Removing build material

1. A system comprising:

a conduit having a first open end to remove build material from a container, when a gas flow is induced in the conduit;

a vacuum pump to create a vacuum in the conduit;

a valve in the conduit; and

a controller to control vacuum pump and valve,

wherein, at the beginning of a cycle, the valve is closed and the vacuum pump is operated for a first period of time to build up a differential pressure between the container and a portion of the conduit separated by the valve from the container;

wherein the valve is then opened to release the differential pressure and to induce a pulsed gas flow into the conduit to remove build material from the container.

2. The system of claim 1 ,

wherein, after a second period of time during which the valve is open, the controller is to start a new cycle by closing the valve to re-establish the differential pressure.

3. The system of claim 2 , wherein the first period of time is longer than the second period of time.

4. The system of claim 1 , wherein the controller is to operate the vacuum pump and the valve in a series of cycles, each cycle comprising

the first period of time wherein the differential pressure is generated in the conduit when the valve is controlled to be closed,

and wherein a pulse of gas flow is induced in the conduit when the valve is controlled to open.

5. The system of claim 1 , wherein the valve is positioned at the conduit closer to a first open end at the container than the vacuum pump.

6. The system of claim 1 ,

wherein the controller is to control the cycle and first period of time based on a parameter of the build material to be removed.

7. The system of claim 6 , wherein the parameter is one of build material type, a desired rate of build material removal, and a temperature of the build material.

8. The system of claim 1 , wherein the container is a replaceable build unit of a 3D printer structured to contain a cake formed by the 3D printer of solidified and un-solidified build material; and

wherein the first open end of the conduit comprises a connector to connect to the build unit to remove un-solidified build material from the build unit.

9. The system of claim 1 ,

wherein the container has a gas inlet so that gas can flow into the container and may cause build material to become airborne when a pulsed gas flow is induced.

10. The system of claim 1 , wherein the conduit has multiple branches and the system comprises multiple valves in the branches of the conduit to be operated by the controller.

11. The system of claim 1 , wherein a time to open the valve is less than 10% of a second period of time during which the valve is left open prior to closing the valve to begin a new cycle.

12. The system of claim 1 , wherein a time to open the valve is less than 10% of a duty cycle comprising the first period of time and a second period of time during which the valve is left open prior to closing the valve to begin a new cycle.

13. A method to remove build material from a container comprising:

connecting, to the container, a conduit having a first open end to remove build material from the container when a gas flow is induced in the conduit;

at the beginning of a cycle, with a valve in the conduit being closed, operating a vacuum pump for a first period of time to build up a differential pressure between the container and a portion of the conduit separated by the valve from the container; and

opening the valve abruptly to release the differential pressure and to induce a pulsed gas flow into the conduit to remove build material from the container thereby aspirating build material from the container.

14. The method of claim 13 , further comprising:

after a second period of time, closing the valve to begin a new cycle; and

with a number of repeated cycles, inducing pulses of gas flow from the container through the conduit connected to the container to aspirate, in pulses, the build material from the container.

15. The method of claim 14 , wherein the second period of time ends and closing the valve is performed when the gas flow from the container through the conduit reaches substantially a steady-state.

16. The method of claim 14 , further comprising:

closing the valve between the container and the conduit for the first period of time of at least one second so that turbulent airflow stops to allow build material to settle in the container; and

opening the valve between the container and the conduit for the second period of time of at least one second so that build material is aspirated from the container.

17. The method of claim 13 , wherein a time to open the valve is less than 10% of a second period of time during which the valve is left open prior to closing the valve to begin a new cycle.

18. The method of claim 13 , wherein a time to open the valve is less than 10% of a duty cycle comprising the first period of time and a second period of time during which the valve is left open prior to closing the valve to begin a new cycle.

19. A non-transitory computer readable storage medium comprising instructions that, when executed by a processor, cause the processor to:

at the beginning of a cycle, with a valve in a conduit that is connecting a container with a vacuum pump, close the valve and operate the vacuum pump for a first period of time to build up a differential pressure between the container and a portion of the conduit separated by the valve from the container; and

then, open the valve to release the differential pressure and induce a pulse of fluid flow into the conduit

so that build material is removed with the pulse of fluid flow flowing from the container through the conduit.

20. The non-transitory computer readable storage medium of claim 19 further comprising instructions that, when executed by a processor, cause the processor to:

control the valve to open and close in a cycle, so that a pulsed fluid flow is induced to remove build material from the container.