This is a plunger intended for solder paste tubes like the one shown below that are normally sold without it. 

The plunger is  printed in one part and you break the support structure of the moving part once it is printed using a small knife.

I have used PLA. The gap (shaft to ring) of the moving part is 0.25 mm. It can be printed with 0.2 or 0.3 layer height and you need to enable supports. I have used the following settings, note that you have to uncheck “support on print plate only”.

The support is the orange part

Update: I have added a second version with a tube shaft (1.2mm wall), it's a bit faster and saves some plastic.

Update2: I have increased the support height to make it easier to detach and also added a 2mm solid part to the base of the shaft tube to improve the connection point with the disk. In addition I have decreased a bit the external diameter of the ring that fits on the solder paste because it was a bit tight.
Also added a 0.35mm gap model in case someone has a problem with the 0.25mm version.