Parameters determining adhesion to the printing base

La correcta adhesión entre la pieza y la base es fundamental para evitar fallos durante la impresión. La unión entre la primera capa de impresión y la superficie de la plataforma cumple dos funciones básicas:

1. Keep the part in a fixed position throughout the printing process to avoid layer displacements or collisions with the printhead.
2. To compensate the stresses caused during the cooling of the part, preventing it from detaching or deforming.

The adhesion between the part and the base is mainly determined by four factors.

1. The physico-chemical affinity between the printing surface material and the printing material.

The physico-chemical affinity between the printing material and the substrate surface is perhaps the most important parameter to take into account. This is why nowadays it is common to use polymeric coatings specifically developed for 3D printing on the surface of the platform. It is very important to use a product that is compatible with the printing material used, as some materials, such as PP or nylon, require specific products. There are mainly two types of solutions:

• Coatings applied on adhesive films.
  They consist of a film or adhesive tape containing a polymeric coating on one of its surfaces. Originally Kapton or similar tapes were used, but nowadays there are a multitude of solutions and products developed specifically for 3D printing. Perhaps the best known example is the polymer films from Buildtak, although similar products can be found from other manufacturers. These are generic solutions compatible with the most common materials, although more specific products can be found, such as PEI-coated films or specific ones for polypropylene or nylon. Their main advantage is that they have a good durability, so they can be used for several prints without replacement, but they lose adhesion with use and do not provide the best performance with certain technical materials. In addition, they usually have a temperature limit for use. To prolong their life span, it is advisable to print each time in a different position on the platform, so that the wear is uniform over the entire surface.
  
  

Image 1: Buildtak adhesive films. Source: buildtak.com

• Liquid coatings.
  These are polymeric suspensions that are applied directly to the surface of the base and which, when dry, form a continuous polymer film similar to that provided by adhesive films. This is the recommended option when working with a variety of technical materials, as there are specific formulations optimised for each of them. They are also easily washable with water or alcohol and can be reapplied quickly, so they can be conveniently replaced when changing printing materials. The main advantage is that material-specific and high-temperature formulations are available for higher performance. They also allow the coating to be regenerated by reapplication before each print, so that adhesion is not lost with use.
  In general, they are available in two formats: stick or spray. In the case of the spray format, it is recommended to always apply it with the base outside the printer to avoid damaging or soiling other components. If it is not possible to remove the base, it is recommended to always use the stick format as it is cleaner. The best known examples are the adhesion solutions from Magigoo or 3D printing lacquers such as 3DLac.
  
  

Image 2: 3D printing stickers by Magigoo. Source: Filament2Print.

2. The contact surface between the part and the base.

A larger contact surface between the part and the base produces greater adhesion. Care must be taken with large parts in printers without a heated chamber, as despite having a larger contact surface, their volume is also greater and therefore the stresses derived from cooling shrinkage. The most commonly used resource to increase the contact surface is to set an edge to the part during lamination. The edge shall be applied only on the first layer or in some cases on the first 2 or 3 layers, increasing the contact surface of the part without increasing the shrinkage stresses. There are other ways to increase the contact surface, such as increasing the extrusion factor during the first layer, although this is not recommended.

Image 3: Edge added to improve the adhesion of the piece. Source: own elaboration.

Some printers also include rough or textured coatings, which is another way to increase the contact surface and therefore adhesion.

3. The temperature of the printing base.

The temperature of the printing base has a double function in terms of adhesion. On the one hand, if it is equal to or slightly higher than the Tg of the printing material, it will prevent shrinkage due to cooling of the first layers, reducing the stresses that tend to lift or peel off the part. On the other hand, it favours the chemical bonding between the printing material and the coating material of the printing surface. It is recommended that the temperature of the base is equal to or slightly higher than the Tg of the printing material, however it is not advisable to set much higher temperatures, as they can reduce adhesion and cause deformations such as elephant foot.

