Lesson 12 MJ and BJ

Material Jetting

Binder Jetting

Process

  1. Powder material is spread over the build platform using a roller

Process

  1. Print head position above build platform
  1. Droplets of material (support and build material) are deposited onto surface from the print head according to design of part, using either thermal or piezoelectric method
  1. Print head deposits the binder adhesive on top of the powder where required.
  1. the build platform is lowered by the model's layer thickness
  1. Deposited material solidify to make the first layer
  1. A levelling blade is then used to smooth the surface by moving across the surface, before the next layer of material is deposited
  1. Another layer of powder is spread over the previous layer. the object is formed where the powder is bound to the liquid
  1. Layers are built on top of the previous layers and the process repeats
  1. Unbound powder remains in position surrounding the object.
  1. Repeat process until entire object has been made. In order for the binder to fully set and for the green part to gain strength, the printed part is left in the powder bed after its completion.
  1. Layers can be harden and cured by UV light

Material Jetting vs Binder Jetting

  1. Post processing is then done; including removal of support by hand or water jetting

Post processing:

  • Removing part from powder bed
  • Removing unbound powder via pressurized air
  • Infiltrating the part with an infiltrant to make it stronger and possibly to impart other mechanical properties

Materials

Challenges

Metals : Stainless steel, aluminum, silver

Polymers : ABS, PA, PC, PMMA

Ceramics : Glass, Sand

Formulation of liquid material

Flow

  1. Suspending particles in carrier liquid
  2. Dissolving materials in solvent
  3. Melting of solid polymer
    4.Mixing formulation of monomer or prepolymer with polymerization initiator

Droplet Formation

Materials must be converted into small discrete droplets from a continuous volume of liquid

depends on material, hardware, and process parameters

Any addition or changes to the material may drastically change the droplet formation behavior

Deposition of droplets

How the droplet drop, impact and substrate wetting or interaction needs to be account for in the movement of print head or substrate

If smaller droplets break off from main droplet during flight, material will be spread over a larger area than intended --> not well defined boundaries

If droplet splashes --> formation of crown

Fabrication of ceramic parts

Material phase changes
1. Solidification
2. Evaporation
3. Curing

Green parts subjected to thermal decomposition to remove polymer binder

Furnace temperature increased until ceramic's sintering temperature

Sometimes an infiltrant is used that reacts to form a ceramic binder

Another possibility is to infiltrate a metal to form a ceramic-metal composite

Set backs of using dry powders

When the phase changes occurs is important as it affects droplet's interaction with substrate and final deposition created

Because non uniform solidification --> warpage and undesirable results

Fine powders does not flow well enough to spread into defect-free layers

Control of depositing material on previous layers

droplets interact differently with own material as compared to metal plate substrate

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BINDER JETTING

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Faster than MJ

Enables material composition that MJ can't achieve easily

Better quality ceramic and metal parts produced

Able to print in colours

No support structures required as parts are self-supporting in powder bed

Increased multiple parts to be produced at one time

Material Jetting

Allows for multiple material parts and colour

capable of using multiple print head to increase printing speed

Lower cost as machine can be assembled from standard parts

Better accuracy and surface finishing than binder jetting

The need for isotactic pressing due to inadequate green part density severely limits the types of part shapes capable of being processed.

Slurry-based working material which is deposited over the build area using ink-jet printing can be used to counteract this.

After the slurry dries, binder is selectively printed to define the part shape.

Repeated step for each layer, --> increase build time.

  1. Each layer must be fully bonded to previous layer
  2. Addition of new layers does not damage previous layer

Can be resolved by smoothen the surface periodically

Maintenance

Another variation for fabricating metal parts

Start with metal oxide powders, following the binder jetting process for ceramics up to the sintering step, a hydrogen atmosphereis introduced while in the furnace

clogging of nozzles can occur

Needs to be monitored and maintained

Purging and Cleaning Cycles

Complex sensing systems to identify and compensate malfunctioning or inconsistent nozzles

Replaceable nozzles

Cause a reaction to occur between hydrogen and oxygen atoms in metal-oxide, which converts it into a metal

Print resolution

After reduction, metal particles are sintered to form a metal part.

Can be improved by producing many small droplets very close together

but

Difficult due to limitations in manufacturing processes and overlapping of thermal and pressure differentials to drive adjacent nozzles

multiple passes over the same area can be done

Weaknesses

Droplet Formation Mode

Poorer accuracies and surface finishes than MJ

Continouos Mode

constant pressure
stream break into droplet
droplets of uniform mass achieved by pertubing jet at fixed frequency close to spontaneous droplet formation rate
charging droplet to deflect and position the droplets by electric field
remaining droplets are collected in gutter and is recycled

infiltration steps needed to fabricate dense parts to ensure good mechanical properties

Not always suitable for structural parts, due to use of binder material

Drop-on-Demand Mode

droplet produced directly from nozzle individually

formed by individual pressure pulses which was created at specific times by thermal, electrostatic, piezoelectric, acoustic or other actuators