Method for manufacturing glass fiber strand

A method for manufacturing a glass fiber strand by which a plurality of glass fiber filaments formed of molten glass being drawn out from a plurality of nozzles are bound together into a glass fiber strand, the method comprising:

• an imaging process of continuously imaging the plurality of glass fiber filaments to generate a plurality of pieces of image data;
• an image processing process of extracting a high brightness object having brightness equal to or greater than a predetermined value from the plurality of pieces of image data;
• and
• a breakage detection process of detecting breakage of any of the glass fiber filaments based on a result of image processing performed in the image processing process, wherein
• in the breakage detection process,
• whether a position of the high brightness object has changed is detected based on the plurality of pieces of image data, and, when a result of the detection indicates that the position of the high brightness object has changed, a determination is made that any of the glass fiber filaments has been broken,
• wherein
• in the breakage detection process,
• a drop velocity of the high brightness object is computed based on the plurality of pieces of image data, and
• when the drop velocity based on the computation is equal to or greater than 1 cm/s, a determination is made that any of the glass fiber filaments has been broken.