Designing the artwork to allow for a gap at the seam of the seat of the pants helps greatly. This is easy to do if you are printing lettering only. Use a smaller squeegee and stroke down the left side and right side of the sweatpant separately. Trying to print directly over the seam may cause some off contact problems.
If your artwork does not lend itself to being separated to both sides of the pant seam and you have to print over the seam, use a lower mesh count screen. If you have off contact problems, try using some shims under the left and right sides of the pant or making several passes with your squeegee. Hot split or cold peel transfers also work well over the seat because of the pressure applied from the heat press.
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