... ,with surface spray coating treatment,with delicate lock.2,Piston:The material is 45#stell,with surface carburization treatment ... shortening afer 300,000times squeezing.At door opening,the spring is compressed,at door closing,the spring is streches to ... door pushing force to the piston and to the spring,and the transform the spring force ino door closing force to realized door closing. Door closer ...