Seam types and printing: what should I check on a finished garment?

What happens

Knitted panels, cut to pattern, are joined into garments on specialised machines. Overlock machines sew and neaten the cut edge in one pass. Cover-stitch machines make the flat, stretchy hems on sleeves and bottoms; lockstitch machines sew details. The work is split into short operations along a sewing line, each timed so that no station holds up the next. Screen printing pushes ink through a fine mesh stencil, one screen per colour, and the print is then heat-cured so that it survives washing. Finally, steam and heat press each garment into shape before inspection and packing.

Why it matters

1. A knit seam must stretch. ASTM D6193 says it should be slightly more elastic than the fabric; with over-tight tension or too few stitches, thread snaps under strain.
2. The wrong needle damages the fabric. A needle that is too thick or has the wrong point breaks knit loops and leaves holes right beside the seam.
3. A needle fragment in a garment can cause scratches or, if a child swallows it, fatal injury. Retailers such as Primark require documented needle and metal control.
4. Plastisol ink must cure through its whole thickness, or it can crack and flake off in the wash. A badly under-cured print can deteriorate after 1–3 washes.

Best practice

• Specify stitch type and density: overlock for knit seams, cover stitch for flat hems, and, per thread maker A&E, about 10–12 stitches per inch for jersey, 14–18 with elastane.coats.com↗ (opens in a new tab)amefird.com↗ (opens in a new tab)
• Use ball-point needles suited to the knit, sized ‘as thin as possible and as thick as necessary’ (Groz-Beckert). If a stretched test seam shows holes, go a size smaller. Replace worn needles.groz-beckert.com↗ (opens in a new tab)
• Swap needles strictly one for one. After a break, match every fragment to a whole needle; if a piece is missing, isolate the garments. Keep records at least a year.primark.a.bigcontent.io↗ (opens in a new tab)
• Cure to the ink data sheet, measuring the ink film, not the dryer setting: about 160 °C for most standard plastisols, about 121–148 °C for low-cure grades. Then stretch- and wash-test.lawsonsp.com↗ (opens in a new tab)avient.com↗ (opens in a new tab)
• Balance the line: time each operation (its SMV) so no station becomes a bottleneck. In one knitwear factory study, line balance rose from 45% to 85% and productivity from 45% to 58%.article.sapub.org↗ (opens in a new tab)

The terms

Overlock (overedge) stitch

A stitch that joins fabric and wraps its cut edge in one pass.

Stitches per inch (SPI)

Number of stitches in one inch of seam; it affects strength, stretch and appearance.

Curing

Heating a print until the whole ink layer fuses and becomes wash-resistant.

Plastisol ink

Screen-printing ink that forms a tough, elastic film when heated.

Standard minute value (SMV)

The time one operation should take, in minutes, set by work study.

Sources

1. ASTM D6193-16(2020): Standard Practice for Stitches and Seams (opens in a new tab)store.astm.org
2. A&E (American & Efird): Selecting the right stitches per inch (opens in a new tab)amefird.com
3. Groz-Beckert: Selecting the right sewing needle (opens in a new tab)groz-beckert.com
4. Lawson Screen Products: General information about plastisol inks (opens in a new tab)lawsonsp.com
5. Primark: Metal Control Policy (opens in a new tab)primark.a.bigcontent.io
6. Coats: Seam types (opens in a new tab)coats.com
7. Avient: Wilflex Product Selection Guide (opens in a new tab)avient.com
8. Nabi, Mahmud, Islam (2015): Improving sewing section efficiency through line balancing, Int. J. Textile Science 4(1) (opens in a new tab)article.sapub.org
9. Coats: Thread Consumption Guide (opens in a new tab)cdn.coats.com