Rapid prototyping
Collective term for modern generative techniques used for incremental manufacture of objects from amorphous raw materials (powders, liquids) or standardised shapes (e.g. milling blocks) using computer-controlled design and manufacture (CAD-CAM) based on three-dimensional datasets (e.g. in STL format).
Originally they were mainly used according to the name for cost-effective fabrication of prototypes and small batch production for testing before completion and/or mass production. Using follow-up techniques (e.g. impression or casting) lost-wax or permanent moulds can be produced in which workpieces are fabricated from other materials.
There is a (decreasing) difference to rapid manufacturing, which is mainly used for the quick production of customised units (e.g. for customised dental laboratory restorations).
Rapid prototyping machines (called "digital fabricators" or "fabbers") are becoming increasingly more efficient, inexpensive and precise. They are now affordable for private users. In the professional sector they are also becoming an increasing part of the digital workflow in dentistry and dental technology.
Basically, there is a difference between subtractive techniques in which the workpiece is carved from a block of material using machine tools (e.g. CNC milling), generally involving a high loss of material, and additive techniques ("3-D printing") in which minute material portions (often resin, plastic or metal) are continuously added to fabricate the virtually designed end product physically. It is now possible to process two or more materials and combine them into one workpiece.
Stereolithography and [MJM] multi jet modelling (also poly jet modelling)
Incremental polymerisation of liquids, light-curing monomers using UV or laser exposure. The workpiece is created by lowering the section already fabricated in the resin/plastic material by a defined slice thickness (usually approx. 100 µm). Undercuts can only be produced using support frameworks, either using the same polymer (must be cut out) or using another material, e.g. thermoplastic wax or plastic (can be melted out). The frameworks must be removed after fabrication of the workpiece. The typical accuracy is about 100 µm to 10 µm. In micro stereolithography (also "RMPD" Rapid Micro Product Development), which is used for the manufacture of microchips for example, slice thicknesses and accuracies of up to 1 µm can be achieved; the possible workpiece diameter is, however, limited to a few centimetres.
Laser sintering, (also selective laser sintering, SLS)
Materials in powder form (plastic, metal or ceramic) are melted/ fused by exposure to a laser beam and gradually built up in thin slices to a solid workpiece. Undercuts are possible, no support framework is required. Rapid Tooling, which is used for the fabrication of metal shapes and tools for processing plastics for small and medium batches/ quantities, is also possible using metal laser sintering.
The following variations of the procedure are differentiated thus:
- Material particles are only partially melted and "baked" together (metals with binders/ sintering additives and plastics).
- Powder particles are completely melted (selective laser melting, SLM) and fuse into one another (metals without binding agents).
- Using laser micro-sintering, e.g. ceramic nanoparticles (diameters of about one µm) can be formed into microworkpieces.
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Composites also composite (from the Latin componere = to compose) are tooth-coloured filling materials with plastic properties used in dental treatment. In lay terms they are often referred to as plastic fillings, also erroneously sometimes confused with ceramic… Composites also composite (from the Latin componere = to compose) are tooth-coloured filling materials with plastic properties used in dental treatment. In lay terms they are often referred to as plastic fillings, also erroneously sometimes confused with ceramic fillings due to their tooth colour. After being placed in a cavity they cure chemically or by irradiating with light or a combination of the two (dual-curing). Nowadays, composites are also used as luting materials. The working time can be regulated with light-curing systems, which is a great advantage both when placing fillings and during adhesive luting of restorations. Dual-curing luting materials are paste/paste systems with chemical and photosensitive initiators, which enable adequate curing, even in areas in which light curing is not guaranteed or controllable. Composites were manufactured in 1962 by mixing dimethacrylate (epoxy resin and methacrylic acid) with silanized quartz powder (Bowen 1963). Due to their characteristics (aesthetics and advantages of the adhesive technique) composite restorations are now used instead of amalgam fillings.
The material consists of three constituents: the resin matrix (organic component), the fillers (inorganic component) and the composite phase. The resin matrix mainly consists of Bis-GMA (bisphenol-A-glycidyldimethacrylate). As Bis-GMA is highly viscous, it is mixed in a different composition with shorter-chain monomers such as, e.g. TEGDMA (triethylene glycol dimethacrylate). The lower the proportion of Bis-GMA and the higher the proportion of TEGDMA, the higher the polymerisation shrinkage (Gonçalves et al. 2008). The use of Bis-GMA with TEGDMA increases the tensile strength but reduces the flexural strength (Asmussen & Peutzfeldt 1998). Monomers can be released from the filling material. Longer light-curing results in a better conversion rate (linking of the individual monomers) and therefore to reduced monomer release (Sideriou & Achilias 2005) The fillers are made of quartz, ceramic and/ or silicon dioxide. An increase in the amount of filler materials results in decreases in polymerisation shrinkage, coefficient of linear expansion and water absorption. In contrast, with an increase in the filler proportion there is a general rise in the compressive and tensile strengths, modulus of elasticity and wear resistance (Kim et al. 2002). The filler content in a composite is also determined by the shape of the fillers.
Minimally-invasive preparation and indiscernible composite restoration
Composite restorations Conclusion |