LEC 5 - Questions

MEX:

Advantages:

• easy to use

• equipment usually not expensive

• variety of materials

Limitations:

• Need of support structures

• high surface roughness compared to other AM techniques

• Anisotropy of properties

BInder Jetting:

Advantages:

• no need of support structures

• huge parts are printable

• variety of materials

Limitations:

• post process needed

• mechanical properties depent on chosen binder

• powder can be harmful

MEX:

• AM Steps:

  • Feed stock material gets heated

  • Fusion/ Melting

  • Extrusion and cooling after deposition

  • Solidification

• Secondary Steps:

  • Green part / body

    • furnace sintering

  • Consolidation (Verdichtung)

  
  

1. Material preparation of the powder

2. Printing

   • Powder coating

   • Binder Printing

   • Heating

     -> Green part

3. Curing:

   • Drying and Curing

   • Strengthen the green part for better handling

     -> cured part

4. Debinding:

   • Single heat treatment

   • Debinding

     -> brown part

5. Sintering:

   • Densification of part

   • Shrinkage to final geometry

     -> Final Part

Steps till Curing are done repetively during build phase

Debinding and Sintering are steps which are only done once after print is finished

• Layer thickness:

  • minimum thickness of material layer

• Powder spread & print speed

• Saturation level:

  • percentage of volume occupied by binder in each layer

  • undersaturation:

    • powder can drop from green part

    • poor surface finish/ dimensional accuracy

  • oversaturation:

    • Migrate to outside the defined area (bleeding)

    • poor surface finish/ dimensional accuracy

• Part orientation

• Feed to powder ratio:

  • Ratio of fresh powder volume supplied in front of the roller to the layer thickness being created

    
    

• concept models:

  • .stl file

• Unprocessed Material:

  • mechanical properties

  • density

• System

  • Nozzle temperature

  • Nozzle diameter

  • envelope temperature

  • Wear resistance (Verschleißfestigkeit) of extruder

• enviromental factors:

  • Humidity

  • Temperature

• Build orientation:

  • x-/y-/ z- direction

• Working parameters:

  • Air gap

  • Raster width/ Raster angle

  • layer thickness

  • Part x-/y-/ shrinkage

• Filament strenght is important, to avoid the Buckling effect.

• Buckling effect (Knickverhalten) occurs when

  • Extrusion pressure > critical buckling stress

• processability of the filament also depends on

  • mechanical properties of filament

  • geometry of filament

  • flow properties of feedstock

  • processing conditions

  • design of printing head

• Both use a powder bed and recoater system which adds a new layer of powder, after finishing to build the previous layer

• Binder Jetting:

  • Powder are joined by Polymerbinder (Polymeradhesion)

  • No phase transformation/ melting occurs in printer

  • Densification & Fusion of powders in secondary Sintering step

  • Powder is 100% reusable, no support needed

• Powder bed fusion:

  • completly fuses and meltes the powder particles of an area

  • Laser as an heat source

  • multiple high heat & consolidation rates

  • heat conduction to avoid overheating, cracks, porosity, delamination

  • needs support

Powder bed - Binder interaction:

• Initial contact:

  • Binder droplets lands on powder bed

• Spreading:

  • Powder spreads over powder surface

  • better spreading = larger contact area = better particle bonding

  • Measures for better wetting beahviour:

    • increase surface tension of solid

    • decrease surface tension of liquid

• Imbition:

  • liquid binder pulled into empty pores between powder

• Drainage:

  • Binder flows from highly saturated regions to nearby dry regions

• Saturation:

  • prencentage of the pore volume filled with binder

  • undersaturation:

    • powder can drop from green body

    • poor surface finish/ dimensional accuracy

  • oversaturation:

    • Binder flows outside the intended printing area (bleeding).

    • poor surface finish/ dimensional accuracy

The powder-binder interaction is important because:

• controls binder distribution

• particle bonding

• good interaction improves green part strength, dimensional accuracy of the part, and surface finsih

• preventing defects like bleeding, weak regions