Embroidery underlay: which type to use and how to set it

What does underlay actually do?

Underlay is a set of light stitches sewn under a satin or fill, as part of the same object, before the cover stitches. It does four jobs, and knowing which job you need tells you which type to choose:

• Stabilize. It stitches fabric and stabilizer together into one layer before the heavy cover stitches start pulling. This is the main defense against puckering and shifting.
• Lift. On knits, fleece and terry it builds a base that holds the cover stitches on top of the surface instead of letting them sink between loops or ribs.
• Define edges. An edge run lays a "fence" along the outline, so the cover stitches land on a firm line and edges stay smooth.
• Add coverage. Underlay laid at a different angle fills the gaps in the cover layer so the fabric color does not grin through, which lets you use a lighter top density.

The trade-off is stitches and stiffness. Every underlay pass adds stitch count, run time and thickness. The aim is the least underlay that does the job for this fabric and this object.

The underlay types and when to use each

Center run

A single line of run stitches along the middle of a satin column. It anchors the column to the fabric and gives the satin a slight crown, without bulking up narrow shapes. It is the default for small lettering and thin borders, where anything wider would show at the edges. On columns under about 1.5 mm, many digitizers skip underlay entirely because even a center run can make the satin lumpy.

Edge run

Runs that trace the inside of the shape's outline, slightly inset. They tack the fabric down around the perimeter, which matters because edges are where pull distortion shows first. Under a satin, edge run gives crisp, straight sides. Around a fill, it holds the boundary so the fill's pull cannot drag the fabric inward and open gaps against neighboring outlines. On its own, it gives little lift, so on textured fabric pair it with zigzag or tatami underlay.

Zigzag and double zigzag

A zigzag underlay is an open, widely spaced satin sewn inside the column. It raises the cover satin, hides the fabric, and on knits it holds the ribs flat so the cover satin sits evenly. Double zigzag sews a second pass back over the first, roughly crossing it, for more lift and stability. That is the standard choice for wide satins on towels, fleece and cap fronts. Keep zigzag spacing open (around 2 to 3 mm as a start); a dense zigzag underlay makes the column hard and raises thread and needle stress without improving the look.

Tatami underlay

A widely spaced fill sewn under a fill, usually at or near 90 degrees to the top fill's angle. The cross-hatch locks the fabric in two directions, so the top fill's pull has something to pull against. It also covers fabric color, which lets you open the top density slightly. For very large fills, an edge run plus tatami underlay is the safe default; on tightly woven, stable fabric with a modest fill, edge run alone can be enough.

Why underlay must be inset from the edges

Underlay is meant to disappear. If it extends to the very edge of the shape, the cover stitches cannot fully hide it, and pull from the cover layer drags the edges inward, leaving underlay peeking out along the sides of satins. That shows up as a ragged edge or a thin line of a different texture, and in a contrasting color it looks like a registration problem.

Starting insets that many digitizers use (hedged, because software defines inset slightly differently):

• Satin edge run or zigzag: about 0.3 to 0.6 mm in from each edge, more for wider columns.
• Fill edge run: about 0.3 to 0.5 mm in.
• Fill tatami underlay: about 0.5 to 1 mm in.

On stretchy fabric, increase insets slightly, because the cover layer will pull the edge further. If you add pull compensation to the cover stitches, the cover widens outward, which also helps hide the underlay.

Underlay by fabric

Knits

Knit fabric stretches in both directions and the ribs let stitches slide sideways. Without underlay, the cover satin pulls the knit together and sinks between ribs, so columns look thin and wavy. Edge run plus zigzag holds the ribs flat before the satin arrives. Pair this with cutaway stabilizer: underlay can only tie fabric to a stabilizer that stays in place, and tearaway on knits loses that support after the first wash. Hooping and stabilizer details are on T-shirts and knits.

Towels and pile fabrics

Loops push up between stitches. Strong underlay crushes and tacks down the loops in the stitched area, and a water-soluble topper above the fabric keeps the top stitches from dropping between them. Some digitizers add a very light knockdown fill under the whole design area for deep pile. See towels and terry.

Caps

A structured cap front is stiff but curved and held away from the needle plate at its edges, which encourages flagging (fabric bouncing up and down with the needle). Underlay that sews from the center outward tacks the front to the backing progressively, pressing the fabric against the frame before the cover stitches. Keep the overall design lighter than you would on a flat jacket. More in caps and hats.

Common underlay mistakes and their symptoms

Underlay visible along satin edges

Inset too small, or no pull compensation on the cover. Increase inset by about 0.1 to 0.2 mm and retest.

Small lettering lumpy and thick

Zigzag or edge run underlay in columns that are too narrow for it. Use center run or none.

Fill puckers despite good stabilizer

Missing tatami underlay, or underlay at the same angle as the cover. Set underlay near 90 degrees to the cover. More causes are on puckering.

Design feels stiff and heavy

Too much underlay plus full top density. Open the underlay spacing or reduce top density, since underlay already adds coverage.

Gaps between fill and outline

No edge run, so the fill pulled the boundary in. Also check overlaps and registration.

How to test underlay settings

1. Make a test sheet. Digitize a few satin columns at 2, 4 and 7 mm wide and a 30 to 40 mm fill square, each with a different underlay option.
2. Sew on the real fabric and stabilizer. Underlay that is perfect on twill can be useless on a pique polo.
3. Inspect edges and coverage. Look for underlay showing at edges, fabric showing through, and wavy or thin columns.
4. Feel the hand. Flex the fabric. A design that crackles or folds like card has too much underlay or density.
5. Record the winners. Save them as a fabric preset or template in your software so the next design for that fabric starts right. Density changes go hand in hand; see stitch density.