My best tip for maintaining pleats- fold clothing article lengthwise- pull into a stocking. Knot top & bottom put in machine on gentle wash. Takeout of stocking- shake – hang to dry- place back in stocking to store.

Take a swatch of your silk and pleat with lightly spraying a mix of white vinegar and water then pressing. (Use more vinegar than water as water can leave spots on silk; you might try all vinegar at first.)

i know your post is 3 years old but i will say if you can get a poly-wool blend fabric, it makes great pleats and will last. wool by itself does have soft pleats unless you get some real professional heat setting treatment (like SiroSet which is specifically used for wool).

On synthetics, no. Washing will remove the pleats. That said, pleating polyester is really hard to do for nice, crisp pleats. There’s nearly always some softness that remains in the pleat folds. And my focus on using vinegar for pleats is for use on historical projects, which are rarely washed but only spot cleaned and aired out. (Vodka spray is recommended for airing out.) The vinegar works best on natural fibers.

If it’s silk taffeta you might not even need the vinegar as taffeta is so tightly woven it can stand up on its own. If you still want to use the vinegar, spray onto a press cloth, lay the cloth on the taffeta pleats then press. Don’t spray the vinegar directly onto the silk as it may spot. Test first, of course. I’d also be hesitant to starch taffeta… again, based simply on the natural stiffness of it and that it is silk. Good luck with your pleats!