984
Void formation during printing , poor adhesion of fibers and polymer matrix , and challenges in continuous fiber printing are all amongst the existing issues in 3D printing of fiber composites .
 concept or principle: Void; machanical property: adhesion; material: fibers; material: polymer; material: continuous fiber; manufacturing process: 3D printing; material: fiber composites
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: 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:
 concept or principle: Void; machanical property: adhesion; material: fibers; material: polymer; material: continuous fiber; manufacturing process: 3D printing; material: fiber composites