To follow up my previous post of stitches per inch, and the importance of getting them correct, for the overall quality and aesthetics of a garment, here is some information for knit SPI.
With knits, seam grinning, and and stitch cracking are common problems.
Jersey T’s/Polos– Around 10-12 SPI, to avoid any needle damage (the factory still needs a proper needle policy).
Fleece– Also around 10-12 SPI, to ensure proper seam coverage.
For medium to heavy sweaters– 8-10 SPI. The more elastic the seam, more SPI, to avoid stitch cracking.
Stretch knits– 14-18, to provide the proper seam elasticity.
Do not overlook, this often overlooked issue in apparel manufacturing. Leaving the decisions to the manufacturer can become an issue. The lower the SPI, the lower the manufacturing costs.