A wavy zip after washing is a common mismatch between two materials. The zip tape and the knit body fabric respond differently to laundering. If the fleece shrinks in length and the tape does not, or the reverse, the fabric bunches along the tape and the placket ripples. Sewing tension and how the zip was set can add to it. A hot dryer can make shrinkage worse, but if the label allows tumble drying, the garment should tolerate it.

Measure the fabric's dimensional change

Wash and dry body fabric and garments under a named procedure such as ISO 6330, or measure garments with AATCC TM150, using the care conditions on your label. Record length shrinkage along the centre front specifically. If the fleece shrinks more in length than the tape, the placket will gather after washing.

Check the zip tape

Ask for the zip specification: tape material and whether it has been pre-shrunk or heat-set by the zip supplier. Wash a length of tape with the fabric specimens and measure it. A tape that changes very little will need a body fabric that is similarly stable, or a pattern allowance that accounts for the difference.

Check the sewing

If the operator stretches the fabric or the tape while setting the zip, the placket can look wavy even before washing, and washing makes it obvious. Ask the factory how the zip is set: stitch type, whether a guide or folder is used, and whether the fabric is stabilised along the placket. Compare new and washed garments from different sewing lines to see whether the problem follows a line.

Specify the fix

Depending on what you find, the fix may be a more stable or pre-shrunk fleece, a placket stabilising tape, a change in how the zip is set, or a combination. Approve a washed sample, not only a new one, and set a maximum length shrinkage at the centre front, tied to the named method. Make sure the care label, including ISO 3758 symbols if used, reflects drying conditions the garment passes.

Then the next order can be checked for the specific dimension that caused the waving, instead of relying on how the hoodie looks on the hanger.