|Hydrophobic Polymers||Hygroscopic polymers that only have cosmetic concerns||Hygroscopic polymers that can also have performance issues|
|PE (LDPE, HDPE)||ABS||PLA|
|POM||Polyimides (PEI, PAI)|
As mentioned before, table 1 just gives a few examples of hydrophobic and hygroscopic polymers which contain no additives. To know exactly which category covers your polymer, please read the supplied technical datasheet or contact the supplier of the polymer. Every technical datasheet should contain information about drying time and temperature. This information is usually based on the maximum moisture content of the polymer, with which it can still be processed successfully. Hygroscopic polymers which fall under the column of cosmetic concerns generally can be processed successfully if the moisture content is below 0.05 or 0.10%. Polymers under the performance issue category should not exceed a moisture level of 0.02%, and it would be even more optimal if the moisture content is contained below 0.005%.
At the 3devo test lab we always pre-dry hygroscopic materials before trying to make filament out of it. After the pre-drying, we also measure the moisture content in a moisture analyzer machine, to check if the material is dry enough to be processed successfully.
5. Polymer shrinkage on cooling
Polymer shrinkage on cooling can cause holes centered in the filament. Filament sections quenched rapidly after leaving the die form a surface skin. As the material continues to cool, the skin continues to thicken. At a certain point, the molten material inside the thick area cannot pull the solidified outer surface in on the part, but it continues to cool and shrink. Since the outside is solidified, the walls cannot be pulled toward the center. Shrinkage causes the molten material on the inside to shrink toward the outside surface, forming a vacuum void in the center. Potential solutions are to cool the filament more slowly, allowing the outside to shrink toward the center. Figure 6 shows a shrinkage void in an extruded piece of filament. Materials that contain additives such as glass or carbon fibre, tend to have this problem very often because of the rigid structure that is generated by the fibres. Figure 7 gives an example of this event has happened during an extrusion test of carbon fibre infused PEKK, here a cross section of the filament is shown.
6. Chart with potential causes and corrective actions
Figure 8 shows a summary of all possible causes of bubbles in the filament in one overview.