POLYMER: Factors affecting Part Properties

POLYMER: Factors affecting Part Properties

1. Pre-Process

   • Material Choice
     • Particle Size
       • Particle size and distribution has major effect on powder packing and porosity of end parts.
       • )Average particle size for Nylon-12 50 micron (range 10-90micron)
       • Larger particle= Poor surface finish lower density, Smaller particle= suffer more from static = poor deposition Smaller particle= susceptible to nitrogen flow in chamber, affecting laser window.
     • Particle Shape
       • 1)Depends on material and production method
       • 2)Different powder-production methods = different shapes. Eg. Grinding gives cornflakes shapes
       • 3)Smooth, spherical powder preferred.
     • Particle Agglomeration
       • 1)Powder may clump together before and during process due to static build-up and moisture absorption.
       • 2)Static build-up can be reduced using additives. Additives help flow but affect properties.
       • 3)Moisture absorption can be alleviated through drying. Moisture absorption differs with material. Some more issues than others.
       • 4)Powder agglomeration can lead to problems with deposition on the build area, van become worse as powder temperature increases(except for when the agglomeration is caused by moisture in which agglomeration maybe reduced with increasing temperature)
       • 5) In gravity-fed systems, powder agglomeration leads to ‘bridging’
         • • Powder will not flow from chute
         • • Forced vibration of delivery chute may help
   • Fillers (Issues)
     • Compromise on properties – obtain favourable property but trade off another
     • Filler ratios – excess fillers may cause deteriorate properties
     • Homogeneity of mixing
       • • Mechanical mixing is simple, but difficult to maintain good distribution of filler both during mixing and storage
       • • More complex methods, e.g. co-extrusion followed bygrinding can offer some improvements (but can affect particle shape…)
   • Thermal History
     • 1) LS is a thermal process. Any unsintered material has been sat in a heated bed temperature for a substantial amount of time. This has effects on its molecular weight.
     • 2)Molecular weight:
       • • Decrease in Melt Flow Rate over successive builds
       • • Increase in molecular weight
     • 3)Common in LS to us nix of virgin and recycled material to reduce costs but maintain good properties
     • 4) Virgin gives great repeatability but recycled giver better properties (elongation at break) 5)Thermal conditioning can improve Elongtaion at break (but need to consider cost and time)

2. In- Process

   • Build Orientation
     • • Anisotropy for parts produced in different orientations
     • • Some processes less affected like FDM
     • • Different degrees for different mechanical properties
   • Part Bed Temperature
     • Preheat build area before sintering.
     • -LS has uneven distribution of temperature in build area (and between systems). Higher at centre of build areas, cooler at the edges (extremeties).
   • Processing Parameters
     • Effects of processing parameters on mechanical properties Energy Density Energy Density = Laser Power/Scan Spacing × Laser Scan speed (doesn’t take into consideration part bed temp. solely focusing on energy input.) Higher energy density = higher ductility

3. Post-Process

   • Infiltration
     • • Improve mechanical properties
     • • Specific to base material
   • End of build actions
     • • E.g. for Duraform EX we must leave heaters and nitrogen on after build is complete - Part warpage -Discoloration
     • • Part removal- parts must be allowed to cool before removing
   • Iso-static pressing
     • • Originally for metals and ceramics
     • • Parts subjected to isostatic pressure by inert gas or liquid solution
     • • Can be performed at high temperature – Hot Isostatic Pressing (HIP)
     • • In casting, simultaneous Hot + high pressure pressing eliminate voids & micro-porosity
     • • E.g. SLS Bronze-nickle : density SLS metals can be low which means low part strength. HIP increases density
     • • E.g. SLS nylon parts: Iso-static at room temperature - Samples coated in liquid PVC @ 160°C to prevent liquid ingress - Parts subjected to iso-static pressure of 2.5 MPa for one minute - Results showed an increase in Impact Strength (~3.44 kJ/m2 to 4.33 kJ/m2) - Negligible effect on tensile properties