So how do we sew balloon fabric together? According to my balloon books quite a few different ways have been used in balloon construction. One of the more popular methods is called a 'flat-fell' seam. This is the type of seam that you see in blue jeans.
Below is a drawing of a flat-fell seam. This type of seam is used to sew together all the panels in my balloon. This picture shows how the seam is sewn with a load tape. Only the vertical seams have a load tape, the horizontal seams do not.
There are other ways of doing a seam. I chose the flat-fell method because it was strong, proven, and hides the cut edges of the fabric. If the cut edge of the fabric were exposed, there would constantly be nylon threads unraveling off the end of it. Balloon Works uses a hot cutting knife to prevent this from happening. This is too much work for me, and the flat-fell seam is just as good.
Below is a diagram showing how the thread comes together in the fabric. The hatched area is the thickness of the fabric. The thread from the lower bobbin and upper needle cross to form a knot. Ideally, this knot will be within the thickness of the fabric. The lower two items (b and c) show what happens when the knot appears above or below the fabric. This weakens the stitch and prevents the stitch from 'locking', so it unravels easier.
If you are wondering how important it is to have the stitch in the proper location, try this experiment. Adjust the tension on your machine so the knot is completely above or below the fabric. Note that one of the threads then goes along the fabric and is never pushed through to make the knot. You can take one end of this thread and pull out up to 6 inches of seam! A lock-stitch will not lock unless sewn properly!
In order to keep the knot in the middle of the fabric, it is necessary to adjust the tension of the thread from time to time. Below is a drawing of a sewing machine much like what I am using. The tension disks are the round things on the head of the machine over the needle. They are disks on a threaded post that you can screw in and out to adjust the tension of the top threads. To adjust the tension of the lower thread, there are small screws on the lower bobbins. Sometimes I also change how the thread is run to adjust the tension as well.