1. Working Process
3D printing creates physical objects from digital design. The material used is power which is located in a build chamber. A printing head deposits liquid binder on to powder, thus it bonded together and forms layer. The build platform is lowered down an amount equal to the layer created. This process repeated to complete the 3D part. SLS principle completely differs from 3DP and works based on sintering principle. The powder material is heated and bonded together by fusion and forms layer. Heat source for sintering process is generated by laser. A Cad model imported to SLS system and completed layer by layer.
2. Process Variations
3DP also offers Jetted Photopolymer System (JPS) and Thermal Phase Change Inkjet Printing. 3DP has different types of printing head, such as single head and multi head printing devices that used on requirement. It also has an advantage to add sintering stage in the system to bond the powders further. SLS also has DMLS (Direct metal laser sintering). It uses precise and powerful laser to micro-weld the raw material. It eliminates tooling time and creates complex shapes. DMSL is completely automated and uses 200 Watt Yb-fiber optic laser. The materials used in MDSL are Stainless steel, Aluminium, Cobalt and Nickel.
3. Processing materials
3DP used powder form of part material and liquid form of binder material. The part material used is bronze, ceramics, stainless steel, moulding sand, plaster and starch. The binder materials are wax, epoxy resin, elatomers and polyurethane. 3DP has limitations in the material selection when compared to SLS printing method. SLS works with variety of materials like plastics, glass, ceramics and metals. Normally aluminium-filled and glass filled Nylon 12 used. Common types of Nylon materials used are PA6, PA66 and PA11. Light weight, high durability, heat resistant and chemical resistant properties makes nylon as an ideal material for SLS products.
4. Design aspects
In SLS, powder materials which are not bonded together can act as support medium for overhanging or undercut features which means there is no need to provide additional support. The typical maximum size of part is 600 x 500 x 400 mm. The tensile strength of parts are depends on material type. SLS is better than 3DP. Very thinner layer / details can be made and has the ability to produce complex features, undercuts which allows engineers to incorporate multiple parts into single component. This eliminates the need of separate fasteners, mounting components which results substantial weight reduction.
5. Quality aspects
3DP parts are fragile direct from the process and can add multiple colours to the model. The tolerances are little higher (varies from ±0.1 to 0.5mm) than other printing methods, that means 3DP is not suitable where accurate fits are required, and the typical roughness is 60 µm Ra. SLS parts offer good design accuracy, water / air tight, heat resistant, good chemical resilience and mechanical stability. Finished parts surfaces are rough and requires painting and coating. The raw material is powder; hence it needs frequent cleaning. Polymer part is thermal distortion which can cause shrinking and wrapping of fabricated parts.
6. Economical aspects
3DP even has fast build speed; it requires finishing which adds cost to parts. However the excess powder material can be reused. When compare to SLS, material usage is high in 3DP; however the material cost and direct labour cost also low. 3DP offers very low initial investment cost and requires skilled labours. Mainly SLS mainly offers low material cost in many applications. This method is suitable for small and precise parts, because of its lower speed. The raw material used here cannot be recycled and requires frequent cleaning. The finished part is rough and requires some post manufacturing activities.