AS ISO ASTM 52911.2:2021 pdf free download – Additive manufacturing -Design Part 2: Laser-based powder bed fusion of polymers

AS ISO ASTM 52911.2:2021 pdf free download – Additive manufacturing -Design Part 2: Laser-based powder bed fusion of polymers.
5 Characteristics of powder bed fusion (PBF) processes
5.1 General
Consideration shall be given to the specific characteristics of the manufacturing process used in order to optimize the design of a part. Examples of the Features of AM processes which need to be taken Into consideration during the design and process planning stages are listed in £2 to 5B.
5.2 SIze of the parts
The size of the parts is limited by the working area/working volume of the PBF-machine. Also, the occurrence of cracks and deformation due to residual stresses limits the maximum part size. Another Important practical factor that limits the maximum part size Is the cost of production having a direct relation to the size and volume of the part. Cost of production can be minimized by choosing part location and build orientation in a way that allows nesting of as many parts as possible. Also, the cost of powder needed to fill the bed to the required volume (part depth bed area) may be a consideration. Powder reuse rules Impact this cost significantly. If no reuse is allowed, then all powder Is scrapped regardless of solidified volume.
5.3 Benefits to be considered in regard to the P1W process
PHF processes can he advantageous for manufacturing parts where the following points are relevant:
— Parts can be manufactured to near-net shape (i.e. close to the finished shape and size), without further post processing tools, in a single process step.
6 Design guidelines for laser-based powder bed fusion of polymers (LB-PBF/P)
6.1 General
The design guidelines in this clause take into account the specific characteristics of LB-PBF/P. In general, the PBF process for polymers Is similar to that for metals as It Includes a thermal source for inducing fusion between the powder particles, a method for limiting powder fusion to a zonal region per layer, as well as mechanisms to add the powder layers. Materials typically used are polyamides (PA 11. PA 12 and their derivatives), although other materials can be processed as well. Some unfused poisders can be recycled in subsequent builds, usually by mixing the recycled powder with virgin powder. In addition, materials can be tilled or mixed with other materials, such as glass and carbon fibres, to Improve strength and thermal, electrical and fire-retardant properties. This clause describes the implications of
— build orientation, positioning and arrangement,
— material properties of fused polymers,
— surface characteristics of Iuscd polymers.
— aspects of post’productlon finishing and
— other design considerationsWl.
6.2 Material and structural characteristics
Different powdered thermoplastics are available for LB-PI3F/P, of which semi-crystalline materials are the most widely used, In polymer PBF, the powder bed is pre-heated and the temperature is maintained a few degrees below the melting temperature of the polymer. The elevated powder bed temperature not only reduces the required energy input From the laser for melting but also prevents the molten polymer from recrystallizing during the build process. Recrystallization during the build process contributes to part shrinkage and warpage. which can lead to a failed build. P1W polymers typically exhibit a melt temperature that is higher than the recrystallization temperature, and the difference defines a processing window that can be exploited by the P1W processlUl. The typically broad softening range of amorphous thermoplastics. on the other hand, impedes this type of process control. Areas exposed to the laser beam solidify rapidly. As a result, the viscous flow associated with fusion and stress relaxation are Impeded, and the parts are characterized by high porosity and low mechanical strength.
Material properties depend on a variety of factors, including the type of polymer, particle size, degreeof powder recycling and processing conditions. In particular, the temperature distribution during thebuild process has a significant effect on material properties. Temperature distribution in the buildplatform is affected by the extent and uniformity of preheating, part density in the build platform, laserenergy density, and the rate of post-build cool down. For these reasons, it is difficult to make blanketstatements about the achievable material structure and properties10.However, large scale studies ofpolyamides have indicated that strengths comparable to injection moulded parts can be achieved withminor variability (approximately 10 %),even for parts oriented orthogonal to the build plane, whereaselongation at break is typically much lower than that for injection moulded parts.