For some hollowed parts, after a period of time, it may have cracked or even broken. What is the cause of this?
Though the parts have reached their final form, you can easily detect that the surface of the part is still tacky and relatively soft. This means that when removed from the printer a post-curing will be required to complete any unfinished chemical reactions. Material properties, such as strength and stability will be improved by UV post-curing. After post-curing, the surface of the part will be harder and drier. The only surface texture that maybe present is a rubber like feel which is partly due to the oxidation process that happens when a print has part cured resin exposed to the air for long periods.
If the inner wall of the part is not cleaned properly, the liquid resin can cause cracking or even break the part after a long period of osmosis and erosion.
The so-called internal stress refers to the stress remaining in the object after the external load is removed. It is caused by uneven volume changes in the structure of the material. In the absence of external forces, due to improper processing and moulding, temperature change, solvent action and other reasons, the material will produce internal stress, resulting in part cracking or even rupture.
Photosensitive resin materials show different properties according to the formula or production method, and are suitable for different applications. In general, tough materials such as ABS-like resin usually have a good balance between strength (the amount of stress a material can withstand) and ductility (the elongation or percentage strain), can withstand higher stress or larger strain and are less prone to be fractured. If the resin toughness is poor, on the contrary, it will easily be fractured.