Speeds and accelerations in 3D printing

The correct setting of speed and acceleration parameters is critical. The maximum print speed is often effectively limited by the maximum volumetric flow rate the hotend can deliver, while there is no exact limit to the speed of head movement, 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 change of direction speed to maximum speed.

• Travelling at constant maximum speed.

• Deceleration up to change of direction speed.

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

Maximum speed: Maximum speed at which the spindle can move in each axis. It is usually configured in the rolling mill and can be different for each element of the part.

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

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

Although print speed is generally considered to have an impact on part quality, the parameters that have the greatest influence in this respect are acceleration and change of direction speed, as high decelerations and change of direction speeds transmit the head energy more quickly to the printer structure, resulting in vibrations and possible losses of pitch to the motors. 

This is not to say that the print speed itself has no impact. The higher the speed, the higher the linear momentum of the printhead, and therefore the more energy will be dissipated in deceleration and change of direction, so high print speeds will also affect print quality.

Speed configuration

In general, manufacturers usually include correct acceleration and shift speed settings in the firmware of their equipment, so it is not recommended to change these settings. The most common setting is based on adjusting the print speed in the lamination software.

Nowadays, most laminating software allows you to modify the speed value for the different elements of the part. This is an important advantage when it comes to optimising printing times, as not all areas of the part require the same print quality. The most common elements on which the print speed can be modified are:

• Perimeters: Large defects in internal perimeters can be reflected on the surface of the part. This is why intermediate values to those used for external perimeters and filling are often used.

• Outer perimeters: Together with the first and last layer, this is the visible part of the part. It is advisable to use medium or low speeds to ensure a good finish. In general, the printing speed is usually reduced by 25-50%.

• Filling: The maximum speed is usually used, as the defects or vibrations that occur in this area are not usually visible on the outside of the part. When using very high fill speeds, it is advisable to use fill overlap values at low perimeters (10 % - 15 %).

• Solid fill: As with fill, it is common to use the maximum speed as defects do not usually impact the quality of the part.

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

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

• Support material: The speed depends to a large extent on whether soluble material is used or whether the supports are made of the same material as the workpiece. In general, somewhat lower speeds are used for substrates, as they are at risk of failure due to their low density. Soluble materials usually require a lower speed due to their low adhesion.

• Bridges: In order to improve the quality of cantilevers on bridges, high velocities are usually selected. Values of 110% or 120% are common.

The following table shows some safe values for printers with light and heavy printheads.

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

Limitations depending on the size and geometry of the part

It may not be possible to use full speed with some parts. This is due to the fact that on short travels it may be necessary to start decelerating before the maximum speed is reached. This occurs mainly in configurations with very low acceleration values and small parts with complex geometries. Generally, in these cases, there is a significant gap between the print time estimated by the laminating software and the actual print time.

Image 1: Acceleration, constant velocity and deceleration profiles for a short and a long run. Source: Filament2print.com

Problems resulting from incorrect speed or acceleration settings

In general, low speeds do not usually lead to problems other than excessively long printing times. Only when speeds are excessively low (5-10 mm/s) can they lead to problems of inconsistent extrusion due to the low speed of the extruder motor, which is unable to provide a constant flow. This problem will not occur in extruders with a gearbox.

However, high speeds are a frequent cause of problems:

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

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

• Motor step loss: The combination of high speeds, coupled with motors powered by low currents, can cause step loss which is reflected in part dimensional errors or layer displacements.

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

• Part separation from the base: High speeds are also a frequent cause of the part or holders detaching from the base. This can be due to vibration, friction of the nozzle on the workpiece or a combination of both.

• Filling and perimeter bonding and perimeter closing: High printing speeds or change of direction can cause a poor bond between the filling and the perimeters or prevent the perimeter from closing properly due to poor adhesion of the end sections of the line. This phenomenon most commonly occurs in the first layer.

Note: This guide discusses concepts in a general way and does not focus on a specific brand or model, although they may be mentioned at some point. There may be important differences in calibration or adjustment procedures between different makes and models, so it is recommended that the manufacturer's manual be consulted before reading this guide.