In some designs, the external surfaces, are defined by another company (the customer or an external designer), another department of the same company, sometimes they come from scanned surfaces, etc.
It is important to check the surface quality before to start to design the plastic part. If this job is done after the part is designed and the original surfaces need to be changed, part of the work will be wasted. So before to start, to check.
Most of CAD software have different tools to analyze the surfaces, zebra analysis, curvature analysis, etc. Apart of this useful tools, it is important to look at the surfaces, their boundaries and isoparmetric curves. Sometimes it is not necessary to use any tool to see that the surfaces are not correct. See the attached picture. Is really necessary to use the zebra analysis to see that the surfaces need to be redone? So before to use special tools, take a look to the file and use the common sense.
An external surface with a lot of patches, small patches, strange shaped patches and isoparametric curves with strange shapes and not continuous through the boundary surfaces is a very bad sign.
To correct the surfaces is not an easy job, the easiest way is to give CAD file to a surf expert to solve the problems using a free form design software.
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