Speeds and accelerations in 3D printing

The correct configuration of speed and acceleration parameters is fundamental. The maximum printing speed is usually effectively limited by the maximum volumetric flow rate that the hotend can provide, while there is no exact limit to the head movement speed, but higher speeds and accelerations tend to decrease print quality.

Speed and acceleration parameters

Straight movements on the axes consist of three stages:

• Acceleration from the direction change speed to maximum speed.

• Travel at constant maximum speed

• Deceleration to direction change speed.

This is why there are three parameters that define speeds and accelerations in the movement of an FFF 3D printer for each of the 4 axes (X, Y, Z, E). These parameters are maximum speed, acceleration, and direction change speed (jerk).

Maximum speed: Maximum speed at which the head can move on each axis. Generally configured in the slicer and may be different for each part element.

Direction change speed: Usually configured directly in the firmware and usually constant for each axis. It is the maximum speed allowed before a direction change.

Acceleration: It is the acceleration value applied to transition from direction change speed to maximum speed and vice versa. Usually configured in the firmware and usually constant for each axis.

Although it is generally considered that printing speed impacts the quality of the piece, the parameters that have the greatest influence in this sense are acceleration and direction change speed, as high decelerations and direction change speeds transmit the head's energy more quickly to the printer's structure, causing vibrations and possible motor steps losses.

This does not mean that printing speed itself does not have an impact. The higher the speed, the greater the linear momentum of the head, and therefore the more energy will be dissipated in deceleration and direction change, so high printing speeds will also affect print quality.

Speed configuration

Generally, manufacturers usually include correct acceleration and direction change speed configurations in the firmware of their equipment, so it is not recommended to modify them. The most common configuration is based on adjusting the printing speed in the slicing software.

Currently, most slicing software allows modifying the speed value for the different parts of the piece. This is an important advantage when optimizing printing times, as not all areas of the piece require the same print quality. The most common elements on which printing speed can be modified are:

• Perimeters: Large defects in internal perimeters can be reflected on the surface of the piece. This is why intermediate values are usually used compared to those used in external perimeters and infill.

• External perimeters: Along with the first and last layer, it is the visible part of the piece. It is recommended to use medium or low speeds to ensure a good finish. Generally, the printing speed is reduced between 25% and 50%.

• Infill: Maximum speed is usually used since defects or vibrations that occur in this area are not usually visible on the outside of the piece. When very high infill speeds are used, it is recommended to use low infill overlap values (10% - 15%).

• Solid infill: Like infill, it is common to use maximum speed since defects do not usually impact piece quality.

• First layer: In order to ensure good adhesion to the print bed, very low speeds are usually used for the first layer. It is most common not to exceed 20 mm/s.

• Last layer: Like in the case of external perimeters, it is common to use speeds with a reduction between 25% and 50% to ensure good quality.

• Support material: Speed depends largely on whether soluble material is used or if the supports are made of the same material as the piece. Generally, slightly lower speeds are used on supports, as their low density carries the risk of failure. Soluble materials usually require lower speeds due to their low adhesion.

• Bridges: In order to improve the quality of overhangs in bridges, high speeds are usually selected. Common values are 110% or 120%.

Below are some safe values for printers with light and heavy heads.

Depending on how stable the structure of the 3D printer is, higher speeds can be used.

Limitations based on the size and geometry of the piece

It is possible that with some pieces, it is not possible to use maximum speed. This is because in short travels it may be necessary to start decelerating before reaching maximum speed. This mainly occurs in configurations with very low acceleration values and small pieces with complex geometries. Generally, in these cases, there is a significant discrepancy between the estimated print time by the slicing software and the actual print time.

Image 1: Acceleration, constant speed, and deceleration profiles for short and long travel. Source: Filament2print.com

Problems derived from a poor adjustment of speeds or accelerations

In general, low speeds usually do not result in problems, beyond excessively long printing times. Only when speeds are excessively low (5-10 mm/s) can inconsistent extrusion problems occur due to the low speed of the extruder motor, unable to provide a constant flow. This problem will not occur in those extruders that incorporate reduction gears.

However, high speeds are often the cause of problems:

• Vibrations: One of the most common problems is the appearance of vibrations. These vibrations are usually reflected in wave patterns on the surface of the piece, generally around the edges.

Image 2: Pattern on the surface of a piece caused by vibrations in the printer. Source: Simplify3D.com

• Motor step losses: The combination of high speeds, together with motors powered by low currents, can cause motor step losses that are reflected in dimensional errors of the piece or layer shifts.

Image 3: Piece with layer shift. Source: Zortrax.com

• Separation of the piece from the base: High speeds are also one of the common causes of the piece or supports detaching from the base. This can be due to vibrations, nozzle friction with the piece, or a combination of both.

• Filling and perimeters joining and perimeter closure: High printing or direction change speeds can cause poor adhesion between the infill and perimeters or prevent the perimeter from closing correctly by producing poor adhesion of the final line sections. This phenomenon occurs more commonly in the first layer.

Note: This guide discusses concepts in a general manner without focusing on a specific brand or model, although they may be mentioned at some point. There may be significant differences in calibration or adjustment procedures between different brands and models, so it is recommended to consult the manufacturer's manual before reading this guide.