Material jetting

Question 1

What is jetting?

Answer 1

There is no direct contact between the nozzle and the substrate (whatever you’re printing on) whereas in extrusion the nozzle is very close to the substrate and material is touching both the nozzle and substrate

Question 2

Where does material jetting come from?

Answer 2

Comes from ink jet printing

Question 3

What are the 5 main mechanisms of jetting droplets?

Answer 3

Piezoelectric 
Thermoelectric (bubble jet)
Electrostatic
Acoustic 
Pneumatic (aerosol)

Question 4

Which jetting mechanisms are droplet on demand (DOD)? And what does this mean?

Answer 4

Piezoelectric and thermoelectric

A single droplet is jetted on demand rather than spraying many

Question 5

Which jetting mechanisms are spray on demand (SoD)? And what does this mean?

Answer 5

Electrostatic
Acoustic
Pneumatic

Continuous flow of a stream of droplets

Question 6

What is the most common material jetting mechanism and why?

Answer 6

Piezoelectric

• Small size of actuators allows for compact manufacturing with 1000s of nozzles
• Feasible economics due to well studied and inexpensive materials
• Voltages required to drive piezoelectric are low when compared to other jetting mechanisms such as electrostatic
• it is a DoD mechanism which enables well defined droplet formation

Question 7

One advantage and one disadvantage of thermoelectric jetting

Answer 7

Common because of cheap micro heating elements

but inks often require more volatile solvents to initiate jetting which makes the inks less stable than those used in piezoelectric

Question 8

One advantage of electrostatic jetting

Answer 8

It is a very fast process which has a potential for large scale coating and the generation of fibrous sheets (e.g. blue face masks)

Question 9

Advantage of Pneumatic jetting

Answer 9

It is very suitable for low and high viscosity inks and it can produce narrow and wide spread of spray, making it suitable for various applications

Question 10

What is the rheology of inks?

Answer 10

Describes the jetability properties and behaviour of a liquid such as viscosity and surface tension

Question 11

Why is it bad to use inks with high viscosity and high surface tension?

Answer 11

Inks with high viscosity tend to tail during jetting and inks with high surface tension tend to be more difficult to jet because of the high cohesion of the surface of the liquid and once jetted it tends to splash and cause satellite droplets

Question 12

What materials can be used for jetting?

Answer 12

Inks with specific rheology
polymers once melted or dissolved in a solvent or dispersion in a liquid
Metal and ceramic particles in a carrier liquid

When printing ink dispersions, carrier liquids and solvents can be evaporated after printing by direct heating, laser, plasma etc…

Question 13

What is droplet on demand jetting and what are the 5 main stages?

Answer 13

Individual droplets are produced directly from the nozzle, an actuator produces the pressure pulses and these cause the fluid to be expelled

1. Droplet ejection
2. Droplet flight
3. Drop impact
4. Drop spreading
5. Drop solidification

Question 14

What are the stages of a continuous flow system?

Answer 14

1. electrochemical device causes pressure oscillations to propagate through the liquid which ejects out of a nozzle, breaks into droplets very rapidly
2. Electrostatic field charges the droplets
3. Charged droplets are directed at their desired location, if nothing is being printed then they are directed to a catcher for recycling back through the system
4. Layers are allowed to harden or are cured by UV light
5. post processing removal of supports

Question 15

What are some comparisons between continuous and DoD?

Answer 15

Continuous mode is mainly used for high speed graphical applications e.g. expiry date printing on production lines. To date no commercial AM process uses this method

DoD is the method of choice for material jetting
High placement accuracy, smaller droplet size, low waste, wider range of materials

Question 16

What is the main application of DoD material jetting?

Answer 16

Creating wax like parts for manufacturing investment casting patterns

Question 17

What is the material jetting architecture for 3D printing?

Answer 17

Print heads jet micro droplets of low viscosity inks
A post processing unit follows the print heads to solidify the printed inks
Layer by layer printing and real time solidification forms a 3D structure

Question 18

What can the jettability of a material be quickly assessed by? And what does it mean?

Answer 18

Using the Reynolds, Webber and dimensionless Ohnesorge numbers

Reynolds = ratio of inertial force to viscous forces
Webber = ration of inertial force to the surface tension forces
Ohnesorge = ratio of viscous forces to the surface tension and inertial forces
Z= jettability 

1 < Z > 10 then a material is considered jettable

Water = 0.4 and if therefore not a jettable ink

Question 19

What is objet printing? and its advantages/disadvantages?

Answer 19

Using jetting to dispense photosensitive material, each layer is cured by UV light

Adv - dissolvable supports, multi-materials, smooth surfaces
Dis - high material cost, wastage of material through roller levelling process

Question 20

What are some general advantages of jetting material?

Answer 20

Good part edge quality
High resolution
Multi-materials are possible with multiple print heads
and therefore dissolvable support structures

Question 21

What are some general disadvantages of jetting material?

Answer 21

limited by rheology (properties of materials used)
high cost process
Materials are photosensitive so they will degrade over time