A chemical reaction between a first chemical reagent and a second chemical reagent occurs in a mixing chamber arrangement; a dispensing chamber; a dispensing piston configured to dispense reacting mixture from the dispensing chamber; and a control unit for controlling the movement of the dispensing piston and operation of the chemical mixing arrangement. The chemical mixing arrangement has a mixing chamber and injection apparatus configured to produce a jet of a first chemical reagent and a jet of a second chemical reagent such that the jets collide within the mixing chamber to produce reacting mixture. The dispensing chamber has an inlet for receiving the reacting mixture from the mixing chamber and an outlet for dispensing the reacting mixture. The control unit co-ordinates the chemical mixing arrangement to mix the chemical reagents while the dispensing piston is retracted at a controlled rate to enlarge the dispensing chamber.