A method of changing material in a plastic foam molding machine comprising a storage bin; a mold; a feed circuit interposed between the storage bin and the mold, and comprising: - a reservoir having a longitudinal axis and comprising a header located at the bottom of the reservoir, having an axis perpendicular to the longitudinal axis of the reservoir and at least one transverse ejector associated with a respective loader to feed material from the reservoir directly into the mold; valve means being provided to control communication between the header and the ejector; the reservoir further having a material inlet located at the top of the reservoir and connected to the storage bin via a material feed pipe; - a two-inlet cyclone separator having a first inlet connected via a pipe to a first two-way valve fitted to the material feed pipe to selectively connect the reservoir to the storage bin and to the cyclone separator; and a second inlet connected via a pipe to a second two-way valve fitted to a pipe to selectively connect the cyclone separator to an axial outlet of the header; - an aspirator connected via a pipe to said second two-way valve to communicate, via a pipe and a valve, with the axial end of the header; the method comprising a cleaning cycle to remove a first batch of a first material from the feed circuit before replacing the first batch with a second batch of a second material different from the first; the cleaning cycle comprising: - a first step of emptying the reservoir by suction to remove any material of the first batch still inside the reservoir; - a second step of pushing any material of the first batch remaining along the feed circuit at least in part outside the feed circuit and at least in part into the header by injecting compressed air into the feed circuit; and - a third step of emptying the header by suction; wherein the second step comprises a first substep of injecting compressed air into the reservoir, expelling the compressed air from the reservoir through the axial outlet of the header by closing the valve means and setting the first and second two-way valves (52, 55) to cause air to flow from the header to the cyclone separator; and a second substep of injecting compressed air into the reservoir, expelling the compressed air from the reservoir along the material feed pipe by maintaining the valve means in a closed position, and setting the first and second two-way valves (52, 55) to cause air to flow from the material feed pipe into the cyclone separator. - a reservoir having a longitudinal axis and comprising a header located at the bottom of the reservoir, having an axis perpendicular to the longitudinal axis of the reservoir and at least one transverse ejector associated with a respective loader to feed material from the reservoir directly into the mold; valve means being provided to control communication between the header and the ejector; the reservoir further having a material inlet located at the top of the reservoir and connected to the storage bin via a material feed pipe; - a two-inlet cyclone separator having a first inlet connected via a pipe to a first two-way valve fitted to the material feed pipe to selectively connect the reservoir to the storage bin and to the cyclone separator; and a second inlet connected via a pipe to a second two-way valve fitted to a pipe to selectively connect the cyclone separator to an axial outlet of the header; - an aspirator connected via a pipe to said second two-way valve to communicate, via a pipe and a valve, with the axial end of the header; the method comprising a cleaning cycle to remove a first batch of a first material from the feed circuit before replacing the first batch with a second batch of a second material different from the first; the cleaning cycle comprising: - a first step of emptying the reservoir by suction to remove any material of the first batch still inside the reservoir; - a second step of pushing any material of the first batch remaining along the feed circuit at least in part outside the feed circuit and at least in part into the header by injecting compressed air into the feed circuit; and - a third step of emptying the header by suction; wherein the second step comprises a first substep of injecting compressed air into the reservoir, expelling the compressed air from the reservoir through the axial outlet of the header by closing the valve means and setting the first and second two-way valves (52, 55) to cause air to flow from the header to the cyclone separator; and a second substep of injecting compressed air into the reservoir, expelling the compressed air from the reservoir along the material feed pipe by maintaining the valve means in a closed position, and setting the first and second two-way valves (52, 55) to cause air to flow from the material feed pipe into the cyclone separator.