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However , 3D printing carbohydrate glass in this way presents several unique challenges : 1 ) the material must resist degradation and crystallization during printing , 2 ) the glass must be hot enough to flow freely during extrusion and fuse to the printed construct , while cooling rapidly to retain its shape upon exiting the nozzle , 3 ) the extruder needs to apply high pressure , with rapid stop and start times and 4 ) the net force that acts on the filament during extrusion must be minimized so that the filament shape is predictable , i.e. , coincides with the path taken by the nozzle .
 3D printing, glass, material, degradation, crystallization, glass, be, flow freely, extrusion, fuse, printed construct, cooling rapidly, nozzle, extruder, pressure, force, filament, extrusion, be, filament, predictable, nozzle
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 3D printing, glass, material, degradation, crystallization, glass, be, flow freely, extrusion, fuse, printed construct, cooling rapidly, nozzle, extruder, pressure, force, filament, extrusion, be, filament, predictable, nozzle