**2. Barriers and challenges of 3D printing**

There are several challenges associated with manufacturing or scaling up of 3D printing mentioned as follows [17]:

**Personal customization vs mass manufacturing:** Currently AM technology is suitable for customized products, and low-volume production whereas for highvolume production, still injection molding is being industrially used because of the high-cycle time of additive manufacturing. However, there are industrial sections where high-cycle time is balanced by high demand for customized and complex geometry products, reduction in material waste, and opportunity to merge parts, such as GE fuels nozzle and customized earphones by Ownphones.

**Material heterogeneity and structural consistency:** For most of the industrial additive manufacturing processes, single materials are used which do not show any heterogeneousness and structural discrepancy at the interface but with the more advancement in the 3D printing, multiple materials are being used to accomplish more complex geometry of the products. Different materials have different behaviors, properties, and functionalities which leads to anisotropic mechanical properties of 3D-printed products due to the interlayer bonding deficiencies which eventually limits the variety of materials used for additive manufacturing. More research needs to be done for analyzing the uncertain behavior of multiple materials and CAD or computer software needs to be redesigned for the adoption of multiple materials in additive manufacturing.

**Building scalability vs layer resolution:** The two parameters scale of building parts and layer resolution are inversely proportional to each other in 3D printing which means that with the increase in the size or layer thickness of 3D-printed parts, the layer resolution decreases which results in the deprived surface quality because of layer stair-stepping effect. Typically layer resolution of 0.1 mm and layer thickness of 25 mm is used for commercial implications of 3D printing. There has been recent advancement with inkjet printing where the researchers have managed to get the layer thickness as low as 0.012 mm but at the cost of high-building time [18]. However, researchers have managed to produce products from nanoscale to macro to large scale using hybrid AM processes, coating, and post-processing subtractive machining.

**Intellectual Property and AM standardization:** To ensure the consistency of additive manufactured product, there is a certain need for the standardization of AM process, material, machine, and file format. ASTM has started to approve AM material standardization for several processes but there is a long way ahead for AM machine manufacturers and researchers. The open access to the downloadable file has opened a new challenge to protect the intellectual property right of researchers and commercial machine manufacturers. There is a need for planned and diverse patent filing for additive manufacturing processes which should include material, process, CAD design, machining, and post-processing modifications.
