This is a bad ball.
They look translucent - seam is clearly visible. Also the two "poles" of each hemisphere are clearly visible. The surface has a fine texture like the 3d printed objects, but not that coarse, is more like the ball would have been brushed. The texture lines are in a plane parallel to the "equator" seam. This texture makes the ball interact differently with the rubber depending on the angle. If we strike the ball in the same direction as the seam almost no spin is created.
When they fall on the table they make a sharp sound, very different from the nittaku 3* plastic or from the old celluloid balls.
The third aspect is that these balls are inconsistent as size. I use them with a robot and it took me a few days to eliminate the bad balls of the bunch ( about 2-3 in the whole box).
The conclusion is that this ball is quite useless as a training ball. A celluloid ball is closer to the premium plastic than these balls.
Reviews for Butterfly Easy 40+ Poly Balls - Pack of 120 (3)
I do not have a lot of experience with ping pong balls, and these are my first poly balls. They seem to be really good for what they are, but my opinion is not very informed. They have a fairly strong plastic smell. I need more balls for my Butterfly Amicus robot, and I am going to buy more Joola Magic instead. I hope my beginner's opinion helps someone make their decision.
It's a bad choice to buy this product even for practice go for 2 star balls will last longer and play better