4. The condition of the printing base

It is also very important to check the condition of our printing base. In spite of complying with the three previous points, there may still be adhesion problems due to poor condition or maintenance of the base. The following points should be checked periodically:

• Surface cleaning: Dust or dirt on the substrate can greatly reduce adhesion. The substrate surface should always be cleaned before every print.
• Surface defects or pitting: A deteriorated surface can also cause adhesion problems. It is important to keep the platform in good condition and to replace it if defects are detected.
• Worn coating: In the case of adhesive films, it must be considered that they lose efficiency with use, so they should be replaced with a new one as soon as we detect that they are worn. Liquid coatings can be reused for several prints, although it is possible that some of the coating may be lost when the part is removed. Before each print, the condition of the coating should be checked and a new application made if necessary.

Image 4: Comparison of a base in good condition and one in bad condition. Source: prusa3d.com

Problems related to bad adhesion

The main consequence of poor adhesion to the printing surface is that the part detaches from the base. This will always result in print failure, however, in some cases, it can also be the cause of equipment failure or breakage.
If the part detaches and moves away from its original position, the printer will continue to print without having a substrate on which to deposit the new material. This will cause the plastic to stick to the nozzle and block, forming a mass that will grow as the printing progresses. If not stopped in time, this mass of molten plastic will grow so large that it can render the hotend unusable.

In order to understand and solve adhesion problems, it must be taken into account that the part can be detached by two mechanisms.

If there is no good adhesion between the printing surface and the first layer, the part will peel off completely due to the friction of the nozzle during printing and vibrations. Generally the failure will occur at the beginning of the printing process. In this case it is advisable to check the following points:

• Distance between nozzle and base
• Height of the first layer
• Condition of the substrate
• Adhesion solution used

It may be that there is a very good adhesion to the base, but the part starts to peel off at the ends. This is because the shrinkage during cooling generates stresses from the outside of the part towards the centre that cause it to deform. In these cases, the adhesion between the part and the base is not sufficient to compensate for the stresses generated by the shrinkage. This phenomenon is known as warping. In this case, the part progressively peels off and the print can fail at any time, even a few minutes before completion.
In this case, attempts can be made to improve adhesion to compensate for warping, but other factors such as lamination parameters, material type or ambient temperature must be considered.

Problems related to excessive adhesion

In some cases, excessive adhesion to the base can make it really difficult to separate the part. In these cases the following should be checked:

• Distance between nozzle and base: It is important to have adequate clearance between the nozzle and the printing base. Too small a distance can damage the printing surface, cause excessive adhesion of the part or cause a jam by not allowing the molten plastic to come out.
• Adhesion solutions: Some materials such as PETG or some flexible materials may have excessive adhesion to adhesive or liquid coatings. In these cases it is advisable not to use coatings on the printing platform or to print on a raft. 
• Excessive extrusion: Excessive extrusion, especially in the first layer, can also increase adhesion.

When a problem of excessive adhesion appears, there are several ways to encourage the removal of the parts:

• Cooling the base: Because the base and the part will have different coefficients of thermal expansion, cooling the base can help the part come off. The best way to do this is to heat the base and then place it on a cold surface, such as an ice pack, so that it contracts sharply.
• Use a sharp spatula: In some cases, excessive adhesion is due to a suction effect. In these cases, it is advisable to introduce a sharp spatula underneath the piece and gradually peel it off. This method has the risk of damaging both the base and the piece.
• Dissolving liquid coatings: This is only possible when using a spray or stick coating. Because these products are easily soluble, placing the base under a stream of water or submerging it can help to remove the part.

When using glass bases it is necessary to be particularly careful, as there is a high risk of breakage. It is not uncommon for a fragment of the surface of a tempered glass base to be torn off when removing pieces that are too tightly glued together.

The use of flexible bases avoids this problem to a large extent and is therefore a highly recommended option.

This guide discusses concepts in a general way and does not focus on a particular make 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.