnecessary retractions

Retraction and coasting settings can help avoid failures. But sometimes it is better to simply avoid the retractions entirely. In doing so, the extruder does not have to reverse direction and can continue a fine consistent extrusion. To adjust these settings, use Simplyfy3D, go the Advanced tab and look for the “Ooze Control Behavior” section. There are a lot of settings in this section that can modify the behavior of the 3D printer. Keep in mind that retractions are mainly used to avoid the nozzle from dripping as it moves between different parts of the object. However, if the nozzle is not crossing an open space, the dripping that occurs is on the inside of the object and not visible from the outside. Therefore, the Simplyfy3D has the “Only retract when crossing open spaces” option enabled to avoid unnecessary retractions.

In “Movement Behavior” section another related setting can be found. If your printer is only retracting when it crosses an open space, it would be beneficial to avoid these open spaces as much as possible. Simplify3D includes a feature that can divert the travel path of the extruder in order to avoid crossing an outline perimeter. And If the extruder is able to avoid crossing the outline by changing the travel path, a retraction is not necessary!
Enable “Avoid crossing outline for travel movement” for this feature.

Non-stationary retractions

Simplify3D also has the ability to perform non-stationary retractions. This is particularly useful because the extruder can build up a lot of pressure inside the nozzle while printing. When the printer then stops extruding, the excess pressure could create  blobs. Simplify3D has added an option that will keep the nozzle moving while it retracts. Therefore, a blob is less likely to occur since the extruder is constantly moving during the process. To enable this option, adjust a few settings.

First, go to “Edit Process Settings” and click on the Extruder tab. Check if the “Wipe Nozzle” option is enabled. This option instructs the printer to wipe the nozzle at the end of each section when it stops printing. For the “Wipe Distance”, enter a value of 5mm as a good starting point.

Next, click the Advanced tab and enable the option “Perform retraction during wipe movement”. It prevents a stationary retraction, because the printer is instructed to wipe the nozzle while it retracts.

Choose the location of your start points

If small flaws are still visible on the surface of the object, try the option that can control the location of these start/stop points. Go to “Edit Process Settings” and select the Layer tab. Usually, the locations of these start points are chosen to optimize the printing speed. However, the placement of the start points or the alignment to a specific location can be chosen. For example, all of the start points can be aligned to the backside of the object so that they are not visible from the front. To do this, enable the “Choose start point that is closest to specific location” option and then enter the XY coordinate where you want the start points to be placed.