VL V - MEX and BJT

Pros MEX

• Easy to use

• Equipment usually not expensive

• Great variety of materials

• Small to large parts printable

• Multi material parts

Cons MEX with highly filled Polymers

• High surface roughness compared to other AM techniques

• Anisotropy of properties

• Support structure

• Accuracy and speed can be low

feeding MEX

• Plunger based

• Filament based

• Screw based

Parts of MEX

• Build Platform

• Built part

• Build material

• Extrusion head

• Build or support material

How can the Process for MEX be split

• Additive Manufacturing process

• secondary process

Whats part of the AM process MEX and happens inbetween

• Feedstock material

  • heating

• Fusion melting

  • Control

• Extrusion

  • cooling

• Solidification

Whats part of the secondary Process MEX + inbetween

• Green part/body

  • Furncace sintering

• Consolidation

Whats the composite material made out of

• Binder system

• Filler(s)

What does the polymeric component consist of

• Main binder component (50-90% V)

• Backbone (0-50% V)

• Additives (0-10%)

Different types of fillers MEX

• Ceramic fillers

• Metal fillers

Ceramic fillers

• SIlicon nitrate

• Fused silica

• Zirconia

• Strontium ferrite

• Tatanium dioxide

Metal fillers

• Stainless steel

• Titanium

• Tungsten carbide-cobalt

• Rare earth magnet

What do filler characteristics influence

• Mechanical and flow properties

Which characteristics Materials or Filaments are not printable (MEX Filler)

• Filaments with low mechanic properties

• Materials with high melt viscosity

What are influencing factors on Filler properties

• Different particle size

• Morphology

• Chemical composition

What do the processability of filaments depent on

• Mechanical properties of filament

• flow properties of feedstock

• processing conditions

• geometry of the filament

• design of printing head

When does Buckling occur

When extrusion pressure exceeds critical buckling stress

Processing parameters MEX

• Build orientation

• Bead width

• Layer/slice thickness

• Contouring

• Raster orientation

• Air gap

• Nozzle or liquefier temp

• Envelope temp

• Feed rate

• Extrusion rate

• Travel speed

Critical buckling stress formula

Pressure drop in the nozzle

Buckling when formula

Path generation MEX

• Destinction between contours and infill area

  • infill between contours

• Different path strategies for contour and infill

Whats debinding MEX

Describes removal of the polymeric binder system

Which critical defects can occur during debinding

• Bloating

• Blistering

• Surface cracking

• Emergence of large internal voids

Three main debinding techniques

• Thermal (most common)

• Solvent

• Catalytic

Why does the binder system has to be removed

Since carbon residues can influence sintering process negatively

Sintering properties MEX

• Thermal treatment that metallic powders into bulk material

• Sintering performed at below melting point (70-90% of melt temp)

What happens during sintering MEX

• Growth of sinter necks between particles

• reduction of porosity

• shrinkage of part and densification

Whats BJT

Liquid binder is deposited on powder materials to bond them together

Pros BJT

• High porductivity

• Wide range of materials availible

• Huge parts are printable

• Shaping occurs at room temp

• Various densities with controlled porosity

• High design freedom

• no support structures

Cons BJT

• Properties dependent on binder used

• Post processing is needed

• Powders can be harmful

• Slightly rough surface

Process steps BJT

• Material preparation

• Printing

• Curing

• Debinding

• Sintering

Feedstock material properties

• In principle, any powder can be processed

• Ratio of material to binder is different for each powder

• Ratio approx. 80:20 for plastic and approx 60:40 for sand

Whats the feedstock made of

• Powder and Binder

Material Powder BJT

• Sand

• Plastic

• Ceramic

• Metal

Material Binder BJT

• Water + glycerol

• Solvent

• Liquid

Requirements of the binder

• Rheology for inflitration and printing

• Stability to be deposited

• Wetting the powder and proper penetration

• Enough binding strength to provide structural integrity

• Thermal characteristics

What are the three types of binders availible BJT

• Acid based binder

• Metals salts binder

• Aqueous based binder

Rheology of Printing Diagramm

Possible Interaction between binder and powder bed

• Initial contact

• Spreading

• Drainage

• Saturation

How is the spreading parameter S calculated

What can be said over the size of S

• When S > 0 complete wetting

• When S < 0 partial wetting

Measures to enhance wetting behavior

• Increase of surface tension of the solid

• Decreasee of the surface tension of the liquid

How can the infiltration or drainige be calculated

Via the Washburn equation (assuming radial symmetry of the liquid and porosity, neglecting finite size pore)

What influences Infiltration BJT

• increases with particle size

• Nano particle decrease inflitration speed

  • Properties of sintered part improve

What process parameters influence printing in BJT

• Layer thickness

• Powder spread and print speed

• Saturation level

• Feed to powder ratio

• Part orientation

What is Binder Saturation

Volume fraction of binder in the voids

What does binder saturation influence

• Dimensional and geometric accuracy

• Surface quality

Whats Undersaturation (Binder BJT)

Inadequate binder saturation

• Powder can drop from green part

• Poor surface finish and dimensional accuracy

Whats oversaturation

Binder saturation too high

• Migrate to outside the defined area

• Poor surface finish and dimensional accuracy

Post processing steps

• Curing cycle

• Depowdering

• Burn out/Debinding

• Infiltration

• Sintering

What Phenomenon can be seen in this picture

Undersaturation

What Phenomenon can be seen in this picture

Oversaturation

Sag effect Sintering

Occurs because unsuppurted material gets close to melting point

Drag effect

Arises from friction between shrinking part and ceramic plate of furnace plate on which it rests during sintering