Scanning and printing are sold together because they look like two halves of a copier, and they are not. A scan produces a mesh of the surface as it currently is, including wear, damage and the operator's fingerprints. Turning that into a part worth making is where the work is, and it is a modelling job rather than a scanning one.
The scanning technologies
Structured light projects a pattern and reads its distortion, giving good accuracy on medium sized objects quickly. Laser scanning is similar and handles a wider range of surfaces. Photogrammetry builds geometry from many photographs and is cheap and less accurate. Computed tomography scans the inside as well as the outside, which is the only route for internal geometry.
What a scan actually gives you
A mesh: millions of triangles describing the surface. It has holes where the scanner could not see, noise, and it faithfully records every dent and scratch on the original. It is not a model in any engineering sense: there are no features, no dimensions, no datums, and nothing that can be edited except by moving triangles.
Mesh repair against reverse engineering
Repairing a mesh and printing it copies the object, damage and all. That is right for an artefact, a sculpture or a shape reference. Reverse engineering fits real geometry to the scan, planes, cylinders, fillets and features, producing a parametric solid model that can be dimensioned, toleranced and changed. That is what is needed for a functional part, and it is an engineering task.
Where the workflow breaks
Expecting a scan of a worn part to produce a correct replacement, when what it produces is a copy of a worn part. Scanning shiny, transparent or dark surfaces, which most scanners cannot see without a temporary coating. Assuming internal geometry is captured when only computed tomography sees it. And underestimating how long the modelling step takes, which is nearly always the longest part of the job.
Questions people ask about 3d scanning and printing services
Can I scan a part and print a copy?
You can copy its shape, including wear and damage. Producing a correct functional replacement needs the scan to be reverse engineered into a proper model.
What is the difference between a mesh and a model?
A mesh is millions of triangles describing a surface. A model has features, dimensions and datums, and can be edited and toleranced. Reverse engineering converts one to the other.
Can scanners capture internal geometry?
Only computed tomography does. Optical and laser scanners see the outside surface only, so internal features must be measured or inferred another way.
Why can my shiny part not be scanned?
Optical scanners cannot read specular, transparent or very dark surfaces. A temporary matte coating is the usual solution and it must be allowed for in accuracy.