2.5. Capable Tolerance Specification for PolyJet Technology

Tolerance specification (tolerance, lower and upper limits) for the dimensions of the 3D printed circular part was chosen based on the general tolerances standards [34] and plastics molded parts tolerances [33]. A tolerance of ±0.1 mm was selected. Based on this specification, the AM system and process capability for PolyJet technology was calculated. The capability indices were compared with a capability target index of 1.67.

Rather than estimating the process capability for a particular tolerance, a capable tolerance and its limit deviations were calculated based on a target capability index. The target capability index was set to 1.67. The index of the process K was calculated using Equation (2) and describes the level by which the process is off target value and represents an appropriate measure of process centering [37,50]. The lower (LSL_T) and upper (USL_T) specification limits of the capable tolerance were calculated based on Equation (3). Upper limit deviation (ULD) and lower limit deviation (LLD) from nominal size were then determined based on Equation (3).

The process mean is positioned between the midpoint of the specifications and one of the required limits if 0< |K|<1. |K|>1 indicates that the process mean is situated outside the required limits.

The capable lower limit deviation and capable upper limit deviation were determined based on the relations LLD_C < LLD_T and ULD_C >ULD_T. The capable tolerance is calculated as follows: T_c = ULD_C – LLD_C. A confirmatory analysis of AM process capability was performed using the determined capable tolerance.