Overlock

Definition and Overview

Overlock is a method of sewing multiple threads crossing with a part called a button loop, stitching so that the fabric edge is wrapped. In many cases, it can cut away excess seam allowance at the same time as sewing, so it is used for edge finishing and for joining knit products.

“Overlock” refers not only to the type of stitch, but is also sometimes used to refer to the overlock machine itself that makes that stitch. When placing an order, it is important to clarify the sewing location and the required finish, rather than the machine name.

Process in Product Making

In general, after cutting, it is done as edge finishing of the seam allowance or as sewing together the cut pieces. In knit products such as T-shirts, cut-and-sewn items, and underwear, it is incorporated into the main assembly process as a stitch that adapts well to stretch.

For woven fabrics, it is common to first sew the side seams and inseams with a lockstitch, then apply overlock as fray prevention at the seam allowance edge. However, in lined products or in construction such as bagged seams where the seam allowance is hidden inside, it may not be necessary.

• Planning and pattern design: assume the textile thickness, stretchability, and seam allowance width.
• Cutting: check cutting accuracy and prevent insufficient or excessive seam allowance.
• Sewing: perform a test stitch and adjust thread tension, feed amount, and overcast width.
• Inspection: check for skipped stitching, thread breakage, excessive notching of the seam allowance, and waviness.

How to Proceed and Practical Points

In the specification sheet, list the areas where overlock is applied, whether it is for joining or edge finishing, how it should look on the front and back, and how the seam allowance is to be handled. If you simply instruct “lock finish,” there may be differences in interpretation of the result, so attach a sewing sample or cross-sectional drawing as needed.

Set the thread, needle, and differential feed to suit the textile. Differential feed is a function that changes the front and back feed amount, and it is adjusted to prevent waviness in stretchy knits and puckering in less stretchy textiles. Thin fabrics, stretch fabrics, and hairy fabrics require especially careful advance checking.

Because the process involves trimming off the seam allowance, restitching is limited. Confirm in advance the seam allowance width on the pattern, the need for pattern matching, and whether dimensional corrections will be needed in later processes, and before mass production evaluate the stitch and post-wash condition with samples.

Ordering and Quality Notes

When requesting a sewing factory, prepare the fabric swatch, sewing specification sheet, pattern, and samples if needed. Since material name alone makes it difficult to judge sewing suitability, also share the textile stretch, thickness, surface slipperiness, and tendency for the edge to fray.

For quality checks, confirm whether the stitch sufficiently wraps the fabric edge, whether there are any skipped stitches, and whether there is puckering or protruding loops caused by uneven thread tension. Also check whether the seam will split during wear or washing, and whether thread breakage occurs in stretch areas.

Differences from Similar Terms

Lockstitch is a representative sewing method that uses an upper thread and a lower thread, and is mainly used for straight seam joining. Overlock is characterized by a structure that wraps the fabric edge, and differs in that it can serve both as edge finishing and joining.

Chain stitch is a general term for stitches that use a button loop, and it is also used for sewing that takes advantage of stretchability. Overlock is a form of chain-stitch structure, with emphasis on overcasting fabric edges. Depending on the application, the two are combined.

Frequently Asked Questions

Do overlock and overlock machine mean the same thing?

In practice, they are often used almost synonymously, but overlock basically refers to the stitch or sewing method that overcasts the fabric edge, while the overlock machine refers to the machine that performs it. When requesting it, adding the target area and the purpose of the stitch reduces misunderstandings.

Is overlock necessary for woven fabrics too?

Woven fabrics often fray easily at cut edges, so overlock is often used as seam allowance edge finishing. However, in bagged seams, piping finishes, or specifications that cover the seam allowance with lining, another finishing method may be chosen.

What defects commonly occur with overlock?

Typical defects include puckering and floating loops caused by poor thread tension, skipped stitches, seam opening, over-trimming of the seam allowance, and waviness in stretchy fabrics. Before mass production, test stitch on the actual textile and check the seam after washing and under tension.