Stitches per Inch
In one of my most recent mentorship calls, someone asked me how to fix an issue with a garment they were reviewing that had puckering and gathering at the seams. Unfortunately, the workmanship from the factory was not of the highest quality, and left room for improvement. As someone new to the industry, they were not quite sure how to advise the factory to fix the issue.
First of all, I want to commend this person for wanting to offer a solution to the problem. I will say from experience that most factories will be able to review a photo that you send them and make their own suggestions. This is definitely a solution you can use if you're not sure how an issue could be fixed. But in this instance, she was absolutely correct that offering a possible solution could get her a much faster result on her next prototype.
Here is what I told her: nine times out of ten, poorly sewn seams that cause puckering and gathering on your garment are caused by incorrect thread tension. Sewing machines are set to specific settings for each fabric and for each garment that is made. If a factory is working quickly through a production run, they might have missed a setting on a machine, and it could be causing the thread to pull at the fabric. Sometimes, it can be good enough just to ask your factory to improve thread tension while sending a photo of the issue.
To go beyond this simpler solution, another feature that you can look at is the stitches per inch throughout your garment. Stitches per inch (or SPI) are most often indicated on tech packs that request decorative stitching in one way or another. For example, when I worked on rodeo apparel, it was very trendy for the denim back pockets to have intricate stitching details to add a unique flair to each piece. We would use extra thick threads with very small SPIs so the threads looked like embroidered yarns.
Many years ago, when I was an intern in the apparel industry, part of my job was counting stitches per inch on all of the garments I was responsible for. I would get out my ruler or measuring tape, line it up against every single row of stitching on my garment, and then manually count out how many stitches per inch were on all of my seams. If you're experiencing puckering or gathering on your seams, you can turn to this technique to advise your factory even further on improvements. If you are seeing puckering on your garment and you count ten stitches per inch on the seams, you can ask your factory to reduce your SPI to seven or eight and you should see an improvement.
The trickiest part of stitches per inch to understand is that the bigger the number is, the smaller the stitches are. If you need to fit twelve stitches into an inch those stitches are going to be much tinier than if you're trying to fit six stitches into that same inch. So reducing your SPI is actually making your stitches bigger, which makes the needle penetrate the fabric less times, resulting in improved seam appearance.