I have always crimped and pinched closed the wrappers of my dumplings by hand, but I was recently in my local Asian food market and saw these plastic clam-shell style dumpling crimpers and thought I would try them out. My take on them is that they produce a decent dumpling with some nice features but I will probably stick to hand crimping.
The circumference of even the larger of the two gadgets is smaller than the standard round dumpling wrapper available in Asian markets, so you can’t really pack a lot of filling into the dumpling. Until you get used to the smaller volume it is hard to keep the edges of the wrapper free of filling and get a clean closure. Also you get a fairly wide fringe of crimped dough around the edge. But if you pan fry the dumplings this fringe of dough fries up nicely and provides a crispy crunch feature to the dumpling. The crimping worked well and the dumplings stayed closed throughout the whole pan frying and steaming process.
The dumplings pictured above were stuffed with tofu, snow pea vine shoots and kim chi.