Watertightness, Strength and Isotropy

FDM printers melt plastic layers on top of layers. This creates mechanical adhesion (not chemical), and means that FDM layer surfaces are not fully adhered to each other. Even when the previous layer is partially melted, the surrounding layers are only partially adhered to their neighbouring layers.

As a result, FDM-printed objects have different mechanical properties based on the direction mechanical stress is impacting them and are less dense than a similar object produced via alternative methods, such as injection moulding. This is also the reason why it is difficult to produce watertight objects via FDM printing: FDM prints are full of microscopic voids and holes.

In SLA prints, there is no difference between the Z-axis and the XY plane in terms of chemical bonds; each continuous part printed on an SLA machine is a continuous polymer network.

As each layer is formed, the resin monomers react and form covalent bonds providing high degrees of lateral strength, but the polymerization reaction is not driven to completion; rather, the print process is modulated in a way that keeps the layer in a semi-reacted state called the “green state.”

This green state differs from the completely cured state in one very important way: there are still polymerizable groups on the surface that subsequent layers can covalently bond to.

As the next layer is cured, the polymerization reaction will also include the groups on the previous layer, thus forming covalent bonds not just laterally, but also with the previous layer. This crosslinking is typical of all SLA printing processes.

That means that on a molecular level, there is little to no difference between the Z-axis and the XY plane in terms of chemical bonds; each continuous part printed on an SLA machine is a single molecule. Since the SLA lines are fully bonded to their neighbors, there are also no voids or microscopic cracks typical in FDM prints; these prints are watertight and fully dense.