Shrinkage of parts in 3D printing and Warping

Note: This guide deals with the concepts in a general way and without focusing 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 to consult the manufacturer's manual before reading this guide.

During printing, high-temperature extruded plastic undergoes volume shrinkage due to cooling. In some materials such as PLA this shrinkage is very low (between 0.3 % and 0.5 %), so it is not usually problematic, however other materials such as nylon 12 can have up to 2 % shrinkage or in the case of PVDF even up to 4 %, causing significant deformations in the parts.

Table 1. Percentage shrinkage of various plastics used in FFF 3D printing. Source: SpecialChem.com

This shrinkage is a percentage of the part size, so it must be taken into account that very large parts printed in low shrinkage materials can be more problematic than small parts in high shrinkage materials. This is why a part with a 20 x 20 cm base made of PLA may have a higher risk of failure than a 5 x 5 cm part made of ABS.

When the cooling of the part is very fast and irregular, mainly due to a high difference between the ambient temperature and the printing temperature, the part shrinks unevenly, causing deformations at the ends of the part. This phenomenon is known as warping. Warping has two basic consequences:
If the adhesion between layers is not good, it causes the layers to separate.
If the adhesion to the base is not good, it causes the part to lift.

Image 1: Explanation of the warping phenomenon. Source: rigid.ink

In any of the above situations, the consequence is that the deformation of the part will cause it to lift or detach, resulting in a collision with the printhead and causing the print to fail.

To compensate for the effect of shrinkage of parts

Shrinkage of parts during cooling is inevitable, however it is possible to avoid or minimise the risk of failure in several ways:

• Avoid shrinkage of the part during the printing process.
  This is the ideal solution, which will guarantee the highest quality parts and prevent deformation. It consists of keeping the part at a temperature equal to or slightly higher than the Tg of the material throughout the printing process, which will prevent the part from beginning to shrink during the printing process. Once finished, it will cool down slowly, achieving a uniform shrinkage of the part without deformation, while avoiding internal stresses. This requires a printer with a heated chamber capable of reaching temperatures suitable for each material.
• To achieve sufficient adhesion at the base to prevent the part from peeling off when shrinking.
  When a heated chamber is not available, it is possible to reduce the risk of failure by increasing the bond between the first layer and the base. If this bond is able to withstand the stresses caused during shrinkage, the part will not detach from the base and the print can be successfully completed. The larger the part size, the higher the stresses and therefore the greater the adhesion required. This is why the risk of failure increases with part size and it is often said that materials prone to warping only allow small parts to be made on printers without a heated chamber. 
• Achieve a good bond between layers.
  If the bonding forces between the first layer and the base are greater than those between layers, a problem of layer separation or delamination may occur.
  
  

Image 2: Layer separation effect due to shrinkage during cooling. Source Geeetech.com

In reality, the size limit when manufacturing any part in FFF will depend on the bond strength between the first layer and the base and the cohesive forces between layers being able to compensate for the tensile, shear, tearing and peeling stresses generated as the part shrinks during cooling.

The magnitude of these stresses will depend on three factors: the volume of the part, the shrinkage coefficient of the material and the ambient printing temperature.

To reduce the risk of failure, it will be necessary to increase as far as possible the adhesion of the part to the base and between layers, and to reduce the stresses generated by the part..

The following strategies can be used to improve the adhesion of the part to the base:

• Improve adhesive bonding: using material-specific adhesive solutions to maximise bond strength.

Image 3: 3D printing specific adhesives. Source: magigoo.com

• Increase the extrusion of the first layer: more material applied at higher pressure will increase the effective contact surface, improving the bond.
• Keep the surface clean and free of defects: The presence of dust or other dirt critically reduces part adhesion.
• Increase the contact surface with the base: Applying a Brim to the part in the first layers will increase the contact surface with the base without increasing the stresses in the part. This will also help to increase the bond strength.
• Avoid corners and rounded edges in contact with the base. Corners are stress accumulation points and are the areas where the part will start to peel off. Rounded edges in contact with the base reduce the contact surface with the base and increase the stresses in the part. It is best to avoid this type of element in the part design, but when this is not possible, applying an edge to the parts will minimise its effects.

The following strategies can be used to reduce the stresses on the workpiece:

• Reduce the volume of material in the part: this can be done by optimising the infill pattern. The percentage of infill in the part can be reduced and the loss of properties can be compensated by using a larger number of perimeters or by optimising the orientation of the pattern.
• Use materials with a low shrinkage coefficient: Materials with a lower shrinkage coefficient should be sought among the materials that meet the technical requirements of the part. Fibre- or particle-loaded materials generally have lower shrinkage coefficients and in many cases offer superior mechanical properties, which also helps to be able to reduce the filler density without losing properties.
• Increasing the ambient printing temperature: Closed or passive chamber printers, although they do not reach temperatures as high as printers with an active heated chamber, can in many cases maintain stable temperatures of between 50 and 70 °C thanks to the heat radiated by the printing base. A common strategy is to heat the base and hold it for 10-15 minutes before printing.

The following strategies can be used to improve the bond between layers:

• Reduce the layer height: Lower layer height means better cohesion between layers.
• Increase the temperature and reduce the speed of the coating fan: Higher temperature and slower cooling will also provide better layer bonding.
• Increasing the flow: This option is not recommended, as it will change the dimensions and tolerances of the parts. However, a higher flow rate has a similar effect to that obtained by reducing the layer height, but without increasing printing times.

How to determine the maximum print volume for each material.

It is possible to determine the maximum safe volume or size when manufacturing parts with a certain material in our printer. To do this, the following steps must be followed:

• Prepare a suitable printing profile. The maximum part size is linked to this particular profile. If a variant of this profile is produced, the maximum volume must be re-determined if any of the following parameters are changed:
• Printing temperature
• Base temperature
• First layer height
• Layer height
• Extrusion flow rate
• First layer speed
• Filler pattern
• Filler density
• Number of perimeters
• Search for the best adhesion surface available. It is advisable to use liquid coatings optimised for the media to be used, and to reapply them before each print.
• Ensure that the temperature and humidity conditions in the room where the printer is located are constant.

Once these preliminary steps have been carried out, it will be necessary to make iterative tests until the maximum size is found. For this purpose, a cube will be used with the edges parallel to the Z axis rounded and a size of approximately half the printing base.

By following this scheme it is possible to determine the maximum safe print size for a material combination and profile on a given printer.

Once the maximum safe volume has been determined, any part contained within this volume should be able to be produced with almost no risk of failure.

To provide a safety margin, it is advisable to activate the border option in the print profile when printing parts of maximum size (this function should not be used during the iterative determination of the maximum size).