AS ISO ASTM 52911.2:2021 – Additive manufacturing – Design Part 2: Laser-based powder bed fusion of polymers
AS ISO ASTM 52911.2:2021 – Additive manufacturing – Design Part 2: Laser-based powder bed fusion of polymers.
— Degrees of design freedom for parts are typically high. Limitations of conventional manufacturing
processes do not usually exist. e.g. for:
— tool accessibility, and
— A wide range of complex geometries can be produced, such as:
— free4orm geometries, e.g. organic structuresl’71,
— topologically optimized structures,
— mull structures, eg honeycomb, sandwich and mesh structures.
— The degree of part complexity is largely unrelated to production costs.
— Assembly and joining processes can be reduced through single-body construction.
— Overall part characteristics can be selectively configured by adjusting process parameters locally.
— Reduction in lead times until part production.
5.4 Limitations to be considered in regard to the PBF process
Certain disadvantages typically associated with AM processes shall be taken into consideration during
— Shrinkage, residual stress and deformation can occur due to local temperature differences.
— The surface quality of AM parts is typically influenced by the layer-wise build-up technique (stairstep effect). Post.processing can be required, depending on the application.
— Consideration shall be given to deviations from form, dimensional and positional tolerances of parts. A machining allowance shall therefore be provided for post-production finishing. Specified geometric tolerances can be achieved by precision post-processing.
— Anisotropic characteristics typically arise due to the layer-wise build-up and shall be taken into account during process planning.
— Not all materials available for conventional processes are currently suitable for PBF processes.
— Material properties can differ from expected values known from other technologies like injection moulding and casting. Material properties can be influenced significantly by process settings and control.
5.5 EconomIc and time efficiency
Provided that the geometry permits a part to be placed in the build space in such a way that It can be manufactured as cost-effectively as possible, various different criteria for optimization are available depending on the number of units p’anned.
— In the case of a one-off production, height is the factor that has the greatest impact on build costs. Parts shall be oriented in such a way that the build height is kept to a minimum, provided that the geometry permits such an orientation.
— If the intention is to manufacture a larger number of units, then the build space shall be used as efficiently as possible. Provided that the part geometry permits such orientation, strategies for reorientation and nesting shall he utilized to maximize the available build space.
— The powder that remains in the system after a build can be reused in some cases. Reuse depends on the application, material, and specific requirements. Powder changes can be inefficient and time consuming. Although they are necessary when changing material type, powders from same- material bLlilds can be reused. It is important to note, however, that recycling of powder can affect the powder size distribution, which in turn affects final part characteristics. The number of times a powder can be recycled is dependent on the machine manufacturer and the material.
5.6 Feature constraints (islands, overhang, stair-step effect)
Since AM parts are built up in successive layers. separation of features can occur at some stage of the build. This depends on the part geometry. The situations In 5.6.2 to 5.6.4 shall be regarded as critical (the level of criticality depends on the PBF technology in focus) in this respect.
Islands (I) are features that connect to form a part (P) only at a later stage of the build process. How this connection will occur shall be taken into consideration at the design stage. Parts that are stable in terms of their overall design can be unstable at some stage of the build process (see Figure 2, left and centre).
NOTE In some circumstances, islands are not protected against mechanical damage during the powder application process. This can lead to deformation of the islands.