Additive Fabrication, Technology

Accuracy and Precision

Accuracy is the difference between the intended and actual finished dimension. There are accuracy specifications for linear dimensions, flatness and whole sizes. This issue is critical and additive technologies have improved the standards. The process is complex because additive methods are all different, the cutting tool is used as a point of reference, there are multiple operations, a complex chemistry and energy exchanges to consider. Subtractive technology is incorporated using additive technologies. This becomes an important part of the process and increases the absolute accuracy. This is because any compounded errors due to the additive process can be corrected using subtractive technology. This is the reason the method based on an inkjet has a distinctive edge. The parts produced are equal to or near CNC accuracy. The versatility of the materials and the speed are the tradeoffs for other RP methods. The majority of the other methods are within approximately a few millimeters. A qualitative idea can be established by beginning with stereolithography because the results are accurate. Powder and LOM methods have less accuracy. The final number depends on the dimensions measured, the part’s geometry, the material and numerous additional factors.

The Finishes

The appearance and finish of the part are related to the accuracy but are dependent on the additive technology. Powder based technology provides a diffuse or sandy appearance and sheet based methods provide a poorer finish. The resolution is the minimum increments of dimension achieved by the system. This partially determines the accuracy, appearance and finish. Some systems have received modifications to produce a finer feature but have limits regarding the size of the parts fabricated. A second material is generally required to paint, sand, polish, fill or infiltrate to decrease the level of porosity. The minimum feature size is the detailing that can be reproduced. The mathematical definitions are based on a minimum curvature. Non-metallic parts are used directly or after finishing operations. Parts made from three dimensional printing are fragile and may not hold up to the normal stresses of shipping and handling. This is counteracted by adding a wax or cyanoacrylate adhesive for added durability. All additive methods require secondary operations prior to the final tolerance and finishes.

The Speed and Basic Costs

All additive technology is slow. It can take days to build a part. This is still quicker than subtractive methods. Complex parts can require a week of machining. Raster based additive methods are quicker but the fastest is 3D printing. Inkjets only require a couple nozzles to make objects with a high resolution. The ancillary secondary operations must be considered for an accurate view of the speed. Secondary finishing operations and file preparation must also be taken into account. Each variable is an important factor into the time required for each part. Professional systems for additive fabrication cost between $3,000 and $800,000. The less expensive machines have a thermoplastic extrusion base. The more expensive machines specialize in laser sintering and stereolithography. The right machine is dependent on numerous factors including material requirements, application and cost.

Additional Costs Include…

The additional costs include maintenance, housing and training. A laser replacement in a stereolithography system can cost in excess of $20,000. This will be necessary at some point in time. Some technologies have complex operating procedures and the capital cost is high. Some companies find considerable savings by purchasing a used system. This type of purchase is often limited because the license for operating the software is not transferrable. This information can be established through the manufacturer. There are also companies specializing in leasing and selling RP systems that have been previously owned. There are additionally certain manufactures that make refurbished equipment available. A fully assembled hobbyist machine sells for between $500 and $3,000. They are not as accurate, have poor support and warranties and make it difficult to use software. They will work well for rudimentary parts and fit models.