Fusing three-dimensional (3D) object layers

1. A method of fusing a 3D object layer comprising:

with a non-scanning, stationary energy system positioned over a work area, simultaneously irradiating the entirety of the work area uniformly with fusing light, wherein a carriage casts a shadow onto a portion of the work area and prevents fusing light from irradiating the portion of the work area when positioned between the fusing light and the work area;

while moving the carriage at a constant speed from a first side to a second side of the work area in one pass, dispensing a liquid fusing agent onto a first layer of a build material in a pattern corresponding to an object slice, such that an exposure time of the patterned build material in the first layer to the fusing light is unequal across the first layer; and

while moving the carriage from the first side to the second side of the work area in another pass at the constant speed, spreading a next layer of the build material over the first layer, such that the exposure time of the patterned build material in the first layer to the fusing light is made equal across the first layer.

2. A method as in claim 1 , wherein the carriage comprises:

a first carriage casting a first shadow and moving at the constant speed for the one pass; and

a second carriage casting a second shadow the same as the first shadow and moving at the constant speed for the other pass.

3. A method as in claim 1 , wherein:

the carriage is a single carriage;

the one pass is a first pass;

the other pass is a third pass; and

the method further comprises returning the carriage to the first side of the work area in a second pass between the first pass and the third pass.

4. A method as in claim 1 , further comprising spreading the first layer over the work area while moving the carriage from the first side to the second side of the work area in the one pass.

5. A method as in claim 4 , further comprising dispensing the liquid fusing agent onto the next layer while moving the carriage from the first side to the second side of the work area in the other pass.

6. A method as in claim 1 , wherein:

the carriage comprises a first carriage for dispensing the liquid fusing agent and a second carriage for spreading the next layer of the build material; and

the first and second carriages move together in the one pass and in the other pass.

7. A fusing system for an additive manufacturing machine, comprising:

a movable platform to support a work area, the platform movable incrementally lower to accommodate a succession of layers of a build material;

a spreader to spread the build material in layers over the work area;

a dispenser to selectively dispense a liquid fusing agent on to the layers of the build material;

a carriage carrying the spreader and the dispenser, the carriage movable back and forth over the work area;

a non-scanning, stationary energy system positioned over the work area to simultaneously irradiating the entirety of the work area uniformly with fusing light, wherein the carriage casts a shadow onto a portion of the work area and prevents fusing light from irradiating the portion of the work area when positioned between the fusing light and the work area; and

a controller programmed to:

while causing the carriage to move at a constant speed from a first side to a second side of the work area in one pass, cause the dispenser to dispense the liquid fusing agent onto a first layer of the build material in a pattern corresponding to an object slice, such that an exposure time of the patterned build material in the first layer to the fusing light is unequal across the first layer; and

while causing the carriage to move at the constant speed from the first side to the second side of the work area in another pass, cause the spreader to spread a next layer of the build material over the first layer, such that the exposure time of the patterned build material in the first layer to the fusing light is made equal across the first layer.

8. A fusing system as in claim 7 , wherein:

the carriage comprises:

a first carriage carrying the dispenser and casting a first shadow; and

a second carriage carrying the spreader and casting a second shadow the same as the first shadow; and

the controller is programmed to:

while causing the first carriage to move at the constant speed from the first side to the second side of the work area in one pass, cause the dispenser to dispense the liquid fusing agent onto the first layer of the build material in the pattern corresponding to an object slice, such that the exposure time of the patterned build material in the first layer to the fusing light is unequal across the first layer; and

while causing the second carriage to move at the constant speed from the first side to the second side of the work area in the other pass, cause the spreader to spread the next layer of the build material over the first layer, such that the exposure time of the patterned build material in the first layer to the fusing light is made equal across the first layer.

9. A system as in claim 7 , wherein the carriage is a single carriage carrying the spreader and the dispenser.

10. A system as in claim 7 , wherein the controller is programmed to cause the spreader to spread the first layer over the work area while the carriage is moving the carriage from the first side to the second side of the work area in the one pass.

11. A system as in claim 10 , wherein the controller is programmed to cause the dispenser to dispense the liquid fusing agent onto the next layer while the carriage is moving from the first side to the second side of the work area in the other pass.

12. A system as in claim 7 , wherein:

the carriage comprises a first carriage carrying the dispenser and a second carriage carrying the spreader; and

the first and second carriages are movable together in the one pass and in the other pass.