In one sweatshirt sample, the lower edge was straight before sewing but visibly rippled after overlocking. The more the operator tried to guide the soft knit, the more obvious the wave became.

The buyer's question was simple: why did we get a wavy hem after overlocking when the panel was cut correctly? The differential feed was not compensating for stretch, and excessive foot pressure was dragging the material through the machine.

We adjusted the differential feed to counter that stretch, reduced the foot pressure while maintaining stable feeding, and stopped pulling the panel during sewing. The revised overlocked edge lay much flatter.

Why You Get a Wavy Hem After Overlocking

An overlock machine trims, feeds, and stitches in one operation, so feeding errors are locked into the cut edge immediately.

Soft jersey, French terry, and fleece can stretch under the foot. If the edge is stitched while extended, the surrounding fabric later relaxes around a longer seam and creates a wave.

Here, the edge was flat before sewing and distorted directly behind the foot. That timing matters in garment quality control because it directs the investigation to sewing rather than cutting.

Differential Feed Was the Main Setting to Correct

An overlock machine uses separate feed-dog sections. Differential feed changes the relationship between their movements.

Brother's technical explanation notes that equal feed-dog movement is a neutral setting, while increased front feeding can gather the fabric slightly and remove rippling from stretch materials (Brother, Differential Feed Guide). In practical terms, the machine must feed a stretch knit in a way that offsets the extension created under the foot.

There is no universal setting. Elasticity, finish, seam direction, stitch length, and machine model affect the result. JUKI's industrial specifications distinguish between gathering and stretching ranges and provide micro-adjustment for the material (JUKI MO-6800S Series).

For custom sweatshirt manufacturing, test the actual bulk fabric and increase anti-stretch compensation only until the edge lies flat. Too much creates gathering.

Presser-Foot Pressure Can Stretch the Fabric

The original foot pressure created excess friction, stretching the knit before it reached the needle.

Reducing pressure helped, but "as low as possible" is the wrong instruction. Too little pressure can let the foot bounce or lose control.

Coats explains that excessive foot pressure can stretch the top ply, while insufficient pressure can let the foot bounce. Its guidance is to use pressure that still provides positive, even feeding (Coats, Eliminating Seam Puckering).

Reduce unnecessary pressure, then confirm that the fabric still tracks evenly at production speed.

Operator Handling Can Override the Machine Setting

Even a correctly adjusted differential feed cannot fully compensate if the operator pulls the panel from behind the machine or holds it back before the needle.

Holding a long knit edge tightly places it under tension while the knife and stitch fix its length. The operator should guide the edge without using the panel to control sewing speed.

If a sweatshirt body hangs off the table, its weight can pull against the feed. During hoodie manufacturing, support the panel so the machine controls its movement.

When only one section waves, operator handling or panel drag is often the first place we look. When the complete edge waves consistently, the differential-feed and pressure settings are more likely to be wrong for that material.

Why Thread Tension Was Not the First Fix

Operators may adjust looper tension first because the defect appears beside the thread. This changes loop formation but cannot correct fabric stretched during feeding.

The stitch must be balanced, but material transport should be corrected before thread tension is used for fine adjustment.

Fabric response also varies by batch and construction. ASTM D2594 treats stretch and growth as separate properties of knitted fabrics, reflecting the fact that a material may extend and then recover by a different amount (ASTM D2594). A machine setting approved on a compact knit should not be copied blindly to a softer or more elastic bulk lot during OEM clothing manufacturing.

How We Corrected the Wavy Overlocked Hem

We used a short seam trial on the actual fabric before sewing another complete garment.

First, we adjusted the differential feed toward the machine's anti-stretch setting until the edge stopped lengthening. We made small changes because excessive compensation would gather the hem.

Second, we reduced foot pressure while maintaining positive feeding and stable stitches.

Third, we supported the full panel on the table and asked the operator to guide rather than pull the fabric. The corrected sample was checked immediately after sewing and again after the knit had relaxed.

The seam became flatter because the material entered and left the stitch area at a controlled length. Pressing was not used as the main correction.

The production lesson is worth remembering: a stretched edge is a feeding problem before it is a pressing problem.

What Buyers Should Check Before Bulk Production

During the apparel sampling process, inspect the overlocked edge before it is folded, coverstitched, or joined to another component. Later operations can hide the original cause and make diagnosis harder.

Before approving a PP sample for private label sweatshirt manufacturing, check:

• Was the seam trial made on the actual bulk fabric and color?
• Does the edge remain the same length before and after overlocking?
• Is the entire hem flat, including the start and end of the seam?
• Was differential feed adjusted for this fabric rather than copied from the previous style?
• Is the presser-foot pressure low enough to avoid drag but high enough for stable feeding?
• Is the operator guiding the panel without stretching it?
• Is the garment body supported instead of hanging from the machine table?
• Does the edge remain flat after relaxation and the required wash test?

Keep the approved seam trial at the line. Record settings by machine model because dial markings differ across equipment.

A wavy edge changes garment drape and is obvious in product photography. Repeated unpicking and retrimming can consume seam allowance or require replacement panels. Confirming settings during sweatshirt quality control is much less disruptive.

Final Takeaway

When you see a wavy hem after overlocking, first check whether the machine and operator stretched the fabric during feeding. Adjust the differential feed to compensate for the knit, reduce excessive presser-foot pressure without losing control, and support the panel so it is not pulled through the machine.

In this sample, the edge became flat after we corrected material transport rather than relying on pressing or thread tension. The best production decision is to approve an overlock seam trial on the actual bulk fabric before the full order reaches the sewing line.