Optimizing Overlock Seam Parameters for Joining Footwear Uppers to Nonwoven Strobel Insoles

Abstract

This study examines the tensile strength of a single-thread overlock seam joining a footwear upper to a
nonwoven Strobel insole intended for special footwear. A three-factor, three-level experimental design varied insole
surface density from 500 to 600 grams per square meter, stitch density from three to seven stitches per centimeter,
and thread designation from three to five. Twenty-seven settings were tested in triplicate, giving 81 force
measurements. A full quadratic response model was fitted to the individual results. The coefficient of determination
was 0.937, and the adjusted coefficient was 0.928. The model predicted a maximum force of approximately 205.4
newtons at 560.9 grams per square meter, 4.67 stitches per centimeter, and thread designation 4.49. However, a lack-
of-fit test against the within-setting replication error was significant, so this interpolated maximum is provisional.
The strongest observed setting used a 550 grams per square meter insole, five stitches per centimeter, and
designation four thread, producing a mean force of 204.3 newtons. At designation four, increasing stitch density
from five to seven stitches per centimeter reduced mean strength at every material density. The observed setting is
a practical starting point for the particular materials and machine studied. Further production runs, other insole
materials, durability tests, and product-level compliance assessments are required before wider application