Plastic parts will never be the 100% same as the CAD design, since part geometry, material characteristics (raw material expands and contracts according to thermal difference), mold construction (CNC machining), processing variability…etc. Therefore, to indicate the area of the part that shall follow the CAD design and the allowed difference is very important. The allowed difference usually relates to dimension, and it is aka tolerance.

During the part design phase, designer shall define the tolerance according to

• Standard specification (such as DIN ISO 20457…etc.)
  It discusses the tolerance from perspective including direction, graft angle, dimension and radius, geometric, raw material…etc. and gives instruction for designer as a clear reference. There is not only this ISO discuss about the same, designers could pick suitable instruction for their own product.
• Tolerance analysis of final product
  Rather than dividing the allowable variability equally over the various features that govern fit and function, allot a greater portion of the total tolerance range to features that are difficult to control. Reserve tight tolerances for features that can accommodate them reasonably.
• Molding knowledge
  such as minimum radius is approximately 0.1~0.2mm, necessary draft angle…etc.
• specify tolerances only when needed, specify tolerance tolerances only when really needed.

Furthermore, here are some suggestions for designers to build parts with more feasibility of mass production

• Use low-shrinkage materials in parts with tight tolerances.
• Avoid tight tolerances in dimensions affected by the alignment of the mold halves or moving mold components such as slides.
• Design parts and assemblies to avoid tight tolerances in areas prone to warpage or distortion.
• Adjust the mold to produce dimensions in the middle of tolerance range at optimum processing conditions for the material.

Designer shall give reasonable tolerance at reasonable location, Development team could also suggest the same when metrology, function test…etc. activity during development. To avoid unnecessary molding costs, as wide as possible is preferred by mass production department. The Manufacturer shall understand its own product and organize its team to build the design with most suitable tolerance(profit).