> How do I sew the corners of silnylon so they are a clean and neat 90 degrees when finished?
This is a well-known problem. You think you are having problems with silnylon? Think what it is like when you are handling 1000 denier Cordura. Or worse still, 12 oz canvas.
I have included a picture with many diagrams here and we will work through it. It may help if you have paper and scissors with you for this, and actually do the steps.
I am assuming you are doing a full hem as shown in A. That creates 3 layers at the side of the hem, but at the corner there are 3 x 3 = 9 layers. The folding gets to be a bit difficult and the fabric tries to escape.
Method 1 is to iron the fabric down and use pins, as suggested. It can work just fine with silnylon, but gets more difficult when the fabric gets heavier.
Method 2 is shown in B. You fold up one hem and sew it down - the sewing is shown by the blue lines. Then you fold the other hem down, with the fold lines shown by the pale dashed lines, and sew it down. This may not look ideal, but it is quite effective, and very strong.
Method 3 involves taking a rectangular notch out at the corner to reduce the overall thickness. You start by changing the border from the thick red line to the thinner orange line shown in C: that means the concealed bit of the hem is just not going to be as wide as the revealed bit. This is pretty normal anyhow. Then you cut out the notch as shown, with the edges in between the two hem lines, as shown in D. First fold is shown in E, second fold is shown in F. Hold this hem down with stitching, as shown in blue. Then fold the other hem down and sew.
Note that you can notch at the fold lines instead. This makes for easier sewing and a much flatter corner, but that does leave a tiy bit of exposed edge at the corner. This may not worry you, especially with silnylon which does not fray very easily. It does work well. You can always put a tiny wipe of silicone sealent across the cut edge too.
Method 4 is often used as it is simpler - although it has variations which need to be considered. The green lines in H show where the inner edges of the hem will be. You take a slice off the corner, as shown by the diagonal dashed green line in I. Then you fold the hem down, using either pins or intermediate sewing (as above) to keep the corner under control. The diagonal corner will end up either concealed (if the snip is small) or visible. The smaller the snip you take off, the thicker the finished hem will be of course.
The important thing with this method is to make sure that the diagonal cut as shown in I does not reach the corner of the inner hem line (2 green lines in H). If you make the cut hit the corner of the inner fold exactly as shown, you will end up with a revealed edge (actually, two edges butting together) at the dashed green line in J. This produces a very flat hem, but you may want to oversew the revealed edge. Please note that the corner as shown in J is definitely NOT as strong as a corner with more overlap.
Experiment with the scissors and paper to see what suits you.